applications of free living plant growth-promoting...
TRANSCRIPT
Applications of free living plant growth-promoting rhizobacteria
M. Lucy, E. Reed and Bernard R. Glick*Department of Biology, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1; *Author forcorrespondence (e-mail: [email protected]; phone: (519) 888-4567 ext. 5208; fax: (519) 746-0614)
Received 4 June 2003; accepted in revised form 30 October 2003
teria
Abstract
Free-living plant growth-promoting rhizobacteria �PGPR� can be used in a variety of ways when plant growthenhancements are required. The most intensively researched use of PGPR has been in agriculture and horticul-ture. Several PGPR formulations are currently available as commercial products for agricultural production. Re-cently developing areas of PGPR usage include forest regeneration and phytoremediation of contaminated soils.As the mechanisms of plant growth promotion by these bacteria are unravelled, the possibility of more efficientplant-bacteria pairings for novel and practical uses will follow. The progress to date in using PGPR in a varietyof applications with different plants is summarized and discussed here.
Introduction
New and novel solutions for plant growth enhance-ments are required to ease the burden imposed on ourenvironment and other resources. Here we look atpotential solutions to these issues by examining someof the research conducted regarding the biologicalapplications of free-living plant growth promotingrhizobacteria �PGPR�. The major applications of bac-teria for improved plant growth include agriculture,horticulture, forestry and environmental restoration.This review presents an overview of informationavailable on these various applications.
Indirect mechanisms used by PGPR include antibi-otic protection against pathogenic bacteria, reductionof iron available to phytopathogens in the rhizo-sphere, synthesis of fungal cell wall-lysing enzymes,and competition with detrimental microorganisms forsites on plant roots. Direct mechanisms of plantgrowth by PGPR include the provision of bioavail-able phosphorus for plant uptake, nitrogen fixation forplant use, sequestration of iron for plants by sidero-phores, production of plant hormones like auxins, cy-
tokinins and gibberellins, and lowering of plantethylene levels �Glick 1995; Glick et al. 1999�.
Applications of PGPR in Agriculture
The most intensively studied application for free liv-ing PGPR is agriculture. Researchers in the formerSoviet Union and India conducted widespread tests inthe early to the mid part of the 20th century studyingthe effects of PGPR on different crops. Though resultsfrom different experiments were not harmonized andwere often inconsistent, up to 50 to 70% yieldincreases were reported. Inconsistency of results wasdue to a lack of quality in experimental design andanalysis of results �Brown 1974; Cooper 1959�.Moreover, during this time an understanding of thedetailed mechanisms of plant growth promotion byrhizobacteria was largely unknown. Nevertheless,these field experiments provided clues concerning theoptimal conditions for bacterial colonization andgrowth promotion of target crops �Brown 1974�.
The results of many studies of the effect of free-living rhizobacteria on various crop plants, conducted
Antonie van Leeuwenhoek 86: 1–25, 2004.© 2004 Kluwer Academic Publishers. Printed in the Netherlands.
1
Key
Review article
words: Agriculture, Forestry, Horticulture, Inoculants, Phytoremediation, Plant growth promoting rhizobac-
over approximately the last twenty-five years, aresummarized in Table 1. Plant growth benefits due tothe addition of PGPR include increases in germina-tion rates, root growth, yield �including grain�, leafarea, chlorophyll content, magnesium content, nitro-gen content, protein content, hydraulic activity, toler-ance to drought, shoot and root weights, and delayedleaf senescence. Another major benefit of PGPR useis disease resistance conferred to the plant, sometimesknown as ‘biocontrol’.
The use of PGPR to increase crop yield has beenlimited due to the variability and inconsistency of re-sults between laboratory, greenhouse and field stud-ies �Mishustin and Naumova 1962�. Soil is anunpredictable environment and an intended result issometimes difficult to obtain �Bashan 1998�. For ex-ample, in a study by Frommel et al. �1993�, poor col-onization of the PGPR on plant roots occurred at onesite due to adverse conditions, including high Verti-cillum infection of the soil, low soil pH, high meantemperature and low rainfall during the growing sea-son. These undesirable growing conditions mostlikely contributed to the low root colonization �Dob-belaere et al. 2001; Klein et al. 1990; Parke 1991;Suslow and Schroth 1982�. Climatic variability alsohas a large impact on the effectiveness of PGPR�Okon and Labandera-Gonzalez 1994� but sometimesunfavourable growth conditions in the field are to beexpected as a normal functioning of agriculture. In-creased yields obtained with wheat inoculated byPseudomonas species in the growth chamber havealso been observed in the field �Weller and Cook1986�. Even though there is a possibility of greatvariability in field results, if a positive effect of aPGPR is seen on a specific crop in greenhouse stud-ies, there is a strong likelihood that those benefits willcarry through to field conditions.
There is a great deal of contradictory informationon the effectiveness of PGPR on plants in soils undervarious conditions of fertilization, especially withAzospirillum species. In the past, the main mechanismof plant growth promotion by Azospirillum wasthought to occur by providing the plant with fixed ni-trogen. In fact, it has been reported that the plantgrowth promotion effect of Azospirillum only occursin nitrogen-limited conditions �Dobbelaere et al.2001; Fallik and Okon 1996�. However, in othercases, much greater increases in plant growth promo-tion have been observed after the addition of fertiliz-ers �Okon and Labandera-Gonzalez 1994�. Anotherway in which Azospirillum species improve plant
growth is through the production of indole acetic acid�IAA�, a plant hormone �Dobbelaere et al. 1999;Okon and Labandera-Gonzalez 1994�.
Different soil types can influence the effectivenessof PGPR �e.g., Kloepper et al. 1980�. In a study withwheat and a pseudomonad, results suggested that theless fertile the soil, the greater the plant growth stim-ulation by the PGPR �De Freitas and Germida 1990�.This is similar to observations in studies conductedwith Azospirillum species, despite the fact thatpseudomonads fix little or no nitrogen. On the otherhand, growth promotion of maize with a strain ofAzospirillum lipoferum has been reported to be inde-pendent of cultivar or soil type in the field �Fages1994�. When choosing an effective PGPR for a plantat a specific site, it is imperative to consider the nu-trient level in the soil and how the intended PGPRwould perform at that location.
Soil moisture content affects the colonization of theplant rhizosphere by the PGPR after inoculation �Burret al. 1978�. Studies in the Soviet Union suggestedthat optimum results were obtained when the soilmoisture was 40% �Brown 1974; Cooper 1959�.However, this may be a function of the type of bac-terium utilized since high moisture content may de-crease the oxygen content of the soil.
In some instances, specific strains of bacteria maypromote bacterial growth only in certain crops. In oneexample, it was found that out of four Pseudomonasstrains that promoted the growth of radish, only onewas effective in promoting the growth of potato�Kloepper et al. 1980�. It has also been observed thatmaximum increases in germination and yield oftenoccur in crops inoculated with PGPR strains isolatedfrom the plants native rhizosphere �Fages and Arsac1991; Favilli et al. 1987�.
During the initial stages of testing in the laboratory,PGPR survival in a microcosm of the field environ-ment should be determined. This is to ensure that anymanipulations conducted on the bacteria are not det-rimental to their growth promotion effect and theircompetitiveness in the field �Tang et al. 1995�. In thefield, the number of PGPR cells applied to the plantis often vital for proper growth promotion �Boddeyand Dobereiner 1988�. Many researchers �Table 1�have applied up to 108 colony forming units �CFU�per seed �De Freitas and Germida 1991; Di Cioccoand Rodriguez-Caceres 1994; Fages 1994; Okon et al.1988; Tran Van et al. 2000; Weller and Cook 1986�or up to 109 CFU/g of inoculant �Barrios et al. 1984;De Freitas and Germida 1990a, 1990b; Fallik and
2
Tabl
e1.
Exa
mpl
esof
free
-liv
ing
plan
tgr
owth
prom
otin
grh
izob
acte
ria
test
edon
vari
ous
crop
type
s
Bac
teri
aPl
ant
Con
ditio
nsR
esul
tsof
addi
tion
ofba
cter
iato
plan
tR
efer
ence
Azo
spir
illu
m�l
ocal
isol
ates
from
Arg
entin
a �W
heat
,M
aize
Fiel
d–
inw
heat
culti
vars
over
seve
nse
ason
s,in
crea
ses
ofyi
eld
from
15to
30%
,an
din
crea
ses
inyi
eld
of50
-60%
whe
nfe
rtili
zed
–ov
ersi
xse
ason
s,in
crea
ses
ofm
aize
yiel
dfr
om15
to25
%ob
serv
ed,
and
with
fert
iliza
tion,
yiel
din
crea
sed
upto
40%
Oko
nan
dL
aban
dera
-Gon
za-
lez
1994
Azo
spir
illu
mbr
asil
ense
Gui
nea
gras
sPe
arl
mill
et,
Dig
itar
iade
cum
bens
Fiel
d–
grea
ter
dry
mat
ter
yiel
dco
mpa
red
toun
inoc
ulat
edco
n-tr
ols
–ap
prox
imat
ely
40kg
/ha
per
year
ofni
trog
enes
timat
edas
save
ddu
eto
inoc
ulat
ion
Smith
etal
.19
78
Azo
spir
illu
mbr
asil
ense
Fing
erm
illet
,So
rghu
m,
Pear
lm
illet
Fiel
d–
aver
age
ofup
to15
%yi
eld
incr
ease
for
finge
rm
illet
–fo
rso
rghu
m,
aver
age
incr
ease
is19
%–
inte
nye
ars
ofst
udy,
Azo
spir
illu
msu
cces
sful
insi
gnifi
-ca
ntly
incr
easi
ngyi
eld
in60
%of
tria
ls
Rao
1986
Azo
spir
illu
mbr
asil
ense
Sorg
hum
Hyd
ropo
nic
syst
emin
gree
n-ho
use
–si
gnifi
cant
incr
ease
sin
dry
mat
ter,
leaf
area
and
grai
nyi
eld
afte
rfo
urw
eeks
–le
afse
nesc
ence
dela
yed
Sari
get
al.
1990
Azo
spir
illu
mbr
asil
ense
Sorg
hum
Gre
enho
use
–in
crea
sed
tota
lnu
mbe
ran
dle
ngth
ofad
vent
itiou
sro
ots
by33
to40
%an
din
crea
sed
hydr
aulic
cond
uctiv
ityby
25-4
0%
Sari
get
al.
