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PGPR: ROLE AND IMPORTANCE
IN AGRICULTURERAWE (0+4)
SPEAKERNEHA TANDON
INSTRUCTORMR. ACHIN KUMARASST. PROF SSCIAS,RGSC, BHU
05/02/2023 2
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
05/02/2023 7
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
9
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
05/02/2023
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
05/02/2023 10
P SOLUBILIZATION BY P SOLUBILIZING BACTERIASource: Insight Microbiology;volume 1;issue 3, 201105/02/2023 12
13
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
05/02/2023
05/02/2023
14
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
15
IMPACT OF MICROBIALLY SECRETED SIDEROPHORES ON PLANT GROWTH
Source: Insight Microbiology;volume 1;issue 3, 2011
05/02/2023
16
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
05/02/2023
17
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
05/02/2023
18
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
05/02/2023
19
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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20
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
05/02/2023
22
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
05/02/2023
23
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
24
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
25
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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26
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
05/02/2023
27
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
05/02/2023
28
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
05/02/2023
29
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
31
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
32
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