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Chapter 7
Plant Growth Promoting Rhizobacteria
and Sustainable Agriculture
Azeem Khalid, Muhammad Arshad, Baby Shaharoona, and Tariq Mahmood
Abstract The diverse groups of bacteria in close association with roots and capable
of stimulating plant growth by any mechanism(s) of action are referred to as plant
growth-promoting rhizobacteria (PGPR). They affect plant growth and develop-
ment directly or indirectly either by releasing plant growth regulators (PGRs) or
other biologically active substances, altering endogenous levels of PGRs, enhanc-
ing availability and uptake of nutrients through fixation and mobilization, reducing
harmful effects of pathogenic microorganisms on plants and/or by employing
multiple mechanisms of action. Recently, PGPR have received more attention for
use as a biofertilizer for the sust ainability of agro-ecosystems. Selection of efficient
PGPR strains based on well-defined mechanism(s) for the formulation of bio-
fertilizers is vital for achieving consistent and reproducible results under field condi-
tions. Numerous studies have suggested that PGPR-based biofertilizers could be used
as effective supplements to chemical fertilizers to promote crop yields on sustainable
basis. Various aspects of PGPR biotechnology are reviewed and discussed.
7.1 Introduction
Sustainability in agricultural systems without compromising the environmental
quality and conservation is one of the major concerns of today’s world. The
excessive use of agro-chemicals (fertilizers and pesticide s) is posing serious threats
M. Arshad (*) and B. Shaharoona
Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad 38040,
Pakistan
e-mail: arshad_ises@yahoo.com; shaha_ss@yahoo.com
A. Khalid and T. Mahmood
Department of Environmental Sciences, PMAS Arid Agriculture University, Rawalpindi 46300,
Pakistan
e-mail: azeemuaf@yahoo.com; qiratm@yahoo.com
M.S. Khan et al. (eds.), Microbial Strategies for Crop Improvement,
DOI: 10.1007/978-3-642-01979-1_7,
#
Springer‐Verlag Berlin Heidelberg 2009
133
to the environment. The gradual reduction in the use of chemical s in agriculture
without affecting yield or quality of the crop produce can only be possible with
a new generation of technologies. Furthermore, the long-term sustainability of
agricultural systems depends most likely on effective handling of the internal/
indigenous resources of agro-ecosystems. During the last couple of decades, the
new biotechnologies have opened new vistas for enhancement of agriculture
productivity in a sustainable manner. Advances in understanding of soil microbiol-
ogy and biotechnology have made possible exploitation of soil microorganisms for
improving crop productivity and, in turn, have offered an economically attractive
and ecologically viable supplement to reduce external inputs to some extent.
The plant rhizosphere is a remarkable ecological environment as myriad micro-
organisms colonize in, on, and around the roots of growing plants. Distinct commu-
nities of beneficial soil microorganisms are associated with the root systems of all
higher plants (Khalid et al. 2006). These rhizobacteria are considered as efficient
microbial competitors in the root zone, and the net effect of plant–microbe associa-
tions on plant growth could be positive, neutral, or negative. Such bacteria inhabiting
plant roots and influencing plant growth positively are often referred to as
plant growth-promoting rhizobacteria (PGPR) (Kloepper et al. 1986; Arshad and
Frankenberger 1998; Zahir et al. 2004). Th ese bacteria, after inoculation, rapidly
colonize onto seeds and roots in response to exudation and, thus, affect plant growth.
A complex matrix of organic and inorganic constituents of soil, particularly
rhizosphere, creates a unique and dynamic environment for the microorganisms
which affect plants and other associative microorganisms. Many microorganisms
are highly dependent for their survival on preformed substrates exuded by plant
roots (Frankenberger and Arshad 1995; Glick et al. 1998; Khalid et al. 2006). In
turn, the soil microflora inhabiting the rhizosphere can cause dramatic changes in
plant growth and development by producing plant growth regulators (PGRs) or
biologically active substances, or by altering endogenous levels of PGRs, and/or by
facilitating the supply and uptake of nutrients and providing other benefits.
A diverse array of bacteria, including species of Rhizobium, Bradyrhizobium,
Pseudomonas, Azospirillum, Azotobacter, Bacillus, Klebsiella, Enterobacter,
Xanthomonas, Serratia and many others, have been shown to facilitate plant growth
by various mechanisms.
Over the years, the PGPR have received worldwide importance and acceptance
for agricultural benefits. These microorganisms are the potential tools for sustain-
able agriculture because they not only ensure the availability of essential nutrients
to plants but also enhance the nutrient use efficiency. Several authors have reported
significant increases in growth and yield of agricultural crops in response to
microbial (PGPR) inoculants both under gree nhouse and field conditions (Kennedy
et al., 2004; Khalid et al. 2004a, 2006; Gravel et al. 2007; Kumar et al. 2007;
Zhuang et al. 2007; Arshad et al. 2008; Banchio et al. 2008; Contesto et al. 2008;
Figueiredo et al. 2008; Mubeen et al. 2008; Naiman et al. 2009). Beside promoting
plant growth, PGPR also enhance efficiency of fertilizers, mitigate abiotic stresses,
manage plant pathogens, and cause the degradation of xenobiotic compounds
(Glick 2003, 2004; Huang et al. 2004; Khan 2005; Arshad et al. 2007; Saleem
134 A. Khalid et al.
et al. 2007; Zhuang et al. 2007; Ahmad et al. 2008; Amor et al., 2008; Dell’Amico
et al. 2008; Lebeau et al., 2008; Masoud and Abbas 2009; Kohler et al. 2009).
Many promising microorganisms have been isolated and marketed as biofertili-
zers; however, their effects on crop yields fluctuate from crop to crop, place to
place, and from season to season, depending on the survival of the introduced
microorganisms on seed, roots, and in soil (Poi and Kabi 1979; Chanway and Holl
1992; Nowak 1998; Khalid et al. 2004a; Hafeez et al. 2006). To make effect ive
utilization of microbial inoculants, accurate and reliable methods for monitoring the
fate of applied PGPR in the rhizosphere/rhizoplane are required to enhance their
efficacy under field conditions. Scientists have used multidisciplinary approaches to
understand the adaptation of PGPR to the rhizosphere, including their mechanisms of
action and root colonization, production of determinants, and biodiversity, etc. They
are trying to manipulate the rhizosphere so that PGPR perform better and help to
increase food production for mankind on a sustainable basis. To achieve this suc-
cessfully, we need to know the players and to understand their interactions with each
other and with the growth substrate, in addition to abiotic factors which otherwise
have drastic effects on PGPR as well as on plant growth under diverse field condi-
tions. In this chapter, the potential mechanisms of action of PGPR, reasons for
inconsistency in their performance, and formulation of effective biofertilizers to be
used as the supplement to chemical fertilizers, are critically reviewed and discussed.
