The fungal ribonuclease-like effector protein CSEP0064/BEC1054 represses plant immunity and interferes with degradation of host ribosomal RNA.

Helen G Pennington,Mark Kwaaitaal,Peggy Luong,Thomas Chandler,Michal Przydacz,Seomun Kwon,James Garnett,Hannah Thieron,Melanie Craze,Ernesto Cota,Ralph Panstruga,Giulia Bonciani,Pietro D Spanu,Sian Natasha Morgan,Emma J Wallington,Sarah Bowden,Tolga Bozkurt,Rhian Jones

doi:10.1371/journal.ppat.1007620

Abstract

The biotrophic fungal pathogen Blumeria graminis causes the powdery mildew disease of cereals and grasses. We present the first crystal structure of a B. graminis effector of pathogenicity (CSEP0064/BEC1054), demonstrating it has a ribonuclease (RNase)-like fold. This effector is part of a group of RNase-like proteins (termed RALPHs) which comprise the largest set of secreted effector candidates within the B. graminis genomes. Their exceptional abundance suggests they play crucial functions during pathogenesis. We show that transgenic expression of RALPH CSEP0064/BEC1054 increases susceptibility to infection in both monocotyledonous and dicotyledonous plants. CSEP0064/BEC1054 interacts in planta with the pathogenesis-related protein PR10. The effector protein associates with total RNA and weakly with DNA. Methyl jasmonate (MeJA) levels modulate susceptibility to aniline-induced host RNA fragmentation. In planta expression of CSEP0064/BEC1054 reduces the formation of this RNA fragment. We propose CSEP0064/BEC1054 is a pseudoenzyme that binds to host ribosomes, thereby inhibiting the action of plant ribosome-inactivating proteins (RIPs) that would otherwise lead to host cell death, an unviable interaction and demise of the fungus.

Highlights

The obligate biotrophic fungus Blumeria graminis causes powdery mildew disease on some small grain cereals and grasses (Poaceae)
We previously assessed the contribution of CSEP0064/BEC1054 to the interaction of barley and its adapted powdery mildew pathogen, B. graminis f.sp. hordei, by Host-Induced Gene Silencing (HIGS; [11])
CSEP0064/BEC1054 contributes significantly to fungal virulence: when the respective gene is silenced by HIGS, the infection success drops to less than half of the value obtained with a control construct

Summary

Introduction

The obligate biotrophic fungus Blumeria graminis causes powdery mildew disease on some small grain cereals and grasses (Poaceae). Some of the RALPH CSEP families contain members for which the RNase domain is not recognised by the current prediction algorithms If the latter are included, the RALPHs comprise the biggest subset of effector candidates within the Blumeria graminis f.sp. Hordei genome, numbering at least 113 of the previously analysed 491 CSEPs. If the latter are included, the RALPHs comprise the biggest subset of effector candidates within the Blumeria graminis f.sp. Their abundance, and their proliferation within a genome that otherwise has lost numerous genes [8], suggests that they play a prominent role during infection [4] This notion is further supported by the fact that some RALPH effectors are recognised by dedicated plant immune receptors and act as avirulence factors in the plant-powdery mildew interaction [9] [10]. These included two genes encoding RALPH effectors: CSEP0064/BEC1054 and CSEP0264/BEC1011, both belonging to family 21 of the predicted CSEPs in this powdery mildew pathogen [4, 11]

Methods

Results

Conclusion