This document discusses the role of plant growth-promoting rhizobacteria (PGPR) in plant health promotion. It defines rhizosphere as the soil area surrounding a plant's root that is influenced by root activity and microbial processes. PGPR are beneficial bacteria that colonize plant roots and improve plant growth. They can fix nitrogen, solubilize phosphorus, produce siderophores, volatile organic compounds, indole-3-acetic acid, and ACC deaminase to facilitate nutrient uptake and stress tolerance in plants. PGPR also produce antibiotics that act as biocontrol agents against phytopathogenic bacteria, fungi and other pathogens. Inoculation of seeds and soil with PGPR can reduce the use of fertil

1. ROLE OF PGPR IN
PLANT HEALTH
PROMOTION
PRESENTED BY
Ms. Sarkar Prapti Partha
M.Sc. Entomology

2. According to the foremost definition given by the German
scientist, Hiltner L., rhizosphere refers to the soil area surrounding
a plant’s root, directly or indirectly influenced by the root and
which has a strong microbial activity
The term of rhizosphere comes etymologically from rhiza (root)
and sphera (surroundings
while the term ‘rhizobacteria’ implies a group of rhizosphere
bacteria competent in colonizing the root environment (Kloepper
et al., 1991)

3. Among the microbes of rhizosphere, bacteria (rhizobacteria) are the most
abundant (Glick, 2012).
The rhizobacteria concentration in the rhizosphere can reach 1012 CFU/g
of soil (Foster, 1988)
These rhizobacteria can affect the plants physiology through different
ways. Thus, the interactions between rhizobacteria and plant can be
beneficial, harmful or neutral.
 Phytopathogenic rhizobacteria (Desulfovibrio, Erwinia, Agrobacterium,
Enterobacter and Chromobacter, etc.)
 Beneficial rhizobacteria (Azospirillum, Pseudomonas, Bacillus, etc.) affect
positively plant growth and yield through various mechanisms of action.
The beneficial rhizobacteria are known under the name ‘PGPR’.

5. PLANT GROWTH PROMOTING RHIZOBACTERIA (PGPR)
The term PGPR was proposed by Kloepper et al. (1980)
PGPR are a group of bacteria capable to actively colonize the plants root system and
improve their growth and yield (Wu et al., 2005).
PGPR represent about 2 to 5% of total rhizospheric bacteria (Antoun and Kloepper,
2001).
 A wide range of species belonging to the genus Pseudomonas, Azospirillum,
Azotobacter, Klebsiella, Enterobacter, Arthrobacter, Burkholderia, Bacillus and
Serratia was reported as PGPR (Saharan and Nehra, 2011).
The PGPR effects depend on ecological and soil factors, plant species, plant age,
development phase and soil type (Werner, 2001).

6. Based on the degree of bacterial proximity
to the root and intimacy of association:
Classified into
1.Extracellular (ePGPR)
2.Intracellular (iPGPR),

7. Examples:
ePGPR iPGPR
Agrobacterium Bradyrhizobium
Arthrobacter Azorhizobium
Azotobacter Allorhizobium
Azospirillum Mesorhizobium
Bacillus
Chromobacterium
Erwinia

8. Inherent distinctiveness of PGPR:
(i) they must be proficient to colonize the root surface
(ii) they must survive, multiply and compete with other
microbiota, at least for the time needed to express their
plant growth promotion/protection activities
(iii) they must promote plant growth (Kloepper, 1994).

10. Role of PGPR in
plant growth

11. Generally, PGPR promote plant growth directly by either
facilitating resource acquisition (nitrogen, phosphorus
and essential minerals) or modulating plant hormone
levels, or indirectly by decreasing the inhibitory effects of
various pathogens on plant growth and development in
the forms of biocontrol agents (Glick, 2012)

13. Nitrogen fixation
 Nitrogen (N) is the most vital nutrient for plant growth and productivity.
 Atmospheric N2 is converted into plant-utilizable forms by biological N2
fixation (BNF) which changes nitrogen to ammonia by nitrogen fixing
microorganisms using a complex enzyme system known as nitrogenase
(Kim and Rees, 1994).
 BNF represents an economically beneficial and environmentally sound
alternative to chemical fertilizers (Ladha et al., 1997).

14. Nitrogen fixing organisms are generally categorized as
(a)symbiotic N2 fixing bacteria including members of the family rhizobiaceae
which forms symbiosis with leguminous plants.
(b)non-symbiotic (free living, associative and endophytes) nitrogen fixing
forms such as cyanobacteria (Anabaena, Nostoc), Azospirillum,
Azotobacter, Gluconoacetobacter diazotrophicus and Azocarus etc.
(Bhattacharyya and Jha, 2012).
 The establishment of the symbiosis resulting in the formation of the
nodules wherein the rhizobia colonize as intracellular symbionts.
 Plant growth-promoting rhizobacteria that fix N2 in non-leguminous
plants are also called as diazotrophs capable of forming a nonobligate
interaction with the host plants.