1992
Azo
spir
illu
mbr
asil
ense
Bea
nH
ydro
poni
cgr
owth
cham
ber
–in
crea
sed
fres
hro
otan
dsh
oot
wei
ghts
Ved
der-
Wei
sset
al.
1999
Azo
spir
illu
mbr
asil
ense
Sorg
hum
,W
heat
,B
arle
y
Fiel
d�t
este
don
over
200,
000
ha�
–co
nsis
tent
posi
tive
effe
cts
with
upto
26%
incr
ease
ofyi
eld
–be
stre
sults
seen
inlig
htsa
ndy
soils
with
inte
rmed
iate
amou
nts
offe
rtili
zers
Dob
bela
ere
etal
.20
01
Azo
spir
illu
mbr
asil
ense
Cd
Foun
tain
gras
sSo
rghu
mSu
dang
rass
Fiel
d–
incr
ease
sin
yiel
dof
11to
24%
aton
elo
catio
nSm
ithet
al.
1984
Azo
spir
illu
mbr
asil
ense
Cd
Mai
ze,
Mill
etSo
rghu
m,
Whe
atFi
eld
–su
mm
ercr
ops
show
sign
ifica
ntyi
eld
incr
ease
sin
75%
oftr
ial
plot
s,w
hile
win
ter
crop
son
lyha
vesi
gnifi
cant
yiel
din
crea
ses
in5-
12%
oftr
ials
Oko
net
al.
1988
3
Tabl
e1.
Con
tinue
d.
Bac
teri
aPl
ant
Con
ditio
nsR
esul
tsof
addi
tion
ofba
cter
iato
plan
tR
efer
ence
Azo
spir
illu
mbr
asil
ense
Cd
Azo
spir
illu
mli
porf
erum
Br-
17
Mai
zeFi
eld
–co
nsis
tent
incr
ease
sof
yiel
dat
inte
rmed
iate
soil
fert
ility
–re
plac
es35
-40%
ofni
trog
enfe
rtili
zers
Oko
nan
dL
aban
dera
-Gon
za-
lez
1994
Azo
spir
illu
mbr
asil
ense
Cd
Mai
zeFi
eld
–si
gnifi
cant
incr
ease
inth
enu
mbe
rof
adve
ntiti
ous
root
s,ro
otle
ngth
,ro
otan
dsh
oot
dry
wei
ght
inth
ree
diff
eren
tre
gion
s
Dob
bela
ere
etal
.20
01
Azo
spir
illu
mbr
asil
ense
Cd
Chi
ckpe
as,
Faba
bean
sG
reen
hous
ean
dfie
ld–
sign
ifica
ntin
crea
ses
inro
otno
dula
tion
byna
tive
rhiz
o-bi
aan
dim
prov
edro
otan
dsh
oot
deve
lopm
ent
–w
hen
irri
gate
dby
salin
ew
ater
,in
ocul
ated
plan
tsar
eno
tas
nega
tivel
yaf
fect
edco
mpa
red
toco
ntro
l–
field
resu
ltssh
owa
sign
ifica
ntin
crea
sein
shoo
tgr
owth
and
crop
yiel
d
Ham
aoui
etal
.20
01
Azo
spir
illu
mbr
asil
ense
Cd,
245
Oat
,M
aize
,So
rghu
m
Gre
enho
use
and
field
–po
sitiv
ein
crea
ses
inm
aize
and
sorg
hum
biom
ass
inbo
thfie
ldan
dgr
eenh
ouse
,bu
tno
tst
atis
tical
lysi
gnifi
cant
–si
gnifi
cant
incr
ease
ofbi
omas
sof
oat
for
one
year
’sfie
ldte
stbu
tno
tfo
rth
ese
cond
year
Dob
bela
ere
etal
.20
01
Azo
spir
illu
mbr
asil
ense
Cd,
Az-
39A
zosp
iril
lum
lipo
feru
mA
z-30
Mill
etFi
eld
–in
crea
sed
yiel
dup
to30
%an
d21
%in
two
year
sof
plan
tgr
owth
Di
Cio
cco
and
Rod
rigu
ez-
Cac
eres
1994
Azo
spir
illu
mbr
asil
ense
Cd,
Az-
39W
heat
Fiel
d–
plan
tyi
eld
incr
ease
d,es
peci
ally
with
Az-
29C
acer
eset
al.
1996
Azo
spir
illu
mbr
asil
ense
NO
40R
ice
Fiel
d–
incr
ease
dyi
eld
by15
-20%
intw
olo
catio
nsO
mar
etal
.19
89
Azo
spir
illu
mbr
asil
ense
Sp24
5A
zosp
iril
lum
irak
ense
KB
Cl
Win
ter
whe
at,
Mai
zeFi
eld
–pl
ant
grow
thpr
omot
ion
effe
ctdi
sapp
eare
dw
hen
plan
tsov
erfe
rtili
zed
with
nitr
ogen
–in
plot
sw
ithlo
wni
trog
en,
high
eryi
elds
not
obta
ined
,lik
ely
due
toad
vers
ew
eath
erco
nditi
ons
atte
stpl
ots
Dob
bela
ere
etal
.20
01
Azo
spir
illu
mbr
asil
ense
Sp-
111
Whe
atFi
eld
–yi
eld
incr
ease
sof
1.3
to2
fold
over
the
five
year
stud
y–
vari
able
resu
ltsdu
eto
clim
actic
cond
ition
sO
kon
and
Lab
ande
ra-G
onza
-le
z19
94
4
Tabl
e1.
Con
tinue
d.
Bac
teri
aPl
ant
Con
ditio
nsR
esul
tsof
addi
tion
ofba
cter
iato
plan
tR
efer
ence
Azo
spir
illu
mbr
asil
ense
Sp-
245,
Sp-1
07st
Whe
atFi
eld
–si
gnifi
cant
incr
ease
sof
grai
nyi
eld
and
plan
tni
trog
enco
nten
t–
stra
inSp
-245
mos
tef
fect
ive
onw
heat
Bod
dey
and
Dob
erei
ner
1988
Azo
spir
illu
mli
pofe
rum
Sunfl
ower
Gre
enho
use
–st
rain
sor
igin
ally
sele
cted
from
plan
trh
izos
pher
e–
posi
tive
grow
thre
spon
ses
with
resp
ect
toge
rmin
atio
nFa
ges
and
Ars
ac19
91
Azo
spir
illu
mli
pofe
rum
CR
T1
Mai
zeFi
eld
–gr
owth
prom
otio
nef
fect
occu
rred
earl
y,de
spite
rapi
dde
crea
seof
bact
eria
lde
nsity
ofin
trod
uced
bact
eria
–pl
ant
heig
ht,
prim
ary
root
leng
than
dro
otfr
esh
wei
ght
all
enha
nced
byth
ead
ditio
nof
the
bact
eria
Jaco
udet
al.
1998
Azo
spir
illu
mli
pofe
rum
CR
T1
Mai
zeFi
eld
–av
erag
egr
ain
yiel
dsan
dN
cont
ent
high
erfo
rth
ein
ocu-
late
dpl
ants
,bu
tno
tst
atis
tical
lysi
gnifi
cant
–la
rger
root
syst
ems,
and
low
ergr
ain
moi
stur
em
easu
red
Dob
bela
ere
etal
.20
01
Azo
spir
illu
mli
pofe
rum
CR
T-1
Mai
zeFi
eld
–po
sitiv
ere
spon
seof
yiel
dto
inoc
ulat
ion
rega
rdle
ssof
culti
var
orso
ilty
peFa
ges
1994
Azo
spir
illu
msp
.M
aize
Fiel
d–
incr
ease
dyi
eld
of6.
7to
75.1
%K
apul
nik
etal
.19
81
Azo
spir
illu
msp
.W
heat
Fiel
d–
chan
ges
inyi
eld
of�
9.6
to14
.8%
Rey
nder
san
dV
lass
ak19
82
Azo
spir
illu
msp
.W
heat
Fiel
d–
chan
ges
inyi
eld
of�
15.8
to31
%B
alda
niet
al.
1987
Azo
spir
illu
msp
.M
illet
Fiel
d–
chan
ges
inyi
eld
of�
12.1
to31
.7%
Klo
eppe
ret
al.
1989
Azo
spir
illu
msp
.M
usta
rdFi
eld
–in
crea
sed
yiel
dof
16to
128%
Klo
eppe
ret
al.
1989
Azo
spir
illu
msp
.R
ice
Fiel
d–
incr
ease
dyi
eld
of4.
9to
15.5
%K
loep
per
etal
.19
89
Azo
spir
illu
msp
.M
aize
Fiel
d–
sign
ifica
ntin
crea
ses
inyi
eld
inlig
htso
ilsan
dat
inte
r-m
edia
teni
trog
enfe
rtili
zatio
n–
infie
lds
with
noni
trog
enfe
rtili
zatio
n,a
defin
iteyi
eld
incr
ease
over
unin
ocul
ated
plan
ts,
but
not
stat
istic
ally
sig-
nific
ant
–in
field
sw
ithhi
ghni
trog
enfe
rtili
zatio
n,no
grow
then
-ha
ncem
ent
effe
ct
Oko
nan
dL
aban
dera
-Gon
za-
lez
1994
5
Tabl
e1.
Con
tinue
d.
Bac
teri
aPl
ant
Con
ditio
nsR
esul
tsof
addi
tion
ofba
cter
iato
plan
tR
efer
ence
Azo
spir
illu
msp
.M
aize
Fiel
d–
noef
fect
ofin
ocul
atio
non
plan
tyi
elds
whe
nso
ilsar
ehe
avy
and
high
inni
trog
enco
nten
t–
inlig
htso
ilslo
win
nitr
ogen
fert
iliza
tion,
yiel
din
crea
seof
11-1
4%
Falli
kan
dO
kon
1996
Azo
spir
illu
msp
.So
rghu
mFi
eld
–in
crea
sed
yiel
dof
20.5
to30
.5%
Kap
ulni
ket
al.
