POSTHARVEST ALTERATION OF IN-VITRO TRANSLATABLE MESSENGER-RNA POPULATION IN ALFALFA (MEDICAGO-SATIVA L)

During harvest and post-harvest handling, alfalfa (Medicago sativa L.) Forage undergoes metabolic changes that result in a rapid, significan t loss of nutritional quality, especially in protein content. In the p resent work, the hypothesis was raised that these changes might be ini tiated by the onset of specific metabolic changes under the control of de novo gene expression. Changes in the population of translatable po ly(A) + RNA from alfalfa leaves after harvest were monitored by in vit ro translation followed by a two-dimensional polyacrylamide gel electr ophoresis (2D-PAGE) analysis of translation products. In a first exper iment, which involved plants grown in a growth chamber, the disappeara nce of 34 of about 120 translation products present before harvest and the appearance of 37 new ones were observed. Comparison of harvest st ress with heat shock, cold, and water deficit, revealed that 14 of the 37 translation products increased specifically after harvest, whereas most of the decreasing polypeptides were common to all treatments. In a second experiment, alfalfa was grown and harvested in open field co nditions. Among the 40 polypeptides found to increase after harvest, 2 3 were common with those induced in the first experiment. These result s suggest that harvest leads to both specific and non-specific stress responses within plant cells, that result not only in the disappearanc e of many mRNA species, but also in the de novo expression of several mRNAs. Further characterization of genes whose expression is specifica lly induced during post-harvest could provide tools for the study of p ost-harvest metabolism and its modification by genetic engineering.