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Microbial & Biochemical Technology

Gupta et al., J Microb BiochemTechnol 2015, 7:2http://dx.doi.or/10.!172/1"!#$5"!#.10001##

esearch Article Open Access

Plant

Growth

Promoting

Rhizobacter

ia (PGPR):

Current and

Future

Prospectsfor

Developme

nt of

ustainable

!griculture

GovindGupta,Shailendra

SinghParihar,NarendraKumarAhirwar,Sunil KumarSnehi andVinod Singh*

Department ofMicrobiology,BarkatullahUniversity,Bhopal, India

Abstract

oil

is

d"

na

mi

c living matri# and it is not onl" a critical

resource in agricultural and food securit"

but it is also towards maintenance of all

life process$ Pathogenic microorganismsaffecting plant health are a ma%or and

chronic threat to sustainable agriculture

and ecos"stem stabilit" worldwide$ &he

chemical fertilizers used in the agriculture

to increase "ields' ill pathogens' pests'

and weeds' have a big harmful impact on

the ecos"stem$ ecause of current public

concerns about the side effects of

agrochemicals' there is an increasing

interest in improving the understanding of

cooperative activities among plants and

rhizosphere microbial populations$ o'

there is an urgent need of biological

agents is accepted worldwide$ &he use of

plant growth promoting Rhizobacteria

(PGPR) is a better alternative to solve

this problem$ &he" pla" an important role

to increase in soil fertilit"' plant growth

promotion' and suppression of

ph"topathogens for development of

ecofriendl" sustainable agriculture$ &his

review provides environment friendl"

approach to increase crop production and

health' development of sustainable

agriculture and commercialization b"

using of plant growth promoting

rhizobacteria with global applicabilit"$

Keywords: Plant

growth promotingrhizobacteria; Bio-

fertilizers;

Biocontrol;Sustainableagriculture

Introduction

In modern

cultivation process

indiscriminate use of

fertilizers,

particularly the

nitrogenous and

phosphorus, has ledto substantial

pollution of soil, air

and water. !cessive

use of these

chemicals e!erts

deleterious effects on

soil microorganism,

affects the fertility

status of soil and also

pollutes environment

"#$. %he application

of these fertilizers on

a long term basisoften leads to

reduction in p& and

e!changeable bases

thus ma'ing them

unavailable to crops

and the productivity

of crop declines. %o

obvia

te

this

probl

em

and

obtai

n

highe

r

plant

yield

s,

farm

ers

have

become

incre

asing

ly

depe

ndent

on

chem

ical

sourc

es of

nitro

gen

andphos

phor

us.

Besid

es

being

costl

y, the production of chemical fertilizers depletes

nonrenewable resources, the oil and natural gas

used to produce these fertilizers, and poses

human and environmental hazards "($.

)ver the last few decades, the agriculture

policy in India has undergone a ma*or change

through diversification and emphasis on

sustainable production system. +hizosphere

researches have been throwing up surprise and

interesting ideas for research ever since the

pleasant environment of microorganisms around

plant root called rhizosphere. %he term rhizosphere

was introduced for the first time by &iltner "$.

%he ma*or influences that the rhizosphere

microorganisms have on plants today become

important tool to guard the health of plants in

ecofriendly manner "$. %hese microorganisms caneffect plant growth often referred to as a plant

growth promotory rhizobacteria "$. %hey are

involved in various biotic activities of the soil

ecosystem to ma'e it dynamic for nutrient turn

over and sustainable for crop production "/$. In

recent years considerable attention has been paid

to P0P+ to replace agrochemicals 1fertilizers and

pesticides2 for the plant growth promotion by a

variety of mechanisms that involve soil structure

formation, decomposition of organic matter,

recycling of essential elements, solubilization of

mineral nutrients, producing numerous plant

growth regulators, degrading organic pollutants,

stimulation of root growth, crucial for soil fertility,

biocontrol of soil and seed borne plant pathogens

and in promoting changes in vegetation "3$. 4n

understanding of plant growth promoting

rhizobacteria and their interactions with biotic and

abiotic factors is indispensable in

bioremediation

techni5ues "6$ energy

generation processes

and in

biotechnological

industries such as

pharmaceuticals,

food, chemical, and

mining. 7urthermore

plant growth

promoting

rhizobacteria can

reduce chemical

fertilizers application

and economically,

environmentally

beneficial for lowerproduction cost as

well as recognize the

best soil and crop

management

practices to achieve

more sustainable

agriculture as well as

fertility of soil "8$.

