ABSTRACT
Leaf senescence is the fi nal stage of leaf development. It is an essential developmental phase in the life of the leaf, because the main purpose of leaf senescence is to mobilize and recycle nutrients (Buchanan-Wollaston et al. 2003). During growth and development, green leaves are packed
Environmental Biotechnology Research Center, Korea Research Institute of Bioscience and Biotechnology (KRIBB), 111 Gwahangno, Yusong-gu, Daejeon 305-806, Republic of Korea, Fax: 82-42-860-4608, E-mail: cefl e@kribb.re.kr (Y-H Kim), sskwak@kribb.re.kr (S-S Kwak) *Corresponding author
with nutrients, and thus, when the leaf is no longer required by the plant, the senescence process is induced and recycling of the nutrients occurs. The fi nal stage of this process is cell death in the leaf, but cell death is actively delayed until all nutrients have been removed from the leaf through the process of developmental leaf senescence. Leaf senescence in diverse plant species involves a coordinated action at the cellular, tissue, and organ level that is under the control of a highly regulated genetic program (Lim et al. 2007). It is characterized by differential gene expression, active generation of cellular structure, recycling of nutrients, degradation of macromolecules, such as proteins, lipids, and nucleic acids, and enhanced metabolism of reactive oxygen species (ROS), which cause severe cellular damage (Lohman et al. 1994, Buchanan-Wollaston et al. 1997, Zimmermann and Zentgraf 2005). ROS derived from oxygen are also thought to play an essential role in leaf senescence (Zimmermann and Zentgraf 2005). Since ROS are highly toxic, the levels of these chemicals have to be tightly regulated. However, at low concentrations, some ROS, particularly H2O2, may serve as signaling molecules (Desikan et al. 2001, Kim et al. 2008). Therefore, the coordinated regulation of the free radical scavenging system, which comprises enzymatic components and non-enzymatic molecules, is essential for the survival of the plant (Mittler 2004, Foyer and Noctor 2005). The increased free radical levels displayed during senescence are not only caused by the elevated production of radicals but also by a loss in antioxidant capacity.
