bacteria in agrobiology: crop ecosystemsthe book entitled bacteria in agrobiology: crop ecosystems...

Bacteria in Agrobiology: Crop Ecosystems

Dinesh K. MaheshwariEditor

Bacteria in Agrobiology:Crop Ecosystems

EditorProf.(Dr.) Dinesh K. MaheshwariGurukul Kangri UniversityDeptt. of Botany and Microbiology249404 Haridwar (Uttarakhand)[email protected]

ISBN 978-3-642-18356-0 e-ISBN 978-3-642-18357-7DOI 10.1007/978-3-642-18357-7Springer Heidelberg Dordrecht London New York

Library of Congress Control Number: 2011926231

# Springer-Verlag Berlin Heidelberg 2011This work is subject to copyright. All rights are reserved, whether the whole or part of the material isconcerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting,reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publicationor parts thereof is permitted only under the provisions of the German Copyright Law of September 9,1965, in its current version, and permission for use must always be obtained from Springer. Violationsare liable to prosecution under the German Copyright Law.The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply,even in the absence of a specific statement, that such names are exempt from the relevant protectivelaws and regulations and therefore free for general use.

Cover illustration: Optical micrograph showing cross sections of intercellular colonization rice calli andregenerated plantlets byA. caulinodans: CS view of root uninoculated control; magnified cross section viewof leaf colonized by A. caulinodans in regenerated rice plant; possible sites of infection and colonization ofrice root (from left to right); see also Fig. 3.1 in “Endophytic Bacteria – Perspectives and Applications inAgricultural Crop Production”, Senthilkumar M, R. Anandham, M. Madhaiyan, V. Venkateswaran, TongMin Sa, in “Bacteria in Agrobiology: Crop Ecosystems, Dinesh K. Maheshwari (Ed.)”

Background: Positive immunofluorescence micrograph showing reaction between cells of the rhizobialbiofertilizer strain E11 and specific anti-E11 antiserum prepared for autecological biogeography studies;see also Fig. 10.6 in “Beneficial Endophytic Rhizobia as Biofertilizer Inoculants for Rice and the SpatialEcology of this Bacteria-Plant Association”, Youssef Garas Yanni, Frank B. Dazzo, Mohamed I. Zidan.in “Bacteria in Agrobiology: Crop Ecosystems, Dinesh K. Maheshwari (Ed.)”

Cover design: deblik, Berlin

Printed on acid-free paper

Springer is part of Springer Science+Business Media (www.springer.com)

Preface

Bacteria are among the most adaptable organisms. Their evolutional passage across

the long timescale, extremely short generation time, and aptitude to adapt to diverse

and often hostile environments, combined with the remarkable power of natural

selection have made these microorganisms the most resilient of life forms on this

planet. As such, bacteria and fungi abound in the soil are the essential contributors

in maintaining the ecological balance.

One of the most remarkable developments of the twentieth century vis-à-vis

microorganisms is the discovery of the plant growth promoting bacteria (PGPB)

that offers a vast array of beneficial attributes to plants, and thereby facilitating

enhancement of crop productivity in a sustainable manner. More than 97% of our

food requirements are realized from terrestrial ecosystems through agricultural

productivity. Diversified populations of bacterial species occur in agricultural fields

and contribute to crop productivity directly or indirectly. Plants provide a substan-

tial ecological niche for microorganisms and below ground (roots) portions of

plants and soil are constantly associated with a larger number of microorganisms

reaping several benefits from such associations. This volume is accordingly con-

ceived to provide consolidated information on the subject.

The book entitled Bacteria in Agrobiology: Crop Ecosystems has chapters thatcover studies on various aspects of bacteria–plant interactions. Better understand-

ings of the challenges in development of PGPB as efficient commercial bioinocu-

lant have met in enhancing crop production. A large number of bacterial genera

interplay with rhizosphere communities in different crops ecosystems, in particular,

the oil-yielding crops, cereals, fruits and vegetables, forest trees, etc. Keeping in

fitness with such important crops, the developmental challenges faced in the

management of growth and soil and seed borne diseases associated with food

crops such as rice, sesame, peanut, along with horticultural, sericultural plant

ecosystems as well as in forestry are aptly covered in this volume. Detection of

PGPR and biocontrol of postharvest pathogens as suitable alternatives to agro-

chemicals for sustainable crop production and protection, and restoration of de-

graded soils has also been duly addressed. I believe that this book will be useful not

v

only for researchers, teachers, and students, but also for those who are interested in

the subjects of applied microbiology, plant protection, ecology, environmental

science, and agronomy.

I would like to express my gratitude to all the authors for their scholarly

contributions. I recognize with credit the continuous support that I received from

my research students Mr. Abhinav Aeron, Mr. Rajat Khillon, Mr. Pankaj Kumar,

and Dr. Sandeep Kumar in the preparation of this volume. I am also thankful to

Council of Scientific and Industrial Research (CSIR), New Delhi; and Director,

Uttarakhand Council of Science and Technology (UCOST), Dehradun, India for

their support in implementation of my research projects on PGPB that served as a

prolog to arrange base for compilation of this book. I extend my earnest apprecia-

tion to Dr. Jutta Lindenborn of Springer for her valuable support to facilitate

completion of the task.

Haridwar, Uttarakhand, India Dinesh K. Maheshwari

vi Preface

Contents

1 Emerging Role of Plant Growth Promoting Rhizobacteriain Agrobiology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Abhinav Aeron, Sandeep Kumar, Piyush Pandey, and D.K. Maheshwari

2 Bacillus as PGPR in Crop Ecosystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Ankit Kumar, Anil Prakash, and B.N. Johri

3 Endophytic Bacteria: Perspectives and Applicationsin Agricultural Crop Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61M. Senthilkumar, R. Anandham, M. Madhaiyan

V. Venkateswaran, and Tongmin Sa

4 PGPR Interplay with Rhizosphere Communities and Effecton Plant Growth and Health . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97Gabriele Berg and Christin Zachow

5 Impact of Spatial Heterogeneity within Spermosphere andRhizosphere Environments on Performance of BacterialBiological Control Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111Daniel P. Roberts and Donald Y. Kobayashi

6 Biocontrol Mechanisms Employed by PGPR and Strategiesof Microbial Antagonists in Disease Control on the PostharvestEnvironment of Fruits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131Anjani M. Karunaratne

7 Plant Growth-Promoting Bacteria Associated with Sugarcane . . . . . 165Samina Mehnaz

vii

8 Use of Plant Growth Promoting Rhizobacteriain Horticultural Crops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189Ahmet Esitken

9 Commercial Potential of Microbial Inoculants for Sheath BlightManagement and Yield Enhancement of Rice . . . . . . . . . . . . . . . . . . . . . . . . 237K. Vijay Krishna Kumar, M.S. Reddy, J.W. Kloepper, K.S. Lawrence

X.G. Zhou, D.E. Groth, S. Zhang, R. Sudhakara Rao, Qi Wang

M.R.B. Raju, S. Krishnam Raju, W.G. Dilantha Fernando, H. Sudini

B. Du, and M.E. Miller

10 Beneficial Endophytic Rhizobia as Biofertilizer Inoculantsfor Rice and the Spatial Ecology of This Bacteria–PlantAssociation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265Y.G. Yanni, F.B. Dazzo, and M.I. Zidan

11 Plant Growth-Promoting Bacteria: Fundamentalsand Exploitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295Clara Pliego, Faina Kamilova, and Ben Lugtenberg

12 PGPR in Coniferous Trees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345Elke Jurandy Bran Nogueira Cardoso, Rafael Leandro de Figueiredo

Vasconcellos, Carlos Marcelo Ribeiro, and Marina Yumi Horta Miyauchi

13 Perspectives of PGPR in Agri-Ecosystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361Meenu Saraf, Shalini Rajkumar, and Tithi Saha

14 Ecofriendly Management of Charcoal Rot and FusariumWilt Diseases in Sesame (Sesamum indicum L.) . . . . . . . . . . . . . . . . . . . . . . 387Sandeep Kumar, Abhinav Aeron, Piyush Pandey, and Dinesh Kumar

Maheshwari

15 Crop Health Improvement with Groundnut Associated Bacteria . . 407Swarnalee Dutta, Manjeet Kaur, and Appa Rao Podile

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431

viii Contents

Contributors

Abhinav Aeron Department of Botany andMicrobiology, Faculty of Life Sciences,Gurukul Kangri University, Haridwar 249404, Uttarakhand, India, abhinavaeron@

gmail.com

Rangasamy Anandham Department of Agricultural Microbiology, AgriculturalCollege and Research Institute, Tamil Nadu Agricultural University, Madurai

625104, Tamil Nadu, India, [email protected]

Gabriele Berg Environmental Biotechnology, Graz University of Technology,Petersgasse 12, 8010 Graz, Austria, [email protected]

Elke Jurandy Bran Nogueira Cardoso Soil Microbiology Laboratory, Depart-ment of Soil Science, Luiz de Queiroz College of Agriculture, São Paulo State

University, Piracicaba São Paulo, Brazil, [email protected]

Frank B. Dazzo Department of Microbiology and Molecular Genetics, MichiganState University, East Lansing, MI 48824, USA, [email protected]

B. Du Department of Microbiology, Shandong Agricultural University, TaianShandong Province, China

Swarnalee Dutta Department of Plant Sciences, School of Life Sciences, Univer-sity of Hyderabad, Hyderabad 500046, India

Ahmet Esitken Department of Horticulture, Faculty of Agriculture, Ataturk Uni-versity, 25240 Erzurum, Turkey, [email protected]

W.G. Dilantha Fernando Department of Plant Science, University of Manitoba,Winnipeg, MB, Canada, [email protected]

ix

D.E. Groth LSU AgCenter, Rice Research Station, Baton Rouge, LA, USA

Bhavdish N. Johri Department of Biotechnology and Bioinformatics Centre,Barkatullah University, Bhopal 462026, Madhya Pradesh, India, bhavdishnjohri@

rediffmail.com

Faina Kamilova Koppert Biological Systems, Veilingweg 14, PO Box 155, 2650AD Berkel en Rodenrijs, The Netherlands, [email protected]