Plant Growth

Promoting

Rhizobacterial

FormsPlant growth

promoting

rhizobacteria can be

classified into

e!tracellular plant

growth promoting

rhizobacteria 1eP0P+2

and intracellular plant

growth promoting

rhizobacteria 1iP0P+2

"#9$. %he eP0P+s may

e!ist in the

rhizosphere, on the

rhizoplane or in the

spaces between the

cells of root corte!

while iP0P+s locates

generally inside the

specialized nodular

structures of root cells.

%he bacterial genera

such as

Agrobacterium,

Arthrobacter,

Azotobacter,

Azospirillum, Bacillus,

Burkholderia,

Caulobacter,

Chromobacterium,

Erwinia,

Flavobacterium,

Micrococcous,

Pseudomonas and

Serratia belongs to

eP0P+ "##$. %he

iP0P+ belongs to the

family of +hizobiaceae

includesAllorhizobium,

Bradyrhizobium,

Mesorhizobium and

hizobium,

endophytes and

Frankia species both

of which can

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symbiotically fi!

atmospheric nitrogen

with the higher plants

"#($.

Plant Growth

Promotion:

Mechanism o

!ction

Plant growth

prom

otion

by

plant

grow

th

prom

otingrhizo

bacteria is a well-'nown phenomenon and this

growth enhancement is due

*Corresponding author: *inod ingh' Department of

+icrobiolog"' aratullah,niversit"' hopal' -ndia' &el:

./00 102 /2..3 45mail: vinodsingh678"ahoo$co$in

eceived 9anuar" 1' 1.203 Accepted +arch 2/'

1.203Published +arch 1'1.20

Citation: Gupta G' Parihar ' !hirwar ;2=?5

0=?$2...2??

Cop!right: @ 1.20

Gupta G' et al$ &his is an

open5access article

distributed under the

terms of the Creative

Commons !ttribution

Aicense' which permits

unrestricted use'

distribution' and

reproduction in an"

medium' provided the

or iginal author andsource are credited
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Citation: Gupta G' Parihar ' !hirwar ;2=?50=?$2...2??

to certain traits of rhizobacteria. %here are a number of

mechanisms used by P0P+ for enhancing plant growth and

development in diverse environmental conditions 17igure #2.4ccording to :loepper and Schroth "#$ plant growth promoting

rhizobacteria mediated plant growth promotion occurs by the

alteration of the whole microbial community in rhizosphere

niche through the production of various substances. 0enerally,

plant growth promoting rhizobacteria promote plant growth

directly 17igure #2 by either often due to their ability for nutrient

supply 1nitrogen, phosphorus, potassium and essential minerals2

or modulating plant hormone levels, or indirectly by decreasing

the inhibitory effects of various pathogens on plant growth and

development in the forms of biocontrol agents, root colonizers,

and environmental protectors "#$.

"irect mechanisms

Plant growth promoting rhizobacteria having directmechanisms that facilitate nutrient upta'e or increase nutrient

availability by nitrogen fi!ation, solubilization of mineral

nutrients, mineralize organic compounds and production of

phytohormones "#,#$.

#itrogen i$ation:itrogen is an essential element for all forms

of life and it is the most vital nutrient for plant growth and

productivity. 4lthough the nitrogen presents 36 < of the atmosphere,

it remains unavailable to the plants. +egrettably no plant species is

capable for fi!ing atmospheric dinitrogen into ammonia and e!pend

it directly for its growth. %hus the atmospheric nitrogen is converted

into plant-utilizable forms by biological nitrogen fi!ation 1B72

which changes nitrogen to ammonia by nitrogen fi!ing

microorganisms using a comple! enzyme system 'nown asnitrogenase "#/$.

