A Myb Transcription Factor of Phytophthora sojae, Regulated by MAP Kinase PsSAK1, Is Required for Zoospore Development

PsSAK1, a mitogen-activated protein (MAP) kinase from Phytophthora sojae, plays an important role in host infection and zoospore viability. However, the downstream mechanism of PsSAK1 remains unclear. In this study, the 3'-tag digital gene expression (DGE) profiling method was applied to sequence the global transcriptional sequence of PsSAK1-silenced mutants during the cysts stage and 1.5 h after inoculation onto susceptible soybean leaf tissues. Compared with the gene expression levels of the recipient P. sojae strain, several candidates of Myb family were differentially expressed (up or down) in response to the loss of PsSAK1, including of a R2R3-type Myb transcription factor, PsMYB1. qRT-PCR indicated that the transcriptional level of PsMYB1 decreased due to PsSAK1 silencing. The transcriptional level of PsMYB1 increased during sporulating hyphae, in germinated cysts, and early infection. Silencing of PsMYB1 results in three phenotypes: a) no cleavage of the cytoplasm into uninucleate zoospores or release of normal zoospores, b) direct germination of sporangia, and c) afunction in zoospore-mediated plant infection. Our data indicate that the PsMYB1 transcription factor functions downstream of MAP kinase PsSAK1 and is required for zoospore development of P. sojae.

PsSAK1是大豆疫霉（Phytophthora sojae）中的丝裂原活化蛋白（MAP）激酶，在宿主侵染与游动孢子活力维持中发挥关键作用。然而，PsSAK1的下游调控机制目前仍未明确。本研究采用3'标签数字基因表达谱（DGE）分析技术，对孢囊阶段及接种至感病大豆叶片组织1.5小时后的PsSAK1沉默突变体进行全局转录组测序。与受体大豆疫霉菌株的基因表达水平相比，Myb家族的多个候选基因在PsSAK1沉默后呈现显著差异表达（上调或下调），其中包含1个R2R3型Myb转录因子PsMYB1。实时荧光定量PCR（qRT-PCR）结果显示，PsSAK1沉默会导致PsMYB1的转录水平显著下调。同时，PsMYB1的转录水平在产孢菌丝、萌发孢囊及早期侵染阶段均出现升高。沉默PsMYB1可引发三种表型：一是细胞质无法裂解为单核游动孢子，且无法释放正常游动孢子；二是孢子囊发生直接萌发；三是丧失游动孢子介导的植物侵染功能。本研究数据表明，PsMYB1转录因子在MAP激酶PsSAK1的下游发挥功能，且是大豆疫霉游动孢子发育所必需的。