Functional Characterization of Calcineurin-Responsive Transcription Factors Fg01341 and Fg01350 in Fusarium graminearum

VmPma1 contributes to virulence via regulation of the acidification process during host infection in Valsa mali

Valsa mali is a destructive phytopathogenic fungus that mainly infects apple and pear trees. Infection with V. mali results in host tissue acidification via the generation of citric acid, which promote invasion. Here, two plasma membrane H⁺-ATPases, VmPma1 and VmPma2, were identified in V. mali. The VmPma1 deletion mutant (∆VmPma1) displayed higher intracellular acid accumulation and a lower growth rate compared to the wild type. In contrast, the VmPma2 deletion mutant (∆VmPma2) showed no obvious phenotypic differences. Meanwhile, loss of VmPma1, but not VmPma2, in V. mali led to a significant decrease in growth under acidic or alkaline conditions compared with WT. More importantly, ∆VmPma1 showed a greater reduction in ATPase hydrolase activity and acidification of the external environment, more sensitivity to abiotic stress, and weaker pathogenicity than ∆VmPma2. This evidence indicates that VmPma1 is the main gene of the two plasma membrane H⁺-ATPases. Transcriptomic analysis indicated that many metabolic processes regulated by VmPma1 are strictly pH-regulated. Besides, we identified two genes (named VmAgn1p and Vmap1) that contribute to the pathogenicity of V. mali by differentially regulating external acidification capacity. Overall, our findings show that VmPma1 plays a pivotal role in pathogenicity by affecting the acidification of V. mali.

The calcium-calcineurin and high-osmolarity glycerol pathways co-regulate tebuconazole sensitivity and pathogenicity in Fusarium graminearum

The calcium-calcineurin and high-osmolarity glycerol (HOG) pathways play crucial roles in fungal development, pathogenicity, and in responses to various environmental stresses. However, interaction of these pathways in regulating fungicide sensitivity remains largely unknown in phytopathogenic fungi. In this study, we investigated the function of the calcium-calcineurin signalling pathway in Fusarium graminearum, the causal agent of Fusarium head blight. Inhibitors of Ca²⁺ and calcineurin enhanced antifungal activity of tebuconazole (an azole fungicide) against F. graminearum. Deletion of the putative downstream transcription factor FgCrz1 resulted in significantly increased sensitivity of F. graminearum to tebuconazole. FgCrz1-GFP was translocated to the nucleus upon tebuconazole treatment in a calcineurin-dependent manner. In addition, deletion of FgCrz1 increased the phosphorylation of FgHog1 in response to tebuconazole. Moreover, the calcium-calcineurin and HOG signalling pathways exhibited synergistic effect in regulating pathogenicity and sensitivity of F. graminearum to tebuconazole and multiple other stresses. RNA-seq data revealed that FgCrz1 regulated expression of a set of non-CYP51 genes that are associated with tebuconazole sensitivity, including multidrug transporters, membrane lipid biosynthesis and metabolism, and cell wall organization. Our findings demonstrate that the calcium-calcineurin and HOG pathways act coordinately to orchestrate tebuconazole sensitivity and pathogenicity in F. graminearum, which may provide novel insights in management of Fusarium disease.

Golgi-localized calcium/manganese transporters FgGdt1 and FgPmr1 regulate fungal development and virulence by maintaining Ca2+ and Mn2+ homeostasis in Fusarium graminearum

