11/04/2023

Department of PLANT PATHOLOGYCCS Haryana Agricultural University, Hisar

Welcome

Plant growth promoting bacteria and their role in disease management

INTRODUCTION

• PGPB includes Rhizoplane and Phylloplane bacteria.

Rhizoplane 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 microbial activity.

• Term Rhizosphere was coined by German agronomist Hiltner in 1904.

• Rhizoplane The external surface of roots together with closely adhering soil particles and debris.

DIVERSITY AMONG PGPRS

Diazotrophic PGPRNitrogen 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.

Bacillus 95% of Gram +ve soil bacilli belong to the genus

Bacillus. The remaining 5% are confirmed to be Arthrobacter

and Frankia. Members form endospores to survive under

adverse conditions.

Pseudomonas Pseudomonas is the most abundant gram –ve genus

in rhizosphere. 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 strains.

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 SDS-PAGE technique and MLEE.

Classification of PGPR

1. Plant part they occupy

Extracellular (e-PGPR, free living

Intracellular (i-PGPR, symbiotics)

On the basis of

Exist inside root cells.Forms root nodules.Ex- Rhizobium

Exist in rhizosphere,on rhizoplane, in intercellular spaces of root cortex.

2. Mechanism of Action

Direct PGPR

Indirect PGPR

Ex. Solubilization of nutrients, nitrogen fixation, production of growth regulators

Ex. Exclusion of pathogens,production of siderophores,lytic enzymes,ISR

Siderophore Production• 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.

Siderophore Organism Bacillibactin Bacillus subtilis Ornibactin Burkholderia cepacia Azotobactin Azotobacter vinelandii Pyoverdine Pseudomonas aeruginosa

pathogens

Microbial Antagonism

AntibioticsPGPR produces antibiotics

and act as antagonistic.Biocontrol based on

antibiosis secretion of molecules that kill target pathogen

Achieved through bacteriocins, antibiotics hydrolytic enzymes,HCN production, SAR, ISR.

Antibiosis ISR Competition

Pathogen

Sr.No

Antibiotic

Source Action against

1. Pyrrolnitrin P. Fluorescens BL915 strain

Prevent the damage of Rhizoctonia solani during damping-off of cotton plants

2. DAPG Pseudomonas spp.

Membrane damage to Pythium spp.

3. Phenazine Pseudomonas spp.

F. oxysporum, Gaeumannomyces graminis

4. Polymyxin, circulin and colistin

Bacillus spp. Pathogenic fungi

5. Zwittermicin A B. cereus UW85 strain

Bio-control of alfalfa damping off

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)

• Production of Gibberellins by PGPR is rare,• However two strains have been reported, Bacillus

pumilis and Bacillus licheniformis.

ROOTS WITHOUT PGPR ROOTS WITH PGPR

Bacteriocin

HCN producing rhizobacteria• HCN is a powerful inhibitor of metal enzymes, especially

cytochrome C oxidases. • HCN production is a common trait within the group

of Pseudomonas. • Include species of Alcaligenes, Bacillus,

Pseudomonas and Rhizobium.

Sr.No Bacteriocin PGPR

1. Pyocins P. pyogenes2. Cloacins Enterobacter cloacae3. Marcescins Serratia marcescens4. Megacins B. megaterium

• 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

Donmez et al., 2011

Cont..

Result:• No significant differences between CD-8, MFD-4,

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 MFD-

81 (25.0%) and T26 (37.5%). Conclusion: 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 with Frankia.

• Non-symbiotic N-fixing rhizospheric bacteria belongs to genera including Azoarcus, Azospirillum, and Pseudomonas

Most studied symbiotic bacteria are Rhizobium, Bradyrhizobium, Sinorhizobium and Mesorhizobium

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 inoculants.

• Plants inoculated with the biocontrol PGPB, P. putida and Serratia marcescens were protected against the cucumber pathogen P. syringae pv. lachrymans. Bashan &Bashan., (2005)

Induced Systemic Resistance

• E. chrysanthemi produces two siderophores

Achromobactin ( iron limiting condition) Chrysobactin (severe iron deficiency)

The role of CB in induction of SAR has been studied in Arabidopsis- Erwinia chrysanthemi system.

Role of siderophore in induction of SAR

Fig: PR1 gene expression and SA production in Arabidopsis leaves following CB treatment (Dellagi et al., 2009).

Cont.

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.

(Hillel, 2005)

111114

Lim et al., 1991

Lim et al., 1991

Cont..

Cont..

Competition and Displacement of Pathogens

• 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 disease.

PGPR Disease promoting traits

References

Pseudomonas fluorescens

IAA, HCN Jeon et al. (2003)

Pseudomonas fluorescens

IAA, Siderophore, Antifungal activity

Dey et al. (2004)

Bacillus subtilis Antifungal activity Cazorla et al. (2007)

Bradyrhizobium spp. IAA,Siderophore, HCN Wani et al. (2007a)

Azospirillum amazonense

IAA, Nitrogenase activity Elisete et al. (2008)

Rhizobium leguminosarum

IAA, Siderophores, HCN,Exopolysaccharides

Ahemad and Khan (2009a)

Pseudomonas, Bacillus

, IAA and Siderophores Wani et al. (2007e)

List of PGPRs

PHYLLOPLANE BACTERIA• Defined as populations that can

survive and multiply on the surface of plants.

• Also called as epiphytic bacteria.• survive in trichomes base,

substomatal chambers, hydathodes, and especially, in between the depressions along the junctions of adjacent epithelial cells.

• They utilize similar mechanism for controlling of pathogens like antibiosis, siderophore production etc.

Location of the epiphytotic PGPB in tomato

P. macerans P. macerans

B. pumilus B. pumilus

control control

Bacterial spot and early blight biocontrol by epiphytotic bacteria in tomato plants

Filho et al., 2010

Filho et al., 2010

Cont…

Conclusion

(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 with PGPB

Challenges in Selection and Characterizati

on of PGPB

Challenges in Commercializati

on of PGPB

Natural variation

Challenges in Field

Application of PGPB

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 soil-borne pathogens

• Selection based on traits known to be associated with PGPB such as root colonization, ACC deaminase activity, antibiotic and siderophore production.

Con… Natural variation 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 of PGPB

CHALLENGE• Lack of consistent

performance in the field due to heterogeneity of abiotic and biotic factors.

REMEDY• Knowledge of factors

optimal concentration, timing and placement of inoculant, and of soil and crop management strategies

• concept of managing the rhizosphere/phyllosphere by manipulation of the host plant, substrates for PGPB, or through agronomic practices.

MANAGEMENT

KNOWLEDGE

CHALLENGELack of better formulations to ensure survival and activity in the field

• REMEDYApproaches include development of improved carriers and application technology

Cont…

Challenges in Commercialization of PGPB• Maintaining quality, stability, and efficacy of

the product. • Factors like shelf life, compatibility

considered while formulation development.• Non-target effects on other organisms

including toxigenicity, allergenicity, pathogenicity.

• Capitalization costs and potential markets must be considered in the decision to commercialize.

CONCLUSION• 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.

11/04/2023 46

“Happiness and bacteria have one thing in common; they multiply by

dividing!”

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