1997
Azo
spir
illu
msp
.M
aize
Fiel
d–
sign
ifica
ntin
crea
sein
dry
mat
ter
yiel
d–
incr
ease
dm
agne
sium
perc
enta
geH
erna
ndez
etal
.19
97
Azo
spir
illu
msp
.So
rghu
mFi
eld
–in
crea
sed
yiel
dof
12to
18.5
%Sa
rig
etal
.19
98
Azo
spir
illu
msp
.M
aize
Gre
enho
use
–in
crea
sed
activ
ityof
glut
amat
ede
hydr
ogen
ase
and
glut
amin
esy
nthe
tase
–in
crea
sed
Nco
nten
tin
leav
esan
dro
ots
Rib
audo
etal
.20
01
Azo
spir
illu
msp
.W
heat
Gre
enho
use
–hi
gher
biom
ass,
grai
nyi
eld,
prot
ein
cont
ent
and
plan
tni
trog
enco
nten
t–
conc
lude
dth
atni
trog
enup
take
isth
em
echa
nism
ofpl
ant
grow
thpr
omot
ion
inth
isca
se
Saub
idet
etal
.20
02
Azo
toba
cter
chro
ococ
cum
Bar
ley
Gro
wth
cham
ber
assa
ys–
incr
ease
ofse
edge
rmin
atio
nan
dse
edlin
gde
velo
pmen
t–
noch
ange
inth
enu
mbe
rof
seed
sge
rmin
ated
,bu
tin
-cr
ease
seen
inth
eex
tens
ion
ofth
egr
owin
gro
ot–
addi
tion
ofni
trat
ede
crea
ses
plan
tst
imul
atio
nef
fect
–so
me
inco
nsis
tent
resu
ltsse
en,
and
auth
ors
conc
lude
that
Azo
toba
cter
isno
ta
relia
ble
inoc
ulan
t
Har
per
and
Lync
h19
79
Azo
toba
cter
sp.
Bac
illu
ssp
.E
nter
obac
ter
sp.
Xan
thob
acte
rsp
.
Ric
eFi
eld
–in
crea
ses
ofto
tal
dry
mat
ter
yiel
d,gr
ain
yiel
d,an
dni
tro-
gen
accu
mul
atio
nby
6to
24%
over
two
year
sof
stud
y–
hypo
thes
ised
that
yiel
din
crea
ses
due
toin
crea
sein
root
leng
th,
leaf
area
and
chlo
roph
yll
cont
ent
Ala
met
al.
2001
Bac
illu
sam
yliq
uefa
cien
sIN
937
Bac
illu
spu
mil
isIN
R7,
SE34
Bac
illu
ssu
btil
isG
B03
Bac
illu
sce
reus
C4
Tom
ato,
Pepp
erFi
eld
–st
atis
tical
lysi
gnifi
cant
incr
ease
sof
plan
tgr
owth
intw
ogr
owin
gye
ars
inst
emdi
amet
er,
stem
area
,le
afsu
rfac
ear
ea,
wei
ghts
ofro
ots
and
shoo
tsan
dnu
mbe
rof
leav
es–
tran
spla
ntvi
gour
and
frui
tyi
eld
impr
oved
–pa
thog
ennu
mbe
rsan
ddi
seas
eno
tre
duce
din
tom
atoe
sor
pepp
erw
ithth
eex
cept
ion
ofre
duct
ion
ofga
lling
inpe
pper
byro
ot-k
not
nem
atod
e
Kok
alis
-Bur
elle
etal
.20
02
6
Tabl
e1.
Con
tinue
d.
Bac
teri
aPl
ant
Con
ditio
nsR
esul
tsof
addi
tion
ofba
cter
iato
plan
tR
efer
ence
Bac
illu
spo
lym
yxa
Whe
atFi
eld
–in
crea
ses
inpl
ant
yiel
dC
acer
eset
al.
1996
Bac
illu
spo
lym
yxa
Bur
khol
deri
asp
.P
seud
omon
assp
.
Suga
rbe
etFi
eld
–si
gnifi
cant
incr
ease
sin
root
yiel
d�6
.1to
13.0
%�,
suga
ryi
eld
�2.3
to7.
8%
�–
yiel
dsfu
rthe
ren
hanc
edby
N,
Pan
dN
Pap
plic
atio
ns
Cak
mak
ciet
al.
2001
Bac
illu
ssp
.So
rghu
mFi
eld
–in
crea
sed
yiel
dof
15.3
to33
%B
road
bent
etal
.19
77
Bac
illu
ssp
.W
heat
Fiel
d–
chan
ges
inyi
eld
of0
to11
4%K
loep
per
etal
.19
77
Bac
illu
ssu
btil
isA
-13
Pean
utFi
eld
–yi
eld
incr
ease
sup
to37
%;
only
two
of24
test
site
spr
o-du
ced
nega
tive
resu
lts–
plan
tre
spon
ses
mos
tpo
sitiv
ew
hen
subj
ecte
dto
stre
sslik
elim
ited
wat
er,
poor
nutr
ition
,co
ldte
mpe
ratu
res
–pl
ant
dise
ase
redu
ced
Tur
ner
and
Bac
kman
1991
Bac
illu
ssu
btil
isB
2O
nion
Gro
wth
cham
ber
–si
gnifi
cant
incr
ease
sin
shoo
tdr
yw
eigh
t�1
2-94
%�,
dry
root
wei
ght
�13-
100%
�an
dsh
oot
heig
ht�1
2-40
%�
over
cont
rols
–rh
izos
pher
epo
pula
tions
ofin
ocul
ated
bact
eria
decr
ease
dth
roug
hout
stud
y,bu
tgr
owth
prom
otio
nef
fect
still
ob-
serv
ed
Red
dyan
dR
ahe
1989
Bei
jeri
ncki
am
obil
isC
lost
ridi
umsp
.B
eet,
Bar
ley,
Whe
at,
Red
radi
sh,
Cuc
umbe
r
Lab
expe
rim
ents
and
gree
n-ho
use
–in
com
bina
tion
with
min
eral
fert
ilize
rs,
incr
ease
ofpl
ant
prod
uctio
nby
1.5
to2.
5tim
es–
posi
tive
incr
ease
sse
enw
ithse
edge
rmin
atio
nra
te,
mea
npl
ant
leng
than
dpl
ant
wei
ght
–di
ffer
ent
cucu
mbe
rcu
ltiva
rsde
mon
stra
ted
vari
abili
tyin
seed
germ
inat
ion
resp
onse
Poly
ansk
aya
etal
.20
00
Bur
khol
deri
avi
etna
mie
nsis
TV
V75
Ric
eO
utdo
orpo
tan
dfie
ldtr
ials
–w
hen
plan
tsin
ocul
ated
and
tran
spla
nted
atda
y24
,in
-cr
ease
sof
shoo
tw
eigh
t�u
pto
33%
�,ro
otw
eigh
t�u
pto
55%
�an
dle
afsu
rfac
e�u
pto
30%
�ob
serv
ed–
end
grai
nyi
eld
incr
ease
of13
-22%
–gr
ain
wei
ght
�ala
te-s
easo
nyi
eld
com
pone
nt�
sign
ifi-
cant
lyin
crea
sed
due
toin
ocul
atio
n,bu
tno
tdu
eto
the
ad-
ditio
nof
nitr
ogen
Tra
nV
anet
al.
2000
Ent
erob
acte
rcl
oaca
eC
AL
3To
mat
o,Pe
pper
,M
ung
bean
Gre
enho
use
–po
sitiv
ese
edlin
ggr
owth
resp
onse
byal
lth
ree
plan
tsp
e-ci
esto
PGPR
trea
tmen
t,es
peci
ally
tom
ato,
whe
reno
exo-
geno
usm
iner
alnu
trie
nts
adde
d–
earl
yst
imul
atio
nef
fect
onse
edlin
gsob
serv
ed
May
aket
al.
2001
7
Tabl
e1.
Con
tinue
d.
Bac
teri
aPl
ant
Con
ditio
nsR
esul
tsof
addi
tion
ofba
cter
iato
plan
tR
efer
ence
Non
-fluo
resc
ent
pseu
dom
onad
isol
ated
from
onio
nrh
izos
hper
e
Pota
toG
row
thch
ambe
r–
plan
tlets
show
edsi
gnifi
cant
incr
ease
sin
root
dry
wei
ght
of44
to20
1%,
stem
leng
th26
to28
%,
ligni
nup
to43
%,
and
enha
nced
stem
hair
form
atio
n55
to11
0%
From
mel
etal
.19
91
OT
HE
RSp
ecie
san
dM
ix-
ture
s
Pse
udom
onad
ssp
p.�fl
uore
s-ce
ntst
rain
s �W
inte
rw
heat
Fiel
d–
bioc
ontr
olef
fect
sse
enag
ains
tta
keal
l�G
aeum
anno
my-
ces
gram
inis
�,27
%yi
eld
incr
ease
due
toin
ocul
atio
nD
eFr
eita
san
dG
erm
ida
1990
Pse
udom
onas
cepa
cia
Pse
udom
onas
fluor
esce
nsP
seud
omon
aspu
tida
Win
ter
whe
atPo
tted
plan
tsin
grow
thch
ambe
r–
stra
ins
dem
onst
rate
bioc
ontr
olag
ains
tR
hizo
cton
iaso
lani
and
Lep
tosp
haer
am
acul
ans
–di
ffer
ent
stra
ins
stim
ulat
edi
ffer
ent
plan
tpa
rts
–gr
owth
ofpl
ants
inle
ssfe
rtile
soil
stim
ulat
ed
De
Frei
tas
and
Ger
mid
a19
90
Pse
udom
onas
cepa
cia
MR
85,
R85
Pse
udom
onas
puti
daM
R11
1,R
105
Win
ter
whe
atFi
eld
–ba
cter
iain
ocul
ated
onpl
ants
able
toov
erw
inte
ron
root
san
dsu
rviv
e;w
ithle
vels
reac
hing
104
to10
8C
FU/g
–w
heat
grai
nyi
elds
incr
ease
dsi
gnifi
cant
lyat
seve
ral
site
sw
ithth
ese
stra
ins,
how
ever
over
all
resu
ltsw
ere
not
sign
if-
ican
tdu
eto
vari
abili
tyof
resu
ltsbe
twee
nso
me
tria
ls
De
Frei
tas
and
Ger
mid
a19
92b
Pse
udom
onas
cepa
cia
R55
,R
85P
seud
omon
aspu
tida
R10
4
Win
ter
whe
atG
row
thch
ambe
r–
anta
goni
smde
mon
stra
ted
agai
nst
Rhi
zoct
onia
sola
ni–
incr
ease
ofdr
yw
eigh
tof
inoc
ulat
edpl
ants
�62-
78%
�gr
own
inR
.so
lani
infe
cted
soil
–dr
yro
otw
eigh
tin
crea
sed
by92
-128
%an
dsh
oot
dry
wei
ght
incr
ease
dby
28-4
8%
De
Frei
tas
and
Ger
mid
a19
91
Pse
udom
onas
cepa
cia
R85
Pse
udom
onas
fluor
esce
nsR
104,
R10
5P
seud
omon
aspu
tida
R11
1
Win
ter
whe
atPo
tted
plan
tsin
grow
thch
ambe
r–
two
soil
type
ste
sted
unde
rsi
mul
ated
fall
cond
ition
s�5
ºC�
–re
spon
seof
whe
atnu
trie
ntup
take
toin
ocul
atio
nde
pen-
dant
onso
ilco
mpo
sitio
n–
grai
nyi
eld
enha
nced
46-7
5%in
mor
efe
rtile
soil
De
Frei
tas
and
Ger
mid
a19
92a
Pse
udom
onas
chlo
rora
phis
2E3,
O6
Spri
ngw
heat
Fiel
dan
dla
bora
tory
–in
crea
sed
emer
genc
eat
two
diff
eren
tsi
tes
by8
to6%
–st
rong
inhi
bitio
nof
Fus
ariu
mcu
lmor
um–
nopr
omot
ion
effe
ctof
inoc
ulat
edpl
ants
evid
ent
inso
ilsfr
eeof
Fus
ariu
min
fect
ion
Kro
ppet
al.
