Detoxification of Mycotoxins In Planta as a Strategy for Improving Grain Quality and Disease Resistance: Identification of Fumonisin-Degrading Microbes from Maize

Fumonisins are mycotoxins produced by Fusarium moniliforme and several related species associated with ear molds in maize. They consist of a long chain amine alcohol with two ester-linked tricarballylate groups, and their biological activity is likely through disruption of sphingolipid biosynthesis. Fumonisins are of concern due to their toxicity to livestock (especially horses) and possible carcinogenicity, and are currently under review for possible regulation by the United States Food and Drug Administration. Fusarium moniliforme is almost ubiquitous in field-grown maize, so the impact of this toxin on the food chain is potentially quite large. Fumonisins are also phytotoxic and some evidence links them to virulence of maize isolates of F. moniliforme. As a first step towards engineering maize to detoxify fumonisins, we have isolated microbes from field-grown, moldy maize kernels and stalk tissue, that are capable of growing on fumonisin B1 as a sole carbon source. Two species, identified as Exophiala spinifera and Rhinocladiella atrovirens, belong to the “black yeasts” found widely in plant debris. Interestingly, several E. spinifera isolates, collected outside the US. from non-maize sources, were also found to metabolize fumonisins. A gram (-) bacterium from stalk tissue, isolate 2412.1, also metabolized fumonisin in liquid culture. Both fungal and bacterial isolates produce 14CO2 from uniformly labeled fumonisin, indicating extensive metabolism of this important toxin. A soluble esterase activity capable of hydrolyzing fumonisin tricarballylate esters can be recovered from culture supernatants or lysates of these microbes. A soluble, heat-labile activity resulting in loss of a free primary amine group from AP1 has also been detected in cell lysates of E. spinifera 2141.10. Work is in progress to clone genes corresponding to these fumonisin degradative enzymes and express them in transgenic maize.