05/02/20231

PGPR: ROLE AND IMPORTANCE

IN AGRICULTURERAWE (0+4)

SPEAKERNEHA TANDON

INSTRUCTORMR. ACHIN KUMARASST. PROF SSCIAS,RGSC, BHU

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CONTENT1. INTRODUCTION

2. RHIZOSPHERE AND PLANT GROWTH PROMOTING RHIZOBACTERIA

3. PGPR FORMS

4. PGPR MECHANISMS

a. DIRECT

b. INDIRECT

5. FUNCTIONS OF PGPR

a. BIOCONTROL PROPERTIES

b. BIOINOCULANT

c. ABIOTIC STRESS RESISTANCE

d. CO-INOCULATION

6. HARMFUL ASPECTS OF PGPR

7. FUTURE PROSPECTS

8. CONCLUSION05/02/2023 3

INTRODUCTION

Indiscriminate use of chemical fertilizers adversely

affects soil microorganism, fertility status of soil and

environment

So, PGPRs are replacing agrochemicals for the plant

growth promotion

Economically, environmentally beneficial for lower

production cost and for sustainable agriculture05/02/2023 4

RHIZOSPHERE AND PLANT GROWTH PROMOTING RHIZOBACTERIA

Term rhizosphere given by Hiltner (1904) and term PGPR given

by Kloepper and Schroth (1981)

Rhizosphere is the narrow zone of soil specifically influenced by

the root system

Populated by microorganisms and the bacteria called

rhizobacteria

Three types: beneficial, deleterious & neutral groups

Beneficial free-living soil bacteria referred to as PGPR

Bacillus and Pseudomonas spp. are predominant among PGPR05/02/2023 5

PLANT GROWTH PROMOTING RHIZOBACTERIAL FORMS

1. EXTRACELLULAR PGPR (ePGPR) In rhizosphere, rhizoplane or between cells of root cortex

Includes Agrobacterium, Arthrobacter, Azotobacter, Azospirillum, Bacillus,

Caulobacter, Chromobacterium, Erwinia, Flavobacterium, Micrococcous,

Pseudomonas and Serratia

2. INTRACELLULAR PGPR (iPGPR) Inside specialized nodular structures of root cells.

Includes Allorhizobium, Bradyrhizobium ,

Mesorhizobium and Rhizobium, endophytes and Frankia 05/02/2023 6

PGPR MECHANISMS1.DIRECT MECHANISM

Providing plant with a compound synthesized by bacterium or facilitating

uptake of nutrients from the environment

2. INDIRECT MECHANISM Reducing or preventing deleterious effects of phytopathogenic

organisms by producing antagonistic substances or by inducing

resistance

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Fig : The possible mode of action used by PGPR towards growth promotion in plants. The flow and location of nitrogen fixation, phosphorus solubilization, and siderophore production are shown

(Vacheron, Desbrosses, Bouffaud, Touraine.,2013)

MODE OF ACTION OF PGPR05/02/2023 8

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DIRECT MECHANISMS

Two mechanisms:-

a. SYMBIOTIC NITROGEN FIXATION

Mutualistic relationship between a microbe and the plant.

Eg. Rhizobium, Bradyrhizobium, Sinorhizobium, Mesorhizobium and Frankia

b. NON-SYMBIOTIC NITROGEN FIXATION

By free living diazotrophs

Eg. Azotobacter, Acetobacter, Azospirillum,, Diazotrophicus, Enterobacter,

Pseudomonas and cyanobacteria

Provides an integrated approach for disease management and maintains nitrogen

level in soil.

1. NITROGEN FIXATION

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The main P solubilization mechanism includes:

a. Release of complex or mineral dissolving compounds

b. Liberation of extracellular enzymes

c. Release of P during substrate degradation

Includes genera Arthrobacter, Bacillus, Beijerinckia,

Enterobacter, Erwinia, Flavobacterium, Microbacterium

Pseudomonas, Rhizobium, Rhodococcus, and Serratia

2. PHOSPHATE SOLUBILIZATION

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11MOVEMENT OF PHOSPHORUS IN SOIL

Source: Insight Microbiology;volume 1;issue 3, 201105/02/2023

P SOLUBILIZATION BY P SOLUBILIZING BACTERIASource: Insight Microbiology;volume 1;issue 3, 201105/02/2023 12

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K is the third major essential macronutrient

PGPR solubilize K rock through production and secretion

of organic acids.

They release K in accessible form from K bearing

minerals in soils

Includes genera Acidothiobacillus ferrooxidans, Bacillus

edaphicus, Bacillus mucilaginosus, Burkholderia,

Paenibacillus sp. and Pseudomonas

3. POTASSIUM SOLUBILIZATION

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Siderophores are low molecular weight iron-chelating

compounds which provide a high affinity to coordinate

ferric ions.

Kloeppar et al. (1980) were the first to demonstrate the

importance of siderophore.

Direct benefit: Take up the labeled iron and chelating

scarcely available iron

Indirect benefit: Enhanced chlorophyll level

4. SIDEROPHORE PRODUCTION

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IMPACT OF MICROBIALLY SECRETED SIDEROPHORES ON PLANT GROWTH

Source: Insight Microbiology;volume 1;issue 3, 2011

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a. Indole Acetic Acid (IAA) Up to 80% of rhizobacteria can synthesize IAA

IAA stimulate cell proliferation, seed germination, resistance

to stressful conditions and enhance uptake of minerals and

nutrients

Pseudomonas, Rhizobium, Bradyrhizobium, Agrobacterium,

Enterobacter and Klebsiella are IAA-producing PGPR

5. PHYTOHORMONE PRODUCTION

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Includes genera Azotobacter sp., Rhizobium sp.,