1996
Pse
udom
onas
corr
ugat
aA
zoto
bact
erch
rooc
occu
mA
mar
anth
uspa
nicu
latu
sE
leus
ine
cora
cana
–pl
ant
grow
than
dni
trog
enco
nten
tin
crea
sed
–hy
poth
esis
edth
atth
egr
owth
prom
otio
nef
fect
isdu
eto
the
stim
ulat
ion
ofna
tive
bact
eria
lco
mm
uniti
es
Pand
eyet
al.
1999
8
Tabl
e1.
Con
tinue
d.
Bac
teri
aPl
ant
Con
ditio
nsR
esul
tsof
addi
tion
ofba
cter
iato
plan
tR
efer
ence
Pse
udom
onas
fluor
esce
nsP
seud
omon
aspu
tida
TL
3,B
K1
Pota
toFi
eld
–la
ckof
grow
thpr
omot
ion
effe
ctan
dba
cter
ial
surv
ival
indr
yso
ils–
inno
rmal
cond
ition
sst
atis
tical
lysi
gnifi
cant
incr
ease
sof
yiel
dof
14-3
3%in
5of
9pl
ots
Bur
ret
al.
1978
Pse
udom
onas
fluor
esce
nsW
inte
rw
heat
Fiel
dan
dgr
owth
cham
ber
–in
grow
thch
ambe
r,se
edlin
ghe
ight
prom
otio
nse
en–
inP
ythi
um-c
onta
min
ated
site
s,si
gnifi
cant
incr
ease
sin
stan
d,pl
ant
heig
ht,
num
ber
ofhe
ads,
and
grai
nyi
eld
Wel
ler
and
Coo
k19
86
Pseu
dom
onas
fluor
esce
ns63
-28,
R17
-FP2
,Q
P5,
R15
-A4
Tom
ato
Gre
enho
use
with
natu
ral
light
ing
–in
favo
urab
lelig
htco
nditi
ons,
frui
tyi
elds
incr
ease
dby
5.6
to9.
4%
–in
unfa
vour
able
light
cond
ition
s,yi
elds
incr
ease
dup
to18
.2%
Gag
néet
al.
1993
Pse
udom
onas
fluor
esce
ns63
-49,
63-2
8,15
Pse
udom
onas
corr
ugat
e13
Serr
atia
plym
thic
aR
1GC
4
Cuc
umbe
rFi
eld
–st
rain
63-4
9si
gnifi
cant
lyin
crea
ses
frui
tnu
mbe
rsby
12%
and
frui
tw
eigh
tby
18%
–st
rain
s13
,15
,R
1GC
4sl
ight
lyin
crea
seyi
elds
–in
Pyt
hium
infe
cted
soils
,yi
eld
incr
ease
dup
to18
%w
ithad
ditio
nof
stra
ins
63-4
9an
d63
-28
McC
ulla
ugh
etal
.20
01
Pse
udom
onas
fluor
esce
nsPf
5B
acil
lus
pum
ilus
Hig
hbus
hbl
uebe
rry
–le
afar
eaan
dst
emdi
amet
erin
crea
ses
deSi
lva
etal
.20
00
Pse
udom
onas
puti
daP
seud
omon
aspu
tida
biov
arB P
seud
omon
asflu
ores
cens
Art
hrob
acte
rci
treu
sSe
rrat
iali
quef
acie
ns
Can
ola
�Bra
ssic
aca
mpe
stri
sL
.an
dB
rass
ica
napu
sL
. �
Fiel
dan
dgr
eenh
ouse
–in
gree
nhou
se,
sele
cted
stra
ins
prod
uce
57%
incr
ease
inyi
eld
–in
field
cond
ition
s,se
lect
stra
ins
incr
ease
seed
ling
emer
-ge
nce
and
vigo
ur–
yiel
din
crea
sefr
om6
to13
%ov
era
two
year
test
pe-
riod
Klo
eppe
ret
al.
1988
Pse
udom
onas
puti
daB
acil
lus
subt
ilis
Ent
erob
acte
rae
roge
nes
Ent
erob
acte
rag
glom
eran
sB
acil
lus
cere
us
Cuc
umbe
rIn
vitr
oan
dgr
eenh
ouse
–m
ost
stra
ins
incr
ease
dro
otle
ngth
inP
ythi
um-i
nfec
ted
plan
tsin
vitr
o–
ingr
eenh
ouse
,in
crea
ses
ofth
ew
eigh
tof
cucu
mbe
rpl
ants
by29
%,
frui
tyi
eld
by14
%an
dfr
uit
num
ber
by50
%by
B.
subt
ilis
Uth
ede
etal
.19
99
Pse
udom
onas
puti
daG
R12
-2C
anol
aG
reen
hous
e–
non-
nitr
ogen
fixin
gm
utan
tspr
ovid
egr
eate
rro
otel
onga
-tio
nef
fect
san
dgr
eate
rph
osph
ate
upta
keL
ifsh
itzet
al.
1987
9
Tabl
e1.
Con
tinue
d.
Bac
teri
aPl
ant
Con
ditio
nsR
esul
tsof
addi
tion
ofba
cter
iato
plan
tR
efer
ence
Pseu
dom
onas
putid
aG
R12
-2C
anol
a,L
ettu
ce,
Tom
ato,
Bar
ley,
Whe
at,
Oat
Gro
wth
cham
ber
–in
dico
tpl
ants
,ro
otel
onga
tion
stim
ulat
edby
the
bact
e-ri
a,bu
tin
mon
ocot
plan
ts,
little
tono
grow
thpr
omot
ion
seen
–di
ffer
ence
due
tose
nsiti
vity
diff
eren
ces
toet
hyle
ne,
this
bact
eria
lst
rain
cont
ains
gene
tore
duce
ethy
lene
synt
hesi
s�i
.e.A
CC
deam
inas
e �
Hal
let
al.
1996
Pse
udom
onas
puti
daW
4P63
Pota
toFi
eld
–in
crea
sed
yiel
dof
10.2
to11
.7%
–po
tato
soft
rot
�Erw
inia
caro
tovo
ra�
supp
ress
edX
uan
dG
ross
,19
86
Pse
udom
onas
sp.
Vari
o-vo
vax
sp.
Agr
obac
teri
umsp
.P
hyll
obac
teri
umsp
.
Can
ola
Gro
wth
cham
ber
–si
gnifi
cant
incr
ease
inro
otdr
yw
eigh
tfr
om11
to52
%–
mos
tim
port
ant
prom
otio
nef
fect
byP
hyll
obac
teri
umsp
.B
ertr
and
etal
.20
01
Pse
udom
onas
sp.
PsJN
Pota
toG
reen
hous
ean
dfie
ldtr
ials
–in
gree
nhou
se,
incr
ease
ofw
hole
plan
tdr
yw
eigh
t;re
sult
not
influ
ence
dby
soil
ster
ility
–in
field
,ea
rly
emer
genc
est
imul
ated
and
sign
ifica
nttu
ber
yiel
din
crea
ses
in3
of4
tria
ls
From
mel
etal
.19
83
Pse
udom
onas
spp.
�fluo
resc
ent
stra
ins �
A1,
B10
,T
L3,
BK
1,E
6
Pota
toG
reen
hous
ean
dfie
ld–
trea
ted
seed
piec
espr
oduc
ela
rger
root
syst
ems
ingr
een-
hous
e–
sign
ifica
ntyi
eld
incr
ease
soc
curr
edin
all
test
field
s,bu
tdi
ffer
ent
stra
ins
prom
ote
plan
tgr
owth
diff
eren
tlyin
diff
er-
ent
soil
type
s–
earl
ypl
ant
resp
onse
sco
rrel
ated
toin
crea
sed
yiel
ds
Klo
eppe
ret
al.
1980
Pse
udom
onas
spp.
�fluo
resc
ent
stra
ins �
A1,
B2,
B4,
E6,
RV
3,SH
5
Suga
rbe
etG
reen
hous
ean
dfie
ldtr
ials
–in
crea
ses
ofse
edlin
gm
ass
byal
lst
rain
sin
gree
nhou
se–
effe
cts
ofin
ocul
atio
npo
sitiv
eon
yiel
din
field
tria
ls,
but
resu
ltsva
riab
lefr
omsi
teto
site
–st
udy
auth
ors
conc
lude
that
prom
otio
nef
fect
due
toan
-ta
goni
smof
plan
tdi
seas
e
Susl
owan
dSc
hrot
h19
82
Pse
udom
onas
spp.
