Biotransformation of the Phytoalexins Brassinin, Brassilexin and Camalexin by Alternaria brassicicola

Abstract

Chemical investigation of the transformation of the crucifer phytoalexins brassinin, brassilexin and camalexin by the phytopathogenic fungus Alternaria brassicicola was carried out. The objectives of this study included:

1) the isolation and characterization of the metabolites of biotransformation of brassinin, brassilexin and camalexin by A. brassicicola;

2) determination of the antifungal activity of these phytoalexins and their metabolites against A. brassicicola.

The phytoalexins were synthesized and characterized using HPLC retention time tR, 1H NMR, 13C NMR, LC-MS and HRMS-ESI data. The metabolites of the biotransformation were also synthesized and characterized similarly. The metabolism of each phytoalexin and their metabolites was studied by analyzing broth extracts by HPLC. The percent inhibition of growth of A. brassicicola was determined by radial growth mycelial assays.

The biotransformation of brassinin by A. brassicicola afforded Nb-acetyl-3-indolylmethylamine via indole-3-methylamine intermediate. Brassilexin was metabolized to 3-(amino)methyleneindoline-2-thione by the reduction of the isothiazole ring. Camalexin did not appear to be metabolized or the metabolism was very slow. The results of biotransformation and bioassay studies established that the metabolism of brassinin by A. brassicicola was a detoxification process. However, these studies using brassilexin did not provide a rigorous conclusion. Camalexin showed strong inhibition of growth against A. brassicicola suggesting its importance in defense against this pathogen.