S-adenosyl-L-methionine metabolism in alfalfa cell cultures following treatment with fungal elicitors

The activities of the enzymes S-adenosyl-L-methionine synthetase (SAM synthetase), S-adenosyl-L-homocysteine hydrolase (SAH hydrolase), adenosine nucleosidase and adenosine deaminase, all of which may be involved in maintaining an active poor of SAM for transmethylation reactions, have been determined in seedlings and cell cultures of alfalfa. The expected reaction products were observed with all the assays, but when SAH formation was determined using C-14-adenosine and homocysteine as substrates, an unknown metabolite derived from SAH was also observed. The specific activities of the enzymes were in the order root > leaf = cultures for SAM synthetase, cultures = root > leaf for SAH hydrolase, root much greater than leaf > cultures for adenosine nucleosidase, with low activities being determined in all tissues with adenosine deaminase. Elicitation of the phytoalexin response in suspension cultures using a yeast cell wan preparation resulted in a sustained six-fold increase in SAM synthetase activity, a rapid transient two-fold increase in SAH hydrolase and adenosine nucleosidase activity, and a delayed two-fold induction in adenosine deaminase activity. The relative importance of these enzymes in the active methyl cycle during the elicitation response in alfalfa is discussed. Copyright (C) 1996 Elsevier Science Ltd