Plant growth promoting
bacteria and their role in
• PGPB includes Rhizoplane and Phylloplane bacteria.
• “Plant Growth Promoting Rhizobacteria”(PGPR).
• Term PGPR was first used by Joseph W. Kloepper
and Schroth in the late 1970s.
• PGPR are root colonizing (rhizosphere) bacteria
benificial to plants.
• Rhizosphere is the region around roots having high
• Term Rhizosphere was
coined by German
agronomist Hiltner in
The external surface of
roots together with closely
adhering soil particles and
DIVERSITY AMONG PGPRS
Nitrogen Fixation is one of the most beneficial
processes performed by rhizobacteria.
Rhizobacteria converts gaseous nitrogen (N2) to
ammonia (NH3) making it available to the host plant.
Nitrogenase enzyme is involved in nitrogen fixation
and requires anaerobic conditions.
Ex- Azospirillum, Bradyrhizobium, Rhizobium,
Serratia, Enterobacter , Burkholderia spp.
95% of Gram +ve soil bacilli belong to the genus
The remaining 5% are confirmed to be Arthrobacter
Members form endospores to survive under
Pseudomonas is the most abundant gram –ve genus
The ecological diversity of this genus is enormous.
• Pseudomonas strains show high
versatility in their metabolic activity.
• Antibiotics, siderophores, HCN are
the metabolites released by these
Taxonomy of PGPB
Earlier bacterial taxonomy relied on phenotypic
traits like cell and colony morphologies.
Taxonomy revolutionized with the discovery of PCR
technique in 1983.
The gene sequences of 16S subunit of rRNA are
used to compare similarities among strains.
Nowadays characteristics of strains are studied
using FAME technique, Protein estimation by SDSPAGE technique and MLEE.
Classification of PGPR
On the basis of
1. Plant part they occupy
Intracellular (iPGPR, symbiotics)
Exist inside root cells.
Forms root nodules.
Extracellular (ePGPR, free living
Exist in rhizosphere,on
rhizoplane, in intercellular
spaces of root cortex.
• Siderophores are high-affinity iron chelating
compounds secreted by microorganisms.
• Siderophores chelate ferric ion with high
affinity, allowing its solubilization and extraction
from most mineral or organic complexes.
• Bacterial siderophores classified into four main
classes carboxylate, hydroxamates, phenol
catecholates and pyoverdines.
Achieved through bacteriocins, antibiotics
hydrolytic enzymes,HCN production, SAR, ISR.
antibiotics and act as
Biocontrol based on
antibiosis secretion of
molecules that kill
Prevent the damage
of Rhizoctonia solani during
damping-off of cotton plants
to Pythium spp.
Zwittermicin A B. cereus UW85 Bio-control of alfalfa damping
Production of Phytohormones
• Phytohormone production by PGPR was first
reported in 1940.
• Auxin and Ethylene are more commonly produced
hormone, Cytokinin is less common.
• Auxin promotes lateral root formation, cell division,
apical dominance etc.
• Among PGPR species, Azospirillum is one of the best
studied IAA producers (Dobbelaere et al., 1999)
ROOTS WITHOUT PGPR
ROOTS WITH PGPR
• Production of Gibberellins by PGPR is rare,
• However two strains have been reported, Bacillus
pumilis and Bacillus licheniformis.
HCN producing rhizobacteria
• HCN is a powerful inhibitor of metal enzymes, especially
cytochrome C oxidases.
• HCN production is a common trait within the group
• Include species
of Alcaligenes, Bacillus, Pseudomonas and Rhizobium.
• Strawberry fruits were harvested and transported
to the laboratory.
• Dipped in a suspension of B. cinerea conidia and
allowed to dry for 1 h.
• Then inoculated with bacterial suspensions.
• Control fruits dipped in conidia, dried and dipped in
nutrient broth diluted with sterile distilled water,
• Fruits were incubated for 4 days at 25°C, and then
observation was recorded.
Donmez et al., 2011
• No significant differences between CD-8, MFD4, MFD-18, MFDÜ-1 and control
• Highest percentage of gray mold infection (79.2%)
was observed in the control and
• Lowest (20.8%) was in MFD-45, followed by MFD81 (25.0%) and T26 (37.5%).
PGPB were effective in biocontrol of Botrytis
cinerea on strawberry fruit.
Fixation of Atmospheric N2
• There are two types of biological fixation:
symbiotic and non-symbiotic.
• The first is the most important mechanism by which
most atmospheric N is fixed.
