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 cel l death 6 (acd6), with elevated defenses, patches of dead and enlarged cell s, reduced stature, and increased resistance to Pseudomonas syringae, The a cd6-conferred phenotypes are suppressed by removing a key signaling molecul e, salicylic acid (SA), by using the nahG transgene, which encodes SA hydro xylase. This suppression includes phenotypes that are not induced by applic ation of SA to wild-type plants, indicating that SA;acts with a second sign al to cause many acd6-conferred phenotypes. acd6-nahG plants show hyperacti vation of all acd6-conferred phenotypes after treatment with a synthetic in ducer 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 I (NPR1) gene-independent pathway that activates defenses and confers r esistance to P. syringae, Surprisingly, BTH-treated acd6-nahG plants develo p many tumor-like abnormal growths, indicating a possible role for SA in mo dulating cell growth.