Chapter 26 - Brassinosteroid Signaling and Complex Interplay of ROS, NADPH Oxidase, and MAPK Mediated Biotic and Abiotic Stress Acclimation in Plants

Abstract

Brassinosteroids (BRs) are a group of naturally occurring polyhydroxylated steroidal plant hormones that are known to have a major role in several growth and developmental processes. Apart from their role in various cellular and physiological processes these steroidal derivatives also function as a key player in stress acclimation. BRs regulate plant growth and stress responses via interacting with several transcription factors through a complex interplay of signaling cascades, which regulate the expression of thousands of downstream genes. BRs are further known to interact with different plant growth hormones by establishing a crosstalk and thus promoting tolerance in plants to wide range of abiotic stresses, including heat, cold, drought, salinity as well as biotic stresses caused by bacterial and fungal pathogens. It is reported that BRs regulate plant stress tolerance via modulation in the reactive oxygen species (ROS) that acts as a signaling molecule to alert the plant for stress adaptations. A stringent regulation between ROS generation and scavenging helps a plant to adaptively utilize ROS as a primary defense molecule against biotic and abiotic stress conditions. A concomitant increase in calcium and H2O2 levels is a major consequence of imbalanced ROS levels. Several reports indicated that increased ROS accumulation and calcium influx may also trigger the activation of mitogen-activated protein kinases (MAPKs). Plant MAPK cascades play pivotal roles in plant defense against pathogen attack and act downstream of H2O2 and are required for the activity of NADPH oxidase during signal amplification. Additionally BRs treated plants were also reported to have elevated H2O2 levels, which also results from enhanced NADPH oxidase activity. Many studies reveal that ROS, especially H2O2 generated by NADPH oxidases encoded by respiratory burst oxidase homolog (RBOH) genes, play important roles in plant response to biotic and abiotic stresses thus further corroborating their critical role in BR-induced stress tolerance. In conclusion ROS, calcium influx, MAPKs, and NADPH oxidases may overall form a positive feedback loop that allows rapid signal propagation, required for systemic signaling during plant responses to a variety of environmental vagaries.