Chapter 5 - Regulation of Leaf Senescence by Macromolecule Degradation and Hormones

Abstract

Leaf senescence is an intricate process that requires time-dependent transitions of cellular physiology and metabolism. It is a coordinated process that includes interaction between cells and organs. It requires the combined functions of hundreds of genes, enzymes, and signaling pathways that desegregate various endogenous and exogenous signals, controlled by many layers of regulation. The timing of senescence is affected by developmental and environmental signals throughout the leaf life span. In plants, the process involves morphological, biochemical, and molecular changes that permit the remobilization of nutrients in a well-synchronized way from senescence-affected leaves toward the growing leaves, developing fruits, seeds, and buds. Significant research has been done to understand leaf senescence by means of genetic and molecular studies. Recently developed omics techniques help in the identification, isolation, and characterization of senescence-associated genes (SAGs) and functions of relative proteins in the last few years. The functioning of SAGs is regulated genetically and epigenetically by plant hormones, nutrients, protein degradation, and nitrogen mobilization. Hormonal regulation and sugar signaling play a central role in leaf senescence, which can influence the plant growth, stress responses and timing of leaf senescence. All known phytohormones to date affect leaf senescence by some means, but the functions of ethylene and cytokinin in the process still need to be explored. It has been hard to study the role of various hormones in regulating the initiation and development of leaf senescence. Some studies have proved that phytohormones play an important role in delaying and accelerating leaf senescence by declining or increasing their levels and site of action. This chapter includes information about the potential biochemical and molecular events that take place during developmental senescence and the prominent roles of hormones that involve developmental aging, initiation, and progression of senescence.