Co-suppression of NbClpC1 and NbClpC2, chaperone subunits in the Clp protease complex, accelerates hypersensitive response and increases disease susceptibility in Nicotiana benthamiana

Abstract

The Clp protease complex, which is the central protein degradation machinery in plastids, plays a role similar to proteasome. ClpC1 and ClpC2 comprise the chaperonic part of the Clp protease and share more than 90% nucleotide sequence similarity with each other. In a previous study, the co-suppression of Nicotiana benthamiana ClpC1 and ClpC2 (NbClpC1/C2) resulted in leaf chlorosis, growth retardation phenotype, disappearance of apical dominance and coarsely packed mesophyll cells. Quantitative real-time PCR analysis revealed that the co-suppression of NbClpC1/C2 upregulated the expression levels of defense-related genes including pathogenesis-related protein 1b (PR1b), glutathione-S-transferase, catalase, ascorbate peroxidase and systemic acquired resistance gene 8.2 (SAR8.2). NbClpC1/C2 co-suppressed leaves accelerated hypersensitive response by the infection with an incompatible pathogen, Pseudomonas syringae pv. tomato T1, but increased susceptibility via the infiltration of a compatible pathogen, P. syringae pv. tabaci. These findings indicate that disruption of the chaperones of ClpC1 and ClpC2 increases the growth of bacterial pathogens and therefore increases hypersensitive response and disease susceptibility.