Plant growth promoting rhizobacteria have the ability to fi!

atmospheric nitrogen and provide it to plants by two mechanisms=

symbiotic and non-symbiotic. Symbiotic nitrogen fi!ation is a

mutualistic relationship between a microbe and the plant. %he

microbe first enters the root and later on form nodules in which

nitrogen fi!ation occurs. +hizobia are a vast group of rhizobacteria

that have the ability to lay symbiotic interactions by the colonization

and formation of root nodules with leguminous plants, where

nitrogen is fi!ed to

ammonia and ma'e it available

for the plant "##$. %he plant

growth promoting rhizobacteriawidely presented as symbionts are

hizobium,

Bradyrhizobium,Sinorhizobium,

and Mesorhizobium with

leguminousplants,Frankiawith

non-leguminous trees and

shrubs "#3$.

)n the other hand, non-

symbiotic nitrogen fi!ation is

carried out by free living

diazotrophs and this can

stimulate non-legume plants

growth such as radish and rice.on-symbiotic itrogen fi!ing

rhizospheric bacteria belonging

to genera includingAzoarcus,

Azotobacter, Acetobacter,

Azospirillum, Burkholderia,

!iazotrophicus, Enterobacter,

"luconacetobacter,

Pseudomonas and cyanobacteria

1Anabaena# $ostoc2 "#(,#6$.

%he genes for nitrogen fi!ation,

called ni% genes are found in

both symbiotic and free living

systems "#8$.itrogenase 1ni%2

genes include structural genes,involved in activation of the 7e

protein, iron molybdenum

cofactor biosynthesis, electron

donation, and regulatory genes

re5uired for the synthesis and

function of the enzyme.

Inoculation by biological

nitrogen fi!ing plant growth

promoting rhizobacteria on crop

provide an integrated approach

for disease management, growth

promotion activity, maintain the

nitogen level in agricultural soil.

Phos%hate solubilization:

Phosphorus is the most

important 'ey element in the

nutrition of plants, ne!t to

nitrogen 12. It plays an

important role in virtually all

ma*or metabolic processes in

plant including photosynthesis,

energy transfer, signal

transduction, macromolecular

biosynthesis and respiration

"(9$. It is abundantly available

in soils in both organic and

inorganic forms. Plants are

unable to utilized phosphate

because 8-88< phosphate

present in the insoluble,

immobilized, and precipitated

form "(#$. Plants absorb

phosphate only in two soluble

forms, the monobasic 1&(P)2

and the diabasic 1&P)(-2 ions

"#($.

Plant growth promoting

rhizobacteria present in the soil

employ different strategies to

ma'e use of unavailable forms of

phosphorus and in turn also help

in ma'ing phosphorus available

for plants to absorb. %he main

phosphate solubilization

mechanisms employed by plant

growth promoting rhizobacteria

include= 1#2 release of comple!ing

or mineral dissolving compounds

e.g. organic acid anions, protons,
http://dx.doi.org/10.4172/1948-5948.1000188http://dx.doi.org/10.4172/1948-5948.1000188http://dx.doi.org/10.4172/1948-5948.1000188

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lantGrothromotinhiobacteria

&G'

irect lant Grothromotion

&Bio3ertilier4ctiit6'

otaium *olubiliation*iderophore roduction

h6tohormone rod&h6totimulation 4c

)ndole 4cetic

4cid .roduction

th6lroduc

"igure #: hematicdigramshowing plant growth promotingbacteria affect plant growthdirectl" and indirectl"$

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hydro!yl ions, >)(, 1(2 liberation of e!tracellular enzymes

1biochemical phosphate mineralization2 and 12 the release of

phosphate during substrate degradation 1biological phosphatemineralization2 "(($.

Phosphate solubilizing P0P+ included in the

genera

Arthrobacter,Bacillus,Bei&erinckia# Burkholderia# Enterobacter#

Erwinia# Flavobacterium# Microbacterium Pseudomonas,

hizobium,

hodococcus, and Serratia have attracted the attention of

agriculturists as soil inoculums to improve plant growth and

yield "#($. &owever, the beneficial effects of the inoculation with

phosphate solubilizing bacteria used alone or in combination

with other rhizospheric microbes have been also reported "($.

