Involvement Of HOG Signaling Pathway And VeA Regulatory System In Regulation Of DON Biosynthesis In Fusarium Graminearum

Fusarium graminearum is the major causal agent of Fusarium head blight (FHB) on wheat, which causes yield loss and affects the quality of grains. The mycotoxins, such as deoxynivalenol (DON) and its derivatives produced by the fungus in infected grains, pose a serious threat to human and animal health. Despite the FHB can be effectively controlled by some fungicides, fungicides application may stimulate toxin production by the fungus. Therefore, the development of novel anti-mycotoxin and antifungal agents is urgently needed for effective management of FHB and mycotoxin contamination in cereals. Previous research has revealed the involvement of two-component histidine kinase signaling pathway (HOG signaling pathway) and VeA regulatory system in the regulation of DON biosynthesis, to get a better insight into the regulation mechanism of DON synthesis in F. graminearum, the biological functions of35genes involved in the HOG signaling pathway and VeA regulatory system were studied, results of our research showed that:1) The response regulator FgRrgl of F. graminearum in the HOG signaling pathway is involved in the regulation of DON and aurofusarin biosynthesis, playing important roles in pathogenicity and adaptation of fungi to a variety of environmental stresses. The FgRRG-1deletion mutant showed increased sensitivity to osmotic stress mediated by NaCl, KCl, sorbitol or glucose, and to metal cations Li+, Ca2+and Mg2+The mutant, however, was more resistant than the parent strain to dicarboximide and phenylpyrrole fungicides. Inoculation tests showed that the mutant exhibited decreased virulence on wheat heads, and produced a significantly low level of DON.2) The type2C protein phosphatases FgPtc3is the negative regulator of the HOG signaling pathway in F. graminearum. FgPtc3is involved in the regulation of DON and aurofusarin biosynthesis, and also plays important roles in regulating many development processes and secondary metabolisms. The FgPTC3deletion mutant exhibited reduced aerial hyphae formation and deoxynivalenol (DON) production, but increased production of conidia. The mutant showed increased resistance to osmotic stress and cell wall-damaging agents, and accumulated a higher basal level of glycerol than the wild-type progenitor. Ultrastructural and histochemical analyses showed that conidia of the mutant contained an unusually high number of large lipid droplets. Pathogencity assays showed that the mutant is unable to infect flowering wheat head. Serial analysis of gene expression in the deletion mutant revealed that FgPtc3is associated with various metabolic pathways. Additionally, the FgPTC3partially rescued the growth defect of a yeast PTC1deletion mutant under various stress conditions, and also reduced the level of phosphorylated Hogl. The FgPtc3has the PP2C phosphatase activity, which may help in exploitation of drug targets based on screening of the protein activity inhibitors.3) The tyrosine phosphatase FgPtp2is not associated with the HOG signaling pathway, but plays important roles in the regulation of DON biosynthesis and secondary metabolisms. The FgPTP2deletion mutant exhibited a significant decrease in mycelial growth and conidiation, but do not change the sensitivity to variety stresses. In addition, the deletion mutant was impaired dramatically in virulence and produced a low level of DON. Serial analysis of gene expression in the mutant revealed that FgPtp2is associated with various metabolic pathways, especially with the synthesis of ribosomal protein.4) The FgVeA and FgVelB in the VeA regulatory system have some common functions in regulation of DON biosynthesis, and various cellular processes. The FgVEA and FgVELB deletion mutant exhibited a reduction in aerial hyphae formation, hydrophobicity, and deoxynivalenol (DON) biosynthesis, but increased the conidial production. Sensitivity tests to various stresses showed that the mutants exhibited increased resistance to osmotic stress and cell wall-damaging agents, but increased sensitivity to iprodione and fludioxonil fungicides, and the mutants contained an unusually high number of large lipid droplets, which is in agreement with the observation that the mutant accumulated a higher basal level of glycerol than the wild-type progenitor. Both FgVeA and FgVe1B play a critical role in pathogenicity,in a variety of biological functions and secondary metabolisms. The LaeA homologous genes are not essential for mycelial growth in F. graminearum, the deletion of the genes caused minor influence in pigment synthesis, and have no effect on DON production.Results of the research indicated that the FgRrgl and FgPtc3in HOG signaling pathway, as well as FgVeA and FgVe1B in VeA regulatory system are involved in pathogenicity and DON biosynthesis, which may represent attractive antimicrobial targets for disease control purposes. The results aslo may help in exploitation of drug targets for the design of new anti-mycotoxin and antifungal agents.