7.2 Mechanisms of Action
PGPR affect growth and development of plants by direct or indirect mechanisms
(Table 7.1). The direct mechanisms include N
2
-fixation, mobilization of nutrients
via production of phosphatases, siderophores, or organic acids, and production of
phytohormones and enzymes (Lucy et al. 2004; Khalid et al. 2004a, b; Gray and
Smith 2005;C¸ akmakc¸i et al. 2006; Tsavke lova et al. 2007). Indirectly, the bacteria
may exert a positive influence on plant growth by lessening certain deleterious
effects of a pathogenic organism by inducing host resistance to the pathogen or by
knocking out the pathogen from root surfaces or by producing chitinases or other
pathogen-suppressing substances (Raj et al. 2003; Guo et al. 2004; Van Loon and
Glick 2004; Van Loon 2007). Although scientists have reported both direct and
indirect methods of growth stimulation by PGPR, but there is no clear separation
between these two mechanisms. Certain bact eria posses multiple traits to affect
plant growth where one trait may dominate the other one (Shaharoona et al. 2006a,
Shaharoona et al. 2008; Hafeez et al. 2006). A bacterium influencing plant growth
by releasing PGRs can also play a role in controlling plant patho gens and diseases,
and vice versa. So, plant response to PGPR is a complex phenomenon, and recent
advances in research at the molecular level have provided a sufficient basis to
understand these mechanisms more precisely. The major mechanisms of PGPR
action involved in the improvement of plant growth and development are discussed
in the following sections.
7 Plant Growth Promoting Rhizobacteria 135
t1:1 Table 7.1 Possible mechanism(s) of action of PGPR for plant growth promotion
Plants PGPR Suggested
mechanism(s)
of action
References
t1:2
Lactuca sativa
L. cv. Tafalla
Pseudomonas mendocina
Palleroni
ACC deaminase
activity
Kohler et al.
(2009)
t1:3
Oryza sativa Methylobacterium sp. strain
NPFM-SB3
Indole-3-acetic
acid, cytokinins
Senthilkumar
et al. (2009)
t1:4
Solanum
tuberosum
Bacillus sp. Auxins Ahmed and
Hasnain
(2008)
t1:5
Arabidopsis
thaliana
Phyllobacterium brassicacearum
STM196, Pseudomonas putida
UW4, Rhizobium leguminosarum
bv. viciae 128C53K,
Mesorhizobium loti MAFF303099
ACC deaminase
activity
Contesto et al.
(2008)
t1:6
Phaseolus
vulgaris L.
Rhizobiumtropici (CIAT899),
Paenibacillus polymyxa (DSM 36),
Rhizobium, P. polymyxa strain
Loutit (L), Bacillus sp.
Indole acetic acid,
cytokinin,
N
2
-fixation
Figueiredo et al.
(2008)
t1:7
Triticum
aestivum L.
Pseudomonas spp., Burkholderia
caryophylli
ACC deaminase
activity,
chitinase
Shaharoona
et al.
(2007b,
2008)
t1:8
Cicer
arietinum L.
Serratia oderifera (J118),
Pantoea dispersa (J112)
and Enterobactor gergoviae
(J107)
ACC
deaminase, P-
solubilization
Shahzad et al
(2008)
t1:9
Malus domestica
Borkh
PGPR strains (OSU-142, OSU-7,
BA-8 and M-3)
Indole acetic
acid, cytokinin
Aslantas et al.
(2007)
t1:10
Brassica rapa Pseudomonas putida UW4 ACC deaminase
activity
Cheng et al.
(2007)
t1:11
Lycopersicon
esculentum
Pseudomonas fluorescens, P.
fluorescens subgroup G strain 2,
P. marginalis, P. putida subgroup
B strain 1 and P. syringae strain1
Indole acetic
acid
Gravel et al.
(2007)
t1:12
Apios americana Pseudomonas fluorescens TDK1 ACC deaminase
activity
Saravanakumar
and
Samiyappan
(2007)
t1:13
Pisum sativum Pseudomonas putida biotype A,
A7’ Acinetobacter calcoaceticus,
M9, P. fluorescens, AM3
ACC deaminase,
ethylene
Shaharoona
et al.
(2007a)
t1:14
Triticum
aestivum L.
Bacillus pumilus Auxin, siderophore Hafeez et al.
(2006)
t1:15
Rubus niveus Bacillus sp. N
2
-fixation,
P-solubilization
Orhan et al.
(2006)
t1:16
Zea mays L.,
Vigna radiata
Pseudomonas spp. ACC deaminase
activity,
chitinase
activity
Shaharoona
et al.
(2006a, b)
t1:17
(continued)
136 A. Khalid et al.
7.2.1 Fixation, Mobilization and Uptake of Nutrients
Nutrients are one of the extremely important factors which influence growth, yield,
and quality of different crops. Soil microorganisms can provide nutrients to plants
either through the fixation of atmospheric N
2
or by enhancing nutrient mobilization/
uptake through their biological activities, such as mineralization and through side-
rophore, organic acid and phosphatase production, etc.
Biological N
2
fixation by rhizobia and associative diazotrophic bacteria is a
spontaneous process and one of the widely studied mechanisms by which plants
benefit from the interacting partners. The bacteria benefit the plants by fixing N
2
in
exchange for fixed carbon either provided directly to the bacteria or indirectly by
t1:18Table 7.1 (continued)
Plants PGPR Suggested
mechanism(s)
of action
References
t1:19
Zea mays L. Azotobacter chroococcum,
Bacillus megaterium,
Bacillus mucilaginous
N
2
-fixation,
P-
solubilization,
K-
solubilization
Wu et al. (2005)
t1:20
Triticum
aestivum L.
PGPR Indole-3-acetic
acid
Khalid et al.
(2004a, b)
t1:21
Lycopersicon
esculentum,
Capsicum
annuum
PGPR Auxins Garci’a et al.
