UC Riverside

Phytophthora sojae is an oomycete pathogen that causes soybean (Glycine max) stem and root rot, the second most destructive soybean disease. P. sojae belongs to the genus Phytophthora which includes other important plant pathogens such as the potato late blight pathogen Phytophthora infestans and sudden oak death pathogen Phytophthora ramorum. These eukaryotic pathogens cause billions of dollars of crop damage each year. However, their pathogenesis is poorly understood.

As many other plant pathogens, Phytophthora utilizes effector proteins to suppress host defense and cause disease. P. sojae is predicted to produce more than 400 effectors that contain the conserved host cell entry motif RxLR-dEER; however, most of these effectors are functionally uncharacterized. In order to understand the mechanisms underlying effector-mediated suppression of host defense, a screen was conducted in our laboratory to identify effectors targeting the RNA silencing pathway. Using a post- transcriptional gene silencing (PTGS) assay, two Phytophthora Suppressors of RNA silencing effectors (PSRs) were indentified. The main goal of my thesis work is to investigate the virulence function of PSRs during pathogen infection. My experiments showed that PSR1 enhances the virulence of Potato Virus X in Nicotiana benthamiana. Furthermore, PSR1 and PSR2 may also enhance the virulence of P. sojae by accelerating the sporulation of the pathogen in soybean roots. Taken together, these results suggest that Phytophthora pathogens have evolved effectors that promote infection by targeting the RNA silencing pathway in plant hosts.

Understanding how these effector proteins target different functional branches of the plant immune systems would advance our understanding of Phytophthora pathogenesis. This knowledge will contribute to the development of disease resistant crops.