Pota
toFi
eld
–ch
ange
sin
yiel
dof
�10
to37
%H
owie
and
Ech
andi
1983
Pse
udom
onas
spp.
Pota
toFi
eld
–ch
ange
sin
yiel
dof
�9
to20
%G
eels
etal
.19
86
Pse
udom
onas
spp.
Pota
toFi
eld
–ch
ange
sin
yiel
dof
�14
to33
%K
loep
per
etal
.19
89
Pse
udom
onas
spp.
Ric
eFi
eld
–in
crea
sed
yiel
dof
3to
160%
Klo
eppe
ret
al.
1989
10
Tabl
e1.
Con
tinue
d.
Bac
teri
aPl
ant
Con
ditio
nsR
esul
tsof
addi
tion
ofba
cter
iato
plan
tR
efer
ence
Pse
udom
onas
spp.
Let
tuce
,C
ucum
ber,
Tom
ato,
Can
ola
Hyd
ropo
nic
grow
thch
ambe
r–
incr
ease
sof
root
and
shoo
tw
eigh
tsfo
ral
lpl
ants
test
ed–
mos
tsi
gnifi
cant
posi
tive
grow
thre
spon
ses
inle
ttuce
,to
mat
oan
dcu
cum
ber
Van
Peer
and
Schi
pper
s19
98
Pse
udom
onas
spp.
7NSK
2M
aize
,B
arle
y,W
heat
Fiel
d–
incr
ease
dyi
eld
of15
to25
%Is
wan
diet
al.
1987
Pse
udom
onas
syri
ngae
pv.
Pha
seol
icol
aB
ean
Gre
enho
use
–po
ores
tabl
ishm
ent
ofte
stpa
thog
enin
plan
tsin
ocul
ated
with
P.sy
ring
ae–
incr
ease
dam
ount
sof
prot
ein
inin
ocul
ated
plan
ts
Als
trom
1995
Pse
udom
onas
W34
Bac
illu
sce
reus
S18
Let
tuce
,To
mat
oPo
tex
peri
men
t–
sign
ifica
ntre
duct
ion
inga
lling
and
enha
nced
seed
ling
biom
ass
inso
ilsin
fest
edw
ithM
eloi
dogy
nein
cogn
ita
–B
.ce
reus
S18
caus
esup
toa
9%yi
eld
incr
ease
com
-pa
red
toth
eco
ntro
l
Hof
fman
n-H
erga
rten
etal
.19
98
Serr
atia
liqu
faci
ens
Pse
udom
onas
sp.
Bac
illu
ssp
.
Mai
zeG
reen
hous
e–
incr
ease
dof
yiel
dof
8to
14%
–S.
liqu
efac
iens
and
Pse
udom
onas
sp.
give
sth
ehi
ghes
tst
imul
atio
nef
fect
indi
ffer
ent
soils
Lal
ande
etal
.19
89
Serr
atia
prot
eam
acul
ans
102
Serr
atia
liqu
efac
iens
2-68
Soyb
ean
Fiel
d–
trea
tmen
tef
fect
sof
both
bact
eria
lst
rain
sno
tsi
gnifi
cant
over
two
stud
yye
ars
Pan
etal
.20
02
Xan
hom
onas
mal
toph
ila
Sunfl
ower
Lab
and
gree
nhou
se–
incr
ease
dge
rmin
atio
nra
teFa
ges
and
Ars
ac19
91
‘Yie
ldIn
crea
sing
Bac
teri
a’�Y
IB�
�Spe
cies
Unk
now
n �R
ice,
Whe
at,
Cor
n,M
illet
,Sw
eet
Pota
to,
Cot
ton,
Bee
t,R
apes
eed,
Wat
erm
elon
Fiel
d�T
otal
area
ofte
stcr
ops
is3.
46m
illio
nha
�–
maj
orst
udy
cond
ucte
din
Chi
nain
vari
ous
prov
ince
s–
aver
age
yiel
din
crea
ses
of10
.0to
22.5
%af
ter
appl
ica-
tion
ofY
IBon
crop
s
Mei
etal
.19
90
11
Okon 1996; Lalande et al. 2002; Paredes-Cardona1988�. While there may be a threshold number ofbacteria that should be inoculated on a given plant,excessively large numbers of bacteria could be detri-mental to the germination and growth of seeds orplants �Chanway 1997�. However, growth promotioneffects can still occur even with lower bacterial popu-lations �Jacoud et al. 1998�. It has been shown undercontrolled experimental conditions, that initial bacte-rial binding to seed, not necessarily the roots aftergermination, is most important for enhanced plantroot elongation �Hong et al. 1991�.
The over-wintering ability of PGPR is fundamen-tal when considering uses in colder climates. There isevidence that Pseudomonas species are able to over-winter in sufficient quantities on the roots of winterwheat �De Freitas and Germida 1990b�. It has alsobeen argued that antifreeze protein activity of manybacterial species may contribute to their survival incolder climates �Sun et al. 1995; Xu et al. 1998�. Onthe other hand, strains of Azospirillum often have lowsurvival rates in soils that are colder �Lifshitz et al.1986�.
There are a few other points of interest that relateto agricultural uses of PGPR. For example, it has beenshown that some PGPR strains are able to counteractirrigation problems by reducing the negative effect ofirrigation of crops with highly saline water �Hamaouiet al. 1996�. This may reflect the lowering of plantethylene levels elevated by salt stress by means of1-amino-cyclopropane-1-carboxylate �ACC� deami-nase-containing PGPR �S. Mayak, T. Tirosh, B.R.Glick, unpublished results�. Also, it has been ob-served that PGPR numbers decline rapidly in therhizosphere after inoculation �Jacoud et al. 1998�, al-though their effects last throughout the growing sea-son. Several studies show that growth promotioneffects are seen early in plant development, and thesesubsequently translate into higher yields �Glick et al.1997; Hoffmann-Hergarten et al. 1998; Kloepper etal. 1988; Polyanskaya et al. 2000�. Evidence of late-season grain weight increases have also been reportedin studies with PGPR and rice �Tran Van et al. 2000�.
Free living PGPRs can be administered to crops insome formulations that are available commercially�Table 2�. The majority of these products are biocon-trol agents which contribute indirectly to the growthpromotion of crops �Chet and Chernin 2002; Glick etal. 1999�. Commercial free living PGPR inoculantsprovide a possible alternative to the use of pesticides
and fertilizers on various crops, though they are notused widely at present �Glick et al. 1999�.
A broad array of methods and materials exist forthe delivery of bacteria to crops in the field. Presentlythe non-free living symbiotic Rhizobium spp. are mostcommonly incorporated into peat as inoculant carri-ers. Peat carriers, although cheap and easily used,have many disadvantages. Peat is generally used as anon-sterile medium and holds a large contaminantload. Also, peat quality can be variable and peat itselfis not necessarily readily available worldwide. Heatsterilization of peat can release substances toxic to thechosen bacteria. Some peat is known to inhibit plantgrowth, probably a reason for some of the negativeeffects of inoculation by PGPR seen in Table 1. Fi-nally, bacteria in peat formulations are vulnerable totemperature fluctuations and have a limited shelf life�Bashan 1998�.
Understanding the mechanisms of plant growthpromotion is important when deciding what type ofbacteria to use with a plant in a given situation. Forexample, Pseudomonas putida GR12-12 contains thegene for ACC deaminase, which inhibits ethylenesynthesis, ethylene being a product of stress. Thismechanism is most effective on plants that are moresusceptible to the effects of ethylene, such as dicoty-ledonous plants �Hall et al. 1996� especially undersuch stress conditions as flooding �Grichko and Glick,2001� drought �S. Mayak, T. Tirosh, B.R. Glick, un-published results� and phytopathogens �Wang et al.2000�.
More recent knowledge of indirect mechanisms ofplant growth promotion by soil bacteria may aid theagricultural production of certain legume crops. Thehydrogen gas that is produced as a by-product of ni-trogen fixation by rhizobia within legume nodulesmay be recaptured and recycled by those rhizobialstrains that contain a hydrogen uptake system �Evanset al. 1987�. It is clearly beneficial to the plant to ob-tain its nitrogen from a symbiotic diazotroph that hasa hydrogen uptake system; however, this trait is notcommon in naturally occurring rhizobial strains�Evans et al. 1987�. The lack of an endogenous hy-drogen uptake system notwithstanding, many soilscontain large numbers of free living microorganismsthat can capture some of the hydrogen gas producedduring nitrogen fixation and thereby indirectly pro-mote the growth of the rhizobia-treated plants �Dongand Layzell 2001; McLearn and Dong 2002�. In theseinstances, the organisms responsible for this plantgrowth promotion effect have not been characterized.
12
Tabl
e2.
Exa
mpl
esof
com
mer
cial
prod
ucts
usin
gfr
ee-l
ivin
gpl
ant
grow
thpr
omot
ing
rhiz
obac
teri
a�C
het
and
Che
rnin
2002
,G
lick
etal
.19
99�
Bac
teri
alC
onte
ntPr
oduc
tIn
tend
edC
rop
Agr
obac
teri
umra
diob
acte
rD
iega
llG
alltr
ol-A
Nog
all
Nor
bac
84C
Frui
t,nu
t,or
nam
enta
lnu
rser
yst
ock
and
tree
s
Azo
spir
illu
mbr
asil
ense
Azo
-Gre
enT
urf
and
fora
gecr
ops
Azo
spir
illu
mbr
asil
ense
Cd
Azo
spir
illu
mli
pofe
rum
Br-
17Z
ea-N
itM
aize
Azo
spir
illu
mli
pofe
rum
CR
T1
AZ
OG
RE
EN
-mM
aize
Bac
illu
sam
ylol
ique
faci
ens
GB
99B
acil
lus
subt
ilis
CB
122
Qua
ntum
4000
Bio
Yie
ldT
MB
rocc
oli,
cabb
age,
cant
alou
pe,
caul
iflow
er,
cele
ry,
cucu
mbe
r,le
ttuce
,or
nam
enta
ls,
pep-
pers
,to
mat
o,an
dw
ater
mel
on
Bac
illu
ssu
btil
isE
pic
HiS
tick
N/T
Kod
iak
Rhi
zo-P
lus
Sere
nade
Subt
ilex
Syst
em3
Bar
ley,
bean
s,co
tton,
legu
mes
pean
ut,
pea,
rice
,an
dso
ybea
n
Bul
khol
deri
ace
paci
aB
lue
Cir
cle
Den
yIn
terc
ept
Alf
alfa
,ba
rley
,be
ans,
clov
er,
cotto
n,m
aize
,pe
as,
sorg
hum
,ve
geta
bles
and
whe
at
Pse
udom
onas
fluor
esce
nsB
light
Ban
A50
6C
onqu
erV
ictu
s
Alm
ond,
appl
e,ch
erry
,m
ushr
oom
,pe
ach,
pear
,po
tato
,st
raw
berr
yan
dto
mat
o
Pse
udom
onas
syri
ngae
Bio
-sav
e10
Citr
usan
dpo
me
frui
t
Stre
ptom
yces
gris
eovi
rdis
K61
Myc
osto
pFi
eld,
orna
men
tal
and
vege
tabl
ecr
ops
13
In other cases, free living bacteria that promote therhizobial-legume symbiosis have been identified andcharacterized �e.g., Andrade et al. 1998; Marek-Kozaczuk et al. 2000; Xu et al. 1994�. In these cases,the free living bacteria are thought to act by decreas-ing the interference in the nodulation process by othersoil microorganisms.