Rhodospirillum rubrum, Pseudomonas fluorescens, Bacillus

subtilis etc

Some strains of phytopathogens also synthesize cytokinins

PGPR produce lower cytokinin levels compared to

phytopathogens

Thus, effect of PGPR on plant growth is stimulatory while

that of pathogens is inhibitory.

b. Cytokinin and Gibberellins

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Lowering of ethylene production by inoculation of PGPR strains induces:-

1. Improved nodule number

2. Improved nodule dry weight

3. Higher grain yield and straw yield

4. Increased nitrogen

Includes genera: Pseudomonas sp., Achromobacter, Agrobacterium,

Azospirillum, Bacillus, Burkholderia, Enterobacter, Ralstonia, Serratia and

Rhizobium etc.

c. Ethylene

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INDIRECT MECHANISMS

One of the most powerful bio control mechanisms

Antibiotics produced:-

By psuedomonads: amphisin, (DAPG), oomycin A, phenazine, tensin,, and

cyclic lipopeptides

By Bacillus, Streptomyces and Stenotrophomonas sp: oligomycin A and

xanthobaccin

Drawback: some phytopathogens may develop resistance to specific

antibiotics due to increased use.

1. ANTIBIOTICS

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PGPRs produce enzymes such as chitinases,

dehydrogenase, β-glucanase, lipases, phosphatases,

proteases etc. exhibiting hyperparasitic activity

Suppression of pathogenic fungi including Botrytis

cinerea, Sclerotium rolfsii, Fusarium oxysporum,

Phytophthora sp., Rhizoctonia solani, and Pythium

ultimum

2. LYTIC ENZYMES

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3. INDUCED AND SYSTEMIC RESISTANCE

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Functions of EP-producing PGPR:-

a. Biofilm formation and root colonization.

b. Holding free phosphorous

c. Circulating essential nutrient to the plant

d. Protecting from foreign pathogens and stress

e. Shielding from desiccation

f. Plant defense response in plant–microbe interactions

4. EXO POLYSACCHARIDE PRODUCTION

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BIOCONTROL PROPERTIES OF PGPRS

Bio control: Process through which a living organism limits the growth or

propagation of undesired organisms or pathogens

Mechanism:-

a. Competition for nutrients

b. Production of antibiotics

c. Production of enzymes to degrade cell wall

d. Production of siderophores

e. Production of metabolites

f. Displacing pathogens05/02/2023

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APPLICATION OF PGPR AS BIOINOCULANT

Bio-fertilizers are defined as “substances that contain living microorganisms

that when applied to seed, plant surfaces, or soil, colonize the plant and

promote its growth by increasing the nutrient availability”

Mechanism:-

a. Increase efficiency of N-fixation

b. Ability to solubilize phosphate

c. Improve availability of Fe and Zn

d. Alter growth of roots and shoots by phytohormones

Eg: strains of Pseudomonas putida & Pseudomonas fluorescens05/02/2023

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ABIOTIC STRESS RESISTANCE THROUGH PGPR

High temperatures lead to increased drought intensity;

reduction in nodule number; infectious events; delay in

nodulation

Heat-tolerant, actively nodulating and

N2 fixing Rhizobium strains identified that play a key

role in normal growth.

1. EXTREME TEMPERATURES

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Soil acidity affects plant growth and cause crop

failures

Some strains of Rhizobium,

Azorhizobium and Bradyrhizobium are low pH

tolerant.

Tolerance to acidity by rhizobia correlated with the

production of extracellular polysaccharide

2. SOIL ACIDITY

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Key pollutants to plants, ecosystem and humans.

Use of recombinant rhizobia plays a major role in

phyto-remediation measures.

Microorganisms with high metal-binding capacity

through metallothionins for enhancing the tolerance,

sequestration of heavy metals widely exploited.

3. HEAVY METAL RESISTANCE

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SYNERGISTIC EFFECTS OF RHIZOBIAL CO-INOCULATION

Inconsistency of beneficial results, when single microbe used Co-inoculation causes synergy by functioning as helper bacteria Best combination of PGP bacteria, rhizobia and host genotype

selected Examples:-a. Azospirillum: In leguminous crops b. A. lipoferum and R. leguminosarum pv. trifolii : White clovers c. Azotobacter d. Bacillus sp.e. Psuedomonas sp.f. Enterobacterg. Serratia

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HARMFUL ASPECTS OF PGPR

Cyanide acts as a growth inhibitor for some plants

High levels of auxin inhibits root growth

Rhizobitoxine produced by Bradyrhizobium elkanii may

have a negative effect on nodulation

Rhizobitoxine can also induce foliar chlorosis in

soybeans.

A select few bacterial species may inhibit growth. 05/02/2023

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FUTURE PROSPECTS

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FUTURE RESEARCH AND DEVELOPMENT STRATEGIES

Need of today’s world – higher yield and enhanced production in an eco-friendly manner

1. New concepts of rhizo-engineering

2. Research in rhizosphere biology (molecular & biotechnological approaches)

3. Integrated management of soil microbial populations

4. Bioinoculants for high value crops like vegetables, fruits, and flowers

5. Application of multi strain bacterial consortium over single inoculation

6. Addition of ice-nucleating plant growth promoting rhizobacteria

7. Comprehensive research on potassium solubilization

8. Biosafety data required for the registration of PGPR

9. Non-phytotoxic PGPR

10. PGPRs tolerant to adverse environmental condition

11. Cost effective PGPR products05/02/2023

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CONCLUSION PGPRs are economically and environmentally beneficial for plant

growth promotion

PGPRs may have a direct or an indirect mode of action

PGPRs may function as biofertilizer, bioinoculant, abiotic stress

resistance inducers, co-inoculants and other growth promoting

activities

New concepts and development strategies regarding PGPRs

need to be constantly developed05/02/2023

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THANK YOU