Potassium solubilization: Potassium 1:2 is the third ma*or

essential macronutrient for plant growth. %he concentrations ofsoluble potassium in the soil are usually very low and more than

89< of potassium in the soil e!ists in the form of insoluble roc's

and silicate minerals "($. ?oreover, due to imbalanced fertilizer

application, potassium deficiency is becoming one of the ma*or

constraints in crop production. @ithout ade5uate potassium, the

plants will have poorly developed roots, grow slowly, produce

small seeds and have lower yields. %his emphasized the search to

find an alternative indigenous source of potassium for plant

upta'e and to maintain potassium status in soils for sustaining

crop production "($.

Plant growth promoting rhizobacteria are able to solubilize

potassium roc' through production and secretion of organic acids

"(/$. Potassium solubilizing plant growth promotingrhizobacteria such as Acidothiobacillus %erroo'idans, Bacillus

edaphicus, Bacillusmucilaginosus, Burkholderia, Paenibacillus

sp. andPseudomonas hasbeen reported to release potassium in

accessible form from potassium-bearing minerals in soils "(3$.

%hus, application of potassium solubilizing plant growth

promoting rhizobacteria as biofertilizer for agriculture

improvement can reduce the use of agrochemicals and support

ecofriendly crop production.

idero%hore %roduction: Iron is an essential micronutrient

for almost all organisms in the biosphere. Aespite the fact that

iron is the fourth most abundant element on earth, in aerobic

soils, iron is not readily assimilated by either bacteria or plants

because ferric ion or 7e

, which is the predominant form innature, is only sparingly soluble so that the amount of iron

available for assimilation by living organisms is e!tremely low

"(6$. ?icroorganisms have evolved specialized mechanisms for

the assimilation of iron, including the production of low

molecular weight iron-chelating compounds 'nown as

siderophores, which transport this element into their cells "(8,

9$. Siderophores are divided into three main families depending

on the characteristic functional group, i.e. hydro!amates,

catecholates and carbo!ylates. 4t present more than 99 different

types of siderophores are 'nown, of which (39 have been

structurally characterized "#$.

Siderophores have been implicated for both direct and

indirect enhancement of plant growth by plant growth promoting

rhizobacteria. %he direct benefits of bacterial siderophores on the

growth of plants have been demonstrated by using radiolabeled

ferric-siderophores as a sole source of iron showed that plants are

able to ta'e up the labeled iron by a large number of plant

growth promoting rhizobacteria including Aeromonas,

Azadirachta,Azotobacter,Bacillus,Burkholderia,Pseudomonas,

hizobium, Serratia and Streptomyces sp. "($ and enhanced

chlorophyll level compared to

un inoculated plants "$.

Phytohormone

%roduction: 4 wide range of

microorganisms found in the

rhizosphere are able to produce

substances that regulate

plant growth and development.

Plant growth promoting

rhizobacteria produce

phytohormones such as au!ins,cyto'inins, gibberellins and

thylene can affect cell

proliferation in the root

architecture by overproduction

of lateral roots and root hairs

with a subse5uent increase of

nutrient and water upta'e "(8$.

Indole !cetic !cid 'I!!(:

4mong plant growth regulators,

indole acetic acid 1I442 is the

most common natural au!in found

in plants and its positive effect on

root growth "$. Cp to 69< ofrhizobacteria can synthesize

indole acetic acid 1I442 colonized

the seed or root surfaces is

proposed to act in con*unction

with endogenous I44 in plant to

stimulate cell proliferation and

enhance the hostDs upta'e of

minerals and nutrients from the

soil "#6$. Indole acetic acid affects

plant cell division, e!tension, and

differentiation; stimulates seed

and tuber germination; increases

the rate of !ylem and root

development; controls processesof vegetative growth; initiates

lateral and adventitious root

formation; mediates responses to

light, gravity and florescence;

affects photosynthesis, pigment

formation, biosynthesis of various

metabolites, and resistance to

stressful conditions "$.

%ryptophan is an amino acid

commonly found in root e!udates,

has been identified as main

precursor molecule for

biosynthesis of I44 in bacteria"/$. %he biosynthesis of indole

acetic acid by plant growth

promoting rhizobacteria involves

formation via indole--pyruvic

acid and indole--acetic aldehyde,

which is the most common

mechanism in bacteria li'e

Pseudomonas, hizobium,

Bradyrhizobium, Agrobacterium,

Enterobacter and (lebsiella "3$.

