Phytophthora infestans RXLR Effectors Target Parallel Steps in an Immune Signal Transduction Pathway.

The potato (Solanum tuberosum) blight pathogen Phytophthora infestans delivers Arg-X-Leu-Arg (RXLR) effector proteins into host cells to subvert plant immune responses and promote colonization. We show that transient expression and stable transgenic expression of the RXLR effector Pi22926 in Nicotiana benthamiana promotes leaf colonization by P. infestans. Pi22926 suppresses cell death triggered by coexpression of the Cladosporium fulvum avirulence protein Avr4 and the tomato (Solanum lycopersicum) resistance protein Cf4. Pi22926 interacts with a potato mitogen-activated protein kinase kinase kinase, StMAP3Kβ2, in the nucleoplasm. Virus-induced gene silencing (VIGS) of the ortholog NbMAP3Kβ2 in N. benthamiana enhances P. infestans colonization and attenuates Cf4/Avr4-induced cell death, indicating that this host protein is a positive regulator of immunity. Cell death induced by Cf4/Avr4 is dependent on NbMAP3Kε and NbMAP3Kβ2, indicating that these MAP3Ks function in the same signaling pathway. VIGS of NbMAP3Kβ2 does not compromise cell death triggered by overexpression of MAP3Kε. Similarly, VIGS of NbMAP3Kε does not attenuate cell death triggered by MAP3Kβ2, demonstrating that these MAP3K proteins function in parallel. In agreement, Pi22926 or another RXLR effector, PexRD2, only suppresses cell death triggered by expression of StMAP3Kβ2 or StMAP3Kε, respectively. Our data reveal that two P. infestans effectors, PexRD2 and Pi22926, promote P. infestans colonization by targeting MAP3K proteins that act in parallel in the same signal transduction pathway.

Ren Y ，Armstrong M ，Qi Y ，McLellan H ，Zhong C ，Du B ，Birch PRJ ，Tian Z ... -  《-》

Potato NPH3/RPT2-Like Protein StNRL1, Targeted by a Phytophthora infestans RXLR Effector, Is a Susceptibility Factor.

Plant pathogens deliver effectors to manipulate host processes. We know little about how fungal and oomycete effectors target host proteins to promote susceptibility, yet such knowledge is vital to understand crop disease. We show that either transient expression in Nicotiana benthamiana, or stable transgenic expression in potato (Solanum tuberosum), of the Phytophthora infestans RXLR effector Pi02860 enhances leaf colonization by the pathogen. Expression of Pi02860 also attenuates cell death triggered by the P. infestans microbe-associated molecular pattern INF1, indicating that the effector suppresses pattern-triggered immunity. However, the effector does not attenuate cell death triggered by Cf4/Avr4 coexpression, showing that it does not suppress all cell death activated by cell surface receptors. Pi02860 interacts in yeast two-hybrid assays with potato NPH3/RPT2-LIKE1 (NRL1), a predicted CULLIN3-associated ubiquitin E3 ligase. Interaction of Pi02860 in planta was confirmed by coimmunoprecipitation and bimolecular fluorescence complementation assays. Virus-induced gene silencing of NRL1 in N. benthamiana resulted in reduced P. infestans colonization and accelerated INF1-mediated cell death, indicating that this host protein acts as a negative regulator of immunity. Moreover, whereas NRL1 virus-induced gene silencing had no effect on the ability of the P. infestans effector Avr3a to suppress INF1-mediated cell death, such suppression by Pi02860 was significantly attenuated, indicating that this activity of Pi02860 is mediated by NRL1. Transient overexpression of NRL1 resulted in the suppression of INF1-mediated cell death and enhanced P. infestans leaf colonization, demonstrating that NRL1 acts as a susceptibility factor to promote late blight disease.

Yang L ，McLellan H ，Naqvi S ，He Q ，Boevink PC ，Armstrong M ，Giuliani LM ，Zhang W ，Tian Z ，Zhan J ，Gilroy EM ，Birch PR ... -  《-》

The Potato MAP3K StVIK Is Required for the Phytophthora infestans RXLR Effector Pi17316 to Promote Disease.

