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Shu-Chen Grace Chen
³¯²Q¯u
Research Fellow
Ph.D., Dept. Biochemistry, Univ. of Arkansas, USA (1976) |
MOLECULAR MECHANISM OF LEAF SENESCENCE
Leaf senescence is an important phase in the plant life cycle during which cells undergo distinct metabolic changes prior to cell death and recycle nutrients to actively growing regions and storage organs. It is a genetically programmed event and involves the induction of expression of specific genes. We have identified numerous novel senescence-associated genes (SAGs) from rice and sweet potato using biochemical and reverse genetic approaches; and functional characterization of the senescence-specific promoters are in progress. DNA microarray of rice SAGs has been constructed to study the global expression profiles during leaf senescence process and under various growth conditions. These studies should lead to the understanding of the regulatory mechanism of leaf senescence, and the application by transgenic biotechnology to improve the quality and yield of crop plants.
The cytological studies with TUNEL and DNA ladder assays indicate that the features of programmed cell death occurring during rice leaf senescence are quite different from features characteristic of apoptosis. Using differential display, PCR-cDNA subtraction and cDNA microarray approaches, a number of SAGs have been isolated from senescing sweet potato and rice leaves. Many SAGs are found to be involved in biomolecule degradation, metabolism, stress and defense responses. Yet numerous SAGs are novel genes with unknown function. Biochemical characterization and functional identification of these SAGs using gene silencing approach, elucidation of the influence of plant growth regulators and the environmental signals on SAG expression during leaf senescence are in progress. Our current studies include the regulatory analysis of the leaf senescence specific promoters for Spg31 encoding cysteine proteinase, the novel gene Spa15 encoding a cell wall-associated protein with unknown function, Osl2 gene encoding mitochondrial GABA transaminase, and Osh69 gene encoding alkaline £\-galactosidase.
OSL2 (pyruvate-dependent GABA transaminase), a key enzyme of mitochondrial GABA shunt, is found to be upregulated during rice leaf senescence. The metabolism of GABA is directly related to glutamate metabolism, and GABA transaminase is thus important in C/N metabolism in the senescing leaves. Furthermore, this enzyme play important role in preventing accumulation of reactive oxygen intermediate and cell death, which is required for plant defense against environmental stress.
OSH69 (alkaline £\-galactosidase) belongs to a novel family of glycosyl hydrolase. Immunoelectron microscopic analysis demonstrated that this enzyme is specifically localized in the chloroplasts of senescing rice leaves. Tos17 insertion mutation of Osh69 gene results in delayed rice leaf senescence. Our findings strongly suggest the important role for this enzyme in the degradation of chloroplast thylakoid membrane galactolipids at the early stage of leaf senescence.
SELECTED PUBLICATIONS
To, K. Y., Cheng, M. C., Chen, L. F. O., and Chen, S. C. G. 1996. Introduction and expression of foreign DNA in isolated spinach chloroplasts by electroporation. Plant J. 10: 737-743.
Cheng, M. C., Wu, S. P., Chen, L. F. O., and Chen, S. C. G. 1997. Identification and purification of a spinach chloroplast DNA-binding protein that interacts specificallty with plastid psaA-psaB-rps14 promoter region. Planta 203: 373-380.
To, K. Y., Suen, D. F., and Chen, S. C. G. 1999. Molecular characterization of ribulose- 1,5-bisphosphatase carboxylase/oxygenase activase in rice leaves. Planta 209: 66-76.
Lee, R. H., Wang, C. H., Huang, L. T., and Chen, S. C. G. 2001. Leaf senescence in rice plants: cloning and characterization of senescence up-regulated genes. J. Exp. Bot. 52: 1117-1121.
Huang, Y. J., To, K. Y., Yap, M. N., Chiang, W. J., Suen, D. F., and Chen, S. C. G. 2001. Cloning and characterization of leaf senescence up-regulated genes in sweet potato. Physiol. Plant 113: 384-391.
Lee, R. H., and Chen, S. C. G. 2002. Programmed cell death during rice leaf senescence is non-apoptotic. New Phytologist 155: 25-32.
Chen, G. H., Huang, L. T., Yap, M. N., Lee, R. H., Huang, Y. J., Cheng, M. C., and Chen, S. C. G. 2002. Molecular characterization of a senescenceassociated gene encoding cysteine proteinase and its gene expression during leaf senescence in sweet potato. Plant Cell Physiol. 43: 984-991.
Yap, M. N., Lee, R. H., Liao, C. J., Huang, Y. J., and Chen, S. C. G. 2003. Molecular characterization of a novel senescence-associated gene SPA15 induced during leaf senescence in sweet potato. Plant Mol. Biol. 50: 471-481.
Lee, R. H., Lin, M. C. and Chen, S. C. G. 2004. A novel alkaline alpha-galactosidase gene is involved in rice leaf senescence. Plant Mol. Biol. 55:281-295.
Ansari, M. I., Lee, R. H. and Chen, S. C. G. 2005. A novel senescence-associated gene encoding GABA: pyruvate transaminase is up-regulated during rice leaf senescence. Physiol. Plant. 123:1-8. |
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