(2003)
t1:22
Brassica rapa,
Vigna radiata
P. putida GR12-2 and
an IAA-deficient mutant
Indole-3-acetic
acid
Patten and
Glick (2002)
t1:23
Hordeum vulgare Arthrobacter mysorens 7,
Flavobacterium sp. L30,
Klebsiella mobilis CIAM880
Indole-3-acetic
acid, ethylene
Pishchik et al.
(2002)
t1:24
Pinus pinea Bacillus spp. Gibberellins Probanza et al.
(2002)
t1:25
Oryza sativa L. Rhizobium, Azospirillum Indole-3-acetic
acid
Biswas et al.
(2000)
t1:26
Oryza sativa L. Rhizobium leguminosarum
(strain E11)
Indole-3-acetic
acid
Dazzo et al.
(2000)
t1:27
Vigna radiata P. putida GR 12-2 (wild type),
GR12-2/acd36 (ACC
deaminase minus mutant),
GR 12-2/aux1 (IAA over
producers)
Indole-3-acetic
acid, ethylene
Mayak et al.
(1999)
t1:28
Vigna radiata Pseudomonas sp. Biocontrol Sindhu et al.
(1999)
t1:29
– Paenibacillus polymyxa cytokinins Timmusk et al.
(1999)
t1:30
Cucumis sativus P. putida, Serratia
marcescens
Biocontrol Wei et al.
(1996)
t1:31
7 Plant Growth Promoting Rhizobacteria 137
releasing carbon as root exudates. A range of bacteria participates in interactions
with different plants, significantly increasing their vegetative growth and grain
yields. However, obtaining maximum benefits on farms from diazotroph PGPR
biofertilizer requires a systematic strategy designed to fully utilize all these benefi-
cial factors, allowing crop yields to be maintained or even increased while fertilizer
applications are reduced (Kennedy et al. 2004). Numerous studies have shown that
different species of bacteria fix atmospheric N
2
and consequently affect growth and
yield of various crops (Sindhu et al. 2002; Bai et al. 2003; Orhan et al. 2006; Afzal
and Bano 2008; Khalequzaman and Hossain 2008; Figueiredo et al. 2008). In
addition to biological N
2
-fixation, PGPR are also known to affect the nutrient
availability to the plant through acidification and redox changes or by producing
iron chelators and siderophores, and/or mobilizing the metal phosphates (Burd et al.
2000;Ro
¨
mkens et al. 2002; Abou-Shanab et al. 2003). Several reports have
suggested that PGPR can stimulate plant growth through their P-solubilizing
activity (Khan et al. 2007; Wani et al. 2007; Afzal and Bano 2008). Furthermore,
Wu et al. (2005) reported increased assimilation of nutrients, such as N, P, and K, in
plants, in response to inoculation with P-solubilizer (Bacillus megaterium) and
K-solubilizer (Bacillus mucilaginous ). Likewise, Orhan et al. (2006) reported that
inoculation with a phosphate-solubilizing Bacillus strain M3 significantly improv ed
P, Fe, and Mn contents of the leaves of raspberry (Rubus idaeus), suggesting that
Bacillus M3 alone or in combination with some other strains had the potential to
increase the nutrition of raspberry plants, in addition to growth and yield.
7.2.2 Production of Plant Growth-Regulating Substances
Plant growth-regulating substances are naturally occurring organic compounds that
influence various physiological processes in plants, such as cell elongation and cell
division. They perform these functions at concentrations far below the levels at
which nutrients and vitamins normally affect plant processes. It is now well
established that the majority of soil microorganisms can produce plant growth-
regulating substances, including phytohormones (auxins, gibberellins, cytokinins,
ethylene, and abscisic acid) and enzymes (Frankenberger and Arshad 1995; Glick
1995; Khalid et al. 2006), which is considered as one of the major mechanisms of
plant growth promotion by PGPR . Production of PGRs by microorganisms is
affected by the presence of suitable substrate(s)/precursor(s) as well as type and
concentration of exudates. The inocula in the presence of a specific physiological
precursor of a PGR and/or inocula that produce physiologically-active concentra-
tions of a phytohormone can be highly effective in promoting plant growth and
enhancing consistency and reproducibility (Frankenberger and Arshad 1995;
Arshad and Frankenberger 1998, 2002).
Microflora capable of producing PGRs in vitro predominates in the rhizosphere
of plants. However, the type and amount of growth-regulating substances released
by such microorganisms are variable. The quantitative or qualitative variations in
138 A. Khalid et al.
plant growth-promoting substances in turn lead to the differences in plant responses
to the PGPR inocula. Several studies have reported the ability of various PGPR to
produce auxins in vitro and in vivo (Almonacid et al. 2000; Khalid et al. 2004a, b;
Gravel et al. 2007). Similarly, plant-associated phototrophic purple bacterium
(Serdyuk et al. 1995) and Methylobacterium sp. (Senthilkumar et al. 2009) have
been reported to be capable of producing cytokinins in vitro. Different bacterial
species such as Proteus mirabilis, P. vulgaris, Klebsiella pneumoniae, Bacillus
megaterium, B. cereus, Escherichia coli and many more have been reported to
synthesize plant growth-promoting substances, including auxin, gibberellin, cyto-
kinin and abscisic acid (Tuomi and Rosenquist 1995; Karadeniz et al. 2006;
Tsavkelova et al. 2007). Soil microbiota is also known to produce the gaseous
phytohormone ethylene in vitro and in vivo (Weingart et al. 1999; Akhtar et al.
2005).
Some PGPR can influence plant growth by altering the synthesis of endogenous
phytohormones through the production of specific enzymes. Among these enzymes,
bacterial 1-aminocyclopropane-1-carboxylate (ACC) deaminase plays a significant
role in the regulation of a plant hormone, ethylene, and thus the modification of the
growth and development of plants (Arshad and Frankenberger 2002; Glick 2005).
Bacterial strains with ACC deaminase can at least partially eliminate the stress-
induced ethylene-mediated negative impact on plants by converting the germinat-
ing seed/root’s ACC into a-ketobutyrate and ammonia (Glick et al. 1998). Since
then, numerous species of Gram-negative and Gram-positive bacteria have been
reported to produce ACC deaminase (Belimov et al. 2005; Pandey et al. 2005;
Sessitsch et al. 2005; Blaha et al. 2006 ; Madhaiyan et al. 2006; Stiens et al. 2006;
Shaharoona et al. 2006a, 2006b, 2007a, 2007b, 2008; Shahzad et al. 2008).