+oot growth promotion by the

free living P0P+ e.g.,Alkaligenes

%aecalis# Enterobacter cloacae#

Acetobacter dizotrophicous,species of Azospirillum#

Pseudomonas and )anthomonas

sp. has been related to low level of

I44 secretion. &owever,

microbially produced

phytohormones are more effective

due to the reason that the

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threshold between inhibitory and stimulatory levels of chemically

produced hormones is low, while microbial hormones are more

effective by virtue of their continuous slow release.

)yto*inins and gibberellins: Several plant growth

promotingrhizobacteria Azotobactersp.,hizobiumsp.,Pantoea

agglomerans, hodospirillum rubrum, Pseudomonas

%luorescens# Bacillus subtilis and Paenibacillus polymy'a can

produce cyto'inins or gibberellins or both can produce either

cyto'inins or gibberellins or both for plant growth promotion

"6$. Some strains of phytopathogens can also synthesizecyto'inins. &owever, it appears that plant growth promoting

rhizobacteria produce lower cyto'inin levels compared to

phytopathogens so that the effect of the plant growth promoting

rhizobacteria on plant growth is stimulatory while the effect of

the cyto'inins from pathogens is inhibitory.

thylene is a 'ey phytohormone has a wide range of biological

activities can affect plant growth and development in a large number

of different ways including

promoting root initiation,

inhibiting root elongation,

promoting fruit ripening,

promoting lower wilting,

stimulating seed germination,

promoting leaf abscission,

activating the synthesis of other

plant hormones "8$. %he high

concentration of ethylene induces

defoliation and other cellular

processes that may lead to

reduced crop performance "#($.

%he enzyme #-

aminocyclopropane-# carbo!ylic

acid 14>>2 is a pre-re5uisite for

ethylene production, catalyzed by

4>> o!idase. I5bal ?4 et al. "9$

reported improved nodule number,

nodule dry weight, fresh biomass,

grain yield, straw yield, and

nitrogen content in grains of lentil

as a result of lowering of the

ethylene production via

inoculation with plant growth

promoting strains of

Pseudomonassp. containing 4>>

deaminase along with *

leguminosarum. >urrently,

bacterial strains e!hibiting 4>>

deaminase activity have been

identified in a wide range of

genera

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such as Acinetobacter# Achromobacter# Agrobacterium#

Alcaligenes#Azospirillum# Bacillus# Burkholderia# Enterobacter#

Pseudomonas# alstonia# Serratia andhizobium etc. "6$.In+direct mechanisms: Phytopathogenic microorganisms are a

ma*or and chronic threat to sustainable agriculture and ecosystem

stability worldwide subverts the soil ecology, disrupt environment,

degrade soil fertility and conse5uently show harmful effects on

human health, along with contaminating ground water. Plant growth

promoting rhizobacteria is a promising sustainable and

environmentally friendly approach to obtain sustainable fertility of

the soil and plant growth indirectly. %his approach ta'es inspire a

wide range of e!ploitation of plant growth promoting rhizobacteria

led to reducing the need for agrochemicals 1fertilizers and pesticides2

for improve soil fertility by a variety of mechanisms that via

production of antibiotics, siderophores, &>, hydrolytic enzymes

etc "#,($.

!ntibiosis: %he production of antibiotics is considered to be one

of the most powerful and studied biocontrol mechanisms of plant

growth promoting rhizobacteria against phytopathogens has become

increasingly better understood over the past two decades "3$. 4

variety of antibiotics have been identified, including compounds

such as amphisin, (,-diacetylphloroglucinol 1A4P02, oomycin 4,

phenazine, pyoluteorin, pyrrolnitrin, tensin, tropolone, and cyclic

lipopeptides produced by pseudomonads "$ and oligomycin 4,

'anosamine, zwittermicin 4, and !anthobaccin produced by Bacillus,

Streptomyces, and Stenotrophomonassp. to prevent the proliferation

of plant pathogens 10enerally fungi2 "$. )ne problem with

depending too much on antibiotic-producing plant growth promoting

rhizobacteria as biocontrol agents is some phytopathogens may

develop resistance to specifc antibiotics due to increased use of thesestrains. %o prevent this from happening, some researchers have

utilized biocontrol strains that synthesize one or more antibiotics

"$. In soils, antibiotic (, -diacetylphloroglucinol 1(, - A4P02

producingPseudomonassp. was reported for biocontrol of disease in

wheat caused by the fungus "aeumanomyces graminis var. tritici

"/$. Bacterization of wheat seeds with P* %luorescens strains

producing the antibiotic phenazine-#-carbo!ylic acid 1P>42 resulted

in significant suppression of ta'e-all in about /9< of field trials "3$.

