Differential expression of two tomato lactate dehydrogenase genes in response to oxygen deficit

Two different cDNAs encoding lactate dehydrogenase (LDH) were isolated from a library of hypoxically treated tomato roots and sequenced. The use of gene-specific probes on northern blots showed that Ldh2 mRNA was predominant in well-oxygenated roots and levels remained stable upon oxygen deficit; in contrast, Ldh1 mRNA accumulated to high levels within 2 h of hypoxia or anoxia. Immunoblot analyses of native gels using a polyclonal antiserum raised against an LDH1 fusion protein indicated that LDH2 homotetramer was the major isoform present in aerobic roots. Levels of both LDH1 and LDH2 subunits increased during an 18 h hypoxic treatment, together with a 5-fold rise in activity. These results suggest that the regulation of ldh1 expression is primarily at the transcriptional level while that of ldh2 is post-transcriptional. Increases in Ldh1 mRNA and LDH activity were not correlated with lactic acid production, which was maximal at the onset of anoxia in unacclimated roots and then declined. Taken together, our results indicate that LDH2 present in aerobic roots is principally responsible for lactic acid production occurring transiently upon imposition of anoxia. Possible physiological roles for LDH1 are discussed.