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Utant, production of FB1, FB2, FB3, and FB4 was reduced by Utant, production of FB1, FB2, FB3, and FB4 was reduced by 70.8 , 84.2 , 87.3 , and 93.6 , respectively, compared with that of the wild type. The FvvelB[FvVELB] strain reverted to wild-type levels of fumonisin production. In the FvvelC mutant, fumonisins were at levels similar to those for the wild type. We then examined transcriptional levels of FUM1 and FUM8 in the FvvelB, FvvelB[FvVELB], Fvve1, and wild-type strains. As shown in Fig. 3B, gene expression of FUM1 and FUM8 was dramatically reduced in the FvvelB and Fvve1 strains, but the reduction was more severe in the Fvve1 strain. Therefore, different components of the velvet complex differentially contribute to fumonisin biosynthesis. As previously shown (8), our data furtherverified that FvVE1 plays an essential role in the regulation of fumonisin biosynthesis, while FvVelB, although not essential for this process, also positively regulates fumonisin production. FvVelC, however, is dispensable for fumonisin biosynthesis. Velvet proteins regulate oxidative stress tolerance. In A. nidulans, VelB and VosA play more important roles than VeA in tolerance against various stresses, including UV and H2O2, although all three velvet proteins contribute to tolerance to these stresses (16). Since ROS (reactive oxygen species) play a key role in plant-pathogen interactions, we examined the sensitivities of each velvet protein null mutant to H2O2 and menadione (a ROS-inducing chemical, 2-methyl-1,4-naphthoquinone, also known as vitamin K3). As shown in Fig. 4, the Fvve1 mutant was hypersensitive to H2O2 and menadione. On solid medium with 25 g/ml menadione, the Fvve1 mutant could not form colonies, while the wild type and the FvvelB strain could. No significant difference in growth inhibition between the FvvelB mutant and the wild type were observed. On solid medium with 3.27 mM H2O2, growth inhibition rates of the Fvve1 mutant and the FvvelB mutant were significantly higher than that of the wild type (Fig. 4A). However, the FvvelB mutant was less sensitive to H2O2 than the Fvve1 mutant: the relative growth inhibition rates of the Fvve1 mutant and the FvvelB mutant were 46.3 and 18.7 , respectively (Fig. 4B). All these observations indicate that FvVE1 plays a more important role in oxidative stress resistance than FvVelB and the relative roles of FvVE1 and FvVelB of F. verticillioides are different from those of A. nidulans in the regulation of oxidative stress tolerance.ec.asm.orgEukaryotic CellVelvet Complex in Fusarium verticillioidesFIG 3 Regulation of fumonisin production by velvet proteins. (A) HPLC-MS chromatogram of fumonisin extracts of indicated strains. (B) Transcriptional levels of FUM1 and FUM8 of the wild-type (WT), Fvve1 deletion mutant, FvvelB deletion mutant, and FvvelB[FvVELB] strains. Relative gene expression levels were calculated relative to the transcriptional level of the wild-type strain. Values shown are means for three replicates. Standard deviations are indicated with error bars.The FvvelC mutant displayed wild-type sensitivities to H2O2 and menadione (Fig. 4). FvVE1 and FvVelB positively regulate FvCAT2 expression. To test how FvVE1 and FvVELB regulate antioxidant activity, transcriptional levels of eight genes involved in ROS detoxification during H2O2 treatment were comparatively analyzed in the Fvve1 deletion mutant, the FvvelB deletion mutant, and the wild-type strain by qRT-PCR. These genes included GST (FVEG_07456; a glutathione S-transferase), GLT2 (FVEG_08420.