Regulation of Leaf Senescence: Role of Reactive Oxygen Species

Abstract

Leaf senescence causes a genetically programmed decline in various cellular processes including photosynthesis and involves hydrolysis of macromolecules including proteins, ­lipids and nucleic acids. Environmental stresses and reproductive structures influence the rate of senescence. The process of senescence and abiotic stress response are associated with the overproduction of reactive oxygen species (ROS) which are highly reactive and toxic compounds, and ultimately result in oxidative stress. ROS contribute to the progression of leaf senescence, as the antioxidant capacity of the leaf declines. Arabidopsis mutants and transgenic plants, in which antioxidant enzymes were manipulated, substantiate direct involvement of the ROS in leaf senescence. Infact, there is an intrinsic link between oxidative damage and leaf senescence and the free radical theory of aging seems to apply to plant senescence. Chloroplasts may play a regulatory role during leaf senescence similar to that of mitochondria during animal programmed cell death. Peroxisomes have a ROS mediated cellular function in leaf senescence and stress response. Reproductive sinks act as a stress leading to higher oxidative damage to proteins, drive the mobilization of nitrogen to the developing seeds and hence regulate the rate of senescence. The photosynthetic organelles are the main targets of ROS linked damage in plants experiencing various environmental stresses and natural senescence with decline in ROS detoxification mechanisms. At the same time, ROS play an important signaling role in plants controlling the processes such as growth, development, senescence, responses to environmental stimuli and programmed cell death. Plants adapt to environmental stresses through the process of acclimation, which involves less ROS production coupled with an efficient antioxidant defence. Among the different ROS, H2O2 appears to be the key regulatory molecule involved both in senescence and stress acclimation. In addition to redox control of chloroplast, a considerable cross-talk is observed in the regulatory networks involving hormones, ROS and transcription factors both in natural and stress induced senescence and abiotic stress responses. In this chapter an attempt has been made to review and analyse the role of ROS in senescence and abiotic stress responses, since both involve oxidative stress.