Change in Gene Expression in Radish Cotyledons during Dark-Induced Senescence

Cotyledons detached from light-grown radish (Raphanus sativus L. cv. Comet) seedlings were used as a model system to study the changes in nuclear gene expression during dark-induced senescence of green leaves. Polyadenylated RNA was prepared from the cotyledons at different times and then translated in a wheat germ system. Approximately 1,000 different polypeptides of the translation products were separated from each other by two-dimensional gel electrophoresis. As judged from the density of autoradiographic spots of the translation products, the induction of senescence by dark treatment involved an increase in 26 species, a decrease in 11 species, and a temporary increase and subsequent decrease in 8 species of translatable mRNA. A similar pattern of change in protein synthesis was also observed in the dark-treated cotyledons when the cotyledons were pulse-labeled with 35S-methionine and the soluble proteins separated by two-dimensional gel electrophoresis, though the polypeptide pattern on the gel did not coincide exactly with those of the cell-free translation products. These findings strongly suggest that the process of leaf senescence is not simply a passive and gradual death of the tissue, but involves a drastic and sequential response of the cells to environmental stimuli with respect to the gene expression of the cells.