Developmental regulation of gene expression during senescence in soybean (Glycine max L.)

Senescence is an endogenously controlled degenerative process involving biochemical and physiological events leading to death of a whole plant, organ, tissue or cell. An understanding of senescence at the molecular level should provide fundamental information that will allow for a greater ability to manipulate the process to increase crop yields and avoid existing problems with post-harvest and post-production losses of crops such as fruits and vegetables. Soybean cotyledons usually proceed through senescence and abscission by 16--20 days after germination. Symptoms of senescence can be reversed by surgical removal of the epicotyl meristem leading to regreening or "rejuvenation" of the cotyledons. The point at which regreening can no longer be induced by removal of the epicotyl is referred as "the point of no return, (PONR)" and it occurs around 14 days after germination under defined conditions. The soybean cotyledon system is an excellent model system to address specific questions concerning gene regulation during the processes of senescence and "rejuvenation". Previous studies in the laboratory of Dr. J. M. Chandlee have identified molecular clones of genes specifically up-regulated during either senescence or "rejuvenation" of soybean cotyledons.; In this study, seven of these senescence up-regulated clones were used as gene probes on northern blots to assess their normal expression patterns during senescence in cotyledon and leaf tissue, and in three situations in which senescence is modified including, genetically determined evergreen mutants, male-sterile mutants, and manually depodded plants. The differential expression patterns of gene clones 1S6 and 3S45 suggest that the regulation of these genes is not identical in cotyledons and leaf, although these two tissues show a similar progression of the syndromes of senescence. The differential expression patterns of all seven clones in response to depodding and in male sterile genetic backgrounds suggest that factors resulting from pod formation and seed development may directly regulate the expression of these senescence-related genes. We also observed differential expression patterns of these genes in response to changes in genetic backgrounds that confer an evergreen phenotype, which indicates that the multiple pathways in the senescence program are likely differentially activated and regulated.