Molecular basis of one-step methyl anthranilate biosynthesis in grapes, sweet orange, and maize
Plants synthesize an array of volatile compounds, many of which serve ecological roles in attracting pollinators, deterring herbivores, and communicating with their surroundings. Methyl anthranilate is an anti-herbivory defensive volatile responsible for grape aroma that is emitted by several agriculturally relevant plants, including citrus, grapes, and maize. Unlike maize, which uses a one-step anthranilate methyltransferase, grapes have been thought to use a two-step pathway for methyl anthranilate biosynthesis. By mining available transcriptomics data, we identified two anthranilate methyltransferases in Vitis vinifera (wine grape), as well as one ortholog in Concord grape. Many angiosperms methylate the plant hormone salicylic acid to produce methyl salicylate, which acts as a plant-to-plant communication molecule. Because the Citrus sinensis (sweet orange) salicylic acid methyltransferase can methylate both anthranilate and salicylic acid, we used this enzyme to examine the molecular basis of anthranilate activity by introducing rational mutations, which identified several active site residues that increase activity with anthranilate. Reversing this approach, we introduced mutations that imparted activity with salicylic acid in the maize anthranilate methyltransferase, which uncovered different active site residues from those in the citrus enzyme. Sequence and phylogenetic analysis revealed that one of the Vitis anthranilate methyltransferases shares an ancestor with jasmonic acid methyltransferases, similar to the anthranilate methyltransferase from strawberry (Frageria sp.). Collectively, these data demonstrate the molecular mechanisms underpinning anthranilate activity across methyltransferases and identified one-step enzymes by which grapes synthesize methyl anthranilate. Significance StatementWhile the two-step pathway responsible for the biosynthesis of the grape aroma molecule, methyl anthranilate, has remained incomplete in Vitis spp., we identified two one-step anthranilate methyltransferases in wine and one in Concord grapes that can methylate the tryptophan pathway intermediate anthranilate. Tracing the molecular basis of anthranilate activity in the maize and sweet orange methyltransferases uncovered distinct active site amino acids that impart substrate specificity.