Regulation of secondary metabolism in phytophathogenic Fusarium spp.

The members of the genus Fusarium are among the most important plant pathogens in the world. Their metabolites are highly complex, often bioactive and therefore associated with virulence on the respective host plant, but can also be harmful to humans and animals when consumed. As Fusarium spp. are aggressive pathogens of agricultural plants, they can cause mycotoxin contamination on cereal grains and other plant-based foods, thereby impairing human and animal health. Fusarium fujikuroi is a notorious pathogen of rice, causing super-elongation, and thereby death of the plants due to the production of the planthormones gibberellic acids. Besides gibberellic acids, F.fujikuroi is able to produce mycotoxins like bikaverin, fumonisin, fusarin C and most likely even more so far unidentified bioactive substances. Although detection of these mycotoxins is of major importance to ensure food and feed safety, understanding the genetic regulation leading to their production is a critical step towards effective and trendsetting pest management. Chapter I will give a general overview about the genus Fusarium with special focus on secondary metabolism and its most common regulation principles. In Chapter II the gene cluster responsible for the production of the cytotoxic secondary metabolite bikaverin has been studied. Besides identification of the first intermediate of the pathway by NMR and MS techniques, several new regulatory mechanisms were identified. Expression studies revealed that all bik genes are repressed by high amounts of nitrogen in an AreA‑independent manner and are subject to a time- and pH‑dependent regulation. Deletion of the pH regulatory gene pacC resulted in partial derepression while complementation with a dominant active allele resulted in repression of bik genes at acidic ambient pH. In Chapter III, a global regulatory network controlling secondary metabolite production, sexual and asexual development and virulence has been characterized in F. fujikuroi. This recently identified Velvet complex containing VeA, the histone methyltransferase LaeA, and several other regulatory proteins is unique to fungi and was shown to be involved in global regulation of secondary metabolism and differentiation in the model organism Aspergillus nidulans. In the course of this work two components of the F. fujikuroi Velvet-like complex, FfVel1 and FfLae1, were characterized. The gene Fflae1 encoding the first reported LaeA ortholog outside the class of Aspergillus-related fungi (Eurotiomycetidae) was found to be upregulated in the ΔFfvel1 knock-out mutant in microarray-studies, and FfLae1 was shown to interact with FfVel1 in the nucleus. Deletion of Ffvel1 and Fflae1 revealed for the first time that Velvet can simultaneously act as positive (gibberellic acids, fumonisins and fusarin C) and negative (bikaverin) regulator of secondary metabolism, and that both components affect conidiation and virulence on rice of F. fujikuroi. Cross genus complementation studies of Velvet complex component mutants between Fusarium, Aspergillus and Penicillium support an ancient origin for this complex which has undergone a divergence in specific functions mediating development and secondary metabolism. In Chapter IV, the investigations were extended on light regulation mechanisms in F. fujikuroi. Light regulation controls essential protection mechanisms of fungi when exposed to light in nature. Deletions of genes coding for key components of a blue-light-sensing white collar complex impair sporulation of the fungus and therefore adjourn spreading. Furthermore, the data gathered suggest an interconnection of the Velvet-like complex with photoreception in F. fujikuroi. Chapter V will summarize the key results of this study and put them into a broader context. The first identification and characterization of a Velvet-like complex in the genus Fusarium sheds new light on regulation of secondary metabolism in conjunction with virulence. This system presents an excellent foundation for further work directed towards a deeper understanding of the pathogen-host-interaction of Fusarium spp..