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

Abstract

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 (ΔSP) mature forms of all proteins fused with the tobacco PR1b signal sequence were generated. The full-length and ΔSP forms of ChpG_Cc and only the ΔSP 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 PF008ΔpCM1 strain, which lacks the intact chpG_Cc gene. The strain with any variants failed to restore the necrosis-causing ability. These results suggest that ChpG_Cc 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. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.