Jasmonic Acid (JA)-Mediated Signaling in Leaf Senescence

Leaf senescence is a complex process regulated by a range of both external and internal factors. The external factors that usually affect leaf senescence include nutrient deficiency, high temperature, water stress, radiation, pathogen attacks, and pollution, while internal factors include age, reproduction, meristem indeterminacy, and levels of plant hormones such as jasmonic acid (JA), ethylene, salicylic acid (SA), and abscisic acid (ABA). JA is one of the most important phytohormones that play a crucial role in the process of leaf senescence, as well as defense against biotic and abiotic stresses. Increased JA content and the expression of its biosynthetic genes in the senescing leaves indicate its vital role in the process of leaf deterioration. In addition to biotic and abiotic stresses, JA induces reactive oxygen species (ROS), which damage chloroplasts in the first target and induce senescence. The starting point of senescence could be chloroplasts, where JA biosynthesis take place. Transcription factors, such as WRKY, TCP, MYC, and NAC, take part in leaf senescence, and these are linked to JA signaling. Chlorophyll breakdown is a feature of JA-induced as well as natural leaf senescence, while the JA-responsive enzyme chlorophyllase is the main enzyme for chlorophyll breakdown. This chapter highlights the crucial role of JA in senescing processes.