Phase-specific transcriptional patterns of the oomycete pathogen Phytophthora sojae unravel genes essential for asexual development and pathogenic processes.

Oomycetes are filamentous microorganisms easily mistaken as fungi but vastly differ in physiology, biochemistry, and genetics. This commonly-held misconception lead to a reduced effectiveness by using conventional fungicides to control oomycetes, thus it demands the identification of novel functional genes as target for precisely design oomycetes-specific microbicide. The present study initially analyzed the available transcriptome data of the model oomycete pathogen, Phytophthora sojae, and constructed an expression matrix of 10,953 genes across the stages of asexual development and host infection. Hierarchical clustering, specificity, and diversity analyses revealed a more pronounced transcriptional plasticity during the stages of asexual development than that in host infection, which drew our attention by particularly focusing on transcripts in asexual development stage to eventually clustered them into 6 phase-specific expression modules. Three of which respectively possessing a serine/threonine phosphatase (PP2C) expressed during the mycelial and sporangium stages, a histidine kinase (HK) expressed during the zoospore and cyst stages, and a bZIP transcription factor (bZIP32) exclusive to the cyst germination stage were selected for down-stream functional validation. In this way, we demonstrated that PP2C, HK, and bZIP32 play significant roles in P. sojae asexual development and virulence. Thus, these findings provide a foundation for further gene functional annotation in oomycetes and crop disease management.

PsHint1, associated with the G-protein α subunit PsGPA1, is required for the chemotaxis and pathogenicity of Phytophthora sojae.

Zoospore chemotaxis to soybean isoflavones is essential in the early stages of infection by the oomycete pathogen Phytophthora sojae. Previously, we have identified a G-protein α subunit encoded by PsGPA1 which regulates the chemotaxis and pathogenicity of P. sojae. In the present study, we used affinity purification to identify PsGPA1-interacting proteins, including PsHint1, a histidine triad (HIT) domain-containing protein orthologous to human HIT nucleotide-binding protein 1 (HINT1). PsHint1 interacted with both the guanosine triphosphate (GTP)- and guanosine diphosphate (GDP)-bound forms of PsGPA1. An analysis of the gene-silenced transformants revealed that PsHint1 was involved in the chemotropic response of zoospores to the isoflavone daidzein. During interaction with a susceptible soybean cultivar, PsHint1-silenced transformants displayed significantly reduced infectious hyphal extension and caused a strong cell death in plants. In addition, the transformants displayed defective cyst germination, forming abnormal germ tubes that were highly branched and exhibited apical swelling. These results suggest that PsHint1 not only regulates chemotaxis by interacting with PsGPA1, but also participates in a Gα-independent pathway involved in the pathogenicity of P. sojae.

PsVPS1, a dynamin-related protein, is involved in cyst germination and soybean infection of Phytophthora sojae.

Plant pathogens secrete effector proteins to suppress plant immunity. However, the mechanism by which oomycete pathogens deliver effector proteins during plant infection remains unknown. In this report, we characterized a Phytophthora sojae vps1 gene. This gene encodes a homolog of the Saccharomyces cerevisiae vacuolar protein sorting gene vps1 that mediates budding of clathrin-coated vesicles from the late Golgi, which are diverted from the general secretory pathway to the vacuole. PsVPS1-silenced mutants were generated using polyethylene glycol-mediated protoplast stable transformation and were viable but had reduced extracellular protein activity. The PsVPS1-silenced mutants showed impaired hyphal growth, and the shapes of the vacuoles were highly fragmented. Silencing of PsVPS1 affected cyst germination as well as the polarized growth of germinated cysts. Silenced mutants showed impaired invasion of susceptible soybean plants regardless of wounding. These results suggest that PsVPS1 is involved in vacuole morphology and cyst development. Moreover, it is essential for the virulence of P. sojae and extracellular protein secretion.