Treatment of cultured parsley (Petroselinum crispum) cells with fungal elicitor rapidly activates transcription of many genes encoding specific steps in pathogen defense-related pathways. We report evidence that three cDNAs corresponding to such genes represent two key enzymes of the activated methyl cycle. Two cDNAs are derived from distinct members of the S-adenosyl-L-methionine synthetase gene family, based on extensive similarity of the deduced polypeptides with authentic enzymes from Arabidopsis thaliana, rat, yeast, and Escherichia coli. The third cDNA exhibits large similarity with a functionally related gene, encoding S-adenosyl-L-homocysteine hydrolase, from rat and a slime mold. Marked differences in the mRNA levels occurred in different organs of parsley plants. Elicitor treatment strongly induced both mRNAs in cultured cells as well as intact leaves and led to marked increases in S-adenosyl-L-homocysteine hydrolase enzyme activity. These results suggest a close metabolic link between pathogen defense and an increased turnover of activated methyl groups.