The Rising of Acylsugar Diversity: Metabolic Innovation in Tomato Trichomes through BAHD Enzyme Promiscuity and Pathway Evolution

Abstract

In cultivated (Solanum lycopersicum) and wild tomatoes, glandular trichomes produce structurally diverse acylsugars. These insect protective metabolites are mixtures of aliphatic esters of sucrose and glucose, with acylchains varying in length from C2 to C11. We are analyzing acylsugar biosynthesis in the cultivated tomato, along with the biochemical and evolutionary mechanisms that generate acylsugar diversity across wild tomatoes (1, 2). Previous studies identified four BAHD family Acylsucrose Acyltransferase (ASAT) enzymes. These catalyze consecutive reactions from sucrose and acyl‐CoA substrates to produce the full set of cultivated tomato acylsucroses in vitro (3). Recently we identified a modified ‘upside down’ acylsugar pathway in the wild tomatoes S. pennellii and S. habrochaites that produces unusual types of acylsucroses, namely triacylsucroses with all three chains on the pyranose ring (P‐type) (4). This is in contrast with the furanose ring acylated acylsucroses (F‐type) produced in cultivated tomatoes and a variety of wild tomato species. Biochemical analysis demonstrates that the acyl acceptor substrate specificities were changed in two S. pennellii ASAT orthologous enzymes, which reversed their enzymatic activity order, leading to the ‘flipped pathway’. A phylogeny‐based structure‐function analysis of the ASAT2 and ASAT3 enzymes led to identification of a small number of amino acid changes that switch their activities between the F‐ and P‐ acylsucrose pathways (4, 5). Investigation of these pathways in other tomato species allowed inference of the evolutionary events giving rise to divergence of F‐ and P‐type acylsucroses across tomato species. This work demonstrates multiple evolutionary mechanisms in biochemical pathway evolution, including gene duplication and loss, amino acid substitution, and how the emergence of enzyme promiscuity restructured a specialized metabolic pathway and led to metabolic product innovation.Support or Funding InformationThis work was funded by National Science Foundation grants IOS‐1025636 and IOS‐PGRP‐1546617 to A.D.J. and R.L.L. Abigail Miller was supported by NSF REU grant DBI‐1358474 in the summer of 2014 and an American Society of Plant Biologists Summer Undergraduate Research Award during the summer of 2015. A.D.J. acknowledges support from the USDA National Institute of Food and Agriculture, Hatch project MICL‐02143.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.