A glycoside hydrolase 12 protein from Cytospora chrysosperma triggers plant immunity but is not essential to virulence

Abstract

Phytopathogens secrete numerous effectors that facilitate their infection and colonization processes. However, the pathogenic mechanism of effectors in Cytospora chrysosperma, the causal agent of canker disease in many woody plants, remains poorly understood. In this study, we identified five glycoside hydrolase family 12 (GH12) effector genes in C. chrysosperma genome, all of which were significantly upregulated during the infection stages. Among them, CcEG1, which contains an additional carbohydrate-binding module family 1 domain (CBM1) at the C-terminal, was selected for further analysis. Transient expression studies showed that CcEG1 was localized to the apoplastic region of Nicotiana benthamiana and acted as an elicitor to induce cell death, and activate the expression of genes involved in salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) signaling pathways. Furthermore, the GH12 domain (position 43–249) was sufficient for cell death-inducing activity, rather than the CBM1 domain. Additionally, the leucine-rich repeat (LRR) receptor-like kinases NbBAK1 and NbSOBIR1 were required for defense responses triggered by CcEG1. Intriguingly, deletion of CcEG1 did not affect fungal pathogenicity, growth, response to hydrogen peroxide and cell wall integrity agents, but affected cellulase utilization. In conclusion, our results suggest that CcEG1 induces coreceptors NbBAK1- and NbSOBIR1- dependent plant immunity, increasing our understanding about fungal pathogenesis during the interaction between C. chrysosperma and its host.