An antifungal compound involved in symbiotic germination of Cypripedium macranthos var. rebunense (Orchidaceae)

Germination of orchid seeds fully depends on a symbiotic association with soil-borne fungi, usually Rhizoctonia spp. In contrast to the peaceful symbiotic associations between many other terrestrial plants and mycorrhizal fungi, this association is a life-and-death struggle. The fungi always try to invade the cytoplasm of orchid cells to obtain nutritional compounds. On the other hand, the orchid cells restrict the growth of the infecting hyphae and obtain nutrition by digesting them. It is likely that antifungal compounds are involved in the restriction of fungal growth. Two antifungal compounds, lusianthrin and chrysin, were isolated from the seedlings of Cypripedium macranthos var. rebunense that had developed shoots. The former had a slightly stronger antifungal activity than the latter, and the antifungal spectra of these compounds were relatively specific to the nonpathogenic Rhizoctonia spp. The level of lusianthrin, which was very low in aseptic protocorm-like bodies, dramatically increased following infection with the symbiotic fungus. In contrast, chrysin was not detected in infected protocorm-like bodies. These results suggest that orchid plants equip multiple antifungal compounds and use them at specific developmental stages; lusianthrin maintains the perilous symbiotic association for germination and chrysin helps to protect adult plants.