Barrett K J, Gold S E, Kronstad J W
Department of Microbiology, University of British Columbia, Vancouver, Canada.
Mol Plant Microbe Interact. 1993 May-Jun;6(3):274-83. doi: 10.1094/mpmi-6-274.
Pathogenicity of the corn smut fungus Ustilago maydis involves the formation of a filamentous, infectious dikaryon by fusion of compatible, yeastlike haploid cells. The mating-type loci, a and b, regulate cell fusion and establishment of the dikaryotic cell type, respectively. On solid medium, compatibility at the mating-type loci, in particular heterozygosity at the b locus, is manifested by the formation of aerial hyphae on colonies formed by mating cells. We have employed this "fuzzy" phenotype to identify haploid mutants that constitutively form hyphal filaments and forego cell division by budding. A total of 125 such mutants have been isolated; characterization of one mutant (termed rem1-1) revealed that it can participate in infection of the host plant, although it must be paired with a compatible, wild-type mating partner. That is, mutation to the mycelial phenotype is not sufficient to allow a haploid strain to be pathogenic by itself. A cosmid has been isolated that restores the ability of an rem1-1 mutant to grow with a budding phenotype. Localization of the complementing region on cosmid DNA allowed the construction of an additional mutation by gene disruption. Coinoculation of plants with two compatible strains, each carrying the disruption mutation, gave greatly reduced disease symptoms. The analysis of the rem1 gene should contribute to an understanding of dimorphic growth and pathogenesis in U. maydis.
玉米黑粉菌Ustilago maydis的致病性涉及通过兼容的酵母样单倍体细胞融合形成丝状感染性双核体。交配型基因座a和b分别调节细胞融合和双核细胞类型的建立。在固体培养基上，交配型基因座的兼容性，特别是b基因座的杂合性，通过交配细胞形成的菌落上的气生菌丝的形成来体现。我们利用这种“模糊”表型来鉴定组成型形成菌丝丝并通过出芽放弃细胞分裂的单倍体突变体。总共分离出125个这样的突变体；对一个突变体（称为rem1-1）的表征表明，它可以参与宿主植物的感染，尽管它必须与兼容的野生型交配伙伴配对。也就是说，向菌丝体表型的突变不足以使单倍体菌株自身具有致病性。已分离出一个黏粒，它恢复了rem1-1突变体以出芽表型生长的能力。通过黏粒DNA上互补区域的定位，通过基因破坏构建了一个额外的突变体。用两个都携带破坏突变的兼容菌株共同接种植物，导致疾病症状大大减轻。对rem1基因的分析应该有助于理解U. maydis中的双态生长和致病性。
