Chapter 5 - Microbial elicitors: Positive and negative modulators of plant defense

Abstract

Elicitors are low-molecular-weight oligomeric chemical compounds that stimulate defense response in plants at low concentration. Microbial elicitors produced during plant-microbe interaction evoke a cascade of signaling events at the cellular level leading to defense responses. The carbohydrate/lipid/glycoprotein/peptide elicitors produced by bacterial, fungal, and viral pathogens are classified as either general or specific elicitors based on the resistance mechanism induced in the host and nonhost plants. Effector proteins evolved in pathogens to combat plant defense responses were later reported to induce resistance by gene-for-gene action (Avr × R) leading to enhanced resistance, by coevolution of R genes in plants. Perception of microbial elicitors by specific receptors on the plant cell membrane initiates a series of signal transduction pathways that leads to defense activation within a few minutes to hours. The activation leads to an increase in ion flux across the plasma membrane, ROS production, changes in protein phosphorylation, lipid oxidation, cell wall reinforcement, and the synthesis of defense enzymes and pathogenesis-related (PR) proteins. Hypersensitive response (HR) activation at the infected site immunizes the distal plant parts through systemic acquired resistance against diverse pathogens. This chapter reviews microbial elicitors and their role as regulators of plant defense and their usefulness in priming for sustainable crop protection and production.