IDENTIFICATION OF A SOLUBLE, HIGH-AFFINITY SALICYLIC ACID-BINDING PROTEIN IN TOBACCO

Salicylic acid (SA) is a key component in the signal transduction path way(s), leading to the activation of certain defense responses in plan ts after pathogen attack. Previous studies have identified several pro teins, including catalase and ascorbate peroxidase, through which the SA signal might act. Here we describe a new SA-binding protein. This s oluble protein is present in low abundance in tobacco (Nicotiana tabac um) leaves and has an apparent molecular weight of approximately 25,00 0. It reversibly binds SA with an apparent dissociation constant of 90 nm, an affinity that is 150-fold higher than that between SA and cata lase. The ability of most analogs of SA to compete with labeled SA for binding to this protein correlated with their ability to induce defen se gene expression and enhanced resistance. Strikingly, benzothiadiazo le, a recently described chemical activator that induces plant defense s and disease resistance at very low rates of application, was the str ongest competitor, being much more effective than unlabeled SA. The po ssible role of this SA-binding protein in defense signal transduction is discussed.