• It is limited to legume plant species and various trees
and shrubs that form actinorrhizal roots
• Non-symbiotic N-fixing rhizospheric bacteria belongs
to genera including
Azoarcus, Azospirillum, and Pseudomonas
Most studied symbiotic bacteria
are Rhizobium, Bradyrhizobium, Sinorhizobium and
Induced Systemic Resistance
• PGPR interact with plant in a restricted area but
response is extended to whole plant.
• Salicylic acid, which plays a protective role in
acquired systemic resistance .
• While acquired systemic resistance is induced upon
pathogen infection, induced systemic resistance can
be stimulated by other agents, such as PGPB
• Plants inoculated with the biocontrol PGPB, P.
putida and Serratia marcescens were protected
against the cucumber pathogen P. syringae pv.
Bashan &Bashan., (2005)
Role of siderophore in induction of SAR
• E. chrysanthemi produces two
Achromobactin ( iron limiting
Chrysobactin (severe iron
The role of CB in induction of
SAR has been studied in
Fig: PR1 gene expression and SA production in Arabidopsis leaves
following CB treatment (Dellagi et al., 2009).
Production of Enzymes
• Hydrolytic enzymes produced by some biocontrol PGPB
lyse specifically fungal cell walls, and thereby prevent
phytopathogens from proliferating .
• Ex. Pseudomonas stutzeri produces chitinase that lyse
cell wall of Fusarium solani.
• Another strategy is the hydrolysis of fungal products
harmful to the plant.
• Ex.Cladosporium werneckii and B. cepacia can hydrolyze
fusaric acid that causes severe damage to plants.
Competition and Displacement of
• Competition for nutrients and suitable niches
among pathogens and is another mechanism of
biocontrol of some plant diseases.
• Ex- high inoculum level of Pseudomonas syringae
protected pears against Botrytis cinerea and
Penicillium expansum .
• Bacteria capable of multiplying on the leaf surface
to form a large population can compete successfully
with pathogens for these sites and often reduce
List of PGPRs
Jeon et al. (2003)
Dey et al. (2004)
Cazorla et al. (2007)
Wani et al. (2007a)
, IAA and Siderophores
Wani et al. (2007e)
IAA, Nitrogenase activity
Elisete et al. (2008)
IAA, Siderophores, HCN,
Ahemad and Khan
• Defined as populations that can
survive and multiply on the surface of
• Also called as epiphytic bacteria.
• survive in trichomes
chambers, hydathodes, and
especially, in between the depressions
along the junctions of adjacent
• They utilize similar mechanism for
controlling of pathogens like
antibiosis, siderophore production etc.
Location of the epiphytotic PGPB in tomato
Bacterial spot and early blight biocontrol by
epiphytotic bacteria in tomato plants
Filho et al., 2010
(I) Paenibacillus macerans and Bacillus pumilus
epiphytic bacteria and benzalkonium chloride reduce
Xanthomonas vesicatoria and Alternaria solani disease
severity in tomato plants.
(II) Epiphytic bacteria are able to inhibit the growth of
tested phytopathogens, and efficiently
colonize the phylloplane of tomato plants.
Challenges in Selection and
Characterization of PGPB
• Lack of proper selection and screening procedure
thus most promising organisms aren’t identified.
• Effective strategies for initial selection and
screening of PGPB isolates are required.
• Selection of PGPB with the potential to control soilborne pathogens
• Selection based on traits known to be associated
with PGPB such as root colonization, ACC
deaminase activity, antibiotic and siderophore
Prediction how an organism will respond
when placed in the field (compared to the
controlled environment of a laboratory.
lack of consistency and many variation in
results that are obtained in field trials
PGPB bacteria will not live forever in a
soil/leaves, there is need to re-inoculate
seeds to bring back populations.
Challenges in Field Application
performance in the field
due to heterogeneity of
abiotic and biotic factors.
• Knowledge of factors
concentration, timing and
inoculant, and of soil and
• concept of managing the
by manipulation of the
host plant, substrates for
ensure survival and
activity in the field
improved carriers and
Challenges in Commercialization of PGPB
Maintaining quality, stability, and efficacy of
Factors like shelf life, compatibility
considered while formulation development.
Non-target effects on other organisms
including toxigenicity, allergenicity,
Capitalization costs and potential markets
must be considered in the decision to
• PGPB has dual role as plant growth
promotion and as bioagent.
• They control the plant pathogen in
direct as well as indirect way.
• PGPB is available in nature but their
screening is not easy.
• It is included in IDM strategy for
controlling several plant pathogens.