Anjani M. Karunaratne Department of Botany, Faculty of Science, University ofPeradeniya, Peradeniya, Sri Lanka, [email protected]

Manjeet Kaur Department of Plant Sciences, School of Life Sciences, Universityof Hyderabad, Hyderabad 500046, India

Joseph W. Kloepper Department of Entomology and Plant Pathology, AuburnUniversity, Auburn, AL, USA

Donald Y. Kobayashi Sustainable Agricultural Systems Laboratory, Henry A.Wallace Beltsville Agricultural Research Center, USDA-ARS, Beltsville, MD

20701, USA; Department of Plant Biology and Pathology, Rutgers University,

New Brunswick, NJ 08901, USA

Sandeep Kumar Department of Botany and Microbiology, Faculty of LifeSciences, Gurukul Kangri University, Haridwar 249404, Uttarakhand, India,

[email protected]

K. Vijay Krishna Kumar Department of Entomology and Plant Pathology, Au-burn University, Auburn, AL, USA; Acharya N G Ranga Agricultural University,

Hyderabad, India

Ankit Kumar Department of Biotechnology and Bioinformatics Centre, Barkatul-lah University, Bhopal 462026, Madhya Pradesh, India, [email protected]

K.S. Lawrence Department of Entomology and Plant Pathology, Auburn Univer-sity, Auburn, AL, USA

Ben J.J. Lugtenberg Sylvius Laboratory, Institute of Biology, Leiden University,Sylviusweg 72, PO Box 9505, 2300 RA Leiden, The Netherlands, Ben.Lugtenberg

@gmail.com

Munuswamy Madhaiyan Department of Agricultural Chemistry, College ofAgriculture, Life and Environment Sciences, Chungbuk National University,

Cheongju, Chungbuk, Republic of Korea

x Contributors

Samina Mehnaz Department of Microbiology and Molecular Genetics, Universityof the Punjab, Quaid-i-Azam Campus, Lahore 54590, Pakistan; Institute of Pharma-

ceutical Biology, Bonn University, Bonn 53115, Germany, [email protected].

pk, [email protected]

M.E. Miller Department of Biological Sciences, Auburn University, Auburn, AL,USA

Tongmin Sa Department of Agricultural Chemistry, College of Agriculture, Lifeand Environment Sciences, Chungbuk National University, Cheongju, Chungbuk,

Republic of Korea, [email protected]

Marina Yumi Horta Miyauchi Soil Microbiology Laboratory, Department of SoilScience, Luiz de Queiroz College of Agriculture, São Paulo State University,

Piracicaba, São Paulo, Brazil

Senthilkumar Murugesan Department of Agricultural Microbiology, Tamil NaduAgricultural University, Coimbatore 641003, Tamil Nadu, India, senthiltnj@

rediffmail.com

Piyush Pandey Department of Biotechnology, S. B. S. P. G. Institute of Biomedi-cal Sciences and Research, Balawala, Dehradun 248161, Uttarakhand, India

Clara Pliego Instituto de Hortofruticultura Subtropical y Mediterrnea “La Mayora”,Universidad de Mlaga – Consejo Superior de Investigaciones Cientı́ficas (IHSM-

UMA-CSIC), Área de Genética, Universidad de Mlaga, Campus de Teatinos s/n,

29071Mlaga, Spain;Division ofBiology,Department of Life Science, Imperial College

London, Imperial College Road, SW7 2AZ London, UK, [email protected],

[email protected]

Appa Rao Podile Department of Plant Sciences, School of Life Sciences, Universityof Hyderabad, Hyderabad 500046, India, [email protected], apparaopodile@

yahoo.com

Anil Prakash Department of Biotechnology and Bioinformatics Centre, BarkatullahUniversity, Bhopal 462026, Madhya Pradesh, India, [email protected]

Shalini Rajkumar Institute of Science, Nirma University, S. G. Highway,Ahmedabad 382481, Gujarat, India

M.R.B. Raju Andhra Pradesh Rice Research Institute, Maruteru, India

S. Krishnam Raju Andhra Pradesh Rice Research Institute, Maruteru, India

Contributors xi

R. Sudhakara Rao Acharya N G Ranga Agricultural University, Hyderabad, India

M. Sudhakara Reddy Department of Entomology and Plant Pathology, AuburnUniversity, Auburn, AL, USA, [email protected]

Carlos Marcelo Ribeiro Soil Microbiology Laboratory, Department of SoilScience, Luiz de Queiroz College of Agriculture, São Paulo State University,

Piracicaba, São Paulo, Brazil

Daniel P. Roberts Sustainable Agricultural Systems Laboratory, Henry A. WallaceBeltsville Agricultural Research Center, USDA-ARS, Beltsville MD 20701, USA;

Department of Plant Biology and Pathology, Rutgers University, New Brunswick,

NJ 08901, USA, [email protected]

Tithi Saha Institute of Science, Nirma University, S. G. Highway, Ahmedabad382481, Gujarat, India

Meenu Saraf Department of Microbiology, Gujarat University, Ahmedabad380009, Gujarat, India, [email protected]

H. Sudini International Crops Research Institute for the Semi-Arid Tropics (ICRI-SAT), Hyderabad, India

Rafael Leandro de Figueiredo Vasconcellos Soil Microbiology Laboratory,Department of Soil Science, Luiz de Queiroz College of Agriculture, São Paulo

State University, Piracicaba, São Paulo, Brazil

V. Venkateswaran Ministry of Food Processing Industries, New Delhi 110049,India

Qi Wang China Agricultural University, Beijing, China

Youssef Garas Yanni Department of Microbiology, Sakha Agricultural ResearchStation, Kafr El-Sheikh 33717, Egypt, [email protected]

Christin Zachow Environmental Biotechnology, Graz University of Technology,Petersgasse 12, A-8010 Graz, Austria, [email protected]

Shouan Zhang Tropical REC, University of Florida, Homestead FL, USA,[email protected]

Mohamed I. Zidan Department of Plant Nutrition, Sakha Agricultural ResearchStation, Kafr El-Sheikh 33717, Egypt

xii Contributors

Chapter 1

Emerging Role of Plant Growth PromotingRhizobacteria in Agrobiology

Abhinav Aeron, Sandeep Kumar, Piyush Pandey, and D.K. Maheshwari

1.1 Introduction

Declining crop productivity due to unsuitable agricultural practices over the years

and a galloping rate of population growth have both put up a severe strain on the

food supply situation in the world. To meet the food requirements of the growing

population, a second green revolution has become imperative due to “loss of

dynamism” in agriculture as pointed out in global economic survey during the

year 2007–2008. This has two obvious objectives, firstly to rejuvenate the agricul-

tural sector and secondly to improve the income of those dependent on it. Pertaining

to massive population pressure, increase in food grain production is an uphill task in

today’s world. The need of the day is sustainable agriculture without harming

the delicate balance of soil ecology as well as unlocking the mystery of biota

influencing plant growth by using plant growth promoting rhizobacteria (PGPR).

PGPR are nowadays applied in a wide array of agro and allied industries in

the form of inoculants in a range of agro-economically important plants including

leguminous and nonleguminous crops, trees and plants of forest, horticulture,

sericulture, medicinal, fodder, oilseed, and cash crops for enhancing their growth

and productivity.

Green revolution was achieved as it resulted in increased yield due to extensive

use of chemical based components. The indiscriminate use of these components

imparted pesticide resistance in pests and made presence in plant produce. Presence

of residual pesticides cause disruption and degradation of agro-ecosystem resulting

in decreased soil fertility. Excessive application of fertilizer for obtaining higher

production was not only undesirable from the economic point of view, but also

A. Aeron, S. Kumar, and D.K. Maheshwari (*)Department of Botany and Microbiology, Gurukul Kangri University, Haridwar 249 404,

Uttarakhand, India

e-mail: [email protected]

P. Pandey

Department of Biotechnology, S. B. S. P. G. Institute of Biomedical Sciences and Research,

Balawala, Dehradun 248 161, Uttarakhand, India

D.K. Maheshwari (ed.), Bacteria in Agrobiology: Crop Ecosystems,DOI 10.1007/978-3-642-18357-7_1, # Springer-Verlag Berlin Heidelberg 2011

1

exerted an adverse effect on the environment and crop quality (Kenny 1982). It led

to nutrient imbalance, whereas inefficient and overuse of chemical fertilizers

resulted in considerable economic loss to the farmers (Ayala and Rao 2002). It is

widely believed that agrochemicals including chemical fertilizers reduce the popu-

lation of beneficial microorganisms thus having an all-embracing effect (Smiley

1981). Reduction in the population of desirable beneficial microbes alters the

topology of top soil and reduces the productivity of fertile soils. Thus, an important

factor in this respect is to maintain the enhancement of soil fertility through

appropriate sustainable technology, which should be achieved to replenish the

nutrients so as to build up the nutrient status of soils (Hera 1996). The challenges

of meeting the food requirements of the burgeoning population and plateauing

productivity of agricultural lands can only be met by a second green revolution or

ever green revolution. Some of the strategies that can be channeled to second green

revolution include micro-irrigation system, organic farming, precision farming,

green agriculture, eco-agriculture, white agriculture, straw revolution, and use of

PGPR and their combinations. The aim of this chapter is to elaborate the need of

PGPR applications in agriculture-based industries for economic development in an

eco-friendly manner.

1.2 Soil and Rhizosphere in Sustainable Agriculture

Agricultural industries are mainly soil based because they extract nutrients from the

soil. Effective and efficient approaches to slowing the removal and returning

nutrients to the soil is required in order to maintain and increase crop productivity

apart from efforts to sustain agriculture for the long term. The overall strategy for

increasing crop yields and sustaining them at high level required natural or artificial

inputs. The soil is managed by both biological and nonbiological factors known to

have a major impact on plant growth, soil fertility, and agricultural sustainability.