7.2.3 Biological Control
Soil-borne pathogens are one of the major limiting factors for low crop productivity
due to their inhibitory effects on plant health. In mode rn agronomic practices, a
huge amount of chemicals (insecticide/fungicide) are used to offset various patho-
gens inflicting severe losses to crop yields. Although these chemicals are vital for
controlling the pathogens on the one hand, on the other hand they can drastically
affect the microbial diversity and functional properties of natural microbial com-
munities of soils, leading thereby to imbalanced agro-ecosystems (Singh et al.
2005; Vig et al. 2006; Mubeen et al. 2008). It is therefore important to discover
the mos t viable and economical means for effective disease management in an
environment-friendly manner. Currently, biopesticides are receiving worldwide
attention and considered important for the sust ainability of the agricultu ral system.
Furthermore, the WTO guideline that suggests that only residue-free agricultural
produce can be exported has further created a great interest and demand for the use
of biopesticides in crop protection systems. In recent times, PGPR have emerged as
potential candidates with wide scope for inducing systemic resistance in crop plants
7 Plant Growth Promoting Rhizobacteria 139
against many pathogens (Jeun et al. 2004; Nakkeeran et al. 2005; Khalequzaman
and Hossain 2008; Mirik et al. 2008; Masoud and Abbas 2009). Various species of
bacteria including Pseudomonas fluorescens, P. putida, P. cepacia, P. aeruginosa,
Bacillus spp., Rhizobium and many other PGPR exhibit biological control activit y
and inhibit pathogens by synthesizing chitinase, and by production of hydrogen
cyanide, protease, siderophores, and cellulase, and/or indirectly by promoting plant
growth and health through any mode of act ion (Zahir et al. 2004; Hafeez et al. 2006;
Narayanasamy 2008).
7.2.4 Multiple Mechanisms of Action
Some PGPR can stimulate plant growth through multifarious activities. Although
PGPR have been reported to influence plant growth by an array of mechanisms, the
specific traits by which PGPR facilitate plant growth, yield, and nutrient uptake
were limited to the expression of one or more of the traits expressed simultaneously
in a given environment of plant–microbe interaction. A PGPR can promote plant
growth by improving plant nutrition, modifying root growth architecture, and by
plant responses to external stress factors, simultaneously (Egamberdiyeva and
Hflich 2004; Dey et al. 2004; Saleem et al. 2007; Shaharoona et al. 2006a, b,
2007a, b, 2008). However, one trait may dominate the other one of the same PGPR
when exposed to certain environmental conditions. For instance, Dey et al. (2004)
reported more than one mechanism of PGPR responsible for growth promotion.
They suggested that, besides ACC deaminase activity, expression of one or more of
the traits such as suppression of phytopathogens, solubilization of tricalcium
phosphate, production of siderophore and/or nodulation promotion by the PGPR
might have simultaneously contributed to the enhancement of growth, yield, and
nutrient uptake of peanut (Arachis hypogaea). Similarly, Shaharoona et al. (2006a, b)
reported plant growth promotion by PGPR employing multiple mechanisms,
such as ACC deaminase activity, chitinase activity, and root colonization. Further-
more, Hafeez et al. (2006) attributed increase in plant growth to multiple traits (such
as produc tion of IAA, and siderophores and P-solubilization) of PGPR. We have
also found that PGPR expressing dual traits, such as ACC deaminase and nitroge-
nase activity or ACC deaminase with phosphatase activity performed better at
enhancing growth and yield of wheat (Triticum aestivum) and maize (Zea mays)
than PGPR possessing a single trait (Arshad et al., unpublished data).
7.3 Application of PGPR in Agriculture
PGPR are commonly used as inoculants for improving the growth and yield of
agricultural crops. They represent an essential and large component of biofertilizer
technology to improve the productivity of agricultural systems in the long run
140 A. Khalid et al.
(Zahir et al. 2004; Khalid et al. 2006; Naiman et al. 2009). Many PGPR have shown
great promise as potential inoculants for agriculture uses and environmental pro-
tection, and can play very critical roles in maintaining the sustainability of agro-
ecosystems. However, the current use of PGPR in agriculture is poor despite
numerous reports on their fair performance under laboratory conditions.
PGPR possess the ability to coloniz e and establish an ongoing relationship with
plants, resulting in better root growth, more biomass, and a substantial increase in
crop yields. In this context, significant eff ects of PGPR have been observed on
various agricultural crops, including legumes, cereals, and noncereals, and other
environmentally important plant species. Furthermore, the impact of PGPR on root
development with the consequent advantage on increasing water and nutrient use
efficiencies has also been observed (Zahir et al. 2005; Ahmad et al. 2008; Arshad
et al. 2008). The potential uses and benefits of PGPR in the improvement of overall
performance of plants are discussed in the following sections.
7.3.1 Effect of PGPR on Plant Growth
The potential of PGPR for improving growth and yields of various crops has been
extensively documented (Table 7.2). However, most of the studies have been
conducted under controlled environments rather than under natural field conditions.
Results of these studies have demonstrated clearly that PGPR carry abundant
potentials to enhance growth and yield of target crops. However, the selection of
a functionally effective PGPR strain is very critical, and the plant responses are
often variable depending upon the bacterial strain, plant genotypes, and experimen-
tal sites. It has also been claimed that the PGPR isolated from a particular crop or
ecological zone are more effective in producing consistent resu lts if reapplied to the
same crop and reused in the same ecological zone (Chanway and Holl 1992; Nowak
1998). This might be due to a greater adaptability of the introdu ced PGPR in the
given rhizosphere, while inconsistency in the responses of same crop to same PGPR
could be attributed to (1) the poor quality of inocula, (2) short shelf life of PGPR,
(3) lack of standard deliver y systems, and/ or (4) failure in maintaining a required
density of PGPR onto seeds or roots. Moreover, the nature and composition of the
material used as a carrier for a PGPR also plays a significant role in producing its
impact on the inoculated plants.
