Stress Responses in Alfalfa (XXI. 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), the first enzyme of the phenylpropanoid pathway, and caffeic acid 3-O-methyltransferase (COMT) and caffeoyl CoA 3-O-methyltransferase (CCOMT), enzymes involved in the synthesis of lignin and wall-esterified phenolic compounds, was strongly activated in elicitor-treated cell-suspension cultures of alfalfa (Medicago sativa L.). However, consequent changes in the extractable activities of COMT and CCOMT were small to nonexistent 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 cell cultures did not accumulate lignin or the products of COMT and CCOMT in the soluble and wall-esterified phenolic fractions. In one alfalfa cell line in which elicitation resulted in very high PAL activity and increased deposition of methoxyl groups in the insoluble wall fraction, there was still no change in COMT and CCOMT activities. Overall, these results indicate that the initial gene transcription events in elicited cells may be less selective than the subsequent metabolic changes, highlighting the importance of posttranscriptional events in the control of phenylpropanoid biosynthesis.