The physical, biological, and chemical characteristics of soil, such as organic

matter content, pH, texture, depth, and water-retention capacity, are factors that

influence soil fertility. A soil’s potential for producing crops is largely determined

by the environment that soil provides for root growth, such as nutrients and the

surrounding microflora that may be beneficial or deleterious. Roots need air, water,

nutrients, and adequate space to develop. Soil quality is defined by capacity to store

water, acidity, depth, and density that determine how well roots developed.

Changes in soil quality affect the health and productivity of the plants and can

lead to lower yields and/or higher costs of production. Organic matter content is

important for the proper management of soil fertility and helps growth by improv-

ing water-holding capacity and drought resistance. Moreover, it permits better

aeration, enhances the absorption and release of nutrients, and makes the soil less

susceptible to leaching and erosion.

The higher plant root system significantly contributes to the establishment of

the microbial population in the rhizosphere. The rhizosphere has attractedmuch interest

2 A. Aeron et al.

as it is a habitat of several biologically important processes and their interactions.

Acidification of the rhizosphere as a result of exudation of organic acids from root

plays a pivotal role in determining the surrounding population (Dakora and Philips

2002). The rhizosphere is populated by diverse microorganisms including bacteria,

fungi, actinomycetes, protozoa, algae, etc.; therefore, modifying plant root systems is

considered as a means of crop improvement targeted toward low-resource environ-

ments, particularly low nutrient and drought-prone agriculture.Microbial processes and

properties in the rhizosphere are crucial to support functional agriculture.

1.3 Beneficial Bacteria

The microbe–plant interaction in the rhizosphere is dynamic and complicated.

Some microbes contribute to plant health by mobilizing nutrients, while some are

detrimental to plant health as they compete with the plant for nutrients or cause

disease and some stimulate plant growth by producing hormones or by suppressing

pathogens. The bacteria useful to plants are characterized into two general types:

bacteria forming a symbiotic relationship with the plant and the free-living ones

found in the soil but are often found near, on, or even within the plant tissues

(Kloepper et al. 1988a; Frommel et al. 1991).

Different authors have found different origins with the classification and defini-

tion of beneficial rhizobacteria. Beneficial free-living soil bacteria that enhance

plant growth are usually referred to as “plant growth promoting rhizobacteria”

(Kloepper et al. 1989) or yield increasing bacteria (YIB) (Tang 1994). PGPRoriginally defined (Kloepper and Schorth 1978) as root-colonizing bacteria (rhizo-

bacteria) cause either growth promotion or biological control of plant diseases.

Bashan and Holguin (1998) proposed that the PGPR can be categorized as biocon-

trol-plant growth promoting bacteria (PGPB) and phytostimulating PGPB.

Root-associated bacteria have a great influence on organic matter decomposition

which in turn is reflected in soil nutrient availability for plant growth (Glick et al.

1994). The phosphorus- and potassium-solubilizing bacteria (PSB) may enhance

plant nutrient availability by dissolving insoluble phosphorus and releasing potas-

sium from silicate minerals (Goldstein and Liu 1987). PGPB often help increase

root surface area to increase nutrient uptake and in turn enhance plant production

(Mantelin and Touraine 2004). The premier example of PGPR agents occur in

many genera including Actinoplanes, Agrobacterium, Alcaligens, Amorphospor-angium, Arthrobacter, Azospirillum, Azotobacter, Bacillus, Burkholderia,Cellulomonas, Enterobacter, Erwinia, Flavobacterium, Gluconacetobacter,Microbacterium, Micromonospora, Pseudomonas, Rhizobia, Serratia, Strepto-myces, Xanthomonas, etc., as stated by several workers (Kloepper et al. 1989;Tang 1994; Okon and Labandera-Gonzalez 1994; Glick et al. 1999; Mayak et al.2001; Lucy et al. 2004; Tahmatsidou et al. 2006; Aslantas et al. 2007; Lee et al.2008; Pedraza et al. 2010).

1 Emerging Role of Plant Growth Promoting Rhizobacteria in Agrobiology 3

One of the dominant genera among PGPR is Pseudomonas spp. reported in thebiological control of different phytopathogenic fungal species such as Rhizoctonia,Fusarium, Sclerotonia, Pythium, Erwinia, Macrophomina, etc. (Defago et al. 1990;Gupta et al. 2001a; Garbeva et al. 2004; Validov et al. 2005). Interestingly, certainrhizobia have also been noticed for the biological control of M. phaseolina (Aroraet al. 2001; Deshwal et al. 2003), F. oxysporum, F. solani, R. solani, Pythium spp.,etc. (Chao 1990). Rhizobia had been reported to produce several secondary metabo-

lites for biocontrol activity, similar to fluorescent pseudomonads in their mode of

action. Pseudomonads confer active role in biocontrol and yield promotion of plants,

and therefore are the most widely used genera among PGPR (Gupta et al. 2001a;Kumar et al. 2005a, b, c). Among free living bacteria, various species of Azotobacterhave been reported for the biological control of different phytopathogens such as

Alternaria,Helminthosporium, Fusarium, etc., under in vitro conditions (Laxmikumariet al. 1975; Joshi et al. 2006a). Free nitrogen-fixing bacteria were probably the first

rhizobacteria used to promote plant growth. Other bacterial genera capable of nitrogen

fixation that may be responsible for growth promotion effect are Azoarcus sp.,Burkholderia sp.,Gluconacetobacter diazotrophicus,Herbaspirillum sp., Azotobactersp., and Paenibacillus polymyxa. These genera have been isolated from a number ofplant species such as rice, sugarcane, corn, sorghum, other cereals, pineapple, and

coffee bean (Vessey 2003). Azoarcus has recently gained attention due to its greatgenetic and metabolic diversity. Because of their competitive advantages in a carbon-

rich, nitrogen-poor environment, diazotrophs become selectively enriched in the

rhizosphere (Reinhold-Hurek and Hurek 2000). Azotobacter spp. is also being appliedas bioinoculant due to its several direct PGP activities including asymbiotic nitrogen

fixation, phosphate solubilization, growth hormones production, and vitamins produc-

tion (Shende et al. 1977). The first reports on Azotobacter appeared in 1902 and it waswidely used in Eastern Europe during themiddle decades of the last century (González

and Lluch 1992). As previously suggested, the effect of Azotobacter and Azospirillumis attributed not only to the amounts of fixed nitrogen but also to the production of plant

growth regulators such as indole acetic acid (IAA), gibberellic acid, cytokinins, and

vitamins (Rodelas et al. 1999). Azotobacter is also known to produce antifungalcompound that inhibits the production of conidia of Botrytis cinerea (Doneche andMarcantoni 1992).

Similarly, Azospirillum is also known to secrete phytohormones, induce root celldifferentiation, and increase water uptake. Azospirillum associates with polysaccha-ride degrading bacteria (PDB) in rhizosphere, establishing a metabolic association

(Bashan and Holguin 1997). The sugar-degrading bacteria produce degradation and

fermentation products that are used by Azospirillum as a carbon source that in turnprovides PDB with nitrogen. In fact, here the symbiosis is extended to multiple

prokaryotic interactions. Other example includes the association between Azospir-illum and Bacillus that degrades pectin, Azospirillum and Cellulomonas degradecellulose, and Azospirillum and Enterobacter cloacae that ferment glucose (Kaiser1995; Khammas and Kaiser 1992; Halsall 1993). Production and release of plant

growth regulators by bacteria causes an alteration in the endogenous levels of the

plant growth regulator. Other growth regulators such as cytokinins are less common

4 A. Aeron et al.

among PGPR, while gibberellin production in high concentrations has only been

described from the genus Bacillus. Different genera of bacteria, such as Proteusmirabilus, P. vulgaris, Klebsiella pneumoniae, B. cereus, Escherichia coli, etc.,produce auxins, cytokinins, gibberellins, and abscisic acid (Griffith and Ewart

1995).

Symbiotic bacteria generally termed as rhizobia for a broad group of nodule-

inhabiting symbionts have been used as inoculants for well over a century. These

organisms were used to enhance nodulation and N-fixation among legumes. Their

roles were limited earlier, but have been extended to an extent as major solubilizers

of inorganic phosphate, making it available for the plants. Further their biocontrol

credentials were proved when the genera Bradyrhizobium and Rhizobium reportedto produce antibiotics effectively controlling fungal pathogens (Chakraborty and

Purkayastha 1984; Briel et al. 1996). Sinorhizobium meliloti showed antagonismtoward F. oxysporum andM. phaseolina regardless of their symbiotic effectivenessin presence of pathogen and increased overall growth of groundnut (Arora et al.

2001). Deshwal et al. (2003) isolated bradyrhizobia from the root nodules that

antagonized M. phaseolina in vitro which increased under iron-limited conditions.One of the mechanisms of biocontrol by rhizobia and bradyrhizobia was established

due to the production of siderophores resulting in increased growth of Arachishypogaea. The adaptability of introduced strains to achieve equilibrium within anaboriginal niche is limited, but identification, screening, and application of local

strain have been advocated (Bashan 1998; Aeron et al. 2010).

Various strains of species B. amyloliquefaciens, B. subtilis, B. pasteurii,B. cereus, B. pumilus, B. mycoides, and B. sphaericus are known as potentialelicitors of induced systemic resistance (ISR) and exhibit significant reduction in

the incidence or severity of various diseases on diverse hosts (Kloepper et al. 2004).

Certain volatile compounds, especially 3-hydroxy-2-butanone (acetoin) and 2, 3-

butanediol, released by the B. subtilis and B. amyloliquefaciens in rhizosphere playa crucial role in the elicitation of ISR (Ryu et al. 2003). More recently, Choudhary

and Johri (2008) have reviewed the significance of ISR by Bacillus spp. in relationto the biological control of pathogenic organisms. Bacillus species are believed toenhance the plant growth through synthesis of plant growth regulators such as

auxins (indole-3-acetic acid) and gibberellic acid (Wipat and Harwood 1999).

