Purification, chemical characterization and host-specificity of the toxin produced by Alternaria brassicae

Abstract

A chromatographic procedure was developed to purify the toxin produced in culture by Alternaria brassicae. The procedure included gel filtration, adsorption on charcoal, silicic acid chromatography and high pressure liquid chromatography. Fast atomic bombardment spectrum of the purified toxin showed protonated molecular ion peak at relative intensity 594. High resolution mass spectrometry gave an exact mass of 593·3791 with a molecular formula C30H51N5O7 (calculated molecular weight = 593·3788). Negative ninhydrin reaction of the toxin and the presence of amino acids in the toxin hydrolysate indicated that the toxin is a cyclic peptide. Molecular weight, ninhydrin reaction, amino acid composition, proton nuclear magnetic resonance spectrum, HPLC retention time, and symptomatology and activity of the toxin on hosts and non-hosts of A. brassicae were identical to those of Destruxin B. Both the fungus and the toxin caused symptoms of different severities on different brassicas, ranging from severe chlorosis and necrosis to almost no visible chlorosis. The order of sensitivity of different brassicas to the toxin was similar to their order of susceptibility to A. brassicae. The toxin was also able to distinguish susceptibility differences within the species Brassica campestris. It did not cause symptoms on nine plant genera which are non-hosts of A. brassicae. The degrees of toxin sensitivity of all plants tested were correlated with their degrees of susceptibility to A. brassicae, suggesting that the activity of the toxin is host-specific. This is the first report on the host-specific nature of a toxin produced by A. brassicae. © 1987.