Another important aspect of these trials is that the effects of PGPR have been
investigated under different fertilizer doses, which has been shown to affect the
efficiency of PGPR, leading to inconsistent performance under different agro-
ecosystems (Shaharoona et al. 2008). Considering the acute demand for food
supply, it is wise to make efforts to improve crop produc tion by using PGPR,
over and above what is achievable with optimum chemical fertilizers. It is also
pertinent that most of the investigations have been focused on diazotrophs, which
were tested under different N application rates. It could be very useful if selected
PGPR were tested under different rates of all the three major nutrients (N, P, and K)
7 Plant Growth Promoting Rhizobacteria 141
t2:1 Table 7.2 Plant responses to inoculation with PGPR
PGPR Plants Comments References
t2:2
Azospirillum brasilense
Az1 and Az2, P.
fluorescens Pf
Oryza sativa L The inoculation increased aerial and root biomass and
grain yield by 12, 40 and 16%, respectively, over
uninoculated control
Naiman et al. (2009)t2:3
A. brasilense, Pantoea
dispersa
Capsicum annuum Inoculation increased the concentration of citric,
ascorbic and succinic acids in green fruit of
sweet pepper compared to noninoculated control
Amor et al. (2008)t2:4
Bacillus sp. Solanum tuberosum Bacterial inoculation caused increment in the growth
of the plants compared to the noninoculated
treatments
Ahmed and Hasnain (2008)t2:5
P. fluorescens, B. subtilis,
Sinorhizobium meliloti,
Bradyrhizobium sp.
Origanum majorana L. Only P. fluorescens and Bradyrhizobium sp. showed
significant increases in shoot length, shoot weight,
number of leaves and node, and root dry weight,
in comparison to control plants or plants treated with
other PGPR. Essential oil yield was also significantly
increased relative to noninoculated plants, without
alteration of oil composition
Banchio et al. (2008)
t2:6
Phyllobacterium
brassicacearum STM196,
P. putida UW4, R.
leguminosarum
bv. viciae 128C53K,
Mesorhizobium loti
Arabidopsis thaliana Root hairs of seedlings inoculated with the ACC
deaminase strains were significantly longer
Contesto et al. (2008)
t2:7
Rhizobiumtropici (CIAT899),
Paenibacillus
polymyxa (DSM 36),
Rhizobium,
P. polymyxa strain Loutit
(L),
Paenibacillus, Bacillus
sp.
Phaseolus vulgaris L. Beans coinoculated with R. tropici (CIAT899) and
P. polymyxa (DSM 36) had higher leghemoglobin
concentrations, nitrogenase activity and N
2
fixation efficiency
and thereby formed associations of
greater symbiotic efficiency. Inoculation with
Rhizobium and P. polymyxa strain Loutit (L)
stimulated nodulation.
PGPR also stimulated specific-nodulation
(number of nodules per gram of root dry weight) and increased
accumulated N
Figueiredo et al. (2008)
t2:8
142 A. Khalid et al.
Bacillus strains Capsicum annuum Stem diameter, root elongation, root dry weight, shoot
dry weight and yield were increased in response to
inoculation in the field experiment by 7.0–20.5, 7.0–17.0,
4.5–23.5, 16.5–38.5, and 11.0–33.0%, respectively
Mirik et al. (2008)t2:9
Pseudomonas spp. Triticum aestivum L. Inoculation significantly increased growth, yield and
nutrient use efficiency of wheat
Shaharoona et al. (2008)t2:10
Serratia oderifera (J118),
Pantoea dispersa (J112),
Enterobactor gergoviae
(J107)
Cicer
arietinum L.
The PGPR in the presence of P-enriched compost resulted
in a highly significant increase in fresh biomass (84%),
number of pods plant
1
(97%), grain yield (79%) and
number of nodules plant
1
(87%) compared to
uninoculated control
Shahzad et al. (2008)
t2:11
PGPR strains OSU-142,
OSU-7, BA-8 and M-3)
Malus domestica Borkh) Inoculation with OSU-142, OSU-7, BA-8 and M-3 PGPR
increased average shoot length by 59.2, 18.3, 7.0 and
14.3% relative to the control and fruit yield by 116.4,
88.2, 137.5 and 73.7%, respectively. Bacterial inoculation
increased shoot diameter from 7.0 to 16.3% compared to control
Aslantas et al. (2007 )
t2:12
P. fluorescens, P. fluorescens
subgroup G strain 2,
P. marginalis, P. putida
subgroup B
strain 1 and P. syringae
strain 1)
Lycopersicon esculentum Pseudomonas putida was shown to improve fruit yields
in rockwool and in organic medium.
The production of IAA was shown as a
possible mechanism for plant growth
stimulation by the bacterium. In addition, roots of tomato
seedlings grown in the presence of increasing concentrations of
IAA were significantly longer when seeds were treated with P.
putida
Gravel et al. (2007)
t2:13
B. megaterium, B. subtilis,
Pseudomonas corrugate
Zea mays L. All three bacterial inoculants resulted in an increment
in grain yield of maize up to 122, 135 and 194%, respectively,
compared to respective control.
The overall beneficial effects of bacterial
inoculations contributed to the colonization and
survival of the introduced bacteria, and to
stimulation of the indigenous microflora in the rhizosphere
Kumar et al. (2007)
t2:14
(continued)
7 Plant Growth Promoting Rhizobacteria 143
t2:15 Table 7.2 (continued)
PGPR Plants Comments References
t2:16
B. subtilis BEB-lSbs (BS13) Lycopersicon esculentum Yield per plant, fruit weight and length were increased
significantly by the Bacillus subtilis BEB-lSbs (BS13) treatment
when compared to the control
Mena-Violante and Olalde-
Portugal (2007)
t2:17
Pseudomonas sp.,
Burkholderia caryophylli
Triticum aestivum L. Both PGPR containing ACC deaminase positively
influenced growth and yield of wheat
Shaharoona
et al. (2007b)
t2:18
B. pumilus 8 N-4 Inoculation of the wheat variety Orkhon with PGPR
B. pumilus 8 N-4 (originated from Mongolia) resulted
in the maximum increase in plant biomass, root length,
and total N and P contents in plants. The isolate was
also capable of producing auxin and siderophore
Hafeez et al. (2006)
t2:19
Cyanobacterial strains Oryza sativa L. Significant increases in grain and straw yield were
observed when rice seedlings were inoculated with
four cyanobacterial strains either applied alone or
in combination with chemical fertilizer. In addition,
a saving of 25 kg Nha
1
was attained through cyanobacterial
fertilization
Jha and Prasad (2006)
t2:20
Pseudomonas sp. Zea mays L., Vigna
radiata
Significant increases in plant height, root weight and
total biomass were observed in response to
inoculation with PGPR containing ACC deaminase. Similarly,
inoculation significantly improved grain
yield of maize in the presence of nitrogenous fertilizers.