What is currently missing from the research ofPGPR in agriculture is a lack of comparative studiesbetween crop types and different species or strains ofrhizobacteria. For example, when Pseudomonasputida GR12-2 is inoculated on various crops, thereare dissimilarities in the plant stimulation betweenmonocot and dicot plants �Hall et al. 1996�. There arealso significant differences in yield between summerversus winter crops following inoculation withAzospirillum brasilense Cd �Okon et al. 1988�. Nev-ertheless, as noted by Okon et al. �1994�, the positiveeffects of PGPR shown for several rhizobacterialtypes on many economically important crops is avalid phenomenon, and these results can act as a ba-sis for the effective utilization of PGPR in a varietyof applications.
Applications of PGPR in Forestry
Research on the use of PGPR in forestry is much lesswidespread than for agricultural applications. PGPRand their effect on angiosperms were the initialresearch focus through the 1980s, however from the1990s to the present there has been more research ofPGPR on gymnosperms �Chanway 1997�. A widerscope of studies of both of these tree types and PGPRcould benefit the commercial forestry sector, as wellas reforestation efforts worldwide. Table 3 summa-rizes many of the studies that have been conductedwith different PGPR and tree species.
There are different considerations that must betaken into account when evaluating the performanceof the inoculation of PGPR on tree species, in con-trast to agricultural crops. Fruit and grain yieldincreases are obviously not an imperative aspect oftree growth but biomass increases due to inoculationare quite important. Aspects such as seedling emer-gence and reduction in seedling transplant injury dur-ing the transfer from nursery to field are alsosignificant �Shishido and Chanway 2000�. Whilesome tree types are very good at rapid and effectivegermination, without inoculation, many have diffi-culty in getting established to grow into an adult tree�Zaady and Perevoltsky 1995�. Soil type may also be
a major consideration when testing PGPR in a forestenvironment. Many forest soils are acidic, especiallythose of conifer forests, and some PGPR are sensitiveto low pH conditions �Brown 1974�.
Winter survival of PGPR is imperative, especiallywith trees intended for the colder regions of the world�e.g., Canada, Scandinavia, Russia� and also particu-larly since trees are perennial plants in contrast tomost agricultural crops. Chanway et al. �2000� hasshown that there is promise for many PGPR, such asstrains of Bacillus polymyxa and Pseudomonas fluo-rescens to over-winter on the roots of field-plantedtrees. In that study, from one year to the next, therewas a decrease of approximately two orders of mag-nitude in the inoculated bacterial populations; how-ever, the benefits of inoculation were seen the nextyear �Chanway et al. 2000�.
The medium in which a bacterial strain is preparedpreceding inoculation may affect the root colonizationpattern of the inoculated bacteria �Zaady et al. 1993�.This study showed that malate-grown bacteria werebetter able to promote the growth of oak, than bacte-rial cells of the same strain grown in fructose-basedmedia. It was found that malate-grown cells have atendency to aggregate, while the fructose cellsdisperse through the soil substrate. Fructose-growncells may be adequate for growth promotion of sur-face-rooted plants like maize, however, they were notsufficient for the growth promotion of trees such asoak with deep tap roots �Zaady et al. 1993�.
Similar to the specificity observed in agriculturalcrops, a specific bacterial strain may promote growthonly in certain tree species. �Enebak et al. 1998; Sh-ishido and Chanway 2000�. Sometimes even the treeecovar is important. For example, while a strain ofbacteria was effective at promoting growth in onetype of pine species it was not effective with anotherpine species �Chanway 1995�. Ecotypes, or trees ofthe same species from different regions or altitudes,also show differential responses to bacterial inocula-tion �Chanway 1995�. For example in one case, Hy-drogenophaga pseudoflava consistently promoted thegrowth of only one of two spruce ecovars �Chanwayand Holl 1993�. However, there are also some broad-range bacterial strains, such Bacillus polymxa L6,which consistently promote the growth of many pinevarieties and other tree species �Chanway 1995; Holland Chanway 1992�. It is interesting to note that thisstrain of bacteria was originally isolated from therhizosphere of perennial ryegrass, and not a tree spe-cies �Holl and Chanway 1992�.
14
Tabl
e3.
Exa
mpl
esof
free
-liv
ing
plan
tgr
owth
prom
otin
grh
izob
acte
ria
test
edon
vari
ous
tree
spec
ies.
Bac
teri
aPl
ant
Con
ditio
nsR
esul
tsof
addi
tion
ofba
cter
iato
plan
tR
efer
ence
Agr
obac
teri
umra
diob
acte
rB
eech
,Sc
otch
pine
Gre
enho
use
–be
ech
biom
ass
incr
ease
sof
upto
235%
–pi
nebi
omas
sin
crea
ses
ofup
to15
%L
eyva
lan
dB
erth
elin
1989
Art
hrob
acte
rci
treu
sP
seud
omon
asflu
ores
cens
Pse
udom
onas
puti
da
Bla
cksp
ruce
,Ja
ckpi
ne,
Whi
tesp
ruce
Gre
enho
use
–he
ight
and
biom
ass
incr
ease
sde
mon
stra
ted
Bea
llan
dT
ippi
ng19
89
Art
hrob
acte
rox
ydan
sP
seud
omon
asau
reof
acie
nsD
ougl
asFi
rG
reen
hous
ean
dfie
ld–
heig
htan
dbi
omas
sin
crea
ses
ofup
to68
%–
also
incr
ease
dbr
anch
and
root
wei
ghts
–so
me
vari
abili
tyin
resp
onse
offir
ecot
ypes
toin
ocul
a-tio
n
Cha
nway
and
Hol
l19
94
Art
hrob
acte
rsp
.Pi
neL
abas
say
–sh
oot
leng
thin
crea
ses
upto
69%
Poko
jska
-Bur
dzie
jet
al.
1982
Azo
spir
illu
mbr
asil
ense
Riv
erO
akG
reen
hous
e–
biom
ass
incr
ease
of90
%R
odri
guez
-Bar
ruec
oet
al.
1991
Azo
spir
illu
mbr
asil
ense
Cd
Oak
Gre
enho
use
–ro
otgr
owth
incr
ease
sof
upto
70%
–th
ese
grow
thpr
omot
ion
obse
rved
only
with
cells
cul-
ture
din
mal
ate
and
not
fruc
tose
Zaa
dyet
al.
1993
,Z
aady
and
Pere
volts
ky19
95
Azo
toba
cter
chro
ococ
cum
Oak
,as
hG
row
thch
ambe
r–
biom
ass
incr
ease
sof
13to
26%
Akh
rom
eiko
and
Shes
tako
va19
58
Azo
toba
cter
chro
ococ
cum
Que
rcus
serr
ata
Potte
dpl
ant
expe
rim
ent
outd
oors
–bi
omas
sin
crea
ses
ofup
to38
%Pa
ndey
etal
.19
86
Azo
toba
cter
chro
ococ
cum
Bac
illu
sm
egat
eriu
mE
ucal
yptu
sPo
tted
plan
tex
peri
men
tou
tdoo
rs–
biom
ass
incr
ease
sof
upto
44%
Moh
amm
edan
dPr
asad
1998
Bac
illu
sli
chen
ifor
mis
CE
CT
5105
Bac
illu
spu
mil
isC
EC
T51
06
Silv
ersp
ruce
Gre
enho
use
–st
atis
tical
lysi
gnifi
cant
incr
ease
inae
rial
plan
tgr
owth
–ro
otsy
stem
deve
lopm
ent
not
affe
cted
–in
crea
sed
plan
tni
trog
enco
nten
t
Prob
anza
etal
.20
02
Bac
illu
sli
chen
ifor
mis
Phy
lo-
bact
eriu
msp
.M
angr
ove
Gre
enho
use
–do
ublin
gof
nitr
ogen
inco
rpor
atio
nin
topl
ant
–in
crea
sed
leaf
deve
lopm
ent
Bas
han
and
Hol
guin
2002
,R
ojas
etal
.20
01
15
Tabl
e3.
Con
tinue
d.
Bac
teri
aPl
ant
Con
ditio
nsR
esul
tsof
addi
tion
ofba
cter
iato
plan
tR
efer
ence
Bac
illu
spo
lym
yxa
Dou
glas
Fir,
Lod
gepo
lePi
ne,
Whi
tesp
ruce
Gro
wth
cham
ber
and
gree
n-ho
use
–fo
rlo
dgep
ole
pine
,si
gnifi
cant
incr
ease
sin
root
dry
wei
ght
�1.3
5-fo
ld�
and
the
num
ber
and
leng
thof
seco
nd-
ary
root
s�1
.44-
fold
and
1.92
-fol
d,re
spec
tivel
y �–
inpi
ne,
root
grow
th,
emer
genc
e,he
ight
and
wei
ght,
root
colla
rdi
amet
erin
crea
ses
pres
ent
for
plan
tsgr
own
inst
er-
ileve
rsus
non-
ster
ilem
edia
–se
edlin
gem
erge
nce
incr
ease
sfo
rw
hite
spru
ce
Cha
nway
etal
.19
91
Bac
illu
spo
lym
yxa
Wes
tern
hem
lock
Gre
enho
use
–in
crea
sed
seed
ling
heig
htan
dbi
omas
sup
to30
%,
due
topl
ant
heig
htan
dw
eigh
tin
crea
ses
of1.
