It has recently been demonstrated that salicylic acid (SA) may serve as an endogenous signal molecule in the induction of systemic acquired resistance in tobacco and cucumber. In addition, SA is an endogenous regulator of heat and odor production in the inflorescence of some thermogenic plants. No information, however, is currently available concerning the mode(s) of action of SA in plant signal transduction. In a search for possible cellular factors that directly interact with SA, we have detected and partially characterized a SA-binding protein in tobacco leaves. The SA-binding activity is both SDS and proteinase sensitive and behaves as a soluble protein with an apparent mass of 650 kDa. The protein has an apparent Kd of 14 microM for SA, which is consistent with the range of physiological concentrations of SA observed for the induction of plant resistance responses. Furthermore, the ability of SA analogues to compete with SA for binding to this soluble protein is strictly correlated with their biological activity to induce the expression of genes associated with disease resistance. Biologically active analogues effectively inhibit SA binding while biologically inactive analogues do not. These results collectively indicate that this SA-binding protein may play a role in perceiving and transducing the SA signal to appropriate response elements, which ultimately activate one or more of the plant disease-resistance responses.