15. PGPR possess the ability to solubilize the soil insoluble phosphate in order to make
available to the plant. These PGPR are referred by the acronym "Phosphate
Solubilizing Bacteria, PSB".
The PSB group contains the genus Pseudomonas, Azospirillum, Bacillus, Rhizobium,
Arthrobacter, Alcaligenes, Serratia, Enterobacter, Acinetobacter, Azotobacter,
Flavobacterium and Erwinia.
The application of phosphate solubilizing microorganisms can reduce phosphorus
application to 50% without affecting the maize seeds yield (Yazdani et al., 2009).
Thus, the plants inoculation with PSB increases the availability of phosphorus in the
rhizosphere and its absorption by the plant.

17.  The solubilization of inorganic phosphorus occurs as a
consequence of the action of low molecular weight
organic acids which are synthesized by various soil
bacteria (Zaidi et al., 2009).
 Conversely, the mineralization of organic phosphorus
occurs through the synthesis of a variety of different
phosphatases, catalyzing the hydrolysis of phosphoric
esters.

19. Siderophores are low molecular weight iron binding protein compounds involved in
the process of chelating ferric iron (Fe (iii)) from the environment.
siderophores act as solubilizing agents for iron from minerals or organic
compounds.
Siderophores also form stable complexes with other heavy metals that are of
environmental concern, such as Al, Cd, Cu, Ga, In, Pb and Zn,
The presence of siderophore-producing PGPR in rhizosphere increases the rate of
Fe3+ supply to plants and therefore enhance the plant growth and productivity of
crop. Further, this compound after chelating Fe3+ makes the soil Fe3+ deficient for
other soil microbes and consequently inhibits the activity of competitive microbes

21. Involved in improving plant growth and induce systemic resistance (ISR) towards
pathogens.
Several bacterial species, from diverse genera including Bacillus, Pseudomonas,
Serratia, Arthrobacter, and Stenotrophomonas, produce VOCs that influence plant
growth.
E.g. Acetoin and 2,3-butanediol synthesized by Bacillus
Some other PGPR strains emit VOCs that can directly and/or indirectly mediate
increases in plant biomass, disease resistance, and abiotic stress tolerance.
 VOC emission directly and/or indirectly mediate increases in plant biomass,
disease resistance, and abiotic stress tolerance.

22.  IAA is associated with the plant defense mechanisms against phyto-
pathogenic bacteria and interferes with the many plant developmental
processes
 Increases root surface area and length, and thereby provides the plant
greater access to soil nutrients
 Rhizobacterial IAA loosens plant cell walls and as a result facilitates an
increasing amount of root exudation that provides additional nutrients to
support the growth of rhizosphere bacteria (Glick, 2012)
 Thus, rhizobacterial IAA is identified as an effector molecule in plant–
microbe interactions, both in pathogenesis and phytostimulators.
,

23.  IAA affects plant cell division, extension, and differentiation;
stimulates seed and tuber germination.
 Increases the rate of xylem and root development and controls
processes of vegetative growth.
 Initiates lateral and adventitious root formation;
 Affects photosynthesis, biosynthesis of various metabolites
and resistance to stressful.

24. Under stress conditions i.e salinity, drought, water logging, heavy metals and
pathogenicity, the endogenous level of ethylene is significantly increased which
negatively affects the overall plant growth. This induces defoliation and other
cellular processes that may lead to reduced crop performance
Plant growth promoting rhizobacteria which possess the enzyme, 1-
aminocyclopropane-1-carboxylate (ACC) deaminase, facilitate plant growth and
development by decreasing ethylene levels, inducing salt tolerance and reducing
drought stress in plants
E.g. Acinetobacter, Achromobacter, Agrobacterium, Alcaligenes, Azospirillum,
Bacillus, Burkholderia, Enterobacter, Pseudomonas, Ralstonia

25.  Such rhizobacteria take up the ethylene precursor ACC and convert it into 2-
oxobutanoate and NH3 Several forms of stress are relieved by ACC deaminase
producers.
 Such as effects of phytopathogenic microorganisms (viruses, bacteria, and fungi
etc.)
 resistance to stress from polyaromatic hydrocarbons, heavy metals, radiation,
wounding, insect predation, high salt concentration, draft, extremes of
temperature, high light intensity, and flooding (Glick, 2012; Lugtenberg and
Kamilova, 2009)
 Effects of seed/ root inoculation with ACC deaminase-producing rhizobacteria
are the plant root elongation, promotion of shoot growth, and enhancement in
rhizobial nodulation and N, P and K uptake as well as mycorrhizal colonization
in various crops.

27. The production of antibiotics is considered to be one of the most
powerful and studied biocontrol mechanisms of plant growth promoting
rhizobacteria against phytopathogens.
Example-
 By psuedomonads: amphisin, (DAPG), oomycin A, phenazine and
cyclic lipopeptides.
 By Bacillus, Streptomyces and Stenotrophomonas sp: oligomycin A
and xanthobaccin.

28. PGPR play a vital role in the management of various fungal diseases.
Examples-
1.P. fluorescens inhibit the growth of stem blight pathogen
Corynespora casiicola.
2.The seed treatment and soil application of P. fluorescens reduce root
rot of black gram caused by Macrophomina phaseolina.
3.Seed and foliar application of P. fluorescens reduce sheath blight
in rice. B. subtilis supplemented with chitin or chitin-containing
materials show control of Aspergillus niger and Fusarium udum in
groundnut and pigeon pea, respectively.

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