19an
d1.
30-f
old
resp
ectiv
ely
–de
gree
ofhe
mol
ock
grow
thpr
omot
ion
diff
eren
tfo
rbi
o-va
rsfr
omdi
ffer
ent
altit
udes
Cha
nway
1995
Bac
illu
spo
lym
yxa
Pse
udom
onas
fluor
esce
nsL
oblo
llypi
ne,
slas
hpi
neG
reen
hous
e–
sign
ifica
ntin
crea
ses
inth
esp
eed
ofse
edlin
gem
erge
nce
and
tota
lbi
omas
s–
post
-em
erge
nce
dam
ping
off
redu
ced
inlo
blol
lypi
ne–
two
bact
eria
lst
rain
ssh
owre
duct
ion
ofbi
omas
sof
both
pine
spec
ies
–lo
blol
lypi
neha
sin
crea
sed
root
leng
thw
ithso
me
stra
ins
Ene
bak
etal
.19
98
Bac
illu
spo
lym
yxa
Pse
udom
onas
fluor
esce
nsH
ybri
dsp
ruce
Fiel
d–
incr
ease
ofsp
ruce
seed
ling
dry
wei
ght
upto
57%
abov
eun
inoc
ulat
edsp
ruce
plan
tsat
five
ofni
nete
stsi
tes
–al
lte
stst
rain
sin
crea
sedr
yw
eigh
tof
spru
ceat
four
ofth
eni
nesi
tes
–so
me
plan
tgr
owth
inhi
bitio
nde
tect
eddu
eto
inoc
ulat
ion
atso
me
site
s
Cha
nway
etal
.20
00
Bac
illu
spo
lym
yxa
Stap
hylo
cocc
usho
min
isH
ybri
dsp
ruce
Gre
enho
use
�usi
ngfie
ldso
il �–
sign
ifica
ntgr
owth
incr
ease
upto
59%
O’N
eill
etal
.19
92
Bac
illu
spo
lym
yxa
L6
Lod
gepo
lePi
neG
reen
hous
e–
som
ese
edlin
gsco
-ino
cula
ted
with
myc
orrh
izae
–no
effe
ctw
ithB
acil
lus
alon
e–
shoo
tan
dro
otbi
omas
sin
crea
ses
Cha
nway
and
Hol
l19
91
Bac
illu
spo
lym
yxa
L6
Lod
gepo
lePi
neG
row
thch
ambe
r–
stat
istic
ally
sign
ifica
ntin
crea
ses
inse
edlin
gbi
omas
sat
6w
eeks
ofpl
ant
grow
th–
mea
nsh
oot
and
root
wei
ghts
incr
ease
dup
to35
%
Hol
lan
dC
hanw
ay19
92
16
Tabl
e3.
Con
tinue
d.
Bac
teri
aPl
ant
Con
ditio
nsR
esul
tsof
addi
tion
ofba
cter
iato
plan
tR
efer
ence
Bac
illu
ssu
btil
isP
seud
omon
assp
p.H
ybri
dsp
ruce
Gre
enho
use
plan
tsou
tpla
nted
tofie
ld–
sign
ifica
ntin
crea
ses
ofpl
ant
biom
ass
byin
ocul
atio
nw
ithP
seud
omon
asst
rain
of10
to23
4%at
all
site
sw
ithty
pica
lin
crea
ses
of28
to70
%–
redu
ctio
nin
seed
ling
shoo
tin
jury
afte
rtr
ansp
lant
–B
acil
lus
stra
inin
effe
ctiv
e
Shis
hido
and
Cha
nway
2000
Hyd
roge
noph
aga
pseu
dofla
vaP
seud
omon
aspu
tida
Hyb
rid
spru
ceFi
eld
–H
.ps
eudo
flava
incr
ease
dse
edlin
gbi
omas
san
dro
otbr
anch
num
bers
upto
49%
intw
osp
ruce
ecot
ypes
test
ed–
one
ecot
ype
show
sro
otgr
owth
prom
otio
n–
P.pu
tida
incr
ease
sse
edlin
gbi
omas
sin
two
tria
lsan
dha
sin
hibi
tory
effe
ctin
othe
rtr
ials
Cha
nway
and
Hol
l19
93
Pse
udom
onas
spp.
App
leG
reen
hous
ean
dfie
ld–
grow
thin
crea
ses
ofse
edlin
gsup
to65
%an
dof
root
-st
ocks
179%
–so
me
bioc
ontr
olse
enag
ains
tpa
thog
enic
fung
i
Cae
sar
and
Bur
r19
87
17
Sterility of the bacterial inoculant carrier can have animpact on the amount of plant growth promotion.Chanway et al. �1991� demonstrated that pine treesinoculated with PGPR in sterilized peat-vermiculitecarrier material promoted growth to a greater extentcompared to when they were inoculated with PGPRin non-sterilized material �Chanway et al. 1991�.Also, many researchers tend to inoculate seedlings orolder plants. This is in contrast to the agricultural useof PGPR, where there is a tendency to inoculateseeds, or the substrate surrounding the seed. This mayhave to do with differential practices in these twodisciplines, since many trees in the forestry industryhave their beginnings in a nursery and are long lived,while crop plants are usually started in the field andare relatively short lived.
There are currently very few research groupsworking in the area of forestry PGPR research. As aconsequence, there is currently no field data for de-ciduous trees and still comparatively little field datafor coniferous trees �Chanway 1997�.
Applications of PGPR for EnvironmentalRemediation
An extension of PGPR technology is the emerginguse of the bacteria with plants for environmental ap-plications. Recent studies in this area include manydifferent uses: for growth promotion of soil stabiliz-ing plants �Bashan et al. 1999�; to counteract flood-ing stress of plants �Grichko and Glick 2001�; to aidplant growth in acidic conditions �Belimov et al.1998a�; to counter high temperature stress �Bensalimet al. 1998�; and the use of PGPR in phytoremedia-tion technologies �Burd et al. 1998; Burd et al. 2000;Huang et al. 2000; Huang et al. 2003a, b�. Besidesenvironmental uses, some of the outcomes of thesestudies may also have an impact on agricultural orforestry applications.
Phytoremediation is the use of plants to extract,degrade or stabilize hazardous substances present inthe environment. �Cunningham and Berti 1993; Cun-ningham et al. 1995; Cunningham and Ow 1996�.Plants used for phytoremediation should be able toaccumulate high amounts of the contaminant, andalso be able to produce a large biomass. However,very often plants can be compromised by growing oncontaminated sites due to the inherent toxicity, soadding PGPR can aid plant growth �Burd et al. 2000�.Clearly, when plants used for phytoremediation are
able to grow well, the site detoxification will begreatly enhanced.
Table 4 shows examples of research that has beenconducted using free-living rhizosphere bacteria withplants and contaminants, to study their potential foruse in phytoremediation technologies. The bacterialisted in this Table include both bacteria with estab-lished plant growth promoting properties as well asbacteria with previously unknown credentials as plantgrowth promoters. Table 4 includes examples ofrhizobacteria which are beneficial to plants by mobi-lization of soil contaminants as well as those whichpromote plant growth. There also exist rhizospherebacteria that degrade contaminants but are not neces-sarily PGPR. In this case, plant roots serve only as asite for contaminant breakdown by the rhizobacteria�Anderson et al. 1993� and these examples have notbeen included in Table 4.
The plant properties that are improved by PGPRduring phytoremediation include biomass, contami-nant uptake, and plant nutrition and health. Grainyield was measured by Belimov et al. �1998b� as anindication of plant health and growth, however thisattribute is obviously not important in plants used forphytoremediation.
Some plants like barley, tomato, canola �Brassicacampestris) and Indian mustard �Brassica juncea� donot accumulate more contaminants �namely metal�per gram of plant material with the addition of PGPR,even though the total biomass increases �Belimov etal. 1998c; Burd et al. 2002; Burd et al. 1998; Nie etal. 2002�. However, Hoflich and Metz �1997� andWhiting et al. �2001� have shown that bacterialinoculation of maize and Thlaspi caerulescens in-creases the uptake of heavy metals by these plants. Inaddition, de Souza et al. �1999� found increased sele-nium accumulation by Brassica juncea after inocula-tion. Upon closer examination, it appears that the verylow level of contaminants used in some studies haveprobably influenced the amount of uptake so that in-creased contaminant accumulation in the presence ofPGPR occurs only at low contaminant concentrationsand not at the higher levels that inhibit plant growth.
When choosing a PGPR to increase metal uptakeby plants, it is important to ensure that the bacteriaused do not cause a reduction in metal uptake. Withbarley, the addition of Azospirillum lipoferum 137significantly reduced the amount of radiolabelled ce-sium uptake per gram of plant dry weight �Belimovet al. 1998c�.
Survival and PGPR success is essential for use inphytoremediation. It has been shown that high con-taminant levels can have inhibitory effects on thegrowth of PGPR. For example, Enterobacter cloacaeCAL2 growth was inhibited by 50% in 20 mM arsen-ate contaminated soils, but was only inhibited by 2%in 2 mM arsenate contamination �Nie et al. 2002�.Also, some bacteria are sensitive to one contaminant,but not another. In a study by Belimov et al. �1998b�,it was shown that Flavobacterium sp. is very sensi-tive to cadmium, but not to lead. In practice, it is es-sential that the bacteria used are at least somewhatresistant to the levels of contaminants endogenous tothe environment to be cleaned up. Thus, preliminaryselection of resistant strains, as performed by Burd etal. �1998� for nickel on Kluyvera ascorbata, isimperative for the practical use of these organisms.
Pairing of PGPR with transgenic plants may be agood way of increasing the efficacy of phytoremedia-tion. In the presence of arsenate, fresh root and shootweights of canola plants were greatly increased dueto the concerted action of the PGPR Enterobactercloacae CAL2 and the canola plants which expressthe stress tolerance gene ACC deaminase �Nie et al.2002�.
Plant exudates are essential for the association ofbacteria with the rhizosphere of the plant as shownby Belimov and Dietz �2000� who demonstrated thatthe addition of an alternate carbon source to the soilcaused an abolition of plant growth promotion in acontaminated site.
