The gain-of-function Arabidopsis acd6 mutant reveals novel regulation and function of the salicylic acid signaling pathway in controlling cell death, defenses, and cell growth.

We isolated a dominant gain-of-function Arabidopsis mutant, accelerated cell death 6 (acd6), with elevated defenses, patches of dead and enlarged cells, reduced stature, and increased resistance to Pseudomonas syringae. The acd6-conferred phenotypes are suppressed by removing a key signaling molecule, salicylic acid (SA), by using the nahG transgene, which encodes SA hydroxylase. This suppression includes phenotypes that are not induced by application of SA to wild-type plants, indicating that SA acts with a second signal to cause many acd6-conferred phenotypes. acd6-nahG plants show hyperactivation of all acd6-conferred phenotypes after treatment with a synthetic inducer of the SA pathway, benzo(1,2, 3)thiadiazole-7-carbothioic acid (BTH), suggesting that SA acts with and also modulates the levels and/or activity of the second defense signal. acd6 acts partially through a NONEXPRESSOR OF PR 1 (NPR1) gene-independent pathway that activates defenses and confers resistance to P. syringae. Surprisingly, BTH-treated acd6-nahG plants develop many tumor-like abnormal growths, indicating a possible role for SA in modulating cell growth.