Phytoalexins and other Elicited Metabolites from Crucifers: Structures, Syntheses and Biosyntheses

Abstract

This thesis focuses on the isolation, syntheses and biosyntheses of phytoalexins and other elicited metabolites from crucifers. Elicitation of plant defense responses increases amounts of numerous constitutive secondary metabolites and induces the de novo production of a group of defense chemicals called phytoalexins. The thesis is divided into four chapters, as described below. Chapter I presents a literature review of topics related to my research work. First, structures and activities of elicited metabolites, including phytoanticipins, phytoalexins, and oxylipins, are discussed. Second, elicitors of plant chemical defenses, including biotic and abiotic elicitors, are reviewed. Finally, recent progress on metabolic engineering of chemical defense pathways, including glucosinolates, cyanogenic glycosides, and phytoalexins, is presented. Chapter II covers my research results, which are divided into three sections. The first section focuses on the investigation of the biosynthetic pathway of the phytoalexin rapalexin A (102). Efforts towards the synthesis of glucorapassicin (118), a glucosinolate previously proposed as the biosynthetic precursor of rapalexin A (102), are reported. Deprotections of N-protected derivatives of glucorapassicin (118) under acidic and basic conditions yield Lossen- and Neber-type rearrangement products, respectively. Based on these results, a biomimetic synthesis of rapalexin A (102) is developed. Additional biosynthetic experiments using an N-protected derivative of glucorapassicin (118) strongly support the hypothesis that rapalexin A (102) is biosynthetically derived from glucorapassicin (118). The second section is about the isolation, syntheses and antifungal activities of phytoalexins and other elicited metabolites from crucifers. Several new phytoalexins from the crucifers Nasturtium officinale and Barbarea verna, including cyclonasturlexin (78), an indole-fused thiazepine scaffold, and nasturlexins, a group of first non-indolyl phytoalexins, are reported. In addition, two groups of elicited metabolites, including phytoalexin and oxylipins are isolated from elicited leaves of the wild crucifer Erucastrum canariense. Tryptanthrin (263), a known indole alkaloid is isolated from elicited leaves of Isatis indigotica and found to be a phytoalexin in this species. The syntheses and antifungal activities of all new metabolites are reported. The third section presents results on the biosyntheses of new phytoalexins in N. officinale and B. verna. First, different isotope-labelled potential precursors and intermediates are synthesized, including glucosinolates, isothiocyanates, dithiocarbamates and nasturlexins. Next, these labelled compounds are administered to the plants and leaf extracts are analyzed by HPLC-DAD-ESI-MS. Finally, based on deuterium incorporations into phytoalexins, the biosynthetic precursors and intermediates of new phytoalexins are proposed. Chapter III discusses overall results and proposes future work arising from these findings. Chapter IV reports methods and experimental procedures used to obtain the data presented in this thesis. Characterization data of new compounds and other supplementary materials are also presented. In summary, a group of novel phytoalexins are discovered and their biosynthetic intermediates are established. In addition, the hypothesis on the biosynthetic pathway of rapalexin A (102) is also supported.