Bacillus amyloli+ue%aciens is 'nown for lipopeptide and poly'etide

production for biological control activity and plant growth promotion

activity against soil borne pathogens "6$. 4part from the production

of antibiotic, some rhizobacteria are also capable of producing

volatile compound 'nown as hydrogen cyanide 1&>2 for biocontrol

of blac' root rot of tobacco, caused by ,hielaviopsis basicola "8$.

Eanteigne et al. "9$ also reported the production of A4P0 and &>

by Pseudomonas contributing to the biological control of bacterial

can'er of tomato.

,ytic enzymes: 0rowth enhancement through enzymatic

activity is another mechanism used by plant growth promoting

rhizobacteria. Plant growth promoting rhizobacterial strains can

produce certain enzymes such as chitinases, dehydrogenase, F-

glucanase, lipases, phosphatases, proteases etc. "#,($ e!hibit

hyperparasitic activity, attac'ing pathogens by e!creting cell wall

hydrolases. %hrough the activity of these enzymes, plant growth

promoting rhizobacteria play a very significant role in plant growth

promotion particularly to protect them from biotic and abioticstresses by suppression of pathogenic fungi including Botrytis

cinerea# Sclerotium rol%sii# Fusarium o'ysporum#Phytophthora sp*#

hizoctonia solani# andPythium ultimum ",$.

umbers of reports have shown the effectiveness of plant

growth promoting rhizobacteria as biocontrol agents including

Pseudomonas%luorescens >&49 suppresses blac' root rot of

tobacco caused by

the fungus ,hielaviopsis basicola

"$,Pseudomonas putidaagainst

Macrophomina phaseolina in

chic'pea and Azotobacterchroococcum against Fusarium

o'ysporum in Sesamum indicum

respectively in field condition "8$.

In recent years, Pseudomonas

%luorescenshas been suggested as

potential biological control agent

due to its ability to colonize

rhizosphere and protect plants

against a wide range of important

agronomic fungal diseases such as

blac' root-rot of tobacco, root-rot

of mustard and damping-off of

sugar beet in field condition "/-

6$. It has been demonstrated thatinoculation of plant with

arbascular mycorrhiza also

improves plant growth. Some

strains of ,richoderma have been

widely used as biological control

agents as well as plant growth

promoters "8$. Inoculation with

,richoderma sp. has been the

preferred choice for novel

biocontrol agents against

Aspergillus niger the causal agent

of collar rot of peanut "/9$. %he

use of multistrain inoculants are

also be a good strategy thatenables organisms to successfully

survive, maintain themselves in

communities. Singh et al. "/#$

recently showed the synergistic

effect of antagonistic fungi

,richoderma with combined

application of Pseudomonas and

rhizobial strains can protect

chic'pea from infection by the

collar rot pathogen

Sclerotium rol%sii.

idero%hore: Iron is an

essential growth cofactor forliving organisms. 7or the soil

microorganisms, availability of

solubilized ferric ion in soils is

limited at neutral and al'aline p&.

Siderophore producing plant

growth promoting rhizobacteria

can prevent the proliferation of

pathogenic microorganisms by

se5uestering 7e in the area

around the root "/($. %hese

siderophores binds with ferric ion

and ma'e siderophore-ferric

comple! which subse5uently

binds with iron-limitation-dependent receptors at the

bacterial cell surface. %he 7erric

ion is subse5uently released and

active in the cytoplasm as ferrous

ion.

