Elicited metabolites from Brassicaceae species and Tropaeolum majus: isolation, structure determination and antifungal activity

Abstract

The order Brassicales contains several economically important plant families of great agricultural value. Species of the families Brassicaceae and Tropaeolaceae are potential sources of novel defenses and antifungal metabolites. Due to the important roles played by chemical defenses such as phytoalexins, it is of interest to find new sources of phytoalexins. Toward this end, the elicited metabolites and phytoalexins produced in elicited leaves of the plant species Lepidium sativum, Neslia paniculata, Barbarea vulgaris, Isatis glauca, Isatis tinctoria and Tropaeolum majus were investigated. Phytoalexins are naturally occurring antimicrobial metabolites produced de novo by plants in response to stress, including microbial attack. The chemical structures of the isolated metabolites were elucidated by analyses of their spectroscopic data and, for some, were confirmed by syntheses. Several elicited metabolites that belong to different structural groups were isolated. Evaluation of the antifungal activity of the elicited metabolites showed that: (1) 7-methoxycamalexin (125) and 6,7-dimethoxycamalexin (124) from N. paniculata are new phytoalexins; (2) nasturlexin C (23), nasturlexin D (24), nasturlexins C sulfoxide (25) and nasturlexin D sulfoxide (26) are new phytoalexins from B. vulgaris; (3) tropalexins A and B (140, 141) are new phytoalexins from T. majus. All these phytoalexins displayed antifungal activity against the economically important crucifer pathogens Alternaria brassicicola, Leptosphaeria maculans and Sclerotinia sclerotiorum. Furthermore, tryptanthrin (80) and indirubin (81) are phytoanticipins from I. glauca and I. tinctoria that display antifungal activity against the pathogens A. brassicicola, L. maculans and S. sclerotiorum. Additionally, several known metabolites from these plant species were isolated and identified.

Moreover, the biosynthesis of new phytoalexins from T. majus was investigated using stable isotope labelling. Isotopically labeled and non-labeled benzylisothiocyanates, benzyldithiocarbamates, 4-hydroxybenzylisothiocyanates and 4-hydroxybenzyldithiocarbamates were synthesized. The synthetic compounds were administered to leaves of T. majus, incubated, extracted and analyzed by HPLC-DAD-ESI-MS and the isotope incorporations into each phytoalexin was calculated. Based on the results of the incorporation, the biosynthetic pathway of these phytoalexins is proposed.