What do penicillin, aflatoxin, and morphine have in common? They are all secondary metabolites, the first two produced by fungi, the last by plants. Secondary metabolites are compounds that are not essential for primary growth, but are produced by all organisms. Each species has a pallet of secondary metabolites that are unique to that species. In secondary metabolism, the benefit to the host organism is not alway clear. Fusarium graminearum produces a vast array of secondary metabolites. We have worked on the polyketides and mutated all 15 polyketide synthase genes to find out what each does. These compounds include pigments and mycotoxins. In the photos across the top, we see the fruiting bodies of F. graminearum. In the center, the perithecia are black, sitting on red hyphae. This is the wild-type strain in culture. If we mutate the polyketide synthase gene that makes the black pigment, we get white perithecia (on the left). If we mutate the gene for the red pigment (aurofusarin), we get flat black perithecia on a black background (on the right).
We have determined the products for 6 of the 15 polyketide synthase genes. Two of them are used to synthesize the mycotoxin zearalenone, which is an estrogen mimick, and a common contaminant of grain. We are now working on determining how zearalenone, aurofusarin and the other polyketides help F. graminearum to survive in the field.