The role of membrane-bound ankyrin-repeat protein ACD6 in programmed cell death and plant defense

Programmed cell death (PCD) is a common defense response in plants against pathogen infection. The recently cloned ACD6 gene was identified in an Arabidopsis mutant, accelerated cell death 6 (acd6), that undergoes PCD in the absence of a pathogen challenge. ACD6 is a founding member of a large family of genes that encode proteins with a short amino-terminal region, nine ankyrin repeats in the middle, and five putative transmembrane domains in the carboxyl-terminal region. Characterization of the original gain-of-function acd6 mutant and a transferred-DNA knockout mutant acd6-T showed that ACD6 is an activator of the defense pathway against bacterial pathogens and plays a role in PCD through regulation of the defense signal salicylic acid (SA). SA mediates not only downstream pathogenesis-related (PR) genes, which encode proteins with antimicrobial activities, but also ACD6, forming a feedback signal amplification loop.