PGPRs in association with plants have an impor-tant role in the phytoremediation of soils but the re-search in this area has been limited. As seen in Table4, there have been no field studies of this work andonly controlled studies in greenhouses and/or growthchambers have been conducted. Also, only rudimen-tary inoculation procedures have been used. In addi-tion, only a few known PGPR and plants types havebeen tested. Despite the lack of extensive data, PGPRinoculation technology has a great deal of potentialin the area of phytoremediation.
Conclusions
PGPR present an alternative to the use of chemicalsfor plant growth enhancement in many different ap-plications. Extensive research has demonstrated thatPGPRs could have an important role in agricultureand horticulture in improving crop productivity. In
18
Tabl
e4.
Exa
mpl
esof
free
-liv
ing
plan
tgr
owth
prom
otin
grh
izob
acte
ria
test
edfo
rph
ytor
emed
iatio
nte
chno
logi
es.
Bac
teri
aPl
ant
Con
tam
inan
tC
ondi
tions
Res
ults
ofpa
irin
gof
bact
eria
and
plan
tR
efer
ence
Agr
obac
teri
umra
diob
acte
r10
Art
hrob
acte
rm
ysor
ens
7A
zosp
iril
lum
lipo
feru
m13
7F
lavo
bact
eriu
msp
.L
30
Bar
ley
Cad
miu
m,
Lea
dPo
tex
peri
men
tsin
gree
nhou
se–
Fla
voba
cter
ium
sp.
L30
very
nega
tivel
yse
n-si
tive
toca
dmiu
m–
Fla
voba
cter
ium
sp.
L30
and
A.m
ysor
ens
7gi
vein
crea
ses
ofgr
ain
yiel
d–
enha
nced
lead
accu
mul
atio
nby
plan
tsin
ocu-
late
dw
ithA
.ra
diob
acte
r10
and
A.
mys
oren
s7
–si
gnifi
cant
grow
thim
prov
emen
tsse
enin
plan
tsin
ocul
ated
byal
lst
rain
sat
high
erca
d-m
ium
conc
entr
atio
ns
Bel
imov
etal
.19
98b
Agr
obac
teri
umra
diob
acte
r10
Art
hrob
acte
rm
ysor
ens
7A
zosp
iril
lum
lipo
feru
m13
7F
lavo
bact
eriu
msp
.L
30
Bar
ley
13
4C
esiu
mPo
tex
peri
men
tsin
gree
nhou
se–
Fla
voba
cter
ium
sp.
L30
incr
ease
s1
34C
sup
-ta
keby
barl
ey,
but
not
sign
ifica
ntly
,du
eto
incr
ease
dpl
ant
biom
ass
–A
.li
pofe
rum
137
sign
ifica
ntly
decr
ease
sth
eto
tal
accu
mul
atio
nof
13
4C
s
Bel
imov
etal
.19
98c
Agr
obac
teri
umra
diob
acte
r10
Art
hrob
acte
rm
ysor
ens
7A
zosp
iril
lum
lipo
feru
m13
7F
lavo
bact
eriu
msp
.L
30
Bar
ley
Cad
miu
mPo
tex
peri
men
tsin
grow
thch
ambe
r–
incr
ease
dab
sorp
tion
ofes
sent
ial
nutr
ient
sfr
omco
ntam
inat
edgr
owth
med
ium
–sl
ight
stim
ulat
ion
ofro
otle
ngth
and
biom
ass
inco
ntam
inat
edgr
owth
med
ium
–A
.li
pofe
rum
137
incr
ease
dco
ncen
trat
ion
ofca
dmiu
min
root
s,bu
tno
chan
gein
cadm
ium
upta
keby
plan
tsin
ocul
ated
with
othe
rst
rain
s
Bel
imov
and
Die
tz20
00
Agr
obac
teri
umsp
.P
seud
omon
assp
.St
enot
roph
omas
sp.
Mai
ze,
Rye
,Pe
a,L
upin
Cad
miu
m,
Cop
per,
Lea
d,N
icke
l,Z
inc,
Chr
omiu
m
Pot
expe
rim
ents
ingr
owth
cham
ber
–ba
cter
iast
imul
ates
grow
thof
mai
zean
din
-cr
ease
sm
etal
upta
keby
mai
ze;
this
effe
ctm
ore
pron
ounc
edon
mor
ew
eakl
ypo
llute
dso
ilsco
mpa
red
tohe
avily
-pol
lute
dso
ils–
grow
than
dm
etal
upta
keof
lupi
n,pe
aan
dry
eno
taf
fect
edby
bact
eria
addi
tion
Hofl
ich
and
Met
z19
97
Azo
spir
illu
mbr
asil
ense
Cd
Ent
erob
acte
rcl
oaca
eC
AL
2P
seud
omon
aspu
tida
UW
3
Tall
fesc
uePo
lycy
clic
arom
atic
hydr
ocar
bons
�PA
Hs �
Pot
expe
rim
ents
ingr
owth
cham
ber
–ac
cele
rate
dan
dm
ore
com
plet
ePA
Hre
mov
alfr
omth
eso
ilw
ithin
ocul
atio
n–
effe
ctiv
enes
sof
PAH
rem
oval
furt
her
en-
hanc
edin
com
bina
tion
with
the
use
ofla
nd-
farm
edso
ilan
din
ocul
atio
nw
ithPA
H-
degr
adin
gba
cter
ia
Hua
nget
al.
2003
a,In
pres
s
19
Tabl
e4.
Con
tinue
d.
Bac
teri
aPl
ant
Con
tam
inan
tC
ondi
tions
Res
ults
ofpa
irin
gof
bact
eria
and
plan
tR
efer
ence
Azo
spir
illu
mbr
asil
ense
Cd
Ent
erob
acte
rcl
oaca
eC
AL
2P
seud
omon
aspu
tida
UW
3
Ken
tuck
ybl
uegr
ass,
Tall
fesc
ue,
Wild
rye
PAH
sPo
tex
peri
men
tsin
grow
thch
ambe
r–
incr
ease
dPA
Hre
mov
alfr
omso
il–
germ
inat
ion
ofal
lth
ree
plan
tty
pes
incr
ease
ddr
amat
ical
lyin
PAH
-spi
krd
soil
with
inoc
ula-
tion
–ro
otbi
omas
ssi
gnifi
cant
lyin
crea
sed
inal
lpl
ant
type
s
Hua
nget
al.
2003
b,In
pres
s
Ent
erob
acte
rca
ncer
ogen
esM
i-cr
obac
teri
umsa
perd
aeP
seud
omon
asm
onte
ilii
Thl
aspi
caer
ules
cens
Thl
aspi
arve
nse
Zin
cPo
tex
peri
men
tsin
grow
thch
ambe
r–
T.ca
erul
esce
nsha
stw
o-fo
ldin
crea
seof
zinc
conc
entr
atio
nin
root
saf
ter
inoc
ulat
ion
and
four
fold
incr
ease
ofzi
ncac
cum
ulat
ion
insh
oots
–T.
caer
cule
scen
sha
shi
gher
shoo
tbi
omas
sw
ithin
ocul
atio
n–
T.ar
vens
eha
sno
incr
ease
dgr
owth
orm
etal
accu
mul
atio
nw
ithin
ocul
atio
n
Whi
ting
etal
.20
01
Ent
erob
acte
rcl
oaca
eC
AL
2C
anol
aA
rsen
ate
Pot
expe
rim
ents
ingr
owth
cham
ber
–sl
ight
inhi
bito
ryef
fect
ofC
AL
2on
germ
ina-
tion
ofca
nola
inpr
esen
ceof
arse
nate
–pa
rtne
red
with
tran
sgen
icpl
ants
,ba
cter
iain
duce
dsi
gnifi
cant
lyhi
gher
root
and
shoo
tw
eigh
tsin
plan
ts–
noin
crea
seof
arse
nate
conc
entr
atio
nby
root
sof
plan
ts
Nie
etal
.20
02
Klu
yver
aas
corb
ata
SUD
165
Can
ola,
Tom
ato
Nic
kel
Pot
expe
rim
ents
ingr
owth
cham
ber
–fo
rto
mat
oan
dca
nola
,bo
thro
ots
and
shoo
tspr
otec
ted
from
toxi
city
with
inoc
ulat
ion
–si
gnifi
cant
dece
ase
inet
hyle
nepr
oduc
tion
bypl
ants
–no
incr
ease
orch
ange
inni
ckel
upta
kein
plan
tm
ater
ial
with
inoc
ulat
ion
Bur
det
al.
1998
Klu
yver
aas
corb
ata
SUD
165/
26,
SUD
165
Indi
anm
usta
rd,
Can
ola,
Tom
ato,
Nic
kel,
Lea
d,Z
inc
Pot
expe
rim
ents
ingr
owth
cham
ber
–bo
thst
rain
sde
crea
seso
me
plan
tgr
owth
inhi
-bi
tion
byth
em
etal
s,bu
tno
tal
way
ssi
gnifi
-ca
ntly
–SU
D16
5/26
decr
ease
spl
ant
grow
thin
hibi
tion
best
–no
incr
ease
ofm
etal
upta
kew
ithei
ther
stra
inov
erno
nino
cula
ted
plan
ts
Bur
det
al.
2000
20
addition, these organisms are also useful in forestryand environmental restoration purposes, though re-search in these areas is minimal. PGPR have beenshown to cause very real and positive effects whenmatched correctly to the right plant and the right en-vironmental situation.
What is needed for the future is a clear definitionof what bacterial traits are useful and necessary fordifferent environmental conditions and plants, so thatoptimal bacterial strains can either be selected orconstructed. Also, it would be very useful to have abetter understanding of how different bacterial strainswork together for the synergistic promotion of plantgrowth. Additional studies need to be conducted onthe effectiveness of different and novel inoculant de-livery systems such as alginate encapsulation. In ad-dition, a better understanding of the factors thatfacilitate the environmental persistence of the PGPRstrains would be very useful. Finally inoculant strainsshould be labelled �e.g., with lux of gfp genes�, sothey can be readily detected in the environment aftertheir release.
Acknowledgements
This review was supported by a strategic grant fromthe Natural Sciences and Engineering ResearchCouncil of Canada to B.R.G.
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