S-adenosyl-L-methionine: Salicylic acid carboxyl methyltransferase (SAMT). An enzyme involved in floral scent and plant defense in Clarkia breweri

Plants employ scent compounds for a variety of functions such as attracting pollinators to their flowers and defending themselves from pathogen attack. Scent compounds may also play important roles in plant ecology and development by acting as volatile or endogenous signaling molecules capable of controlling cascades of reactions both within and between plants. Such reactions and the enzymes that catalyze them are likely intimately intertwined with primary metabolic functions and have surely contributed to the evolutionary success of plants. Methylsalicylate and its derivative floral scent volatiles play a widespread role in plant and insect ecology. Here I present a study of the enzyme that catalyzes the formation of methylsalicylate from salicylic acid in the flowering plant Clarkia breweri. Since methylsalicylate is a volatile emitted by flowers of C. breweri, I hypothesized that an enzymatic activity present in flowers would be involved in the conversion of salicylic acid to methylsalicylate. I describe the discovery of the enzyme that converts salicylic acid to methylsalicylate, designated S-adenosyl-L-methionine: salicylic acid carboxyl methyltransferase (SAMT) in various C. breweri flower parts. I describe the purification of SAMT from C. breweri flower petals, and its biochemical characterization. I also describe the cloning of a C. breweri cDNA encoding this enzyme. The crystal structure of the C. breweri SAMT has been determined, and critical active site residues involved in substrate binding and stabilization have been identified. Finally, one of the major unsolved questions in plant defense signaling centers around whether salicylic acid or methylsalicylate is the ultimate signaling molecule in the induction of defense gene expression. Therefore, I tested whether methylsalicylate could function independently of salicylic acid in the induction of gene expression in C. breweri. I present an mRNA differential display analysis of the variation in gene expression in C. breweri tissues after treatment with either salicylic acid or methylsalicylate. My results suggest that both salicylic acid and methylsalicylate regulate gene expression via the same mechanism.