Phytophthora Suppressor of RNA Silencing 2 Is a Conserved RxLR Effector that Promotes Infection in Soybean and Arabidopsis thaliana

Abstract

The genus Phytophthora consists of notorious and emerging pathogens of economically important crops. Each Phytophthora genome encodes several hundreds of cytoplasmic effectors, which are believed to manipulate plant immune response inside the host cells. However, the majority of Phytophthora effectors remain functionally uncharacterized. We recently discovered two effectors from the soybean stem and root rot pathogen Phytophthora sojae with the activity to suppress RNA silencing in plants. These effectors are designated Phytophthora suppressor of RNA silencing (PSRs). Here, we report that the P. sojae PSR2 (PsPSR2) belongs to a conserved and widespread effector family in Phytophthora. A PsPSR2-like effector produced by P. infestans (PiPSR2) can also suppress RNA silencing in plants and promote Phytophthora infection, suggesting that the PSR2 family effectors have conserved functions in plant hosts. Using Agrobacterium rhizogenes-mediated hairy roots induction, we demonstrated that the expression of PsPSR2 rendered hypersusceptibility of soybean to P. sojae. Enhanced susceptibility was also observed in PsPSR2-expressing Arabidopsis thaliana plants during Phytophthora but not bacterial infection. These experiments provide strong evidence that PSR2 is a conserved Phytophthora effector family that performs important virulence functions specifically during Phytophthora infection of various plant hosts.

Phytophthora effector targets a novel component of small RNA pathway in plants to promote infection

“…23 and 24. Disease severity was evaluated at 3 dpi using a disease index based on hyphae extension (24).…”

“…We therefore investigated the role of PINP1 in plant defense during Phytophthora infection. For this purpose, we used a pathosystem with Arabidopsis as the host and the Phytophthora capsici isolate LT263 as the pathogen (23,24). A drastic enhancement of susceptibility was observed from PINP1-silenced plants (Fig.…”

Phytophthora infestans effector Pi14054 is a novel candidate suppressor of host silencing mechanisms

“…In the related soybean pathogen Phytophthora sojae two members of the RxLR class of effectors, PSR1, PSR2 (PsAvh18 and PsAvh146) were recently discovered to act as suppressors of host RNA silencing and promote Phytophthora infection (Qiao et al 2013). A homolog of PSR2 has been identified in P. infestans (PITG_15152) and several other Phytophthora species (Xiong et al 2014;de Vries et al 2017).…”

“…More specifically, PSR1 was shown to down regulate the accumulation of microRNAs and small interfering RNAs in Arabidopsis, via binding of a host RNA helicase, whilst PSR2 specifically decreases the accumulation of host siRNAs (Qiao et al 2013;Qiao et al 2015). The overexpression of both of these P. sojae effectors as well as PITG_15152 from P. infestans enhances virulence of both these oomycetes and of viruses (Qiao et al 2013;Xiong et al 2014). Thus the secretion of silencing suppressors is likely to be a general counter defense mechanism employed by oomycete pathogens.…”

“…2c) suggesting the importance of the effector in silencing suppression activity during pathogenesis. RxLR class effectors that can act to suppress host plant RNA silencing mechanisms have been described from P. sojae, with PSR2 orthologs being present in a variety of Phytophthora species, including P. infestans (Xiong et al 2014). Here we have shown that P. infestans possesses an additional predicted secreted effector with silencing suppression activity.…”