The Pepper Mannose-Binding Lectin Gene CaMBL1 Is Required to Regulate Cell Death and Defense Responses to Microbial Pathogens

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Abstract
Plant mannose-binding lectins (MBLs) are crucial for plant defense signaling during pathogen attack by recognizing specific carbohydrates on pathogen surfaces. In this study, we isolated and functionally characterized a novel pepper (Capsicum annuum) MBL gene, CaMBL1, from pepper leaves infected with Xanthomonas campestris pv vesicatoria (Xcv). The CaMBL1 gene contains a predicted Galanthus nivalis agglutinin-related lectin domain responsible for the recognition of high-mannose N-glycans but lacks a middle S-locus glycoprotein domain and a carboxyl-terminal PAN-Apple domain. The CaMBL1 protein exhibits binding specificity for mannose and is mainly localized to the plasma membrane. Immunoblotting using a CaMBL1-specific antibody revealed that CaMBL1 is strongly expressed and accumulates in pepper leaves during avirulent Xcv infection. The transient expression of CaMBL1 induces the accumulation of salicylic acid (SA), the activation of defense-related genes, and the cell death phenotype in pepper. The G. nivalis agglutinin-related lectin domain of CaMBL1 is responsible for cell death induction. CaMBL1-silenced pepper plants are more susceptible to virulent or avirulent Xcv infection compared with unsilenced control plants, a phenotype that is accompanied by lowered reactive oxygen species accumulation, reduced expression of downstream SA target genes, and a concomitant decrease in SA accumulation. In contrast, CaMBL1 overexpression in Arabidopsis (Arabidopsis thaliana) confers enhanced resistance to Pseudomonas syringae pv tomato and Alternaria brassicicola infection. Together, these data suggest that CaMBL1 plays a key role in the regulation of plant cell death and defense responses through the induction of downstream defense-related genes and SA accumulation after the recognition of microbial pathogens.