No abstract

The fungus Fusarium fujikuroi causes “bakanae” disease of rice due to its ability to produce gibberellins (GAs), but it is also known for producing harmful mycotoxins. However, the genetic capacity for the whole arsenal of natural compounds and their role in the fungus' interaction with rice remained unknown. Here, we present a high-quality genome sequence of F. fujikuroi that was assembled into 12 scaffolds corresponding to the 12 chromosomes described for the fungus. We used the genome sequence along with ChIP-seq, transcriptome, proteome, and HPLC-FTMS-based metabolome analyses to identify the potential secondary metabolite biosynthetic gene clusters and to examine their regulation in response to nitrogen availability and plant signals. The results indicate that expression of most but not all gene clusters correlate with proteome and ChIP-seq data. Comparison of the F. fujikuroi genome to those of six other fusaria revealed that only a small number of gene clusters are conserved among these species, thus providing new insights into the divergence of secondary metabolism in the genus Fusarium. Noteworthy, GA biosynthetic genes are present in some related species, but GA biosynthesis is limited to F. fujikuroi, suggesting that this provides a selective advantage during infection of the preferred host plant rice. Among the genome sequences analyzed, one cluster that includes a polyketide synthase gene (PKS19) and another that includes a non-ribosomal peptide synthetase gene (NRPS31) are unique to F. fujikuroi. The metabolites derived from these clusters were identified by HPLC-FTMS-based analyses of engineered F. fujikuroi strains overexpressing cluster genes. In planta expression studies suggest a specific role for the PKS19-derived product during rice infection. Thus, our results indicate that combined comparative genomics and genome-wide experimental analyses identified novel genes and secondary metabolites that contribute to the evolutionary success of F. fujikuroi as a rice pathogen.

Summary Besides industrially produced gibberellins (GAs), Fusarium fujikuroi is able to produce additional secondary metabolites such as the pigments bikaverin and neurosporaxanthin and the mycotoxins fumonisins and fusarin C. The global regulation of these biosynthetic pathways is only poorly understood. Recently, the velvet complex containing VeA and several other regulatory proteins was shown to be involved in global regulation of secondary metabolism and differentiation in Aspergillus nidulans. Here we report on the characterization of two components of the F. fujikuroi velvet-like complex, FfVel1 and FfLae1. The gene encoding this first reported LaeA ortholog outside the class of Eurotiomycetidae is upregulated in ΔFfvel1 microarray-studies and FfLae1 interacts with FfVel1 in the nucleus. Deletion of Ffvel1 and Fflae1 revealed for the first time that velvet can simultaneously act as positive (GAs, fumonisins and fusarin C) and negative (bikaverin) regulator of secondary metabolism, and that both components affect conidiation and virulence of F. fujikuroi. Furthermore, the velvet-like protein FfVel2 revealed similar functions regarding conidiation, secondary metabolism and virulence as FfVel1. 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.

SummaryFusarium secondary metabolites are structurally diverse, have a variety of activities and are generally poorly understood biosynthetically. The F. fujikuroi polyketide synthase gene bik1 was previously shown to be responsible for formation of the mycelial pigment bikaverin. Here we present the characterization of five genes adjacent to bik1 as encoding a putative FAD-dependent monooxygenase (bik2), an O-methyltransferase (bik3), an NmrA-like protein (bik4), a Zn(II)2Cys6 transcription factor (bik5) and an MFS transporter (bik6). Deletion of each gene resulted in total loss or significant reduction of bikaverin synthesis. 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. Transcription of all bik genes in strains lacking bik1, bik2 or bik3 was essentially eliminated, while transcription of some bik genes was detected in strains lacking bik4, bik5 or bik6. Thus, bikaverin synthesis is regulated by a complex regulatory network. Understanding how different factors influence the synthesis of this model secondary metabolite will aid understanding secondary metabolism in general.