Pathogen infection induces an array of sophisticated defense responses in plants, including gene expression reprogramming and activation of defense signal transduction. Among thousands of genes affected by pathogen infection, only a few are known to play a direct role in effecting plant defense while most are only diagnostic for defense responses. The major challenge in the field is to identify defense-related genes, decipher their roles in defense regulation, and determine how they interact to coordinate a complex defense network.

Salicylic acid (SA) is a small phenolic compound that plays a central role in defense signaling. However, it is not well understood how SA mediated signaling is generated and transduced. ACCELERATED CELL DEATH 6 (ACD6), encoding a plasma membrane protein with cytoplasmic ankyrin repeats, is one of a few genes known to regulate SA-mediated defense in Arabidopsis. acd6-1, a gain of function mutant caused by an amino acid substitution in the transmembrane domain of ACD6, exhibits constitutive defense and dwarf phenotypes. Interestingly, reduced SA levels and/or blocked SA signaling renders the acd6-1 plants more susceptible to pathogen infection but larger in size. This defense dependent size change makes acd6-1 a powerful tool in a genetic study to identify new mutants affected for SA-mediated defense, and hence novel defense related genes. Based on this unique feature of acd6-1, we have used both forward and reverse genetic approaches combining molecular and cell biology methods to uncover and characterize previously unknown defense-related genes in Arabidopsis.