?any plants can use various

bacterial siderophores as iron

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sources, although the total concentrations are probably too low to

contribute substantially to plant iron upta'e. Garious studies showed

isolation of siderophore producing bacteria belonging to the

Bradyrhizobium,

Pseudomonas, hizobium, Serratia and Streptomyces genera

from therhizosphere "/$.

Induced systemic resistance 'IR(: Induced resistance may

be defined as a physiological state of enhanced defensive

capacity elicited in response to specific environmental stimuli

and conse5uently the plantDs innate defenses are potentiatedagainst subse5uent biotic challenges "/$. Biopriming plants with

some plant growth promoting rhizobacteria can also provide

systemic resistance against a broad spectrum of plant pathogens.

Aiseases of fungal, bacterial, and viral origin and in some

instances even damage caused by insects and nematodes can be

reduced after application of plant growth promoting rhizobacteria

"/$. ?oreover, induced systemic resistance involves *asmonate

and ethylene signaling within

the plant and these hormones

stimulate the host plantDs

defense responses against a

variety of plant pathogens "$.

?any individual bacterial

components induce induced

systemic resistance such as

lipopolysaccharides 1EPS2,

flagella, siderophores, cycliclipopeptides, (, -

diacetylphloroglucinol,

homoserine lactones, and

volatiles li'e, acetoin and (, -

butanediol "//$.

-$o %olysaccharides

%roduction or bioilm

ormation: >ertain bacteria

synthesize a wide spectrum of

multifunctional polysaccharides

including intracellular

polysaccharides, structural

polysaccharides, and e!tracellular

polysaccharides. Production of

e!o polysaccharides is generally

important in biofilm formation;

root colonization can affect theinteraction of microbes with roots

appendages. ffective

colonization of plant roots by

PS-producing microbes helps to

hold

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the free phosphorous from the insoluble one in soils and

circulating essential nutrient to the plant for proper growth and

development and protecting it from the attac' of foreignpathogens. )ther innumerable functions performed by PS

producing microbes constitute shielding from desiccation,

protection against stress "/3$, attachment to surfaces plant

invasion, and plant defense response in plantHmicrobe

interactions "/6$. Plant growth promoting rhizobacterial

producing e!o polysaccharides are highly important in promoting

plant growth due to wor' as an active signal molecule during

beneficial interactions, and provide defense response during

infection process "/8$. Some plant growth promoting

rhizobacterial producing e!o polysaccharides can also bind

cations, including asuggesting a role in mitigation of salinity

stress by reducing the content of aavailable for plant upta'e

"(8$.

)ommercialization o PGPR

%he success and commercialization of plant growth

promoting rhizobacterial strains depend on the lin'ages between

the scientific organizations and industries. umerous wor' done

showed different stages in the process of commercialization

include isolation of antagonist strains, screening, fermentation

methods, mass production, formulation viability, to!icology,

industrial lin'ages, 5uality control and field efficacy "39$.

?oreover, commercial success of P0P+ strains re5uires

economical and viable mar'et demand, consistent and broad

spectrum action, safety and stability, longer shelf life, low capital

costs and easy availability of career materials.

Plant growth %romotory bioormulations

Bioformulations are best defined as biologically active

products containing one or more beneficial microbial strains in

an easy to use and economical carrier material. ?ost

bioformulations are meant for field application, it is essential that

suitable carrier materials are used to maintain cell viability under

adverse environmental conditions. 4 good 5uality formulation

promotes survival of bacteria maintaining available population

sufficient to e!ude growth promoting effects on plants "3#$. Plant

growth promoting rhizobacterial bioformulation refers to

preparations of microorganism that may be partial or complete

substitute for chemical fertilization, pesticides, offer an

environmentally sustainable approach to increase crop

production and health.

Formulation design

7ormulation is the crucial issue for inoculants containing an

effective bacterial strain and can determine the success or failure

of a biological agent. %he use of inoculant formulations

involving carrier materials for the delivery of microbial cells to

soil or the rhizosphere is an attractive option. >arrier materials

are generally intended to provide a protective niche to microbial

inoculants in soil, either physically, via the provision of a

protective surface or pore space or nutritionally, via the provision

of a specific substrate.

