STRESS RESPONSES IN ALFALFA (MEDICAGO-SATIVA L) .16. ANTIFUNGAL ACTIVITY OF MEDICARPIN AND ITS BIOSYNTHETIC PRECURSORS - IMPLICATIONS FOR THE GENETIC MANIPULATION OF STRESS METABOLITES

The alfalfa (Medicago sativa L.) phytoalexin medicarpin and four biosynthetic intermediates immediately preceding medicarpin in the phytoalexin pathway were tested for antimicrobial activity against eight phytopathogenic fungi. Medicarpin, vestitone, 2′-hydroxyformononetin (2′-OHF), formononetin and daidzein were tested at concentrations from 0·1 mm to 0·5 mm in agar plate assays measuring inhibition of linear fungal mycelial growth. Medicarpin at 0·1 mm caused little or no inhibition of the growth of the four known alfalfa pathogens. Unlike the other alfalfa pathogens, Phytophthora megasperma f.sp. medicaginis (Pmm) showed a very significant sensitivity to 0·1 mm vestitone and 2′-OHF. Pmm was strongly inhibited by medicarpin, vestitone, and 2′-OHF at 0·5 mm. Phoma medicaginis was strongly inhibited by 0·5 mm medicarpin and vestitone, whereas 2′-OHF was not a significant inhibitor. Three strains of Nectria haematococca, varying in their phytoalexin detoxification capabilities, were inhibited to different extents by medicarpin and its precursors. The only growth inhibition observed at 0·1 mm was of strain 156-2-1 by medicarpin. At 0·5 mm, 2′-OHF and vestitone caused weak to moderate inhibition of the growth of all three strains; in contrast, 0·5 mm medicarpin greatly inhibited the growth of strain 156-2-1 compared to the other two strains. These results are discussed in relation to a potential strategy for testing the role of phytoalexins in disease resistance based upon transgenic plants blocked in the formation of vestitone from 2′-OHF. © 1992.