Alan J. Castle

Regulation of Parasitic Development of the Smut Fungus, Ustilago violacea, by Extracts from Host Plants

Ustilago violacea grows as yeastlike cells on laboratory media, and its parasitic mycelial stage has so far been restricted to its host plants, members of the Caryophyllaceae Aqueous or acetone extracts of all tested host species induced the formation and growth of mycelia on laboratory media and also enhanced the growth and development of conjugation pegs during mating. Extracts from most tested nonhost species had no effect on U. violacea. Races of U. violacea isolated from different caryophyllaceous hosts varied in response, but all races responded to all host extracts under some growth conditions; therefore, there was no evidence that each race is induced to form parasitic hyphae by the specific products of its particular host species. Ustilago violacea is thus able to restrict development of its parasitic phase to conditions when it is exposed to a particular product or group of products (termed silenins) present in its hosts. Only cells carrying both mating types (conjugated a1 + a2 cells or diploid a1a2 cells) responded to silenin by developing infection hyphae. Cells of a single mating type were not affected in growth rate or morphology. Silenin appears to be a member of a new class of plant compounds, termed mycoboethins, which are defined as plant products exploited by particular pathogens to induce highly specific morphogenetic changes leading to parasitism.

The Effect of Host Extracts on Differentiation in the Genus Ustilago

Ustilago violacea grows as yeastlike sporidia on nutritive media. Leaf extracts from all of 38 tested plant species that host a species of U stilago induced hyphae and inhibited sporulation in U. violacea. However, extracts from most (33 of 39) of the tested plant species that do not host species of smut fungi were inactive. Ustilago scabiosae and U. utriculosae responded similarly to U. violacea to the same range of plant extracts. Species of Ustilago from monocotyledonous hosts formed hyphae on nutritive media in the absence of extracts. Plant extracts inhibited sporulation in these species but did not noticeably affect hyphal growth Farysia olivacea, like U. violacea, remained strictly sporidial in the absence of host extracts. This species, however, was induced to form hyphae by different plant extracts from those active on U. violacea.

Diploid Derivatives of Ustilago violacea with Altered Mating-Type Activity. I. Isolation and Properties

Freshly isolated a1a2 diploid cells of Ustilago violacea grow in the vegetative phase (VP) by budding at temperatures above 20 C on complete medium but differentiate into sexual-precursor-phase (SPP) cells at temperatures below 20 C. At frequencies as high as 0.5%, strains arise spontaneously which lack the ability to differentiate into SPP cells under low temperatures. The strains are called opaque because their VP colonies are opaque as opposed to the translucent colonies of SPP cells. Three types of opaque strains are described: op-N (neutral in mating), op-a1 (a1 mating ability), and op-a2 (a2 mating ability). Analysis of all three types by mitotic haploidization indicates that they are still diploid and heterozygous for all marked chromosomes. Only the mating-type locus (and perhaps chromosome) appears to be altered during their formation. The op-N strains vary, some producing haploids of both mating types and others producing only one type of haploid (mainly a2). These op-N were still capable of differentiation into SPP but only under altered conditions of temperatures and nutrient. At least some op-N were solopathogenic, like the original diploid. The op-a1 and op-a2 strains yielded only the corresponding mating-type alleles on haploidization, could not develop into SPP cells under any conditions, and were not solopathogenic. Complementation tests indicated that the genes for SPP development were not impaired in either of these strains. Although the overall level of opaque formation was much higher, ultraviolet light induced opaque formation to the same extent as it induced mitotic crossing-over near a variety of genetic markers. It is, therefore, likely that op-a1 and op-a2 strains represent homozygous a1a1 and a2a2 strains formed by mitotic recombination. Possible mechanisms for the origin of opaques are discussed.

Diploid Derivatives of Ustilago violacea with Altered Mating-Type Activity. III. Constitutive Mating Strains

Freshly isolated diploids of Ustilago violacea, heterozygous for mating type (a1/a2), give rise to frequent (5 x 10-3) derivative forms that appear to form after mitotic crossing-over near the mating-type locus. One such derivative, termed op-C, constitutively produces mating tubes at temperatures below 20 C on complete medium or 25 C on minimal medium but buds vegetatively at higher temperatures. Op-C strains remain diploid and have no detectable alterations, compared with the original diploid, in all the marked chromosomes except for the mating-type locus. Op-C strains yield segregants, which are either a1 in mating type (both diploids and haploids) or haploids that produce mating tubes constitutively. The op-C phenotype arises after alterations that affect the a2 allele in a1/a2 diploids and that therefore change the development of these cells. Op-C and other opaque-derivative forms provide valuable strains to study the action of the mating-type locus as a developmental master switch.