Enzymic O-methylation of isoliquiritigenin and licodione in alfalfa and licorice cultures.

S-Adenosyl-L-methionine (SAM): isoliquiritigenin (2',4,4'-trihydroxychalcone) 2'-O-methyltransferase (CHMT) of alfalfa (Medicago sativa) catalyses the formation of 4,4'-dihydroxy-2'-methoxychalcone, which is the most potent inducer of nodulation-genes of Rhizobium meliloti, the symbiont of alfalfa which forms nitrogen-fixing nodules. SAM: licodione 2'-O-methyltransferase (LMT) is involved in the biosynthesis of a retrochalcone in cultured licorice (Glycyrrhiza echinata) cells and has been shown to be induced as a defence response of the cells. Because licodione exists in an equilibrium mixture of tautomeric 2',4,4',beta-tetrahydroxychalcone (major) and 1-(2,4-dihydroxyphenyl)-3-(4-hydroxyphenyl)-1,3-propanedione (minor), the apparent mode of action of both enzymes is very similar. In this study, cultured alfalfa cells were shown to exhibit rapid and transient increases in the extractable activities of both CHMT and LMT after treatment with yeast extract (YE). Treatment of solution-cultured alfalfa seedlings with YE also resulted in a similar induction of both CHMT and LMT activities in the roots, but no activity was detected in the shoots. These activities were attributed to a single gene product, the CHMT protein, as extracts of Escherichia coli transformed with the CHMT cDNA exhibited both CHMT and LMT activities. In contrast, in G. echinata cells, LMT was induced after YE treatment, but no CHMT activity was observed. It is concluded that alfalfa CHMT and licorice LMT are distinct enzymes, the former displaying the wider substrate specificity.