Involvement of ZFR1 of Fusarium verticillioides in kernel colonization and the regulation of FST1, a putative sugar transporter gene required for fumonisin biosynthesis on maize kernels

Fumonisins comprise a class of carcinogenic mycotoxins produced by Fusarium verticillioides during colonization of maize kernels. In previous work, we identified ZFR1, which is predicted to encode a Zn(II) 2Cys6 zinc finger transcription factor required for fumonisin B-1 (FB1) production during growth on kernels. In this study, we characterized the role of ZFR1 in colonizing maize kernels and inducing FB1 biosynthesis. The ZFR1 deletion strain (Delta zfr1) grew approximately 2.5-fold less than the wild-type on endosperm tissue and a variety of other carbon sources, including glucose and amylopectin. However, the Delta zfr1 strain displayed higher alpha-amylase activity and expression of genes involved in starch saccharification than the wild-type, thus indicating that the reduced growth of the Delta zfr1 strain was not due to inhibition of amylolytic enzymes. In the wild-type strain, expression of six genes encoding putative sugar transporters was significantly greater on endosperm tissue than on germ tissue, and expression of at least three of the six genes was negatively affected by disruption of ZFR1. Intriguingly, disruption of FST1 had no effect on growth, kernel colonization or kernel pH but decreased FB1 production by approximately 82% on maize kernels. Based on these findings, we hypothesize that ZFR1 controls FB1 biosynthesis by regulating genes involved in the perception or uptake of carbohydrates.