Plant pathogens deliver effectors to manipulate processes in their hosts, creating a suitable environment for invasion and proliferation. Yet, little is known about the host proteins that are targeted by effectors from filamentous pathogens. Here, we show that stable transgenic expression in potato (Solanum tuberosum) and transient expression in Nicotiana benthamiana of the arginine-any amino acid-leucine-arginine effector Pi17316 enhances leaf colonization by the late blight pathogen Phytophthora infestans Expression of Pi17316 also attenuates cell death triggered by the pathogen-associated molecular pattern Infestin1 (INF1), indicating that the effector suppresses pattern-triggered immunity. However, this effector does not attenuate cell death triggered by a range of resistance proteins, showing that it specifically suppresses INF1-triggered cell death (ICD). In yeast two-hybrid assays, Pi17316 interacts directly with the potato ortholog of VASCULAR HIGHWAY1-interacting kinase (StVIK), encoding a predicted MEK kinase (MAP3K). Interaction in planta was confirmed by coimmunoprecipitation and occurs at the plant plasma membrane. Virus-induced gene silencing of VIK in N. benthamiana attenuated P. infestans colonization, whereas transient overexpression of StVIK enhanced colonization, indicating that this host protein acts as a susceptibility factor. Moreover, VIK overexpression specifically attenuated ICD, indicating that it is a negative regulator of immunity. The abilities of Pi17316 to enhance P. infestans colonization or suppress ICD were compromised significantly in NbVIK-silenced plants, demonstrating that the effector activity of Pi17316 is mediated by this MAP3K. Thus, StVIK is exploited by P. infestans as a susceptibility factor to promote late blight disease.

Murphy F ，He Q ，Armstrong M ，Giuliani LM ，Boevink PC ，Zhang W ，Tian Z ，Birch PRJ ，Gilroy EM ... -  《-》

U-box E3 ubiquitin ligase PUB17 acts in the nucleus to promote specific immune pathways triggered by Phytophthora infestans.

Ubiquitination regulates many processes in plants, including immunity. The E3 ubiquitin ligase PUB17 is a positive regulator of programmed cell death (PCD) triggered by resistance proteins CF4/9 in tomato. Its role in immunity to the potato late blight pathogen, Phytophthora infestans, was investigated here. Silencing StPUB17 in potato by RNAi and NbPUB17 in Nicotiana benthamiana by virus-induced gene silencing (VIGS) each enhanced P. infestans leaf colonization. PAMP-triggered immunity (PTI) transcriptional responses activated by flg22, and CF4/Avr4-mediated PCD were attenuated by silencing PUB17. However, silencing PUB17 did not compromise PCD triggered by P. infestans PAMP INF1, or co-expression of R3a/AVR3a, demonstrating that not all PTI- and PCD-associated responses require PUB17. PUB17 localizes to the plant nucleus and especially in the nucleolus. Transient over-expression of a dominant-negative StPUB17(V314I,V316I) mutant, which retained nucleolar localization, suppressed CF4-mediated cell death and enhanced P. infestans colonization. Exclusion of the StPUB17(V314I,V316I) mutant from the nucleus abolished its dominant-negative activity, demonstrating that StPUB17 functions in the nucleus. PUB17 is a positive regulator of immunity to late blight that acts in the nucleus to promote specific PTI and PCD pathways.

He Q ，McLellan H ，Boevink PC ，Sadanandom A ，Xie C ，Birch PR ，Tian Z ... -  《-》

Phytophthora infestans RXLR effector PexRD2 interacts with host MAPKKK ε to suppress plant immune signaling.

King SR ，McLellan H ，Boevink PC ，Armstrong MR ，Bukharova T ，Sukarta O ，Win J ，Kamoun S ，Birch PR ，Banfield MJ ... -  《-》