Ethylene and leaf senescence

Leaf senescence is currently considered the last stage of leaf development and is a genetically programmed process, highly regulated with recycling of reserves from the leaves to other storage organs (seeds, trunk, branches). Leaves of deciduous plants perceive the changing of the seasons in autumn and activate all the processes for the preparation for winter. The signs that initiate natural leaf senescence are the photosynthesis reduction and shortening of photoperiod (Smart 1994; Yoshida 2003). The leaves of evergreens have a turnover regulated by an internal clock that is correlated with the photosynthetic machinery health status. Leaf cells undergo an organized selfdestruction process that involves protein degradation (Lutts et al. 1996), and altered turnover, nucleic acid degradation, lipid degradation (Buchanan- Wollaston 1997; Thompson et al. 1998; Buchanan-Wollaston et al. 2003), membranes disruption (Trippi and Thiamann 1983), and leaf pigments breakdown (Matile et al. 1997; Fang et al. 1998). The metabolites and nutrients are reallocated in storage organs such as branches and trunk in deciduous trees, which will use the reserve for making a new leaf area in spring, or in seeds in annual species that will use the reserve for growing a new plant the next year (Noodén and Guiamét 1989; Gan and Amasino 1995). The leaf senescence is activated at the molecular level when leaves are fully expanded (Fig. 3.1). At this stage, many processes are induced and many others are turned off. The leaf senescence can be subdivided into three phases. The first is named the initiation phase, which may be induced by hormones, environment, age or pathogens. At the physiological level, the metabolic thresholds are crossed, the redox state is altered, and the signalling cascades are activated. The second phase can be influenced by hormones and environment. During this stage, the leaf cells are reorganizing for responding to the degeneration by activation of savage pathways, shifting the metabolism from autotrophic to heterotrophic, detoxification and reversible organelle redifferentiation. The last phase of leaf senescence is characterized by antibiotic accumulation, release of free radicals, elimination of remaining metabolites, and irreversible loss of cell integrity and viability. Finally, the leaf senescence process is visible with necrosis of leaf cells or discoloration caused by chlorophyll degradation and is concluded with the death of the leaf cells (Dangl et al. 2000). The leaf senescence can also be a defense response to pathogen infection. In this case, the plant reacts to the infection, inducing the death of leaf cells that are located around the infection isolating the pathogen and its damage. However, there are other forms of leaf senescence that can be induced by abiotic stresses, such as mechanical damage (wounds) or harvest. Among the leaf senescence inducers the ethylene plays an important role. Ethylene is a simple gaseous plant hormone that regulates many diverse plant processes, during the whole plant development stages. The physiological ethylene effectiveness depends from environmental and biological factors, which may affect positively or negatively on the ethylene action.