Two genes encoding GH10 xylanases are essential for the virulence of the oomycete plant pathogen Phytophthora parasitica

Plant cell walls are pivotal battlegrounds between microbial pathogens and their hosts. To penetrate the cell wall and thereby to facilitate infection, microbial pathogens are equipped with a wide array of cell wall-degrading enzymes to depolymerize the polysaccharides in the cell wall. However, many of these enzymes and their role in the pathogenesis of microbial pathogens are not characterized, especially those from Oomycetes. In this study, we analyzed the function of four putative endo-beta-1,4-xylanase-encoding genes (ppxyn1-ppxyn4) from Phytophthora parasitica, an oomycete plant pathogen known to cause severe disease in a wide variety of plant species. All four genes belong to the glycoside hydrolase family 10 (GH10). Recombinant proteins of ppxyn1, ppxyn2, and ppxyn4 obtained from the yeast Pichia pastoris showed degrading activities toward birch wood xylan, but they behaved differently in terms of the conditions for optimal activity, thermostability, and durability. Quantitative RT-PCR revealed upregulated expression of all four genes, especially ppxyn1 and ppxyn2, during plant infection. In contrast, ppxyn3 was highly expressed in cysts and its close homolog, ppxyn4, in germinating cysts. To uncover the role of ppxyn1 and ppxyn2 in the pathogenesis of P. parasitica, we generated silencing transformants for these two genes by double-stranded RNA-mediated gene silencing. Silencing ppxyn1 and ppxyn2 reduced the virulence of P. parasitica toward tobacco (Nicotiana benthamiana) and tomato plants. These results demonstrate the crucial role of xylanase-encoding ppxyn1 and ppxyn2 in the infection process of P. parasitica.