Quorum Sensing Molecules of Rhizobacteria: A Trigger for Developing Systemic Resistance in Plants

Abstract

Induced systemic resistance (ISR) is a widespread phenomenon by which plants develop resistance against various pathogens. A number of plant growth-promoting rhizobacteria are reported to evoke ISR in plants through their surface components, secretion of metabolites, or production of volatile compounds. These compounds in return activate the signaling pathway in plant and allow plants to withstand pathogen attack. Quorum sensing (QS), which is defined as the intercellular communication process, is a crucial feature of rhizobacteria to sense the ecological niche and distribute their population. Signaling process involves the exchange of diffusible signal molecules that serve as autoinducers. The concentration of these QS molecules is a key factor in mediating the gene expression for EPS production, biofilm formation, extracellular enzyme production, etc. and helps bacteria to adapt in a particular environmental condition. In general bacteria have a conserved QS system with central components such as LuxR-type regulator and LuxI-type protein as receptors. At low population density, bacteria produce a low level of QS signals, which are then released in the environment. N-acyl homoserine lactones (AHLs) are the major signaling molecules in Gram-negative bacteria, and cyclic peptides serve as signaling molecules in Gram-positive bacteria. Recent studies revealed that AHL molecules play important role in plant growth and defense. In this chapter, we will discuss the role of different signaling molecules in inducing plant defense and their mechanism.