Alfalfa (Medicago sativa L.) cell suspension cultures accumulated high 
concentrations of the pterocarpan phytoalexin medicarpin, reaching a maximum 
within 24 hours after exposure to an elicitor preparation from cell walls of the 
phytopathogenic fungus Colletotrichum lindemuthianum. This was preceded by 
increases in the extractable activities of the isoflavonoid biosynthetic enzymes 
l-phenylalanine ammonia-lyase, cinnamic acid 4-hydroxylase, 4-coumarate coenzyme 
A-ligase, chalcone synthase, chalcone isomerase, and isoflavone 
O-methyltransferase. Pectic polysaccharides were weak elicitors of phenylalanine 
ammonia-lyase activity but did not induce medicarpin accumulation, whereas 
reduced glutathione was totally inactive as an elicitor in this system. The 
fungal cell wall extract was a weak elicitor of the lignin biosynthetic enzymes, 
caffeic acid O-methyltransferase and coniferyl alcohol dehydrogenase, but did 
not induce appreciable increases in the activities of the hydrolytic enzymes 
chitinase and 1,3-beta-d-glucanase. The results are discussed in relation to the 
activation of isoflavonoid biosynthesis in other legumes and the development of 
the alfalfa cell culture system as a model for studying the enzymology and 
molecular biology of plant defense expression.