The barley powdery mildew effectors CSEP0139 and CSEP0182 suppress cell death and promote B. graminis fungal virulence in plants

Xue Li,Jiankun Xie,Qian-Hua Shen,Wanting Huang,Fang Liu,Cong Jin,Hongbo Yuan,Renchun Fan

doi:10.1186/s42483-021-00084-z

Abstract

The powdery mildew fungi secrete numerous Candidate Secreted Effector Proteins (CSEPs) to manipulate host immunity during infection of host plants. However, the function of most of these CSEPs in cell death suppression has not yet been established. Here, we identified several CSEPs from Blumeria graminis f. sp. hordei (Bgh) that have the potential to suppress BAX- and NtMEK2DD-triggered cell death in Nicotiana benthamiana. We further characterized two effector candidates, CSEP0139 and CSEP0182, from family six and thirty-two, respectively. CSEP0139 and CSEP0182 contain a functional signal peptide and are likely secreted effectors. Expression of either CSEP0139 or CSEP0182 suppressed cell death triggered by BAX and NtMEK2DD but not by the AVRa13/MLA13 pair in N. benthamiana. Transient overexpression of CSEP0139 or CSEP0182 also inhibited BAX-induced cell death and collapse of cytoplasm in barley cells. Furthermore, overexpression of either CSEPs significantly increased Bgh haustorial formation in barley, whereas host-induced gene silencing (HIGS) of the CSEP genes reduced haustorial formation, suggesting both CSEPs promote Bgh virulence in barley. In addition, expression of CSEP0139 and CSEP0182 reduced size of the lesions caused by the necrotrophic Botrytis cinerea in N. benthamiana. Our findings suggest that CSEP0139 and CSEP0182 may target cell death components in plants to promote fungal virulence, which extends the current understanding of the functions of Bgh CSEPs and provides an opportunity for further investigation of fungal virulence in relation to cell death pathways in host plants.

Highlights

Powdery mildew fungi are widespread and important fungal pathogens that colonize many plant species, including economically important cereal crops, such as wheat and barley (Glawe 2008; Dean et al 2012; Takamatsu 2013)
We selected some Candidates for Secreted Effector Proteins (CSEPs) for further study based on their relatively high expression in haustoria with respect to epiphytic tissues (Pedersen et al 2012), and whether the CSEPs suppressed cell death in N. benthamiana triggered by BAX or NtMEK2DD, a constitutively active mutant of NtMEK2 (Yang et al 2001)
We further characterized CSEP0139 and CSEP0182, two effector genes from family six and thirty-two respectively (Menardo et al 2017), which are highly expressed in haustoria but are not members of the RNase-like proteins associated with haustoria (RALPH) family, and expression of them suppressed cell death induced by BAX and NtMEK2DD in N. benthamiana (Additional file 1: Table S1)

Summary

Introduction

Powdery mildew fungi are widespread and important fungal pathogens that colonize many plant species, including economically important cereal crops, such as wheat and barley (Glawe 2008; Dean et al 2012; Takamatsu 2013). It has been shown that ~ 63% of the identified Bgh CSEPs are predicted to contain an N-terminal signal peptide (SP), and share an N-terminal Y/F/WxC motif that is speculated to have a function in effector translocation to host cells (Godfrey et al 2010; Spanu et al 2010; Pedersen et al 2012). 25%) of the Bgh CSEPs are RNase-like proteins associated with haustoria (RALPH effectors) that may act as pseudoenzymes with critical function (Pedersen et al 2012; Spanu 2017; Pennington et al 2019). The mechanisms by which Bgh CSEPs manipulate barley immune responses and enhance fungal virulence in host cells remain largely unknown

Methods

Results

Discussion

Conclusion