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 differen t aqueous suspensions of smut sporidia. When inoculum consisted of ind ividual haploid strains, no mating or infection structures were observ ed. When stigmas were inoculated with sporidia that had compatible all eles at both mating loci (a and b), sporidia mated in pairs using a co njugation tube, and each pair formed a dikaryotic infection hypha that grew rapidly across the stigma surface, developed an appressorium, an d 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 in fection structures regardless of the b alleles they carried. Diploid s poridia with compatible a and b alleles did not mate but formed infect ive hyphae directly. Diploid and dikaryotic hyphae formed hyaline, sli ghtly swollen appressoria over epidermal cell-wall junctions. Penetrat ing hyphae arose from appressoria and entered stigmas by growing betwe en epidermal cells. Subsequent growth in stigmas was intracellular, in that hyphae penetrated the walls of host cells, but transmission elec tron micrographs showed that host-cell plasma membranes remained intac t around the hyphae. Hyphae in stigmas were multinucleate, with septa delimiting only vacuolate posterior portions of hyphae from the cytopl asm-filled hyphal tips. Stigma infections were readily effected under the greenhouse growth conditions described and may provide a convenien t system for investigating some aspects of this host-pathogen interact ion.