Figure 2: Various components of induced systemic tolerance. PGPB may help associated plant reduce effect of stressors through reducing level of stress ethylene due to presence of ACC deaminase activity, release of antioxidants, volatile organic compounds and plant hormone cytokinin.
PGPB equipped with ability to synthesize ACC deaminase (ACCD) reduce level of stress ethylene produced in plants in response to various biotic and abiotic stressors. ACCD degrades ACC, an immediate precursor of ethylene, to α-ketobutyrate and ammonia (Yang et al., 2009). In addition to ACC deaminase mediated IST, other mechanisms also exist to confer IST in response to stresses. In salt stress, level of Na+ elevates, which decreases plant growth and productivity. The ion transporter high-affinity K+ transporter 1 (HKT1) regulates Na+ import in roots. VOC of Bacillus subtilis GB03 confer salt tolerance by down- and up-regulating HKT1 in roots and shoots respectively, and result in low Na+ accumulation throughout the plant in comparison to control. Other PGPB mediated IST include production of cytokinin which affects abscicic acid (ABA) signaling of plants during stress and augmented production of antioxidant catalase (Yang et al., 2009).