However, more recently representatives of B. subtilis/B. amyloliquefaciens grouphave been shown to produce substances with IAA-like activity; reasonable amount

of IAA was produced by B. amyloliquefaciens FZB42 when fed with tryptophan(Idris et al. 2004). Based on studies of wheat rhizosphere colonization by Bacillusspecies, it seems that rhizosphere competent genotypes occur in this bacterium

(Milus and Rothrock 1993; Mavingui et al. 1992; Maplestone and Campbell 1989;

Juhnke et al. 1987). Enhancement of plant growth by root-colonizing species of

Bacillus and Paenibacillus is well documented and PGPR members of the genusBacillus can provide a solution to the formulation problem encountered during thedevelopment of biocontrol agents to be used as commercial products, due in part to

their ability to form heat- and desiccation-resistant spores (Kloepper et al. 2004;

Emmert and Handelsman 1999).


The organic forms of phosphorous are estimated to comprise between 30 and

50% of total soil phosphorous. This reservoir can be mineralized by microorgan-

isms, making it available to the plant as soluble phosphates. Different PGPR genera

are capable of solubilizing phosphate and include Pseudomonas, Bacillus, Rhizo-bium, Burkholderia, Achromobacter, Agrobacterium, Micrococcus, Aerobacter,Flavobacterium, Chryseobacterium, and Erwinia. The solubilization of phosphateoccurs due to the involvement of organic acids and/or by releasing phosphatases

responsible for releasing phosphate groups bound to organic matter. Most of these

bacteria are able to solubilize the Ca–P complex, and there are others that operate

on the Fe–P, Mn–P, and Al–P complexes. Results with PGPR capable of solubiliz-

ing phosphate are sometimes erratic, probably due to soil composition. While the

inorganic forms occur in minerals as insoluble calcium, iron, or aluminum phos-

phates, organic phosphates are derived from the decaying plants, animals, and

microorganisms. Organic matter is an important reservoir of immobilized phos-

phate that accounts for 20–80% of soil phosphorus (Goldstein 1986) and only a

small portion (0.1%) is available to plants. Phosphatases are known to play a key

role in transforming organic forms of phosphorous into plant available inorganic

forms. Conversion of the insoluble forms of phosphorous to a form accessible by

plants such as orthophosphates is an important criterion. Plant may poorly/not

possess an innate ability to acquire phosphorus directly from soil phytate which is

a major phosphorous source. The production of enzyme phytase leads to an increase

in the availability of phosphorus to plants and in turn the plant uptake (Gyaneshwar

et al. 2002). It is known to be secreted by many microorganisms and is involved in

the stepwise degradation of phytate to lower phosphate esters. Although plants are

known to produce phytase, they display poor activity in roots and other plant organs

(Greiner and Alminger 2001).

As zinc is a limiting factor in crop production, study on zinc solubilization by

bacteria has an immense application in zinc nutrition to plants. Zinc-solubilizing

potential of few bacterial genera has been studied along with mobilization of

potassium (Sarvanan et al. 2003; Sperberg 1958). Potassium (K) is an essential

soil nutrient that performs a multitude of important biological functions to maintain

plant growth and quality. Although silicon (Si) is still not recognized as an essential

element for plant growth, the beneficial effects of this element on the growth,

development, yield, and disease resistance have been observed in a wide variety

of plant species. However, plants cannot directly use mineralic K and Si unless they

are released by weathering or dissolved in soil water. Studies have documented the

release of K and Si during the degradation of silicate minerals by bacteria (Barker

et al. 1998; Welch and Vandevivere 1994; Sheng and He 2006).

Iron in the Earth’s crust is present in a highly insoluble form of ferric hydroxide

(Fe3+), and thus unavailable to microorganisms and plants. Some bacteria have

developed iron uptake systems (Neilands and Nakamura 1991). These systems

involved a siderophore – an iron binding legend – and an uptake protein needed to

transport iron into the cell. Siderophores are low molecular weight (~400–1,000 Da)

iron-chelating compounds that bind Fe3+ and transport it back to the cell and make it

available for the microbial cells (Briat 1992). The secreted siderophore molecules

6 A. Aeron et al.

have a very high affinity (kd ¼ 10�20 to 10�50) for iron and bindmost of the Fe3+ thatis available in the rhizosphere and prevent the pathogens present in immediate vicinity

from proliferation because of lack of iron (O’Sullivan and O’Gara 1992). Antagonists

can prevent the proliferation of fungal phytopathogens by producing siderophores that

bind most of the Fe3+ in the rhizosphere. The resulting lack of the iron prevents any

fungal pathogen from proliferating in this immediate vicinity. Kloepper et al. (1980)have supported this mechanism and stated that the production of siderophores that

chelate and thereby scavenge the ferric iron in the rhizosphere may result in growth

inhibition of other microorganisms whose affinity for iron is lower. It has been

suggested that the ability to produce specific siderophores and/or to utilize a broad

spectrum of siderophores may contribute to the root-colonization ability of biocontrol

strains. In addition, siderophores also mediated the iron uptake by plant roots in iron-

limiting conditions (Wang et al. 1993).Root colonization is an important first step in interaction of PGPR group of

bacteria with host plant (Kloepper and Beauchamp 1992). Effective colonization of

plant roots by PGPR plays an important role in growth promotion irrespective of the

mechanism of action, i.e., production of metabolites, antibiotics against pathogens,

or ISR or even nutrient uptake (Bolwerk et al. 2003). It is now common knowledge

that bacteria in natural environments persist by forming biofilms (Davey and

O’Toole 2000). The use of microorganism for biological control is a nonhazardous

strategy to reduce crop damage caused by plant pathogens. The antagonistic

microorganisms are ideal biocontrol agents, as the rhizosphere provides the front-

line defense for roots against infection by the pathogens (Lumsden et al. 1987).

Biocontrol research has gained considerable attention and appears promising as a

viable alternative to chemical control strategies (Rebafka et al. 1993). The protec-

tion of root from the attack of the pathogen was due to the production of diverse

metabolites like siderophore (Arora et al. 2001) and antifungal metabolites such as

rhizobitoxine (Chakraborty and Purkayastha 1984). One of the most effective

mechanisms, which antagonists employ to prevent proliferation of phytopathogens,

is the synthesis of antibiotics. A large number of antibiotics have been reported

from different fluorescent pseudomonads including agrocin-84, agrocin-434, 2, 4-

diacetyl phloroglucinol, herbicolin, oomycin, phenazine, pyoluteorin, pyrrolnitrin,

pyrroles, etc., and they have a role to play in inhibition of pathogens (Colyer and

Mount 1984; Gutterson et al. 1986; James and Gutterson 1986). Many Bacillusstrains are considered as natural factories of cyclic lipopeptides, including iturins,

fengycins, and surfactins, and their involvement in control of plant microbial

diseases has been proved (Li et al. 2007; Romero et al. 2007; Yoshida et al.

2001; Asaka and Shoda 1996).

Recently, the hydrolytic enzymes have received considerable attention because

they play a role in controlling diseases due to plant pathogens. Microorganisms

capable of lysing other organisms are widespread in natural ecosystems. The

enzymatic digestion or deformation of cell wall components of phytopathogenic

fungi by the enzymes chitinase, b-1, 3-glucanase produced by antagonistic bacteriaand the hyperparasitism, lysis of phytopathogen propagules present in soil is one

of the few logical methods of biological control of soil-borne plant pathogens


(Vaidya et al. 2001). Hyperparasitism occurs when a fungus exists in intimateassociation with another fungus which derives some nutrients while conferring no

benefit in return. Hyperparasitism and lysis of propagules in soil is a logically

satisfying method of biological control of soil-borne plant pathogens by microbial

antagonists. The production of cell wall degrading or lytic enzymes, such as chitinase,

chitosanase, b-1, 3-glucanase, b-1, 4-glucanase, b-1, 6-glucanase, proteases, andlipase (Fridlender et al. 1993; Lim and Kim 1995; Dunne et al. 1997; Vaidya et al.2001; Vivekanathan et al. 2004), degrades fungal cell walls, resulting in lysis of wallmaterial, leading to cell death. Induction of the systematic resistance against many

pathogens has also been reported inducing long-lasting and broad-spectrum systemic

resistance against disease-causing agents (Zehnder et al. 2001). Plants do not have an

immune system but have evolved a variety of potent defense mechanisms, including

the synthesis of low-molecular-weight compounds such as proteins and peptides that

have antifungal activity.

1.4 Crop Ecosystem

PGPR can influence plant growth directly but may differ from species to species

and even at strain level. Symbiotic plant colonizers and certain free-living bacteria

contribute to plant growth by nitrogen fixation. The symbiotic bacteria form a host-

specific symbiosis with legumes. Molecular signal molecules (lipo-oligosaccha-

ride) secreted by these bacteria play a critical role in this process (Lange 2000;

Spaink 2000; Perrot et al. 2000). Species of Bacillus are common inhabitantsamong the resident microflora of inner tissues of various species of plants, including

cotton, grape, peas, spruce, and sweet corn, where they play an important role in

plant protection and growth promotion (Berg et al. 2005; Shishido et al. 1999; Bell

et al. 1995; McInroy and Kloepper 1995; Huang et al. 1993; Hallaksela et al. 1991;

Misaghi and Donndelinger 1990).

Little work has been done to date concerning the beneficial relationship of

Rhizobium and nonleguminous plants, although Wiehe and Hoflich (1995) demon-strated that rhizobia can multiply and survive under field conditions as well as in the

rhizosphere of nonhost legumes. The attachment of bacteria with maize, wheat,

rice, oat, sunflower, mustard, and asparagus has been reported along with improved

growth of certain nonlegumes when inoculated with rhizobia (Planziski et al. 1985;Terouchi and Syono 1990; Yanni et al. 1995; Biswas et al. 2000a, b; Peng et al.

2002). Recently, Chandra et al. (2007) reported that a successful rhizospheric

competentMesorhizobium lotiMP6 induced root hair curling, inhibited Sclerotiniasclerotiorum, and enhanced growth of mustard.