Effect of PGPR was also positive on nodulation of mung
bean (Vigna radiata)
Shaharoona
et al. (2006a, b)
t2:21
Pseudomonas, Azotobacter,
Azospirillum
Triticum aestivum L. Significant positive effects of inoculation on germination and
growth of wheat were observed
Shaukat et al. (2006)t2:22
A. chroococcum,
B. megaterium,
B. mucilaginous
Zea mays L. The application of PGPR significantly increased the plant
growth and resulted in the highest biomass and seedling
height. Inoculation not only increased the nutritional
assimilation of plant (total N, P, and K), but also improved soil
properties, such as organic matter content and total N in soil
Wu et al. (2005)
t2:23
144 A. Khalid et al.
Pseudomonas spp. Arachis hypogaea L. Seed inoculation with PGPR containing ACC deaminase resulted in
a significantly higher pod yield than the control, in pots, during
rainy and post-rainy seasons. PGPR also significantly enhanced
pod yield (23–26,
24–28, and 18–24%, respectively), haulm yield and nodule dry
weight over the control under field
conditions
Dey et al. (2004)
t2:24
B. licheniformis CECT 5106,
B. pumilus CECT 5105
Quercus ilex ssp. ballota Only B. licheniformis promoted the growth of Q. ilex seedlings.
Furthermore, B. licheniformis inhibited
fungal growth as revealed by ergosterol/chitin analysis
Domenech et al. ( 2004)t2:25
PGPR isolates Triticum aestivum L. Peat-based seed inoculation with selected PGPR strains capable of
producing auxins exhibited stimulatory
effects on grain yields of tested wheat cv. in pot
(up to 14.7% increase over control) and field
experiments (up to 27.5% increase over control); however, the
response varied with cv. and PGPR
strains. It was concluded that the strain which
produced the highest amount of auxins in nonsterilized
rhizosphere soil also caused maximum increase in
growth and yield of both the wheat cultivars
Khalid et al. (2004a)
t2:26
PGPR Quercus ilex ssp. ballota,
Pinus pinea
All strains significantly increased stem length, neck
diameter and shoot dry weight of the inoculated plants
Garci´a et al. (2003)t2:27
Enterobacter cloacae, P.
putida,
P. fluorescens
Brassica rapa Inoculation significantly enhanced root elongation
of canola under gnotobiotic conditions
Penrose and Glick (2003)t2:28
Rhizobacteria Brassica
juncea
A significant increase in growth was observed in the inoculated
seedlings. The PGPR were also capable of producing IAA
Asghar et al. (2002)t2:29
P. putida Am2, P. putida
Bm3,
Alcaligenes xylosoxidans
cm4,
Pseudomonas sp. Dp2
Brassica
juncea L.
Significant increase in root elongation of phosphorus-sufficient
seedlings of rapeseed in a growth-pouch
culture experiment was observed in response to inoculation
Belimov et al. (2002)
t2:30
(continued)
7 Plant Growth Promoting Rhizobacteria 145
t2:31 Table 7.2 (continued)
PGPR Plants Comments References
t2:32
P. putida GR12-2 and an
IAA-deficient mutant
Brassica rapa,
Vigna radiata
Primary roots of canola seeds treated with wild-type strain
were 35–50% longer than the roots from seeds treated with the
IAA-deficient mutant and the roots from uninoculated seeds.
Exposing mung bean cuttings to
high levels of IAA by soaking in a suspension of the
wild-type strain stimulated formation of many adventitious roots
Patten and Glick (2002)
t2:33
Arthrobacter mysorens 7,
Flavobacterium sp. L30,
Klebsiella mobilis
CIAM880
Hordeum vulgare All the PGPR actively colonized barley root system
and rhizosphere, and significantly stimulated root
elongation up to 25%
Pishchik et al. (2002)
t2:34
B. licheniformis CECT 5106,
B. pumilus CECT 5105
Pinus pinea Both Bacillus strains promoted the growth of
P. pinea seedlings
Probanza
et al. (2002)
t2:35
Rhizobium, Azospirillum Oryza sativa L. Inoculation with diazotrophs had significant
growth-promoting effects on rice seedlings
Biswas et al. (2000 )t2:36
R. leguminosarum (strain
E11)
Oryza sativa L. Growth-promoting effects of inoculation on rice seedlings
were observed under axenic conditions
Dazzo et al. (2000)t2:37
Azotobacter Zea mays L. Inoculation with strains efficient in IAA production had
significant growth-promoting effects on maize seedlings
Zahir et al.
(2000)
t2:38
P. putida GR 12-2 (wild-
type),
GR12-2/acd36 (ACC
deaminase
minus mutant), GR 12-2/
aux1
(IAA over producers)
Vigna radiata Only the wild-type strain produced longer roots Mayak et al. (1999)
t2:39
146 A. Khalid et al.
instead of N only. Moreover, the soil fertility status should also be considered while
using PGPR along with specific doses of N, P, and K fertilizers. Therefore, the
proper understanding of mechanism(s) of action of chosen PGPR can substantially
help in obtaining consistent responses in terms of improved growth and yield of
bioprimed plants/crops. If all these factors are considered properly, the agriculture
industry can draw significant benefits from the PGPR-based biotechnological
approaches. Another aspect which requires urgent attention is the quality improve-
ment of the agricultural produce following PGPR inoculation. Since PGPR exert
their influence through different mechanism(s), it is very likely that they can affect
quality of the produce; unfortunately, most of the studies have reported the effects of
PGPR on growth and yield of inoculated plants in quantitative terms, but very little is
known about the quality parameters of produce modified by the PGPR inoculations.
Future efforts should, hence, be directed to assess how the quality of food produce is
influenced by PGPR, besides their role in growth and development of plan ts.
7.3.2 PGPR in Stress Agriculture
Agricultural crops are exposed to many stresses that are induced by both biotic and
abiotic factors. These stresses invariably affect plant growth and yield of crops
depending on the type and intensity of stress. Under stress conditions, such as
salinity, drought, waterlogging, heavy metals and pathogenicity, the produc tion of
ethylene in plants at substantially accelerated rates is a very common feature, which
adversely affects the root growth and, consequently, the development of the plants.
As described earlier (see Sect. 2.2), certain PGPR lower ethylene synthesis by
metabolizing ACC (an immediate precursor of ethylene biosynthesis in higher
plants) into a-ketobutyrate and ammonia, and, thus, mitigate the negative impact
of both biotic and abiotic stresses on plants (Saleem et al. 2007). Recently, several
authors have documented profound effects of inoculation with PGPR containing
ACC deaminase on plan t growth under stress conditions (Table 7.3).
