Ultrastructural in situ evidence for the role of glyceollin in the expression of soybean resistance to fungal infection.

Abstract

When soybean (cv. Harosoy 63) hypocotyls were inoculated with Phytophthora capsici, a nonpathogen of soybean, the fungus grew in the hypocotyls until 8 to 10hr after inoculation but then growth was strongly inhibited. This growth inhibition was accompanied by rapid accumulation of a phytoalexin glyceollin. No fungal growth inhibition was observed, however, in hypocotyls treated with blasticidin S, in which glyceollin accumulation was greatly suppressed, and a completely susceptible-type disease symptom appeared in the treated hypocotyls.Ultrastructure of P. capsici hyphae in resistant-responding hypocotyls at 12 or 15hr after inoculation exhibited the characteristic plasma membrane abnormalities that were indistinguishable from those observed in the fungus treated with glyceollin in vitro. Typical plasma membrane flattening and separation from cell walls with small vesicles in the intervening space as well as disappearance of plasma membrane from some portions of fungal cytoplasmic periphery were observed. Such plasma membrane distortion in the fungus was not observed, however, in the corresponding hypocotyls treated with blasticidin S, nor in the untreated hypocotyls at 5hr after inoculation when glyceollin accumulation had not been initiated. These results appear to support the contention that glyceollin is indeed active in situ in inhibiting fungal growth during the expression of resistance in soybean hypocotyls.