How the Pathogenic Fungus Alternaria alternata Copes with Stress via the Response Regulators SSK1 and SHO1

The tangerine pathotype of Alternaria alternata is a necrotrophic fungal pathogen causing brown spot disease on a number of citrus cultivars. To better understand the dynamics of signal regulation leading to oxidative and osmotic stress response and fungal infection on citrus, phenotypic characterization of the yeast SSK1 response regulator homolog was performed. It was determined that SSK1 responds to diverse environmental stimuli and plays a critical role in fungal pathogenesis. Experiments to determine the phenotypes resulting from the loss of SSK1 reveal that the SSK1 gene product may be fulfilling similar regulatory roles in signaling pathways involving a HOG1 MAP kinase during ROS resistance, osmotic resistance, fungicide sensitivity and fungal virulence. The SSK1 mutants display elevated sensitivity to oxidants, fail to detoxify H2O2 effectively, induce minor necrosis on susceptible citrus leaves, and displays resistance to dicarboximide and phenylpyrrole fungicides. Unlike the SKN7 response regulator, SSK1 and HOG1 confer resistance to salt-induced osmotic stress via an unknown kinase sensor rather than the “two component” histidine kinase HSK1. SSK1 and HOG1 play a moderate role in sugar-induced osmotic stress. We also show that SSK1 mutants are impaired in their ability to produce germ tubes from conidia, indicating a role for the gene product in cell differentiation. SSK1 also is involved in multi-drug resistance. However, deletion of the yeast SHO1 (synthetic high osmolarity) homolog resulted in no noticeable phenotypes. Nonetheless, our results show that A. alternata can sense and react to different types of stress via SSK1, HOG1 and SKN7 in a cooperative manner leading to proper physiological and pathological functions.

链格孢菌（Alternaria alternata）的橘致病型是一种坏死营养型真菌病原体，可在多种柑橘品种上引发褐斑病。为更好地解析介导氧化、渗透胁迫响应及柑橘真菌侵染的信号调控动态，本研究对酵母SSK1应答调节因子同源物进行了表型表征。研究确定，SSK1可响应多种环境刺激，并在真菌致病过程中发挥关键作用。针对SSK1缺失所产生表型的实验显示，在活性氧（Reactive Oxygen Species, ROS）抗性、渗透抗性、杀菌剂敏感性及真菌毒力相关的信号通路中，SSK1基因产物可能行使类似的调控功能，该通路涉及HOG1丝裂原活化蛋白激酶。SSK1突变体对氧化剂的敏感性升高，无法有效清除过氧化氢（H₂O₂），仅能在易感柑橘叶片上诱导轻微坏死，且对二羧酰亚胺类与苯基吡咯类杀菌剂具有抗性。与SKN7应答调节因子不同，SSK1与HOG1通过未知的激酶传感器（而非“双组分”组氨酸激酶HSK1）赋予菌株对盐诱导渗透胁迫的抗性。SSK1与HOG1在糖诱导渗透胁迫中发挥中等程度的调控作用。本研究同时证实，SSK1突变体丧失了从分生孢子产生芽管的能力，表明该基因产物参与细胞分化过程。SSK1还参与多药耐药性调控。然而，酵母SHO1（合成高渗性）同源物的敲除未产生可观测的表型。尽管如此，本研究结果表明，链格孢菌可通过SSK1、HOG1与SKN7的协同作用感知并响应不同类型的胁迫，从而维持正常的生理与病理功能。