In the present scenario of several biotic and abiotic stresses to which agriculture
is confronted, the role of PGPR containing ACC deaminase could be crucial for
sustainable crop production. However, some beneficial aspects of these PGPR
under salinity, drought, waterlogging, biocontrol, temperature, and n utritional
stresses, and in the cut-flower industry and in nodulation in legumes have not been
thoroughly exploited. Glick (2006 ) reported that transgenic tomato (Lycopersicon
lycopersicum), canola (Brassica napus), and tobacco (Nicotiana tabacum) plants
that express ACC deaminase exclusively in their roots behaved physiolo gically
similarly to nontransformed plants treated with ACC deaminase-containing
PGPR, both in the presence and absence of various stresses. Consequently, there is
no apparent advantage to the use of ACC deaminase transgenic plants compared to
treating the roots of the plants with ACC deaminase-containing PGPR. However,
genetic modification of all plant species is not possible due to many limitations,
such as proprietary rights and international trade agreements on genetically
7 Plant Growth Promoting Rhizobacteria 147
t3:1 Table. 7.3 Effect of PGPR on plant growth under various abiotic and biotic stresses
Plant species PGPR Plant responses under stress References
t3:2
Lactuca sativa
L. cv. Tafalla
Pseudomonas mendocina
Palleroni
The inoculated plants had significantly greater shoot biomass
than
the control plants at low and high salinity levels. At the
highest salinity level, the water content was greater in leaves
of plants
treated with P. mendocina. The plants also showed higher
concentrations of foliar K and lower concentrations of foliar
Na under high salt conditions
Kohler et al. (2009)
t3:3
Sorghum bicolor,
Zea mays L.
Pseudomonas spp. Inoculation with PGPR containing ACC deaminase significantly
improved fresh biomass under water-deficient field conditions
Arshad and
Khalid (2008)
t3:4
Pisum sativum L Pseudomonas spp. The inoculation partially eliminated the effects of water stress on
growth, yield and ripening of P. sativum L., both in pot and
field trials
Arshad et al. (2008),
Zahir et al. (2008)
t3:5
Brassica rapa P. putida UW4 Induced salt tolerance of plants by lowering the synthesis of
salt-induced stress ethylene and promoted the growth of
canola
in a saline environment
Cheng et al. (2007)
t3:6
Apios americana P. fluorescens TDK1 The PGPR strain enhanced the saline resistance in the plants and
increased yield as compared to strains lacking ACC
deaminase activity
Saravanakumar and
Samiyappan (2007)
t3:7
Zea mays L. Unidentified PGPR Significantly increased plant growth under salinity stress
conditions
Nadeem et al. (2006,
2007)
t3:8
Vitis vinifera L. Burkholderia
phytofirmans PsJN
Inoculation enhanced plant growth and physiological activity at
both ambient (26
C) and low (4
C) temperatures. Inoculation
also increased root growth and plantlet biomass. Moreover,
the bacterium significantly improved plantlet cold tolerance
compared to that of the nonbacterized control, which was
more sensitive to exposure to low temperatures
Barka et al. (2006 )
t3:9
148 A. Khalid et al.
Solanum tuberosum PGPR PGPR were capable of antagonizing at least one of the two potato
pathogens Ralstonia solanacearum and Rhizoctonia solani
Rasche et al. (2006)t3:10
Pisum sativum Pseudomonas sp. Inoculation with bacteria counteracted the Cd-induced inhibition
of nutrient uptake by roots
Safronova
et al (2006)
t3:11
Brassica napus PGPR Increases (up to 31%) in root elongation of inoculated rape
seedlings compared to the control plants were observed.
Inoculation with the isolates was found to increase root dry
weight (ranging from 8 to 20%) and shoot dry weight (ranging
from 6 to 25%) of rape in cadmium-amended soil in pot
experiments. The bacterial isolates were also able to colonize
and develop in the rhizosphere soil of rape after root
inoculation
Sheng and Xia (2006)
t3:12
Brassica juncea L. Variovorax paradoxus,
Rhodococcus sp.
Plant growth was improved in Cd
2+
-supplemented media in
response to inoculation
Belimov et al. ( 2005)t3:13
Pisum sativum Variovorax paradoxus 5C-2 Inoculated plants gave more seed yield (25–41%), seed number
and seed nitrogen accumulation than uninoculated plants
under moisture stress and watering conditions
Dodd et al. (2005)t3:14
Chamaecytisus proliferus P. fluorescens The bacterium showed positive effect in antagonizing the growth
of Fusarium oxysporum and Fusarium proliferatum in
growth medium.
Donate-Correa et al.
(2005)
t3:15
Mimosa pudica Burkholderia sp. The bacterium exhibited antagonistic activity against Rhizoctonia
solani and Sclerotinia sclerotiorum
Pandey et al. (2005)t3:16
Phragmites australis Pseudomonas asplenii AC Inoculation resulted in normal plant growth under high levels
of Cu
2+
and creosote
Reed et al. (2005)
t3:17
Lycopersicon esculentum Achromobacter piechaudii Inoculation significantly increased the fresh and dry weights of
tomato seedlings grown in the presence of NaCl salt
concentration up to 172 mM. The bacterium also significantly
increased the fresh and dry weights of both tomato and pepper
seedlings exposed to transient water stress
Mayak et al. (2004a, b)
t3:18
(continued)
7 Plant Growth Promoting Rhizobacteria 149
t3:19 Table. 7.3 (continued)
Plant species PGPR Plant responses under stress References
t3:20
Glycine max B. subtilis NEB4, NEB5, B.
