INFECTION OF MAIZE STIGMAS BY USTILAGO-MAYDIS - LIGHT AND ELECTRON-MICROSCOPY

Infection of maize stigmas (silks) by the smut fungus Ustilago maydis was documented with micrographs. Stigmas were inoculated with different aqueous suspensions of smut sporidia. When inoculum consisted of individual haploid strains, no mating or infection structures were observed. When stigmas were inoculated with sporidia that had compatible alleles at both mating loci (a and b), sporidia mated in pairs using a conjugation tube, and each pair formed a dikaryotic infection hypha that grew rapidly across the stigma surface, developed an appressorium, and entered the stigma. When sporidia were compatible at a but not at b, mating occurred irregularly, and resultant hyphae grew slowly and did not enter stigmas. Sporidia incompatible at a did not mate or form infection structures regardless of the b alleles they carried. Diploid sporidia with compatible a and b alleles did not mate but formed infective hyphae directly. Diploid and dikaryotic hyphae formed hyaline, slightly swollen appressoria over epidermal cell-wall junctions. Penetrating hyphae arose from appressoria and entered stigmas by growing between epidermal cells. Subsequent growth in stigmas was intracellular, in that hyphae penetrated the walls of host cells, but transmission electron micrographs showed that host-cell plasma membranes remained intact around the hyphae. Hyphae in stigmas were multinucleate, with septa delimiting only vacuolate posterior portions of hyphae from the cytoplasm-filled hyphal tips. Stigma infections were readily effected under the greenhouse growth conditions described and may provide a convenient system for investigating some aspects of this host-pathogen interaction.