Comparison of fumonisin B1 biosynthesis in maize germ and degermed kernels by Fusarium verticillioides

Fusarium verticillioides produces a group of mycotoxins known as fumonisins in maize kernels. Fumonisins are associated with a variety of mycotoxicoses in humans and animals; thus, their presence in food is a considerable safety issue. This study addressed fumonisin B1 (FB1) production in two components of the maize kernel, namely the germ tissues and the degermed kernel. Growth of F. verticillioides was similar in colonized germ tissue and degermed kernels, but FB1 production was at least five times higher in degermed maize kernels than in germ tissue. Expression of the fumonisin polyketide synthase gene, FUM1, as measured by beta-glucuronidase (GUS) and Northern blot analysis, followed the same pattern as FB1 production. Also correlated to FB1 was a concomitant drop in pH of the colonized degermed kernels. A time course experiment showed that degermed kernels inoculated with F. verticillioides became acidified over time (from pH 6.4 to 4.7 after 10 days of incubation), whereas colonized germ tissue became alkaline over the same period (from pH 6.5 to 8.5). Because conditions of acidic pH are conducive to FB1 production and alkaline pH is repressive, the observed correlation between the acidification of degermed kernels and the increase in FB1 provides one explanation for the observed differences in FB1 levels.