| Abstract |
The mycotoxin deoxynivalenol (DON) is a secondary metabolite produced by Fusarium graminearum (teleomorph synonym: Gibberella zeae), which causes Fusarium head blight disease in barley and wheat. Although many studies have explored the photobiological responses (from blue to red in the visible wavelength range) of fungi that produce mycotoxins, few have focused on the growth and DON production of F. graminearum exposed to short-wave blue light at approximately 400 nm. In this study, conidia of G. zeae JCM 9873 were exposed to light at wavelengths of 385 or 405 nm for 1, 1.5, and 2 h, with the irradiance adjusted to 19 (low) or 53 (high) mW/cm2. The effects of wavelength and irradiance on mycelial growth, colony morphology, DON yield, and the levels of four transcripts (Tri4, Tri5, Tri6, and Tri101) of genes involved in DON biosynthesis were investigated. In addition, accumulation of reactive oxygen species (ROS) was involved in DON biosynthesis. Conidia treated with 385 or 405 nm light at low and high irradiance for 2 h formed colonies after a 2-day cultivation (no colonies after 1 day), and the radial mycelial growth enlarged, eventually becoming comparable to unirradiated conidia. Colony observations confirmed the induction of an intense orange color after 405-nm irradiation with high irradiance only. The colonies derived from conidia exposed to 385-nm light at low irradiance for 2 h showed the lowest DON yield (0.104 ± 0.008 mg/g fungal biomass) among tested light conditions, corresponding to a yield of 43 % of the control samples. Quantitative real-time PCR analysis of transcript levels indicated that all four DON-associated genes were downregulated. The suppressive effect of 385-nm light treatment on DON production may be useful for constructing light treatments that are economical with weak irradiance. By contrast, all four Tri genes were upregulated after treatment with 405-nm light at high irradiance for 2 h, resulting in a high DON yield 1.49-times that of the control. Microscopic observations using a fluorescent ROS probe suggested that accumulation of intracellular ROS after irradiation with 405-nm light stimulates DON biosynthesis. Therefore, DON production in response to light exposure varies depending on the wavelength and irradiance within the short-wave blue light range. A better understanding of the photobiological responses to light in this range, as well as to light in the visible wavelength range, could facilitate the development of photo-technologies to control contamination by mycotoxins, including DON.
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