A conserved extracellular effector protein Ssh1296 from Scleromitrula shiraiana triggers cell death and regulates plant immunity

The plant apoplast is a key battleground in the initial stages of interaction between the plant and pathogen. Despite its importance, few apoplastic effectors have been characterized to date. Here, we identified Ssh1296, a conserved apoplastic effector from Scleromitrula shiraiana. Ssh1296 and its homologous proteins, prevalent among fungi and oomycetes, possess the ability to induce cell death and enhance resistance against pathogens in Nicotiana benthamiana. Fragments containing conserved motifs 1-3 elicit more pronounced cell death responses than the full-length Ssh1296 protein. Furthermore, cysteine residues at positions C38, C52, C84, and C89 are essential for inducing sufficient cell death. The cell death response mediated by Ssh1296 depends on the RECEPTOR-LIKE PROTEIN REQUIRED FOR CSP22 RESPONSIVENESS (NbCSPR/RE02), SUPPRESSOR OF BIR1-1 (SOBIR1), and BRI1-ASSOCIATED KINASE-1 (BAK1). Constitutive expression of Ssh1296 and its homologous protein in Arabidopsis thaliana activates plant immunity but concurrently inhibits growth and development. These findings suggest that Ssh1296 and its homologous proteins can be recognized by plants as pathogen-associated molecular patterns (PAMPs). In conclusion, Ssh1296 holds promise as a potential inducer of plant immunity.