| Glucosinolates are secondary metabolites found throughout the Brassicales, including the model plant Arabidopsis thaliana, which play an important role in defending the plants against a variety of pests, as well as influencing the flavor and nutritional benefits of many vegetables and spices. Most biological activities associated with these phytochemicals are due to the products of catabolic activation by the myrosinase enzyme system. Myrosinase catalyzed glucosinolate hydrolysis generates an unstable intermediate which rearranges into the product structures, isothiocyanates, simple nitriles, epithionitriles, or thiocyanates. The structural outcome of glucosinolate activation depends on the particular structural specifier proteins present. In a study of six accessions of Arabidopsis, I show that structural outcomes are controlled by interactions among plant age, planting density, tissue type and natural genetic variation. Using mutants previously identified as affecting glucosinolate activation in rosette leaves, I demonstrate the requirement for additional myrosinases and accessory proteins to explain the structural outcomes observed in roots and seedlings. This work provided evidence for a novel simple nitrile structural specifier protein. In a study of 411 Bayreuth by Shadara recombinant inbred lines at three different ages, I identified several QTL, QTL by age interactions, and a large epistatic network centered on epithiospecifier locus. Analysis of contrasting inbred lines validated eight loci and all of the QTL x age interactions tested. The identified structural outcomes QTL showed extensive collocation with gene expression QTL mapped in this population. Finally, I show that two of the QTL are the result of whole-genome duplications of a glucosinolate activation gene cluster. This work reveals complex age dependent regulation of structural outcomes and suggests that transcriptional regulation is associated with a significant portion of the ontogenic variation and the epistatic interactions controlling structural outcomes. As a result of this work, as well as several other recent developments, the structural outcomes of glucosinolate activation is rapidly becoming one of the best described quantitative traits, and presents several opportunities to address long standing questions about the molecular basis and evolution of natural variation in complex traits. |
