Sexual specific functions of Tub1 beta-tubulins require stage-specific RNA processing and expression in Fusarium graminearum

Unveiling the A-to-I mRNA editing machinery and its regulation and evolution in fungi

A-to-I mRNA editing in animals is mediated by ADARs, but the mechanism underlying sexual stage-specific A-to-I mRNA editing in fungi remains unknown. Here, we show that the eukaryotic tRNA-specific heterodimeric deaminase FgTad2-FgTad3 is responsible for A-to-I mRNA editing in Fusarium graminearum. This editing capacity relies on the interaction between FgTad3 and a sexual stage-specific protein called Ame1. Although Ame1 orthologs are widely distributed in fungi, the interaction originates in Sordariomycetes. We have identified key residues responsible for the FgTad3-Ame1 interaction. The expression and activity of FgTad2-FgTad3 are regulated through alternative promoters, alternative translation initiation, and post-translational modifications. Our study demonstrates that the FgTad2-FgTad3-Ame1 complex can efficiently edit mRNA in yeasts, bacteria, and human cells, with important implications for the development of base editors in therapy and agriculture. Overall, this study uncovers mechanisms, regulation, and evolution of RNA editing in fungi, highlighting the role of protein-protein interactions in modulating deaminase function.

Sexual stage-specific A-to-I mRNA editing is mediated by tRNA-editing enzymes in fungi

A-to-I RNA editing catalyzed by adenosine-deaminase-acting-on-RNA (ADARs) was assumed to be unique to metazoans because fungi and plants lack ADAR homologs. However, genome-wide messenger RNA (mRNA) editing was found to occur specifically during sexual reproduction in filamentous ascomycetes. Because systematic characterization of adenosine/cytosine deaminase genes has implicated the involvement of TAD2 and TAD3 orthologs in A-to-I editing, in this study, we used genetic and biochemical approaches to characterize the role of FgTAD2, an essential adenosine-deaminase-acting-on-tRNA (ADAT) gene, in mRNA editing in Fusarium graminearum. FgTAD2 had a sexual-stage-specific isoform and formed heterodimers with enzymatically inactive FgTAD3. Using a repeat-induced point (RIP) mutation approach, we identified 17 mutations in FgTAD2 that affected mRNA editing during sexual reproduction but had no effect on transfer RNA (tRNA) editing and vegetative growth. The functional importance of the H352Y and Q375*(nonsense) mutations in sexual reproduction and mRNA editing were confirmed by introducing specific point mutations into the endogenous FgTAD2 allele in the wild type. An in vitro assay was developed to show that FgTad2-His proteins purified from perithecia, but not from vegetative hyphae, had mRNA editing activities. Moreover, the H352Y mutation affected the enzymatic activity of FgTad2 to edit mRNA but had no effect on its ADAT activity. We also identified proteins co-purified with FgTad2-His by mass spectrometry analysis and found that two of them have the RNA recognition motif. Taken together, genetic and biochemical data from this study demonstrated that FgTad2, an ADAT, catalyzes A-to-I mRNA editing with the stage-specific isoform and cofactors during sexual reproduction in fungi.

FgCsn12 Is Involved in the Regulation of Ascosporogenesis in the Wheat Scab Fungus Fusarium graminearum

Fusarium head blight (FHB), caused by the fungal pathogen Fusarium graminearum, is a destructive disease worldwide. Ascospores are the primary inoculum of F. graminearum, and sexual reproduction is a critical step in its infection cycle. In this study, we characterized the functions of FgCsn12. Although the ortholog of FgCsn12 in budding yeast was reported to have a direct interaction with Csn5, which served as the core subunit of the COP9 signalosome, the interaction between FgCsn12 and FgCsn5 was not detected through the yeast two-hybrid assay. The deletion of FgCSN12 resulted in slight defects in the growth rate, conidial morphology, and pathogenicity. Instead of forming four-celled, uninucleate ascospores, the Fgcsn12 deletion mutant produced oval ascospores with only one or two cells and was significantly defective in ascospore discharge. The 3'UTR of FgCsn12 was dispensable for vegetative growth but essential for sexual reproductive functions. Compared with those of the wild type, 1204 genes and 2240 genes were up- and downregulated over twofold, respectively, in the Fgcsn12 mutant. Taken together, FgCsn12 demonstrated an important function in the regulation of ascosporogenesis in F. graminearum.

