DIFFERENTIAL ACCUMULATION OF S-ADENOSYLMETHIONINE SYNTHETASE TRANSCRIPTS IN RESPONSE TO SALT STRESS

NaCl stress causes the accumulation of several mRNAs in tomato seedlin gs. An upregulated cDNA clone, SAM1, was found to encode a S-adenosyl- L-methionine synthetase enzyme (AdoMet synthetase). Expression of the cDNA SAM1 in a yeast mutant lacking functional SAM genes resulted in h igh AdoMet synthetase activity and AdoMet accumulation. We show that t omato plants contain at least four SAM isogenes. Clones corresponding to isogenes SAM2 and SAM3 have also been isolated and sequenced. they encode predicted polypeptides 95% and 92% identical, respectively, to the SAM1-encoded AdoMet Synthetase. RNA hybridization analysis showed a differential response of SAM genes to salt and other stress treatmen ts. SAM1 and SAM3 mRNAs accumulated in the root in response to NaCl, m annitol or ABA treatments. SAM1 mRNA accumulated also in leaf tissue. These increases of mRNA level were apparent as soon as 8 h after the i nitiation of the salt treatment and were maintained for at least 3 day s. A possible role for AdoMet synthetases in the adaptation to salt st ress is discussed.