Chapter 11 - Understanding plant-plant growth-promoting rhizobacteria (PGPR) interactions for inducing plant defense

Abstract

Rhizobacteria fostering plant growth have received considerable attention in modern agriculture as they are capable of enhancing growth of the plants and are also a chemical fertilizer replacement. Besides enhancing growth, many PGPRs are recognized to induce plant defenses while in contact with the host plant. The plants have a nonspecific and broad-spectrum immune system to protect themselves from the diverse array of phytopathogens compared to innate immune system of animals. Depending on the type of interaction, plants cope with the invader attack through the activation of different defense mechanisms. In locally and systemically induced resistance responses, the main activator is salicylic acid (SA). However, studies have demonstrated that both ethylene and jasmonic acid (JA) are the main signaling molecules for induced systemic resistance (ISR) mediated by the rhizobacteria. For generating systemic resistance, different rhizobacteria exploit different mechanisms like some activate SAR (SA-dependent) pathway, while others activate ISR (ethylene/JA-dependent) pathway. Interestingly, coactivation of the ethylene/JA-dependent and the salicylic acid-dependent pathways has been shown to result in a synergistic effect on the acquired induced resistance. Few reports have suggested toward adaptive immune responses in plants and existence of immunological memory. The importance of PGPR in initiating plant defense against biotic stress, plant-PGPR interactions, and the PGPR significance in defense priming are discussed in this chapter.