Page 628 - e-Book

which metabolizes ACC to ethylene, carbon dioxide, and cyanide. Environmental stresses

including extreme temperature, changed light, flooding, drought, effect of toxic metals and
organic pollutants, radiation, wounding, insect predation, high salt, and various pathogens

including viruses, bacteria, and fungi cause rise in ethylene (Morgan et al., 1997). The term
“stress ethylene”, describes the increase in ethylene synthesis. This suggests that ethylene

level rises as a response when plants exposed to stress.

2.2 Indirect Mechanisms of Action
PGPR produces secondary metabolites and hence these secondary metabolites are capable of

control of phytopathogenes. Phytopathogenes are a chronic threat to agriculture. PGPR
produces various antibiotics, siderophores, HCN and hydrolytic enzymes which lead to

control of phytopathogenes and reducing the need for chemical pesticides.

2.2.1 HCN Production
Some soil bacteria produce CN. HCN is a secondary metabolite that is volatile and controls

growth of surrounding microorganisms (Akhtar et al., 2006). Many different bacterial genera
have shown to produce HCN, including species of Alcaligenes, Aeromonas, Bacillus,

Pseudomonas and Rhizobium (Bhuiyan et al., 2008). HCN inhibits enzyme systems,
especially cytochrome oxidase. HCN inhibits electron transport which disrupts energy supply

leading to the death of the cell. Several studies have shown that volatile compounds may also

contribute to inhibition of different plant diseases (Gagné et al., 1991).
2.2.2 Antibiotic Production

PGPR which mostly belongs to Pseudomonas and Bacillus spp. are sometimes called

biopesticides or biocontrol PGPR (Whipps, 1997) produce antibiotics. Bacillus biocontrol
strains produce a wide range of antibiotics. Bacillus cereus strain UW85 (Handelsman et al.,

1996) and P. fluorescens strains CHA0 and Pf5 (Bender et al., 1999) produce multiple
antibiotics which have different degrees of activity against specific pathogenic fungi. A wide

vary of antibiotics are known together like amphisin, 2,4-diacetylphloroglucinol (DAPG),
oomycin A, phenazine, pyoluteorin, pyrrolnitrin, tensin, tropolone, and cyclic lipopeptides

synthesized by pseudomonads (Loper et al., 2007), and oligomycin A, kanosamine,

zwittermicin A, and xanthobaccin made by Bacillus, Streptomyces, and Stenotrophomonas
sp. to forstall the proliferation of plant pathogens (Compant et al., 2005). Several strains of P.

fluroscens produce DAPG which has antibacterial, anthelminthic and phytotoxic properties

including activity against a wide range of plant pathogenic fungi (Thomashow et al., 1996).

618