Abstract
Sexual spores (ascospores) of Fusarium graminearum, a homothallic ascomycetous fungus, are believed to be the primary inocula for epidemics of the diseases caused by this species in cereal crops. Based on the light requirement for the formation of fruiting bodies (perithecia) of F. graminearum under laboratory conditions, we explored whether photoreceptors play an important role in sexual development. Here, we evaluated the roles of three genes encoding putative photoreceptors [a phytochrome gene (FgFph) and two white collar genes (FgWc-1 and FgWc-2)] during sexual development in F. graminearum. For functional analyses, we generated transgenic strains lacking one or two genes from the self-fertile Z3643 strain. Unlike the wild-type (WT) and add-back strains, the single deletion strains (ΔFgWc-1 and ΔFgWc-2) produced fertile perithecia under constant light on complete medium (CM, an unfavorable medium for sexual development) as well as on carrot agar (a perithecial induction condition). The expression of mating-type (MAT) genes increased significantly in the gene deletion strains compared to the WT under both conditions. Deletion of FgFph had no significant effect on sexual development or MAT gene expression. In contrast, all of the deletion strains examined did not show significant changes in other traits such as hyphal growth, mycotoxin production, and virulence. A split luciferase assay confirmed the in vivo protein-protein interactions among three photoreceptors along with FgLaeA, a global regulator of secondary metabolism and fungal development. Introduction of an intact copy of the A. nidulans LreA and LreB genes, which are homologs of FgWc-1 and FgWc-2, into the ΔFgWc-1 and ΔFgWc-2 strains, respectively, failed to repress perithecia formation on CM in the gene deletion strains. Taken together, these results demonstrate that FgWc-1 and FgWc-2, two central components of the blue-light sensing system, negatively regulate sexual development in F. graminearum, which differs from the regulation pattern in A. nidulans.
Highlights
Fusarium graminearum is an economically important plant pathogen that causes diseases on major cereal crops such as maize, wheat, and barley [1]
In contrast to F. graminearum, the darkness is favorable for sexual development of Aspergillus nidulans; blue/visible light inhibited the sexual cycle compared to cultures grown in darkness, the cultures grown under white light still produced cleistothecia [12]
Comparative analysis with photoreceptor homologs in other fungi showed that FgWc-1, FgWc-2, and FgFph were highly conserved within species of the subphylum Pezizomycotina of the Ascomycota than in the phyla Oomycota, Basidiomycota, and the subphylum Saccharomycotina (S1 Fig.). Quantitative real-time PCR (qPCR) analysis revealed that these three genes were constitutively expressed under both light and dark conditions, indicating that they did not show stage-specific expression
Summary
Fusarium graminearum is an economically important plant pathogen that causes diseases on major cereal crops such as maize, wheat, and barley [1]. In contrast to F. graminearum, the darkness is favorable for sexual development of Aspergillus nidulans; blue/visible light inhibited the sexual cycle compared to cultures grown in darkness, the cultures grown under white light still produced cleistothecia [12]. Together, these observations suggest that fungi including F. graminearum and A. nidulans have evolved intricate molecular mechanisms to detect and respond to light; these mechanisms vary among fungal species [9, 11]
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