Novel genes of the plant defense response pathway in Arabidopsis thaliana

The plant defense response to bacteria] pathogens is an active process that is initiated by plant cells that have encountered and recognized the presence of invading bacteria. The plant defense response pathway is induced by a complex and intricate balance of signals which are only now being identified and their importance being assessed. One key signal required for many aspects of plant defense is the phenolic molecule salicylic acid (SA). It has previously been shown that SA is required for some defenses (i.e. bacterial resistance and the induction of pathogenesis related (PR) genes). For other defenses, including programmed cell death, it has been shown to be necessary but not sufficient to induce these defenses.;Two Arabidopsis mutants of the defense response pathway, acd6 (a&barbelow;ccelerated c&barbelow;ell d&barbelow;eath 6) and agi2 (a&barbelow;berrant G&barbelow;ST1 i&barbelow;nduction 2), were identified for their constitutive activation of defenses in the absence of pathogenic stimulation. These two novel mutations act at different locations in the defense response pathway. Both mutations play a role in the induction of defenses; however, the way they interact with SA to induce these defenses is also different. Genetic and physiological experiments show that in acd6 SA is required for all defenses but acts with a second signal to induce a subset of defenses. ACD6 encodes a membrane-associated ankyrin repeat protein suggesting it plays an important role in signal transduction. In contrast to acd6, only a subset of agi2 defenses show a SA requirement. In addition to the defense phenotypes, agi2 showed abnormal cell expansion. An unexpected finding was that SA depletion in agi2 exacerbated this cell expansion defect. This abnormal cell expansion may mimic certain pathogenic infections and cause SA-dependent responses. The role of SA may include not only the defense response pathway, but pathways in which SA has not been previously implicated, such as a role in cell growth and expansion.