Isolation of chalcone synthase and chalcone isomerase cDNAs from alfalfa (Medicago sativa L.): highest transcript levels occur in young roots and root tips.

Flavonoids are involved in several different interactions between plants and microorganisms. In the Rhizobium-legume symbiosis, they play an important role as inducers of rhizobial nodulation (nod) genes. We have identified from an alfalfa cDNA library four clones for chalcone synthase (CHS) and two clones for chalcone isomerase (CHI); CHS and CHI are key enzymes in flavonoid biosynthesis. In Medicago sp., CHS is encoded by 8-12 genes, and CHI is encoded by 1-2 genes. Here we report the DNA sequence of these clones as well as their relatedness to other legume CHS and CHI clones. In addition, we report on the expression patterns of two CHS gene family members as well as the CHI gene in M. sativa cv. Iroquois. While CHS and CHI transcript levels are high in root tips and entire young roots, they are low in effective nodules elicited by wild-type strains of Rhizobium meliloti and very low in aerial portions of the plant (stems, leaves, flowers). However, wounding the cotyledons results in a rapid increase in transcript levels of both chalcone synthase and chalcone isomerase genes in these organs.