Specific rhizobacteria have the ability to improve plant growth and/or root health

(Kloepper et al. 1980; Suslow and Schrolh 1982; Schippers et al. 1987; Sikora1988; Weller 1988). A key factor of all PGPR is that they colonize seed and root, or

behave as endophytes. Such traits are desirable for considering them suitable for

biocontrol activity (Lugtenberg and Bloemberg 2004; Compant et al. 2005).

8 A. Aeron et al.

Further, the phenomenon of chemotaxis, flagellar mobility, lipopolysaccharides

(LPS) structure, the outer membrane protein OprF and to a lesser extent,

presence of pili, all are important for competitive root colonization (Lugtenberg

and Bloemberg 2004). However, in field soil, environmental conditions and

competition or displacement by the myriad of microorganisms present in the

rhizosphere limit colonization. Certainly, use of mutants and promoter probe

techniques are the beginning to identify genes in bacteria that are important to

root colonization and these are often related to nutrient uptake (Roberts et al.

1997; Rediers et al. 2005).

Variation for interaction with PGPR is often dependent on environmental con-

ditions. For example, phosphorus deficiency provokes a differential response

to Rhizobium inoculation among common bean cultivars (Vadez et al. 1999;Christiansen and Graham 2002). Such phenotypic variation among cultivars may

be, in part, the result of genetic variation and suggests genetic improvement of the

host as an approach for development of superior plant growth promoting (PGP)

strategies in conjunction with rhizosphere microbial inoculants.

1.5 PGPR in Agrobiology

PGPR are most commonly used in agriculture, and their application in various

crops resulted in an average approximate increase of 20–40% in yield across

multiple crops all over the world when various reports were combined over last

decade. In general, PGPR-carried plant growth benefit owing to increase in seed

germination rates, root growth, leaf area, chlorophyll, proteins, and hydraulic

activity, fluid movement within the plant, tolerance to drought, low temperature,

delayed leaf senescence, disease resistance, and finally enhanced grain size and

crop yield of crop, as elaborated for some of the crops in this chapter.

1.5.1 Cereals

In recent years, crop roots association with bacteria and their proliferation in the

rhizosphere has been found to be beneficial in most of the cereals (Terouchi and

Syono 1990; Biswas et al. 2000a, b; Peng et al. 2002). However, the selection of

effective strains is of prime importance for the growth promotion of cereals

(Westcott and Kluepfel 1993; Siddiqui and Ehteshamul-Haque 2000).

In rice, endophytic strains of Rhizobium leguminosarum br. trifoli E11 and E12increased grain yield of rice in field inoculation experiment (Yanni et al. 1997).

Biswas et al. (2000a, b) reported that rhizobial inoculation increased rice grain yield

at different N rates. The benefit of early seedling development could carry over to a

significant increase in grain yield at maturity. Earlier, Datta et al. (1982) reported

that a P-solubilizing and IAA-producing strain of B. firmus increased the grain yield


and P-uptake of rice in a P-deficient soil amended with rock phosphate. Similarly,

increased yield was obtained in wheat, sorghum, and barley due to application of

A. brasilense (Okon and Labandera-Gonzalez 1994; Dobbelaere et al. 2001; Saubidetet al. 2002) and with Beijerinikia mobilis and Clostridium sp. in wheat and barley,respectively (Polyanskaya et al. 2000). It was interesting to note that Pseudomonasspp. increased yield of winter wheat by 27% (de Frietas and Germida 1990).

Application of P. cepacia, P. fluorescens, and P. putida in winter wheat exhibitedantagonism against Rhizotonia solani and Leptosphaera maculans and enhancedyield indirectly in soil reported to be relatively infertile (De Frietas and Germida

1990; 1992). Inoculation of P. cepacia R55, R85 and P. putida R104 increased rootand shoot dry weight of winter wheat in R. solani infested soil (de Freitas andGermida 1991). On the other hand, P. chlororaphis 2E3 and 06 when applied onspring wheat showed increased emergence of seedlings. Both strains could also

inhibit a dreaded pathogen F. culmorum (Kropp et al. 1996). In another study onwinterwheat, application ofP. fluorescens enhanced length of seedling and significantincrease in plant height and grain yield in Pythium infested soil (Weller andCook 1986). Iswandi et al. (1987) observed an increased yield in wheat along with

maize and barley by Pseudomonas spp. 7NSK2.Rice, wheat, corn, millet, sweet potato, cotton, etc., also showed average yield

increase by 10–22.5% after application of YIB (Mei et al. 1990). Chabot et al.

(1993) demonstrated R. trifolii inoculation on increased yield in maize by reducingdose of phosphorous fertilizers. Seed treatment with rhizobacteria or their formula-

tions increased the growth of maize (Jacoud et al. 1999), wheat (Khalid et al. 2004),

rice (Yanni et al. 1997), and several other crops (Vidhyasekaran and Muthamilan

1995; Rabindran and Vidhyasekaran 1996; Vidhyasekaran et al. 1997a, b; Podile

and Dube 1988; Kloepper et al. 1991). Recently, Ashrafuzzaman et al. (2009)

isolated bacterial strains with successful root colonizer and increased plant height,

root length, and dry matter production rice seedlings. Some novel efforts were made

to elucidate the molecular responses of rice to P. fluorescens treatment throughprotein profiling (Kandasamy et al. 2009). However, the mechanism underlying

such promotional activity is not yet fully understood clearly.

Application of several genera, such as B. licheniformis RC02, Rhodobactercapsulatus RC04, P. polymyxa RC05, P. putida RC06, Bacillus OSU-142,B. megaterium RC01, and Bacillus M-13, showed increased root and shoot weightalong with nutrient uptake in barley (Cakmacki et al. 1999). Similarly, Bacillusobserved increase in yield of rice (Sudha et al. 1999), barley (Sahin et al. 2004),

wheat (de Freitas 2000), canola (de Freitas et al. 1997), and maize (Pal 1998; Pal

et al. 2001). Lalande et al. (1989) observed increased yield in maize by using

Serratia liquifaciens, Pseudomonas spp., and Bacillus sp., but B. megateriuminduced yield in rice and barley (Cakmacki et al. 1999; Khan et al. 2003). Gholami

et al. (2009) reported maize seeds inoculated with bacterial strains significantly

increased plant height, seed weight, number of seed per ear and leaf area along with

significant increase in ear and shoot dry weight of maize.

Recently, more efforts have focused on beneficial rhizobacteria in cereals that

are endophytic in nature especially in the regions where legume crop season is

10 A. Aeron et al.

followed by cereals (Ashrafuzzaman et al. 2009). Nodule-inhabiting bacteria are

now known to colonize the cereals such as rice, wheat, and sorghum. Several field

inoculation trials have been conducted to assess the agronomic potential of rhizo-

bial group in nonlegumes (Chandra et al. 2007). More efforts are required to focus

on Rhizobium–cereal associations under field conditions, with the long-term goal ofidentifying, developing, and implementing superior PGPR inoculants for the

growth promotion of rice and wheat productivity in real-world cropping systems

while reducing their dependence on nitrogen fertilizer inputs.

1.5.2 Oilseeds

The importance of PGPR applications in oil seed crops production was demon-

strated by several workers. The growth promotion and health of canola, sesame, and

peanut were supported by using different genera of PGPR (Kumar et al. 2005a;

Chandra et al. 2007; Bhatia et al. 2008; Kumar et al. 2009). Pseudomonas putida,P. fluorescens, Arthrobactro citreus, Azospirillum spp., and Serratia liquefaciensdemonstrated growth promotion of canola (Brassica campestris and B. napus) infield conditions (Kloepper et al. 1988a, b; 1989). Kloepper et al. (1989) observed

an increase in the yield of mustard with the application of Azospirillum spp.Selected bacterial strains showed increased seedling emergence, vigor, and

yield. Non-nitrogen fixing mutants provide greater root elongation effects and

greater phosphate uptake in canola (Lifshitz et al. 1987), while P. putida inocu-lation increased yield of canola. Van Peer and Schippers (1998) found that

inoculation of Pseudomonas spp. increased root and shoot weight in canolaunder hydroponic growth chamber. Belimov et al. (2001) observed that inocula-

tion of B. napus seeds with Alcaligenes sp., B. pumilus, Pseudomonas sp., andVariovorax paradoxus showed vigorous growth. Bertrand et al. (2001) observedsignificant increase in root dry weight due to aggressive effect of Phyllobacteriumsp. apart from Variovorax sp. and Agrobacterium sp. It was demonstrated thatMethylobacterium fujisawaense promoted root elongation in canola (Madhaiyanet al. 2006). Earlier, Ghosh et al. (2003) observed that B. circulans DUC1,B. wrmus DUC2, and B. globisporus DUC3 enhanced root and shoot elongationin B. campestris.