Bioformulation of plant growth promoting rhizobacteria should

be composed of a superior carrier material such as high water-holding capacity, high water retention capacity, no heat production

from wetting, nearly sterile, chemically uniform, physically uniform,

nonto!ic in nature, easily biodegradable, nonpolluting, nearly neutral

p& 1or easily ad*ustable p&2, and supports bacterial growth and

survival. 4part from these materials, many other synthetic and inert

materials, such as vermiculite, ground roc' phosphate, calcium

sulfate, polyacrylamide gels, and alginate have also been evaluated

"3($.

Arying is a part of many

procedures for development of

formulation of microbial

inoculants. +emar'ably lowpercentage of endospore formers

was observed that survived after

drying "$. )ne factor which

can have a detrimental effect on

dried microorganisms over the

long term is humidity in the

environment; increasing

moisture content of the dried

sample compromises viability.

Storage under vacuum or in an

inert atmosphere can prevent

this but is costly and unwieldy.

%he use of each type of

inoculant depends upon mar'etavailability, choice of farmers,

cost, and the need of a particular

crop under specific

environmental conditions "3$.

Future Research and

"e.elo%ment trategies

or ustainable

Technology

%he need of todayDs world is

high output yield and enhanced

production of the crop as well asfertility of soil to get in an eco-

friendly manner. &ence, the

research has to be focused on

the new concept of

rhizoengineering based on

favorably partitioning of the

e!otic biomolecules, which

create a uni5ue setting for the

interaction between plant and

microbes "3$. 7uture research

in rhizosphere biology will rely

on the development of

molecular and biotechnological

approaches to increase our'nowledge of rhizosphere

biology and to achieve an

integrated management of soil

microbial populations. 7resh

alternatives should be e!plored

for the use of bioinoculants for

other high value crops such as

vegetables, fruits, and flowers.

%he application of multi strain

bacterial consortium over single

inoculation could be an effective

approach for reducing the

harmful impact of stress on

plant growth. %he addition of

ice-nucleating plant growth

promoting rhizobacteria could

be an effective technology for

enhancing plant growth at low

temperature "$.

+esearch on nitrogen

7/26/2019 PGPR word

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fi!ation and phosphate solubilization by plant growth promoting

rhizobacteria is progress on but little research can be done on

potassium solubilization which is third ma*or essential

macronutrient for plant growth. %his will not only increase the

field of the inoculants but also create confidence among the

farmers for their use. In addition, future mar'eting of

bioinoculant products and release of these transgenics into the

environment as eco-friendly alternations to agrochemicals will

depend on the generation of biosafety data re5uired for the

registration of plant growth promoting rhizobacterial agents. 4part from that future research in optimizing growth condition and

increased self life of P0P+ products, not phytoto!ic to crop

plants, tolerate adverse environmental condition, higher yield and

cost effective P0P+ products for use of agricultural farmer will

be also helpful.

e$erences

1. Boussef ++!' 4issa +F+ (1.2) iofertilizers and their role in

management of plant parasitic

nematodes$ 4 9 iotechnol

Pharm Res 0: 256$

2. 9oshi

7/26/2019 PGPR word

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J Microb Biochem Technol %olume 7&2': 0"($102&2015' $ 100


7/26/2019 PGPR word

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promoting +esorhizobium on the performance of chicpea grown in

insecticide stressed alluvial soils$ 9 Crop ci iotechnol 21: 1275111$

7. ivasahti ' ,sharani G' aranra% P (1.2) iocontrol potentialit" of

plant growth promoting bacteria (PGPR)5 Pseudomonas fluorescence

and acillus subtilis: ! review$ !frican 9ournal of !gricultural Research

=: 2160521//$

8. agar ' Dwivedi !' Badav ' &ripathi +'

7/26/2019 PGPR word

17/20

40. -Ebal +!'

7/26/2019 PGPR word

18/20

J Microb Biochem Technol %olume 7&2': 0"($102&2015' $ 101


7/26/2019 PGPR word

19/20


for ustainable !griculture: Fundamentals and Recent !dvances$ -n: !rora

;

7/26/2019 PGPR word

20/20

Growth Promoting Rhizobacteria (PGPR): Current

and Future Prospects for Development of

ustainable !griculture$ 9 +icrob iochem &echnol

/: .=652.1$ doi:2.$2/1>2=?50=?$2...2??

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