STRESS RESPONSES IN ALFALFA .13. ACTIVATION OF CAFFEIC ACID 3-O-METHYLTRANSFERASE AND CAFFEOYL COENZYME-A 3-O-METHYLTRANSFERASE GENES DOES NOT CONTRIBUTE TO CHANGES IN METABOLITE ACCUMULATION IN ELICITOR-TREATED CELL-SUSPENSION CULTURES

Transcription of genes encoding L-phenylalanine ammonia-lyase (PAL), t he first enzyme of the phenylpropanoid pathway, and caffeic acid 3-O-m ethyltransferase (COMT) and caffeoyl CoA 3-O-methyltransferase (CCOMT) , enzymes involved in the synthesis of lignin and wall-esterified phen olic compounds, was strongly activated in elicitor-treated cell-suspen sion cultures of alfalfa (Medicago sativa L.). However, consequent cha nges in the extractable activities of COMT and CCOMT were small to non existent compared with a 15- to 16-fold increase in PAL activity. Only low levels of COMT and CCOMT transcripts were reflected in the total and polysomal RNA fractions compared with PAL transcripts. Elicited ce ll cultures did not accumulate lignin or the products of COMT and CCOM T in the soluble and wail-esterified phenolic fractions. In one alfalf a cell line in which elicitation resulted in very high PAL activity an d increased deposition of methoxyl groups in the insoluble wall fracti on, there was still no change in COMT and CCOMT activities. Overall, t hese results indicate that the initial gene transcription events in el icited cells may be less selective than the subsequent metabolic chang es, highlighting the importance of posttranscriptional events in the c ontrol of phenylpropanoid biosynthesis.