Differential response of sesame under influence of indigenous and nonindige-

nous rhizosphere competent fluorescent pseudomonads were observed recently by

Aeron et al. (2010). The results of root colonization stated the difference of using

indigenous and a nonindigenous strain and the successful colonization by

fluorescent pseudomonads in sesame rhizosphere promoted growth which

proved efficacy of indigenous microflora over nonindigenous microflora

(Table 1.1). Integrated use of organic and inorganic biofertilizers has been

reported to sustain productivity of sesame by improving soil physical conditions

and also reduce the costly inorganic fertilizer needs (Duhoon et al. 2001;


Tab

le1.1

Plantgrowth

promotingattributesoffluorescentpseudomonadsandtheireffect

onplantgrowth

param

etersofsesame

Treatments

PGPattributes

Plantgrowth

param

eters

Isolate

Rhizospheric

origin

IAA

(mg/m

l)

ACC

deaminase

P(m

gof

P/m

l)

SU/m

l/h

HCN

(OD

at625nm)

Antagonism

(%)

G(%

)RDW

(gm)

SDW

(gm)

RL

(cm)

SL

(cm)

G/P

12

P.aeruginosa

GRC1rif+

tet+

Potato

+(31)

–+(67)

+(29)

–+(55)

–78.8**

16.2**

43.2**

26.2*

170.3**

42.6**

P.aeruginosa

PS2str+

Groundnut

+(36)

–+(55)

+(17)

+(0.09)

+(61)

–76.3**

13.5**

40.7**

24.7*

168.9**

38.8**

P.aeruginosa

PS(II)

neo+

Sunflower

+(30)

–+(71)

+(12)

+(0.05)

––

77.9**

14.1**

42.5**

25.6*

173.8**

39.2**

P.aeruginosa

LES4tet+

Tomato

+(42)

–+(75)

+(22)

–+(72)

+(68)

80.3**

14.7**

44.3**

26.2*

180.7**

41.3**

P.aeruginosa

PRS4gen+

Velvet

Bean

+(40)

–+(68)

+(15)

–+(78)

–79.1**

12.4**

39.8**

22.8*

165.3**

40.8**

P.aeruginosa

PSI5

azi+

kan+

Sesam

e+(41)

++(76)

+(19)

–+(70)

+(63.3)

83.5**

17.3**

45.6**

27.1*

185.9**

47.3**

Control

na

na

na

na

na

na

na

61

10.1**

18.6**

21.6

141.3

26.3

PGPattributesaremeanofthreeindependentexperim

ents;Field

dataisameanoftwoyeartrials;Values

aremeanof15randomly

selected

plants

GGermination,R

DW

Rootdry

weight,SD

WSeedlingdry

weight,RLRootLength,SL

ShootLength,G

/Pseed

yieldperplant;+attributepositive;�attribute

negative;

IAAIndole

acetic

acid;PPhosphatesolubilization;SidSiderophore;ACC1-aminocyclopropane-3-carboxylicacid;1Macrophominaphaseolina;

2Fusariumoxysporum;(%

)pathogen

inhibitionpercentage(control–treatm

ent/control�1

00)[A

daptedfrom

Aeronet

al.(2010)]

*Significantat

P>

0.01level

ofANOVA

**Significantat

0.01level

ofLSD

ascompared

tocontrol

12 A. Aeron et al.

Kumar et al. 2009). Siddiqui et al. (2001) reported inoculation of P. fluorescensalong with chemical fertilizers is an effective way to reduce the infestation of

Meloidogyne spp. in sesame.Groundnut (Arachis hypogaea L.) is a major oilseed and food crop of the

semiarid tropics. The late leaf spot disease of groundnut caused by the fungus

Cercosporidium personatum almost co-exists with the crop and contributes tosignificant loss in yield throughout the world. Leaf spots can cause up to 53%

loss in pod yield and 27% loss in seed yield (Patel and Vaishnav 1987). Smith

(1992) reported pod loss of 10–50% by late leaf spot disease. Control of this disease

mainly depends on fungicides, although considerable effort has been invested in

developing biocontrol methods (Meena et al. 2002). On the other hand, Jadhav et al.

(1994) reported a Rhizobium isolate that increased plant growth and chlorophyllcontent in groundnut. Earlier, Howell (1987) explained in part the rhizobia-

enhanced mineral uptake in groundnut tissues. Pal et al. (2000) reported increased

pod yield following seed treatment with Pseudomonas sp. Gupta et al. (2002) foundreduced disease incident, better vegetative growth parameters, and ultimately

enhanced grain yield in peanut by the addition of P. aeruginosa GRC2 inM. phaseolina-infested field soil. Recently, Bhatia et al. (2008) reported increasedseed germination, growth promotion, and suppression of charcoal rot due to

M. phaseolina with fluorescent pseudomonads.Earlier, Arora et al. (2001) observed enhanced seed germination, seedling

biomass, and nodule weight with reduced disease incidence in groundnut. Simi-

larly, Meena et al. (2006) applied P. fluorescens for plant growth and in biocontrolof late leaf spot caused by C. personatum in groundnut. Seed treated withP. fluorescens strain Pf1 recorded the highest seed germination percentage andthe maximum plant height with significantly controlled late leaf spot disease of

groundnut resulting in increased pod yield. In another study, B. subtilis strain AF1,isolated from soils suppressive to pigeon pea (C. cajan) wilt caused by F. udum,was presumed to induce resistance against Aspergillus niger on peanut. Strongexperimental evidence that AF1 elicited ISR came from the findings of Sailaja et al.

(1997) who reported a noteworthy reduction in the incidence of crown rot of peanut

caused by A. niger corresponding to the increase in lipoxygenase activity, aphenomenon associated with ISR.

1.5.3 Fruits, Vegetables, and Cash Crops

Several workers have reported successful management of plant disease and

increased yield in various horticultural crops such as strawberry (Tahmatsidou

et al. 2006; Pedraza et al. 2010), chillies (Bharathi et al. 2004), mango (Vivekanathan

et al. 2004), tobacco (Pan et al. 1991), pea (Chang et al. 1992), potato (Geels et al.

1986), red pepper (Lee et al. 2008), banana (Gunasinghe and Karunaratne 2009), and

apple (Karlidag et al. 2007; Aslantas et al. 2007) with the application of PGPR.

Increased seedling growth was observed in sugar beet with the application of


Pseudomonas spp. (Williams and Asher 1996). Reddy et al. (2001) reported foliarapplication of PGPR bioformulation which promoted plant growth besides effectively

controlling tomato bacterial spot, cucumber angular leaf spot, tobacco blue mold, and

wild fire. Sarvanankumar et al. (2007) used Pseudomonas and Bacillus bioformulationagainst blister blight disease of tea caused byExobasidium vexanswhile Chakraborthyet al. (2009) studied talc-based bioformulation ofOchrobacterium anthropi TRS-2 forplant growth promotion and management of brown root rot disease of tea. Recently,

Karakurt and Aslantas (2010) investigated the effects of Agrobacterium rubi A-18,B. subtilis OSU-142, B. gladioli OSU-7, and P. putida BA-8 on growth and leafnutrient content of apple cultivars and found interesting variations that support the

application of PGPR.

The role of PGPR in vegetative crops production has got less attention in

comparison to that of other crops. Raupach and Kloepper (2000) reported that

seed treatment of cucumber with B. amyloliquefaciens IN937a, B. subtilis GB03,and a mixture of the two strains resulted in significant increases in plant growth and

reductions in disease severity. Han and Lee (2005) reported PSB B. megaterium andpotassium solubilizing bacteria (KSB) B. mucilaginosus inoculated in nutrient-limited soil planted with eggplant. Inoculation of these bacteria in conjunction

with amendment of its respective rock P or K materials increased the availability

of P and K in soil and enhanced N, P, and K uptake, and growth of eggplant.

The early seedling emergence and significant increase in yield of potato was

observed with the application of PGPR. Increase in yield of potato was reported by

several workers after the application of different species of Pseudomonas (Howieand Echandi 1993; Geels et al. 1986; Kloepper et al. 1989). Kloepper et al. (1980)

demonstrated larger root system and significant increase in yield in different soil

types. Frommel et al. (1993) found the application of Pseudomonas strain Ps JNincreased whole plant dry weight. Suppression of Erwinia caratovora causing softrot in potato was seen after the inoculation with P. putida W4P3 (Xu and Gross1986). Raupach and Kloepper (2000) demonstrated the effect of B. amyloliquefa-ciens In937a and B. subtilis GB03 individually as well as in combination for plantgrowth promotion and reduction of disease severity on seeds of cucumber treated

with these antagonists. A nonfluorescent Pseudomonas of onion rhizosphereshowed significant increases in root dry weight, stem length, and lignin and

enhanced stem hair formation (Frommel et al. 1991). A disease complex by

Meloidogyne incognita and F. oxysporum was suppressed by the application offluorescent pseudomonads in tomato (Santhi and Sivakumar 1995; Kumar et al.

2005a) (Table 1.2). Ekin et al. (2009) applied Bacillus sp. OSU-142 as comparedto three different levels of N fertilization. The beneficial effect of Bacillus sp. OSU-142 on tuber yield of potato was reported in two successive years over fertilizers.

Several strains of P. fluorescens, P. cepacia, and P. aeruginosa have been used forthe biological control of several plant diseases in a wide range of horticultural crops

(Weller 1988; Chandel et al. 2010). Yusran et al. (2009) reported biological control

of F. oxysporum f. sp. radicis-lycopersici that causes crown and root rot in tomato.The inoculation also increased the N yield and fixed N in association with banana

roots subsequently increased the yield, improved the physical attributes of fruit

14 A. Aeron et al.

quality, and initiated early flowering. More recently, PGPR proved effective as a

bioenhancer and biofertilizer for banana cultivation (Mia et al. 2005).

B. subtilis S499 is involved in suppression of gray mold disease caused byBotrytis cinerea on wounded apple fruits (Ongena and Jacques 2007; Jacqueset al. 1999). Recently, Romero et al. (2007) showed the involvement of iturin and

fengycin antibiotics from four B. subtilis strains UMAF6614, UMAF6616,UMAF6639, and UMAF8561 in suppression of powdery mildew of cucurbits

caused by Podosphaera fusca. Arrebola et al. (2010) reported the production ofiturin from B. amyloliquefaciens PPCB004 which inhibited seven different post-harvest pathogens of citrus, avocado, and mango fruits. Recently, Choudhary and

Johri (2008) implicated the mechanisms and role of Bacillus species as inducers ofsystemic resistance in relation to plant–microbe interactions and explicated the

pathways involved in their regulation.

1.5.4 Legumes

A unique relationship was observed between two bacterial isolates Burkholderiasp. MSSP and Sinorhizobium meliloti PP3, where commensalisms between themresulted in increased IAA production in mixed-species culture and significant

increase in seedling length and weight of pigeon pea (Cajanus cajan) (Pandeyand Maheshwari 2007a). When wheat-bran-based bioformulation comprising con-

sortium of PGPR was applied in field trials, significant improvement in growth and

yield of pigeon pea was obtained (Pandey and Maheshwari 2007b). Kumar et al.

(2010) obtained wilt disease management and enhancement of growth and yield of

Cajanus cajan (L) var. Manak by bacterial combinations using root nodulatingSinorhizobium fredii KCC5 and P. fluorescens LPK2 isolated from nodules of hostplant and disease suppressive soil of tomato rhizosphere, respectively (Table 1.3).

