ACD6, an important positive regulator of plant defense responses, is a pioneering member of a large protein family in Arabidopsis. There are over 30 ACD6-like genes, whose predicted protein products share similarities to ACD6 in amino acid sequence and protein domain organization. However, none of the ACD6-like genes has been characterized to date. Why are there so many ACD6-like genes? Are some of them redundant in function? Which ones, like ACD6, may also play a role in controlling innate immunity in plants? Of the many unanswered questions related to the ACD6 gene family, the one that interests us most is this last one. We are using a reverse genetic approach to isolate loss of function mutants defective in the ACD6-like gene functions, and we will test these mutants for resistance to Pseudomonas syringae infection. A mutant displaying enhanced disease susceptibility upon infection would identify the corresponding ACD6-like gene as necessary for disease resistance. Such an ACD6-like gene will be further studied to elucidate its mechanism in defense activation.