thuringiensis NEB17,
Bradyrhizobium japonicum
Coinoculation exhibited consistent and significant increases in
nodule number, nodule weight, shoot weight, root weight,
total biomass, total nitrogen, and grain yield at low (25, 17,
and
15
C) root zone temperatures
Bai et al. (2003)
t3:21
Brassica juncea L. Pseudomonas sp., Alcaligenes
sp., Variovorax paradoxus,
B. pumilus, Rhodococcus sp.
The bacteria were tolerant to Cd
2+
toxicity and stimulated
root elongation of rape seedlings in the presence
of 300mM CdCl
2
in the nutrient solution
Belimov
et al. (2001)
t3:22
Lycopersicon esculentum P. putida UW4, Enterobacter
cloacae CAL2, P. Putida
Inoculated plants showed substantial tolerance to flooding
stress
Grichko and
Glick (2001)
t3:23
Brassica juncea L.,
Lycopersicum esculentum
Mill
Kluyvera ascorbata Toxic effects of heavy metals (Ni
2+
,Pb
2+
and Zn
2+
) were not
pronounced in inoculated plants
Burd et al. (2000)t3:24
Cucumis sativus P. putida UW4 The bacterial strains were effective in biocontrol of
Pythium ultimum
Wang et al. (2000)t3:25
Solanum tuberosum Burkholderia phytofirmans PsJN PGPR helped potato plants in maintaining normal
growth under heat stress
Bensalim et al. (1998)t3:26
150 A. Khalid et al.
modified crops and restrictions in the use of genetically modified microorgan-
isms or plants. The use of PGPR containing ACC deaminase activity along with
other traits, such as the ability to synthesize nitrogenases, phosphatases, and
chitinases could prove to be a cost-effective and environment-friendly strategy
to ensure sustainable agricultu re.
7.3.3 PGPR for Bioremediation
Recently, the application of PGPR in association with plants has been expanded to
remediate contaminated soils (Khan et al. 2009). The addition of PGPR increases
the removal of pollutant most likely by enhancing germination, and by stimulating
plant growth including root biomass and survival of plants, in soils that are heavily
contaminated (Huang et al. 2004; Reed et al. 2005; Safronova et al. 2006; Arshad
et al. 2007). Some rhizobacteria can enhance phytoremdiation by promoting plant
growth through the synthesis of siderophores, phytohormones, enzymes, and anti-
biotics (Pattern and Glick 1996; Burd et al. 2000; Khalid et al. 2006; Arshad et al.
2007; Wani et al. 2008a), and/or through stimulation of certain metabolic pathways,
such as nitrogen fixation and the uptake of N, P, S, Mg, Ca, and other nutrients
(Belimov and Dietz 2000). Similarly, PGPR can increase the tolerance of plants to
contaminants; the PGPR–plant system cannot survive in comparatively extreme
environments such as with high concentrations of heavy metals (Wani et al. 2008a).
Although micro bial communities in polluted soils have been studied, little is known
about the composition of microbial commun ity in the plant rhizosphere growing on
highly polluted soils. Usually, rhizosphere soil is more conducive to remediation
due to high concentrations of nutrients exuded from the roots and dense bacterial
populations. Many workers have reported bioremediation of both organic
(Narasimhan et al. 2003; Huang et al. 2004, 2005; Villacieros et al. 2005; Muratova
et al. 2005) and inorganic contaminants in the environment by using PGPR
(Hallberg and Johnson 2005; Kao et al. 2006; Umrania 2006; Bur d et al. 2000;
Kamnev et al. 2005; Abou-Shanab et al. 2006; Wu et al. 20 06; Zaidi et al. 2006;
Sheng and Xia 2006; Wani et al. 2008b).
7.4 Formulations of Effective Biofertilizers
A number of steps are involved in developing effective PGPR-based biofertilizers
for achieving consistent results in terms of crop productivity under field conditions
(Fig. 7.1). The most critical steps involved in the developm ent of biofertilizers
include isolation of bacterial strains from the same habitat and/or crop followed by
screening of PGPR under axenic conditions by conducting repeated trials. The
strains showing better results under controlled conditions should be tested further
for their performance under natural conditions by conducting pot and field trials.
7 Plant Growth Promoting Rhizobacteria 151
Finally, the PGPR strain selected for biofertilizer formulation should be investi-
gated thoroughly to maintain its quality. Quality of bioferti lizers is one of the most
critical factors which determine their success or failure and acceptance or rejection
by end users, the farmers. The functionality of selected PGPR must be defined well
before using it as a candidate for biofertilizer formulation, which could be achieved
by employing biochemical and molecular tests in the laboratory and then determin-
ing correlations between PGPR traits with the growth promotion of inoculated
plants under axenic and natural conditions. However, there is no established basis
for acceptance of a formulation as an effective biofertilizer. Since PGPR based
biofertilizers contain living entities, which are very sensitive to environmental
conditions, the consistency in effectiveness cannot be as good as observed in the
case of chemical fertilizers. Howeve r, maintaining a particular population of a
selected PGPR strain could help in enhancing the consistency of biofertilizer
effects. A strict control over quality is the only answer to avoid failure of biofer-
tilizers in different agron omic regions of the world.
The production of biofertilizer and its acceptance by farming communities are
closely linked. For their use to expand globally at the farmers’ end, quality
management is essential and must be perform ed consistently in order to supply
contaminant-free bioproducts to the users. For this, skilled personnel are required
who know how to work with these materials and be able to respond to the modern
conditions of agricultural production. In addition, they should be well aware of the
sustainability and environmental protection measures. Furthermore, proper guide-
lines for the production and commercialization of biofertilizers should be framed in
order to popularize the use of such bioagents for maintaining the sustainability of
agro-ecosystems across the globe.
7.5 Conclusion
Enhancement in the use of PGPR is one of the newly emerging options for meeting
agricultural challenges imposed by the still-growing aggregate demand for food.
Moreover, this biotechnology is also likely to ensure conservation of our environments.
Isolation of bacteria Screening
Trials under axenic conditions
Pot trials
Trials under field conditions
Commercial biofertilizer
Standardization
(Quality control)
Fig. 7.1 Steps involved in the
development of an effective
biofertilizer product
152 A. Khalid et al.
However, before PGPR can contribute to such benefits, scientists must learn
more about them and explore ways and means for their better utilization in the
farmers’ fields.
Future research should focus on managing plant–microbe interactions, particu-
larly with respect to their mode of actions and adaptability to conditions under
extreme environments for the benefit of plants. Furthermore, scientists need to
address certain issues, like how to improve the efficacy of biofertilizers, what
should be an ideal and universal delivery system, how to stabilize these microbes
in soil systems, and how nutritional and root exudation aspects could be control led
in order to get maximum benefits from PGPR application. Biotechnological and
molecular approaches could possibly develop more understanding about PGPR
mode of actions that could lead to more successful plant–microbe interaction.
Efforts should also be directed towards the use of PGPR to reduce pesticide
applications. In brief, PGPR biotechnology provides an excellent opportunity to
develop environment-friendly biofertilizer to be used as supplements and/or alter-
natives to chemical fertilizers.
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