Calcium and manganese transporters play important roles in regulating Ca²⁺ and Mn²⁺ homeostasis in cells, which is necessary for the normal physiological activities of eukaryotes. Gdt1 and Pmr1 function as calcium/manganese transporters in the Golgi apparatus. However, the functions of Gdt1 and Pmr1 have not been previously characterized in the plant pathogenic fungus Fusarium graminearum. Here, we identified and characterized the biological functions of FgGdt1 and FgPmr1 in F. graminearum. Our study shows that FgGdt1 and FgPmr1 are both localized to the cis- and medial-Golgi. Disruption of FgGdt1 or FgPmr1 in F. graminearum caused serious defects in vegetative growth, conidiation, sexual development and significantly decreased virulence in wheat but increased deoxynivalenol (DON) production. Importantly, FgGdt1 is involved in Ca²⁺ and Mn²⁺ homeostasis and the severe phenotypic defects of the ΔFggdt1 mutant were largely due to loss of FgGdt1 function in Mn²⁺ transportation. FgGdt1-mCherry colocalizes with FgPmr1-GFP at the Golgi, and FgGDT1 exerts its biological function upstream of FgPMR1. Taken together, our results collectively demonstrate that the cis- and medial-Golgi-localized proteins FgGdt1 and FgPmr1 regulate Ca²⁺ and Mn²⁺ homeostasis of the Golgi apparatus, and this function is important in modulating the growth, development, DON biosynthesis and pathogenicity of F. graminearum.

A C2H2 Zinc Finger Protein PlCZF1 Is Necessary for Oospore Development and Virulence in Peronophythora litchii

C₂H₂ zinc finger is one of the most common motifs found in the transcription factors (TFs) in eukaryotes organisms, which have a broad range of functions, such as regulation of growth and development, stress tolerance and pathogenicity. Here, PlCZF1 was identified to encode a C₂H₂ zinc finger in the litchi downy blight pathogen Peronophythora litchii. PlCZF1 is conserved in P. litchii and Phytophthora species. In P. litchii, PlCZF1 is highly expressed in sexual developmental and early infection stages. We generated Δplczf1 mutants using the CRISPR/Cas9 method. Compared with the wild type, the Δplczf1 mutants showed no significant difference in vegetative growth and asexual reproduction, but were defective in oospore development and virulence. Further experiments revealed that the transcription of PlM90, PlLLP and three laccase encoding genes were down-regulated in the Δplczf1 mutant. Our results demonstrated that PlCZF1 is a vital regulator for sexual development and pathogenesis in P. litchii.

The Transcription Factor FgAtrR Regulates Asexual and Sexual Development, Virulence, and DON Production and Contributes to Intrinsic Resistance to Azole Fungicides in Fusarium graminearum

Simple Summary

Fusarium graminearum is a devastating plant pathogen that can cause wheat head blight. Azole fungicides are commonly used chemicals for control of this disease. However, F. graminearum strains resistant to these fungicides have emerged. To better understand the azole resistance mechanism of F. graminearum, we identified and characterized the Zn(II)2-Cys6 transcription factor FgAtrR in F. graminearum. We found that FgAtrR played critical roles in vegetative growth, conidia production, perithecium formation, and virulence on wheat heads and corn silks. FgAtrR was also involved in the resistance to azole antifungals by regulating the expression of the drug target FgCYP51s and efflux pump transporters. These results broadened our understanding of the azole resistance mechanisms of F. graminearum.

Abstract

Fusarium graminearum is the predominant causal agent of cereal Fusarium head blight disease (FHB) worldwide. The application of chemical fungicides such as azole antifungals is still the primary method for FHB control. However, to date, our knowledge of transcriptional regulation in the azole resistance of F. graminearum is quite limited. In this study, we identified and functionally characterized a Zn(II)2-Cys6 transcription factor FgAtrR in F. graminearum. We constructed a FgAtrR deletion mutant and found that deletion of FgAtrR resulted in faster radial growth with serious pigmentation defects, significantly reduced conidial production, and an inability to form perithecia. The pathogenicity of the ΔFgAtrR mutant on wheat spikes and corn silks was severely impaired with reduced deoxynivalenol production, while the tolerance to prochloraz and propiconazole of the deletion mutant was also significantly decreased. RNA-seq indicated that many metabolic pathways were affected by the deletion of FgAtrR. Importantly, FgAtrR could regulate the expression of the FgCYP51A and ABC transporters, which are the main contributors to azole resistance. These results demonstrated that FgAtrR played essential roles in asexual and sexual development, DON production, and pathogenicity, and contributed to intrinsic resistance to azole fungicides in F. graminearum. This study will help us improve the understanding of the azole resistance mechanism in F. graminearum.