Abstract

The potato blight agent Phytophthora infestans secretes a range of RXLR effectors to promote disease. Recent evidence indicates that some effectors suppress early pattern-triggered immunity (PTI) following perception of microbe-associated molecular patterns (MAMPs). Phytophthora infestans effector PiSFI3/Pi06087/PexRD16 has been previously shown to suppress MAMP-triggered pFRK1-Luciferase reporter gene activity. How PiSFI3 suppresses immunity is unknown. We employed yeast-two-hybrid (Y2H) assays, co-immunoprecipitation, transcriptional silencing by RNA interference and virus-induced gene silencing (VIGS), and X-ray crystallography for structure-guided mutagenesis, to investigate the function of PiSFI3 in targeting a plant U-box-kinase protein (StUBK) to suppress immunity. We discovered that PiSFI3 is active in the host nucleus and interacts in yeast and in planta with StUBK. UBK is a positive regulator of specific PTI pathways in both potato and Nicotiana benthamiana. Importantly, it contributes to early transcriptional responses that are suppressed by PiSFI3. PiSFI3 forms an unusual trans-homodimer. Mutation to disrupt dimerization prevents nucleolar localisation of PiSFI3 and attenuates both its interaction with StUBK and its ability to enhance P.infestans leaf colonisation. PiSFI3 is a 'WY-domain' RXLR effector that forms a novel trans-homodimer which is required for its ability to suppress PTI via interaction with the U-box-kinase protein StUBK.

Highlights

  • The plant immune system may be activated following the detection of conserved microbial molecules by cell surface pattern recognition receptors (PRRs)

  • Recent evidence indicates that some effectors suppress early pattern-triggered immunity (PTI) following perception of microbe-associated molecular patterns (MAMPs)

  • We employed yeast-two-hybrid (Y2H) assays, co-immunoprecipitation, transcriptional silencing by RNA interference and virus-induced gene silencing (VIGS), and X-ray crystallography for structure-guided mutagenesis, to investigate the function of PiSFI3 in targeting a plant U-box-kinase protein (StUBK) to suppress immunity

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Summary

Introduction

The plant immune system may be activated following the detection of conserved microbial molecules (microbe-associated molecular patterns, MAMPs) by cell surface pattern recognition receptors (PRRs). Pathogen effectors serve to suppress this pattern-triggered immunity (PTI), or otherwise manipulate processes in the host to promote susceptibility. RXLR effectors manipulate a range of host processes by direct interaction with diverse plant proteins (Whisson et al, 2016). Some host proteins targeted by P. infestans RXLR effectors are positive regulators of immunity that confer posttranslational modifications (PTMs). The RXLR effector AVR3a targets CMPG1, a host ubiquitin E3 ligase required for cell death triggered by perception of the PAMP infestin-1 (INF1; Bos et al, 2010) and a range of additional pathogen elicitors perceived at the cell surface (Gilroy et al, 2011). RXLR effector PexRD2 targets the kinase domain of the host protein MAP3Kɛ, which mediates signal transduction following perception of Cladosporium fulvum apoplastic effector Avr by the tomato Cf4 resistance protein (King et al, 2014)

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