Making Sense of Senescence (Molecular Genetic Regulation and Manipulation of Leaf Senescence).

Leaf senescence is the final stage of leaf development. In forests of deciduous trees, the autumn colors that develop during leaf senescence are of great aesthetic value. This process is also of great practical value because during leaf senescence, nutrients are recycled to other parts of the plant. For example, nitrogen from leaves of deciduous trees is used for the synthesis of storage proteins in stems that will support growth during the following spring (Clausen and Apel, 1991). However, in an agricultural setting, leaf senescence may limit yield in certain crops. Senescence also contributes to the postharvest loss of vegetable crops. Therefore, studying leaf senescence will not only contribute to our knowledge about this fundamental developmental process, but may also lead to ways of manipulating senescence for agricultural applications. There have been many physiological, biochemical, and molecular studies of leaf senescence. These studies show that during senescence leaf cells undergo highly coordinated changes in cell structure, metabolism, and gene expression. The earliest and most significant change in cell structure is the breakdown of the chloroplast, the organelle that contains up to 70% of the leaf protein. Metabolically, carbon assimilation (photosynthesis) is replaced by catabolism of chlorophyll and macromolecules such as proteins, membrane lipids, and RNA so that some of the released nutrients can be recycled. At the molecular level, these changes are accompanied by, or perhaps driven by, changes in gene expression. In this Update, we summarize physiological and biochemical studies that have contributed to the present understanding of leaf senescence, then we discuss current molecular investigations into the regulatory mechanism(s) underlying leaf senescence, and, fi-