Substrate Preference and Function of Salicylic Acid Methyltransferase in Dicots

Abstract

Salicylic acid carboxyl methyltransferase (SAMT) and benzoic acid carboxyl methyltransferase (BAMT) are enzymes in plants which catalyze the methylation of small carboxylate containing molecules: salicylic acid (SA) and benzoic acid (BA). Both of these small molecules function in various essential plant pathways. These pathways aid in critical functions, such as plant defense against abiotic and biotic stressors, and in pollinator attraction. The evolution of SAMT from BAMT was positively selected for to increase the specificity and control of crucial pathways. The functionality of SAMT has been maintained across studied descendant families of dicots. RNA extracted from Daucus carota subsp. Sativus, Ipomea alba, Viola mandshurica, and Punica granatum was used in enzyme functional assays in order to determine SAMT substrate preference. The SAMT enzymes in these previously unstudied families were all found to prefer SA over BA despite variation in the active site. This preference matches those of the SAMT enzymes in other experimentally-determined dicots. The maintenance of the SAMT gene and substrate preference of SA over BA across families of dicots suggests that SAMT does not lower organism fitness. This suggests that the increased specialization of enzymes for SA and BA methylation is advantageous in plant fitness. Understanding how best to aid plant fitness may provide insights in how best to protect produce naturally and effectively.