Enzyme Functional Evolution of Single Amino Acids in Beta Genus (Sugar Beet) Promotes Multiple Differential Specialized Enzymes

Abstract

Beta vulgaris (sugar beet) has been selectively cultivated for hundreds of years to produce traits such as more effective disease resistance. Volatiles produced from plant secondary metabolism are important for many types of ecological interactions, including defense responses. Salicylic acid (SA) and its volatile methyl ester, methyl salicylate (MeSA), play an important role in defense responses. The SABATH gene family is a family of methyltransferases (MTs), specifically, salicylic acid methyltransferases (SAMT), that have been shown to have a high catalytic efficiency for salicylic acid. However, SAMTs can be involved in the formation of both MeSA and methyl benzoate (MeBA). The purpose of this study was to test the hypothesis that the target amino acid, histidine (H), tyrosine (Y) or methionine (M) present before the conserved WLS, tryptophan (W), leucine (L), serine (S) amino acid sequence will determine which substrate, SA or benzoic acid (BA), will be methylated by SAMT. It was also hypothesized that because Beta vulgaris contains multiple copies of SAMT-like genes, which are SAMT-like duplicate genes characterized by possessing the same target amino acid before the WLS sequence, that each copy would exhibit specialization of substrate preference.