Mishra et al. (2009) studied application of several potential PGPR strains on Cicerarietinum. All isolates showed significant increase in shoot length, root length, anddry matter of seedlings. Even the application of P. cepacia caused an early soybean

Table 1.2 Effect of Pseudomonas EP10 on root disease complex and growth of tomato after 60days

Treatment Plant length

(cm)

Plant fresh

weight (g)

Shoot dry

weight (g)

Root Knot

Index

Infection (%) of

F. oxysporum

F. oxysporum 26.5 18.0 4.5 6.0 100Pseudomonas

EP10

55.0** 51.0** 15.0** 3.0 –

PseudomonasEP10 +

F. oxysporum

58.5** 55.0** 17.5** 2.5 4

Control 32.8 27.5 6.5 7.0 –

**P < 0.01, Values average of ten replicates during three trials (Modified and adapted fromKumar et al. 2005b)


growth and enhanced seed germination (Cattelan et al. 1999). Ma et al. (2003)

reported R. leguminosarum bv. viciae 128C53K enhanced nodulation in pea (Pisumsativum L.). Moreover, Peña-Cabriales and Alexander (1983) found that strainsof rhizobia and bradyrhizobia grew readily in the presence of germinating seeds

and developing root systems of soybean (Glycine max (L.) Merr.), kidney bean(Phaseolus vulgaris L.), red clover (Trifolium pretense L.), and cowpea (Vignaunguiculata L.), but Pseudomonas sp. and Bradyrhizobium sp. increased growthand promoted nodulation in mung bean (Shaharoona et al. 2006). Wiehe and

Hoflich (1995) demonstrated that R. leguminosarum bv. trifolii can multiply andsurvive under field conditions in the rhizosphere of nonhost legumes (Lupinus albusL.and Pisum sativum L.) and nonlegumes such as corn, rape, canola, and wheat; somestrains ofB. subtilis that have been integrated into pest management strategies, such asbiocontrol strain GB03 of B. subtilis, could inhibit the fusarial wilt caused byFusarium species more effectively on semiresistant cultivar of chick pea than onsusceptible variety (Jacobson et al. 2004; Hervas et al. 1998).

There are several reports which reveal that efficacy of rhizobia could be

enhanced by co-inoculation with PGPB. Co-inoculation with symbiotic and rhizo-

sphere bacteria may improve nodulation by a number of mechanisms. Different

mechanisms for such activity by Gram-positive and Gram-negative bacteria include

siderophore chelating insoluble cations, LPS, flavonoids, phytoalexins, antibiotics,

and colonization of root surfaces by outcompeting pathogenic organisms, and thus

increase nodulation and growth (Garcia Lucas et al. 2004; Parmar and Dadarwal

2000). A common attribute, although, is efficient colonization of roots by PGPR

strain to reduce the ethylene concentration inside the plant. That is so if it is able to

utilize ACC as a sole nitrogen source, thereby increasing the root surface in contact

with soil. Therefore, it is highly expected that presence of PGPR containing ACC

deaminase on the roots of legume could suppress accelerated endogenous synthesis

of ethylene during the rhizobial infection and thus may facilitate nodulation. So,

co-inoculation of legumes with competitive rhizobia and PGPR-containing ACC

deaminase could be an effective and novel approach to achieve successful and

dense nodulation in legumes. It is highly expected that inoculation with rhizobac-

teria containing ACC-deaminase hydrolyzed endogenous ACC into ammonia and

alpha-ketobutyrate instead of ethylene. Consequently, root and shoot growth of the

Table 1.3 Effect of S. fredii KCC5, P. fluorescens LPK2, bacterial consortium (KCC5 þ LPK2)on post harvest parameters of C. cajan var. Manak, after 120 days of sowing

Treatments *Pods

plant�1Grain yield

(Kg ha�1)A soluble

protein (mg g�1)Stover yield

(Kg ha�1)Harvest

index (%)

KCC5 115.4** 962.1** 211.1* 4,230* 18.53

LPK2 116.3** 955.2** 197.2* 4,200* 18.52

Consortium

(KCC5 þ LPK2)118.6** 988.3** 212.3* 4,280* 18.76

Control 51.2 710.0 179.1 3,150 18.03

Average value of ten plants from each treatment; a soluble protein content of seed (g�1)*Significant at 5% (ANOVA)

**Significant at 1% as compared to control (ANOVA) [Modified and adapted from Kumar et al.

(2010)]

16 A. Aeron et al.

legume plant aswell as nodulation can be promoted (Garcia Lucas et al. 2004; Remans

et al. 2007). Earlier, the use of specific PGPR mutant strains has indicated that

bacterial indole-3-acetic-acid production and 1-aminocyclopropane-1-carboxylate

deaminase activity play an important role in the host nodulation response. Tittabutr

et al. (2008) conducted such a study to evaluate effect of ACC deaminase on nodula-

tion and growth of Leucaena leucocephala. Further, Remans et al. (2007) examinedthe potential of ACC deaminase producing PGPR to enhance nodulation of common

bean (Phaseolus vulgaris). Shaharoona et al. (2006) observed that co-inoculation withPseudomonas and Bradyrhizobium species significantly improved root length, totalbiomass, and nodulation in mung bean. Belimov et al. (2009) evaluated the effect of

root-associated bacterium containing ACC deaminase on pea (Pisum sativum) plantsgrown in dry soil.

Huang and Erickson (2007) tested the effectiveness of R. leguminosarum forimproving growth and yield of pea and lentil. They found improved seedling

growth, nodule biomass, and shoot and root biomass in peas as we observe in

velvet bean. Similarly, the effect of different methods of rhizobial inoculation on

yield, root nodulation, and seed protein contents of two lentil varieties and improve-

ment in nodulation was observed in peanut by inoculation with Rhizobium species(Ahmad et al. 2008; Dey et al. 2004). Rhizobia and other microorganisms employ

various mechanisms to acquire essential nutrients such as iron, which includes

production of iron-chelating molecules known as siderophores. Despite their

efficient nitrogen-fixing potential, most of the times they fail to increase plant

yield under field trials in agricultural soils. This has been attributed to their ineffi-

ciency to successfully colonize the rhizosphere. Iron availability is one of the

limiting factors for poor rhizospheric colonization. The successful performance of

rhizobial inoculant strain depends upon their capability to outcompete the indige-

nous soil bacteria, survive, propagate, and enter into effective symbiosis with host

plant. Many studies have indicated that efficient utilization of siderophores by

rhizobia is a positive fitness factor with respect to its soil survival (Carson et al.

2000). Further, Joshi et al. (2009) observed increase in nodule occupancy and

higher rhizospheric colonization by pigeon pea-nodulating rhizobia expressing

engineered siderophore cross-utilizing abilities. Since survival under iron limita-

tions in soils is an important quality which every biofertilizer strain must possess,

the iron sufficiency of any organism therefore largely depends on its ability to

utilize siderophores present in large and small concentrations in its vicinity that

may be of plant, microbial, or soil origin (Carson et al. 2000; Joshi et al. 2009; Joshi

et al. 2008; Joshi et al. 2006b; Khan et al. 2006). Thus, iron availability is one of the

major factors determining rhizospheric colonization. This fact is further evidenced

by work of Mahmoud and Abd-Alla (2001). They showed that co-inoculation

of siderophore-producing PGPR significantly enhanced nodulation and nitrogen

fixation of mung bean compared to plants infected with rhizobial strain alone. Thus,

siderophore plays an important role in the competition between microorganisms

and may act as growth promoters as the rhizosphere is heavily populated with

siderophore-producing microorganisms. There are more reports that specific side-

rophore producing microorganisms stimulated the nodulation, nitrogen fixation,


and plant growth of leguminous plants (Grimes and Mount 1987; Omar and Abd-

Alla 1994; Shenker et al. 1999). A nodulated legume has an increased need for iron

compared to non-nodulated plant since this metal is a constituent of key proteins

involved in nitrogen fixation such as nitrogenase and leghemoglobin. Although

scientists have reported both direct and indirect ways of growth stimulation by

PGPR, there is no clear separation between these two mechanisms. A bacterium

influencing plant growth by regulating synthesis of plant hormones can also play a

role in controlling plant pathogens and diseases and vice versa. The presence of

PGPR in the root vicinity may also improve ability of rhizobia to compete with

indigenous populations for nodulation. Parmar and Dadarwal (2000) reported

that increase in root growth provides more number of active sites and access

to nodulation for rhizobia in chickpea. Co-inoculation of Bradyrhizobium withP. striata has also been observed to enhance biological nitrogen fixation insoybean (Dubey 1996). Rosas et al. (2006) studied the promising action of two

phosphate solubilizing Pseudomonas strains on the symbiosis of rhizobialstrains (S. meliloti and B. japonicum) with alfalfa and soybean. Further, differ-ential effects on chick pea plant growth were also observed under co-inoculation

with a PSB (Pseudomonas) strain and rhizobia alone (Valverde et al. 2006).There is a great advantage of using PSB in co-inoculation with rhizobia. This is

because increased P mobilization in soil alleviates P deficiency. Deficit

P severely limits plant growth and productivity particularly with legumes,

where both plants and their symbiotic bacteria are affected. This may have a

deleterious effect on nodule formation, development, and function (Robson

et al. 1981). Similarly, dual inoculation of rhizobia with PGPR promoted

nodulation, plant growth, and N2 fixation in Vigna radiate (Gulati et al. 2001;Gupta et al. 2003).

1.5.5 Forestry

Worldwide efforts to increase green cover and reforestation of abandoned, barren,

and wasteland can benefit from a wider application of PGPR in both angiosperm and

gymnosperm. There are currently very few reports on forestry-PGPR research. As a

consequence, there is currently no field data for deciduous trees and still compara-

tively little field data for gymnosperms (Chanway 1997). Hindrance in successful

application of PGPR in forestry includes aspects such as low pH conditions of forest

soil, perennial nature of trees, soil type, forest environment, and survival of PGPR

with trees in colder regions. In contrast to agricultural crops, the inoculation of

PGPR on tree species and with special reference to their effect on seedling emer-

gence, reduction in seedling transplant injury during the transfer from nursery to

field, biomass increase due to inoculation apart from raising disease free plantlets in

nursery, and increased strength of plantlets to withstand storm of antagonists in the

form of pathogens have scope for investigation.

