PURIFICATION AND CHARACTERIZATION OF A SOLUBLE SALICYLIC ACID-BINDINGPROTEIN FROM TOBACCO

Previously, we identified a soluble salicylic acid (SA)-binding protei n (SABP) in tobacco whose properties suggest that it may play a role i n transmitting the SA signal during plant defense responses. This SA-b inding activity has been purified 250-fold by conventional chromatogra phy and was found to copurify with a 280-kDa protein. Monoclonal antib odies capable of immunoprecipitating the SA-binding activity also immu noprecipitated the 280-kDa protein, indicating that it was responsible for binding SA. These antibodies also recognized the 280-kDa protein in immunoblots of the partially purified SABP fraction or the crude ex tract. However, when the crude extract was prepared in the presence of antioxidants, only a 57-kDa protein was recognized. Since the SABP ha s a native molecular mass of 240 kDa, it appears that the SABP is a co mplex which contains a 57-kDa subunit and perhaps one or more addition al proteins which are covalently crosslinked in the absence of antioxi dants. The ability of a variety of phenolic compounds to compete with SA for binding to the SABP was both qualitatively and quantitatively c orrelated with their biological activity in inducing defense-related g enes. Moreover, the inducibility of the pathogenesis-related (PR)-1 ge nes by SA was proportional to the abundance of the SABP in different o rgans. These correlations are consistent with a role for the SABP in p erceiving and transducing the SA signal in plant defense.