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Biology and Biotechnology of Environmental Stress Tolerance in Plants, Volume 3

Metal chelation is another attribute of PGPR under extreme heavy metal

contamination (Glick et al., 2007). Under Zn contamination plants inocu­

lated with Pseudomonas aeruginosa showed higher gene expression for

antioxidant enzymes like catalase and SOD than those un-inoculated plants

(Noctor & Foyer, 1998). It was also reported that some PGPR strains can

accumulate metals thus reducing their availability in the soil rhizosphere

(Naddem et al., 2013). In Table 3.2, some of PGPRs and their effects on

plant stress tolerance are listed.

3.5.2 PGPR-MEDIATED BIOTIC STRESS MANAGEMENT IN PLANTS

Inhibition of different plant pathogens by PGPR enabled their use as a poten­

tial biocontrol agent. PGPR increases host resistance by several mechanisms,

including secreting different cell wall degrading lytic enzymes, improved

nutrients availability, and inducing systemic resistance (Bhattacharyya &

Jha, 2012). In some studies, the biocontrol activity of PGPRs is well estab­

lished in the plant tomato and pepper (Mayak et al., 2004). Pseudomonas

polymyxa can inhibit fungal pathogens like Fusarium sp. and Botrytis sp.

which are responsible for several destructive diseases in many crops (Kumar

et al., 2019). In another report it was found that, Bacillus amyloliquefaciens

can inhibit wheat pathogen Fusarium gaminearum by reducing the produc­

tion of deoxynivalenol (DON) a mycotoxin produced by the fungus (Shi

et al., 2014). This PGPR was also found to be effective in eggplant against

Fusarium sp. (Chakraborty et al., 2021). Another aspect of is the inhibition of

pathogens by exhibiting antagonistic mechanism. Rhizobacteria can produce

many toxic and protective secondary metabolites which help to inhibit

disease progression. Though this mechanism is not clearly understood but

it was suggested that most of the protective secondary metabolites are non-

ribosomal peptides and polyketides (Mongkolthanaruk, 2012). For instance,

Bacillus and Paenibacillus exhibited antagonistic mechanism by producing

secondary metabolites in many crop species (Cawoy et al., 2013).

3.6 ARBUSCULAR MYCORRHIZAL FUNGI (AMF) AND PLANT

GROWTH

Arbuscular mycorrhizal fungi (AMF) are the microorganism present in the

soil showing obligate symbiotic relationship with plants. AMF are the most

pervasive microorganisms that are found in almost 90% of plants and crop