Induction of Salicylic Acid β-Glucosidase in Tobacco Leaves by Exogenous Salicylic Acid

Salicylic acid (SA) has been proposed to be an endogenous signal for systemic acquired resistance to infection by pathogens in plants. In general, most SA is found in an inactive form as SA β-glucoside (SAG). SAG seems to be a storage form of SA from which bioactive SA can be generated. Recent reports indicate that β-glucosidase might be involved in regulating the signaling activity of phytohormones. Therefore, it seems likely that SA β-glucosidase, the enzyme that hydrolyzes SAG to yield free SA, might also play an important role by regulating the level of free SA. Since hydrolysis of SAG seems to occur in intercellular spaces, we attempted to isolate SA β-glucosidase activity from the intercellular spaces of SA-treated tobacco leaves, where we found considerable amounts of the enzymatic activity. Furthermore, increased levels of SA and SA β-glucosidase activity were found in the leaves after treatment with exogenous SA. The role of SA β-glucosidase in plant defense systems is discussed.