STRUCTURE-DEPENDENT PHYTOTOXICITY OF FUMONISINS AND RELATED-COMPOUNDS IN A DUCKWEED BIOASSAY

Fumonisins A1, A2, B1, B2, and B3 are a series of secondary metabolite analogues produced by Fusarium moniliforme. The A series fumonisins differ from B series by possession of a N terminal acetyl group. Hydrolytic removal of the propanetricarboxylic acid moieties from fumonisins B1 and B2 yields the aminoalcohols HB1 and HB2, respectively. AAL-toxin is a chemically related phytotoxin (only one propanetricarboxylic acid) produced by Alternaria alternata. In a duckweed (Lemna pausicostata) bioassay, AAL-toxin and the B series fumonisins at 1 muM caused pronounced cellular leakage of electrolytes and photodegradation of chlorophylls. These four compounds also caused the most marked reductions in duckweed growth. HB1 at 1 muM was a moderate growth inhibitor and caused a low level of cellular leakage. The other compounds were inactive at this concentration. Significant effects on cellular leakage were measured at 0.04 muM for both AAL-toxin and fumonisin B1, the two most active analogues. The propanetricarboxylic acid groups of fumonisins and AAL-toxin are necessary for significant herbicidal activity of this series of compounds, whereas acetylation of the terminal amine group greatly reduces their activity. The structurally related sphingolipids, phytosphingosine and sphingosine, were about two orders of magnitude less phytotoxic than fumonisin B1; however, the phytotoxicity symptoms were similar.