18 A. Aeron et al.

Earlier, Pokojska-Burdziej (1982) and Beall and Tipping (1989) demonstrated

increase in height and biomass in black spruce, jack pine, and white spruce by using

Arthrobacter citreus, P. fluorescens, and P. putida under greenhouse conditions,while Chanway and Holl (1994) used A. oxydans and P. aureofaciens in DouglasFir and recorded improved height and biomass. On the other hand, Arthrobactersp. increased the shoot length of pine. Leyval and Berthelin (1989) found that a

strain of Agrobacterium radiobacter increased biomass of beech and pine. Accord-ing to one of the reports from authors group, Pinus roxburghii was found to showluxuriant growth due to application of Bacillus subtilis BN1. Seed treatment resultedin significant increase in seed germination, early seedling emergence, increase in

biomass besides reduction in charcoal root rot in chir-pine seedlings (Singh et al. 2008;

Singh et al. 2010) (Table 1.4), and reduction in the total, pre- and postemergency

mortality of the P. radiata seedlings in nursery trial (Valiente et al. 2008).Inoculation of A. brasilense Cd increased root growth while inoculation

of A. chroococcum increased biomass in oak (Akhromeiko and Shestakova1958; Zaady et al. 1993; Zaady and Perevoltsky 1995), A. brasilense in riveroak (Rodriguez-Barrueco et al. 1991), A. chroococcum in ash (Akhromeiko andShestakova 1958), Quercus serrata (Pandey et al. 1986), and Eucalyptus(Mohammad and Prasad 1988). Enebak et al. (1998) studied inoculation of

B. polymyxa and P. fluorescens in loblolly pine and slash pine under greenhouse conditions. A significant increase in seedling emergence was observed

along with total biomass. The postemergence damping-off was reduced in lob-

lolly pine. B. polymyxa and Staphylococcus hominis significantly increasedgrowth of hybrid spruce (O’Neill et al. 1992). Chanway et al. (2000) observed

that B. polymyxa and P. fluorescens overwinter on the roots of field-planted treessuch as spruce. Shishido and Chanway (2000) observed a significant increase in

plant biomass of hybrid spruce when inoculated with Pseudomonas strain at allsites apart from reduction in seedling injury after transplant. B. licheniformisCECT5105 and B. pumilis CECT5106 led to increase in the plant growth andnitrogen content in silver spruce (Porbanza et al. 2002). Mafia et al. (2009)

reported B. subtilis and Pseudomonas sp. in controlling mini-cutting rot ofeucalyptus caused by Cylindrocladium candelabrum and R. solani. In fact, treeswith mycorrhizal associates, with associative, or symbiotic N2-fixers, or with

Table 1.4 Effect of P. aeruginosa strain PN1 on the growth of chir-pine (90DAS/Pot assay)

Treatments Germination

(%)

Shoot length

(cm)

Root length

(cm)

Fresh weight (g)

Shoot Root

P. aeruginosa PN1 84* 7.2ns 10.1ns 0.92ns 0.266ns

P. aeruginosaPN1 þ M. phaseolina

72* 6.5ns 9.1ns 0.727ns 0.236ns

M. phaseolina 54ns 5.9ns 8.1ns 0.52ns 0.182ns

Control 66 6.2 8.8 0.625 0.212

Values are the mean of triplicates; ns non-significant

*Significant at 5% LSD

Modified and adapted from Singh et al. 2010


rock-weathering capacities have significant impacts on biogeochemical processes,

affecting recovery of degraded ecosystems and forest sustainability.

1.5.6 Mulberry (Sericulture)

For sericulture industry, mulberry is food plant of silkworm (Bombyx mori) grownin 1, 70,000 ha in India under different agro-climatic conditions. The sustainable

leaf production, silkworm rearing, and cocoon production are dependent on soil

fertility of mulberry gardens maintained through periodical application of either

organic manures or chemical fertilizers in required quantity. Former is an approach

wherein crop can be raised without imparting any adverse effect on soil and other

beneficial microbial ecology. Therefore, a shift toward nonchemical strategies

has to be evolved. The biofertilizers enriched with bacteria and fungi have proven

to be great importance in improving the yield and quality of mulberry (Morus albaL.) More than three decades ago, Vasantharajan and Bhat (1967) studied the

interactions of beneficial microorganisms and mulberry and reported an increase

in shoot length and root length of seedlings and saplings due to the application of

different genera of PGPR such as Pseudomonas spp., Acetobacter, Flavobacterium,Achromatobacterium, Micrococus, Bacillus, Arthrobacterium, etc. Later, Kasivis-wanathan et al. (1977) observed inoculation of Azotobacter to soil proved beneficialto increase the growth and yield of mulberry. Vijayan et al. (2007) evaluated the

effect of biofertilizers Azotobacter and Azospirillum on establishment of mulberryand revealed that inoculants improved the mulberry growth and development over

control in saline conditions. Even the P-solubilizing Bacillus megaterium, Bacillussp., Aspergillus awamuri, and A. niger enhanced the growth and yield parameters ofmulberry (Nagendra Kumar and Sukumar 2001). Gangwar and Thangavelu (1992)

isolated the nitrogen-fixing bacteria Azotobacter and Beijerinckia from phyllo-sphere and rhizosphere of mulberry. The first pair of leaf was inoculated with

both the bacterial isolates and showed the airborne nature of the nitrogen fixers.

In rhizosphere, the population of both the genera increased corresponding to

increase in the age of the plant. Yadav and Nagendra Kumar (1989) observed

reduction of nitrogen fertilizer to half or one-third dose along with Azospirillum,which improved the mulberry plant growth and leaf yield at par with full dose of

nitrogen fertilizer. Umakant and Bagyaraj (1998) reported improvement of plant

growth in mulberry nursery with dual inoculation of Azotobacter chroococcum andGlomus fasciculatum.

During several studies, Das et al. (1990, 1994) reported that biological nitrogen

fixation mediated through Azotobacter is considered to be the potential system formulberry cultivation for economizing up to half dose of N fertilizer in different

mulberry cultivars without any reduction in leaf yield and quality. Rangarajan and

Santhanakrishnan (1995) demonstrated the combined effect of P. fluorescensand Azospirillum more superior than that of single inoculation or uninoculatedcontrols. This enhanced the quality of mulberry leaf and consequently improved

20 A. Aeron et al.

the silkworm growth and silk production. Chandrashekar et al. (1996) studied the

effect of co-inoculation in mulberry with Acaulospora sevis, B. megaterrium var.phosphaticum, and A. brasillense using two sources of phosphorous and attainedimprovement of leaf growth, yield, and quality. Gupta et al. (2008) reported the

sustainability of mulberry leaf production by reducing the application of chemical

fertilizers and biofertilizers, such as Azotobacter, Azospirillum, PSB, and mycor-rhizae. Baqual and Das (2006) demonstrated that dual inoculation of mulberry with

Azotobacter and VAM could curtail use of fertilizers by 50% besides improving theleaf yield, cocoon production, and quality. Azotobacter inoculation to the rootscaused better increase in root and shoot weight in comparison to that of leaf

inoculation. The proliferation of Azotobacter in nutrient solutions was highlystimulated in the presence of mulberry plants, but the similar stimulation in the

natural condition was not observed.

Sudhakar et al. (2000a, b) studied the role of biotic and abiotic (seasonal

variation) factors in contributing toward population buildup of diazotrophs and

other microorganisms both on phylloplane and rhizosphere of mulberry. It was

concluded that rainy season and the shoot age of 30–40 days after pruning appear to

be ideal for the increase of diazotrophs both in phylloplane and rhizosphere by

foliar application of Azotobacter and Beijerinckia. Similarly, PGPR proved to be aneffective tool for increase in biomass production in som (Machilus bomycina),which in turn has an impact on the growth of silkworms to produce more silk fiber

of good quality. Unni et al. (2008) isolated and exploited PGPR from rhizosphere of

som plants. Muga silkworm larvae fed on some leaves of the plant treated with

PGPR showed growth-promoting activities in plants. The shell weight of the

cocoons formed from the larvae fed with treated som leaves was significantly

higher than that of the control. Such cocoons used for fiber estimation showed

considerable increase in fiber content which were not only longer but had higher

nonbreakable filament length (Unni et al. 2008). B. subtilis strain Lu144 wasisolated as an endophyte from the surface sterilized leaves of mulberry (Ji et al.

2008). Strain Lu144 exhibited strong in vitro antagonistic activity against Ralstoniasolanacearum which causes bacterial wilt on mulberry plants and displayed reduc-ing the disease incidence.

1.6 Limitations Associated with PGPR

In fact, the inconsistent response of field-grown crops to PGPR has limited com-

mercial development. In natural ecosystems, the behavior of introduced bacterial

inoculants (e.g., PGPR) and the subsequent expression of PGP represent a complex

set of multiple interactions between introduced bacteria, associated crops, and

indigenous soil microflora. These interactions are, in turn, influenced by multiple

environmental variables such as soil type, nutrition, moisture, and temperature

(Kloepper et al. 1989; Glick 1995). Thus, the ability of a bacterial inoculant to

promote plant growth can only be fully evaluated when they are tested in


association with all of the components of the rhizosphere (Schroth and Weinhold

1986). Inconsistent responses to beneficial bacteria are frequently reported (Brown

1974; Broadbent et al. 1977; Schroth and Hancock 1982; Howie and Echandi 1983;

Schroth and Weinhold 1986; Schippers et al. 1987; Kloepper et al. 1988a, b; de

Frietas and Germida 1990, 1991, 1992a, b). Moreover, i

bacteria in agrobiology: crop ecosystemsthe book entitled bacteria in agrobiology: crop ecosystems...

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