Mayorga M E, Gold S E
Department of Plant Pathology, University of Georgia, Athens, GA 30602, USA.
Mol Microbiol. 1999 Nov;34(3):485-97. doi: 10.1046/j.1365-2958.1999.01610.x.
Ustilago maydis, the causal agent of corn smut disease, displays dimorphic growth in which it alternates between a budding haploid saprophyte and a filamentous dikaryotic pathogen. We are interested in identifying the genetic determinants of filamentous growth and pathogenicity in U. maydis. To do this, we have taken a forward genetic approach. Previously, we showed that haploid adenylate cyclase (uac1) mutants display a constitutively filamentous phenotype. Mutagenesis of a uac1 disruption strain allowed the isolation of a large number of budding suppressor mutants. These mutants are named ubc, for Ustilago bypass of cyclase, as they no longer require the production of cAMP to grow in the budding morphology. Complementation of one of these suppressor mutants led to the identification of ubc3, which is required for filamentous growth and encodes a MAP kinase most similar to those of the yeast pheromone response pathway. In addition to filamentous growth, the ubc3 gene is required for pheromone response and for full virulence. Mutations in the earlier identified fuz7 MAP kinase kinase also suppress the filamentous phenotype of the uac1 disruption mutant, adding evidence that both ubc3 and fuz7 are members of this same MAP kinase cascade. These results support an important interplay of the cAMP and MAP kinase signal transduction pathways in the control of morphogenesis and pathogenicity in U. maydis.
玉米黑粉病菌（Ustilago maydis）是玉米黑粉病的致病因子，具有双态生长特性，在出芽单倍体腐生菌和丝状双核病原体之间交替。我们对鉴定玉米黑粉病菌丝状生长和致病性的遗传决定因素感兴趣。为此，我们采用了正向遗传学方法。此前，我们发现单倍体腺苷酸环化酶（uac1）突变体表现出组成型丝状表型。对uac1缺失菌株进行诱变，使得我们能够分离出大量出芽抑制突变体。这些突变体被命名为ubc，即环化酶旁路的黑粉菌，因为它们不再需要产生cAMP来以出芽形态生长。对其中一个抑制突变体进行互补分析，从而鉴定出ubc3，它是丝状生长所必需的，编码一种与酵母信息素反应途径中的MAP激酶最为相似的MAP激酶。除了丝状生长外，ubc3基因对于信息素反应和完全致病性也是必需 的。先前鉴定的fuz7 MAP激酶激酶中的突变也能抑制uac1缺失突变体的丝状表型，这进一步证明ubc3和fuz7都是同一MAP激酶级联反应的成员。这些结果支持了cAMP和MAP激酶信号转导途径在控制玉米黑粉病菌形态发生和致病性方面的重要相互作用。
