| Fusarium verticillioides is a ubiquitous fungal pathogen of agriculturally important commodities throughout the world. During the colonization of maize (Zea mays) kernels, F. verticillioides produces toxic secondary metabolites known as fumonisins. Fumonisins cause a range of species-specific health effects when ingested and are suspected to cause cancer and birth defects in humans. Despite enormous economic impacts associated with fumonisin contamination of grain, very little is known about how fumonisin biosynthesis is regulated during kernel colonization. My research focused on elucidating the interaction between F. verticillioides and maize that results in the accumulation of fumonisins in maize kernels. To this end, I identified FCK1, a gene encoding a cyclin-dependent kinase in F. verticillioides. Disruption of FCK1 resulted in pleiotropic morphological defects, including reduced growth, conidiation, fumonisin B1 (FB 1) production, and enhanced pigmentation. Additionally, I identified amylopectin, a component of maize kernel starch, as a key inducer of FB 1 biosynthesis during kernel colonization. Kernels lacking starch due to physiological immaturity did not accumulate FB1, and a mutant strain of F. verticillioides with a disrupted alpha-amylase gene was impaired in its ability to produce FB1 on kernels containing starch. Both the wild-type and mutant strains produced significantly less FB1 on a high-amylose kernel mutant of maize. When grown on a defined medium with amylose as the sole carbon source, the wild-type strain produced only trace amounts of FB1, but it produced large amounts of FB 1 when grown on amylopectin or dextrin, a product of amylopectin hydrolysis. Finally, I determined that disruption of ZFR1, a putative transcriptional regulator, blocked the induction of fumonisin biosynthesis by amylopectin and affected the transcription of genes involved in starch hydrolysis and carbohydrate uptake. From these findings, a new model has been developed to explain how FB1 biosynthesis is regulated during kernel colonization. The model highlights new avenues to unravel the complex interactions underlying colonization of maize kernels by F. verticillioides and could give rise to innovative strategies to control fumonisin accumulation in maize. |
