Expression of a gene encoding mitochondrial aldehyde dehydrogenase in riceincreases under submerged conditions

It is known that alcoholic fermentation is important for survival of plants under anaerobic conditions. Acetaldehyde, one of the intermediates of alco holic fermentation, is not only reduced by alcohol dehydrogenase but also c an be oxidized by aldehyde dehydrogenase (ALDH). To determine whether ALDH plays a role in anaerobic metabolism in rice (Oryza sativa L. cv Nipponbare ), we characterized a cDNA clone encoding mitochondrial ALDH from rice (Ald h2a). Analysis of sub-cellular localization of ALDH2a protein using green f luorescent protein and an in vitro ALDH assay using protein extracts from E scherichia coli cells that overexpressed ALDH2a indicated that ALDH2a funct ions in the oxidation of acetaldehyde in mitochondria. A Southern-blot anal ysis indicated that mitochondrial ALDH is encoded by at least two genes in rice. We found that the Aldh2a mRNA was present at high levels in leaves of dark-grown seedlings, mature leaf sheaths, and panicles. It is interesting that expression of the rice Aldh2a gene, unlike the expression of the toba cco (Nicotiana tabacum) Aldh2a gene, was induced in rice seedlings by subme rgence. Experiments with ruthenium red, which is a blocker of Ca2+ fluxes i n rice as well as maize (Zea mays), suggest that the induction of expressio n of Adh1 and Pdc1 by low oxygen stress is regulated by elevation of the cy tosolic Ca2+ level. However, the induction of Aldh2a gene expression may no t be controlled by the cytosolic Ca2+ level elevation. A possible involveme nt of ALDH2a in the submergence tolerance of rice is discussed.