Metacaspases, which belong to the
cysteine-type C14
protease family, are most structurally similar to mammalian
caspases than any other
caspase-like
protease in plants. Atmc9 (Arabidopsis thaliana metacaspase 9) has a unique domain structure, and distinct biochemical characteristics, such as Ca²⁺ binding, pH, redox status, S-nitrosylation and specific
protease inhibitors. However, the
biological roles of Atmc9 in plant-pathogen interactions remain largely unknown. In this study, a metacaspase gene present as a single copy in the pepper genome, and sharing 54% amino acid sequence identity with Atmc9, was isolated and named Capsicum annuum metacaspase 9 (Camc9). Camc9 encodes a 318-amino-acid
polypeptide with an estimated molecular weight of 34.6 kDa, and shares approximately 40% amino acid sequence identity with known type II metacaspases in plants. Quantitative reverse transcription-polymerase chain reaction analyses revealed that the expression of Camc9 was induced by
infections of Xanthomonas campestris pv. vesicatoria race 1 and race 3 and treatment with
methyl jasmonate. Suppression of Camc9 expression using virus-induced gene silencing enhanced
disease resistance and suppressed cell death symptom development following
infection with virulent bacterial pathogens. By contrast, overexpression of Camc9 by transient or stable transformation enhanced
disease susceptibility and pathogen-induced cell death by regulation of
reactive oxygen species production and defence-related gene expression. These results suggest that Camc9 is a possible member of the metacaspase gene family and plays a role as a positive regulator of pathogen-induced cell death in the plant kingdom.