CHANGES IN THE GENE-EXPRESSION OF GRAPES IN RESPONSE TO HYPOXIA

Grape berries and cell cultures were evaluated for the induction of RNAs which encode products involved in anaerobic metabolism. Berries and cultured cells exhibited different responses. In contrast to other plants studied to date, berries did not exhibit detectable induction of RNAs encoding three different enzymes involved in fermentative metabolism: that is, ADH (alcohol dehydrogenase), ALD (fructose-1,6-bisphosphate aldolase), or GAPDH (glyceraldehyde-3-phosphate dehydrogenase), and of HSP70 RNA encoding a 70 kD heat shock protein. RNAs encoding these proteins decreased in berries upon prolonged hypoxia. A few RNA species of unknown identity and function do increase several-fold in vitro during grape anaerobiosis. High ADH enzyme activity and ADH RNA level were observed in non-treated berries. During anaerobiosis, decreases in ADH protein and enzyme activity assayed in vitro paralleled the presence of ADH RNA, but could not be explained solely on the basis of RNA steady-state levels. Rather, some mechanism must be operating in vivo that mediated either selective translation, protein turnover rates, or post-translational modifications which could account for the magnitude of the observed changes in ADH enzyme activity. It is possible that stress endured in the fields by grapes such as heat stress, know to induce ethanol production in aerobic conditions, have precluded a secondary hypoxic response in the laboratory. The presence of HSP70 RNA and of ADH RNA in mature berries is consistent with the occurrence of stress under field conditions. In contrast to grape berries, but in accordance to other plant species, RNA encoding ADH or GAPDH were induced two to three-fold in hypoxia-stressed grape cell cultures. A transient induction of ADH activity was also observed and paralleled the presence of ADH RNA, although the magnitude of enzyme induction was much greater than RNA accumulated. Cultured cells appear to adapt to hypoxia but cannot support anaerobiosis for as long a period as intact berries.