A Putative Apoplastic Effector of Clavibacter capsici, ChpG_Cc as Hypersensitive Response and Virulence (Hrv) Protein in Plants

Clavibacter bacteria use secreted apoplastic effectors, such as putative serine proteases, for virulence in host plants and for hypersensitive response (HR) induction in nonhost plants. Previously, we have shown that Clavibacter capsici ChpG(Cc) is important for the necrosis development in pepper (Capsicum annuum) leaves. Here, we determine the function of ChpG(Cc), along with three paralogous proteins, for HR induction in the apoplastic space of a nonhost plant, Nicotiana tabacum. The full-length and signal peptide-deleted (Delta SP) mature forms of all proteins fused with the tobacco PR1b signal sequence were generated. The full-length and Delta SP forms of ChpG(Cc) and only the Delta SP forms of ChpE(Cc) and Pat-1(Cc), but none of the ChpC(Cc), triggered HR. Based on the predicted protein structures, ChpG(Cc) carries amino acids for a catalytic triad and a disulfide bridge in positions like Pat-1(Cm). Substituting these amino acids of ChpG(Cc) with alanine abolished or reduced HR-inducing activity. To determine whether these residues are important for necrosis development in pepper, alanine-substituted chpG(Cc) genes were transformed into the C. capsici PF008 Delta pCM1 strain, which lacks the intact chpG(Cc) gene. The strain with any variants failed to restore the necrosis-causing ability. These results suggest that ChpGCc has a dual function as a virulence factor in host plants and an HR elicitor in nonhost plants. Based on our findings and previous results, we propose Clavibacter apoplastic effectors, such as ChpG(Cc), Pat-1(Cm), Chp-7(Cs), and ChpG(Cm), as hypersensitive response and virulence (Hrv) proteins that display phenotypic similarities to the hypersensitive response and pathogenicity (Hrp) proteins found in gram-negative bacteria. Copyright (c) 2024 The Author(s).