INDUCTION OF SALICYLIC-ACID BETA-GLUCOSIDASE IN TOBACCO-LEAVES BY EXOGENOUS SALICYLIC-ACID

Salicylic acid (SA) has been proposed to be an endogenous signal for s ystemic acquired resistance to infection by pathogens in plants. In ge neral, most SA is found in an inactive form as SA beta-glucoside (SAG) . SAG seems to be a storage form of SA from which bioactive SA can be generated. Recent reports indicate that beta-glucosidase might be invo lved in regulating the signaling activity of phytohormones. Therefore, it seems likely that SA beta-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 interc ellular spaces, we attempted to isolate SA beta-glucosidase activity f rom the intercellular spaces of SA-treated tobacco leaves, where we fo und considerable amounts of the enzymatic activity. Furthermore, incre ased levels of SA and SA beta-glucosidase activity were found in the l eaves after treatment with exogenous SA. The role of SA beta-glucosida se in plant defense systems is discussed.