Phytotoxicity of Australifungin, AAL-toxins and fumonisin B₁ to Lemna pausicostata

AAL-toxins and fumonisin B₁ are phytotoxic to susceptible plants by inhibiting the enzyme ceramide synthase. Australifungin, which is structurally unrelated to these toxins, inhibits the same enzyme in the sphingolipid biosynthesis pathway of animals. In duckweed (Lemna pausicostata) cultures, 5 μM australifungin caused accumulation of the sphingolipid precursors, phytospingosine and sphinganine, although less so than AAL-toxin T(A) or FB₁ at 1 μM. Phytosphingosine and sphinganine began to accumulate after 12 h, followed by increased electrolyte leakage at 24 h. Electrolyte leakage with 5 μM australifungin was somewhat less than 1 μM fumonisin B₁ or AAL-toxin T(A). Morphological effects were not identical; fumonisin B₁ and AAL-toxin T(A) mainly caused bleaching, while australifungin caused clumping of duckweed fronds. Our study shows that australifungin inhibits sphingolipid synthesis in plants, but is about eight times less potent than AAL-toxin T(A) or fumonisin B₁.