Leaf Senescence and Transformation of Chloroplasts to Gerontoplasts

Abstract

Senescence of green leaves brings about several structural and functional alterations in the cells. The process causes modifications in mitochondrial cristae, condensation of the nucleus, shrinkage of chloroplasts and extensive alteration of thylakoid structure. Senescence-induced changes in chloroplasts are extensive. These changes during senescence result in transdifferentiation of a chloroplast into a gerontoplast, a plastid form with unique structural features and physiology. During leaf senescence, the cells lose essential macromolecules including proteins, lipids and nucleic acids. The stroma proteins and lipids of the thylakoid membrane are the major targets for degradation. In addition to macromolecular degradation, the process causes loss in the photosynthetic pigments, namely chlorophylls and carotenoids. The enzymes that participate in chlorophyll degradation and their regulation are now known. However, the mechanism of degradation of carotenoids still remains a mystery. Macromolecular degradation and mobilization of the breakdown products that participate in the nutrient recycling mechanism are mediated by up-regulation of senescence-related genes. These genes are known as senescence-associated genes (SAGs), many of which have been cloned and characterized. Data are now available on signaling systems associated with expression of SAGs. Down-regulation of photosynthetic genes, cellular sugar-sensing mechanisms, phytohormones and reactive oxygen species are likely to play major roles in the signal transduction pathway in the initiation, progression and termination of the senescence process. Leaf senescence is directly related to plant productivity and therefore its implication in the area of agricultural biotechnology is important. However, its biotechnological application will be possible only when some of the outstanding fundamental questions relating to the process are addressed