Biological Characteristics of Verticillium dahliae MAT1-1 and MAT1-2 Strains

Verticillium dahliae is a soil-borne plant pathogenic fungus that causes Verticillium wilt on hundreds of dicotyledonous plant species. V. dahliae is considered an asexually (clonal) reproducing fungus, although both mating type idiomorphs (MAT1-1 and MAT1-2) are present, and is heterothallic. Most of the available information on V. dahliae strains, including their biology, pathology, and genomics comes from studies on isolates with the MAT1-2 idiomorph, and thus little information is available on the MAT1-1 V. dahliae strains in the literature. We therefore evaluated the growth responses of MAT1-1 and MAT1-2 V. dahliae strains to various stimuli. Growth rates and melanin production in response to increased temperature, alkaline pH, light, and H₂O₂ stress were higher in the MAT1-2 strains than in the MAT1-1 strains. In addition, the MAT1-2 strains showed an enhanced ability to degrade complex polysaccharides, especially starch, pectin, and cellulose. Furthermore, several MAT1-2 strains from both potato and sunflower showed increased virulence on their original hosts, relative to their MAT1-1 counterparts. Thus, compared to MAT1-1 strains, MAT1-2 strains derive their potentially greater fitness from an increased capacity to adapt to their environment and exhibit higher virulence. These competitive advantages might explain the current abundance of MAT1-2 strains relative to MAT1-1 strains in the agricultural and sylvicultural ecosystems, and this study provides the baseline information on the two mating idiomorphs to study sexual reproduction in V. dahliae under natural and laboratory conditions.

FgPal1 regulates morphogenesis and pathogenesis in Fusarium graminearum

Ascospores are the primary inoculum in Fusarium graminearum, a causal agent of wheat head blight. In a previous study, FgPAL1 was found to be upregulated in the Fgama1 mutant and important for ascosporogenesis. However, the biological function of this well-conserved gene in filamentous ascomycetes is not clear. In this study, we characterized its functions in growth, differentiation and pathogenesis. The Fgpal1 mutant had severe growth defects and often displayed abnormal hyphal tips. It was defective in infectious growth in rachis tissues and spreading in wheat heads. The Fgpal1 mutant produced conidia with fewer septa and more nuclei per compartment than the wild type. In actively growing hyphal tips, FgPal1-GFP mainly localized to the subapical collar and septa. The FgPal1 and LifeAct partially co-localized at the subapical region in an interdependent manner. The Fgpal1 mutant was normal in meiosis with eight nuclei in developing asci but most asci were aborted. Taken together, our results showed that FgPal1 plays a role in maintaining polarized tip growth and coordination between nuclear division and cytokinesis, and it is also important for infectious growth and developments of ascospores by the free cell formation process.

An expanded subfamily of G-protein-coupled receptor genes in Fusarium graminearum required for wheat infection

The cAMP-PKA and MAP kinase pathways are essential for plant infection in the wheat head blight fungus Fusarium graminearum. To identify upstream receptors of these well-conserved signalling pathways, we systematically characterized the 105 G-protein-coupled receptor (GPCR) genes. Although none were required for vegetative growth, five GPCR genes (GIV1-GIV5) significantly upregulated during plant infection were important for virulence. The giv1 mutant was defective in the formation of specialized infection structures known as infection cushions, which was suppressed by application of exogenous cAMP and dominant active FST7 MEK kinase. GIV1 was important for the stimulation of PKA and Gpmk1 MAP kinase by compounds in wheat spikelets. GIV2 and GIV3 were important for infectious growth after penetration. Invasive hyphae of the giv2 mutant were defective in cell-to-cell spreading and mainly grew intercellularly in rachis tissues. Interestingly, the GIV2-GIV5 genes form a phylogenetic cluster with GIV6, which had overlapping functions with GIV5 during pathogenesis. Furthermore, the GIV2-GIV6 cluster is part of a 22-member subfamily of GPCRs, with many of them having in planta-specific upregulation and a common promoter element; however, only three subfamily members are conserved in other fungi. Taken together, F. graminearum has an expanded subfamily of infection-related GPCRs for regulating various infection processes.

Thioredoxin Reductase Is Involved in Development and Pathogenicity in Fusarium graminearum

Fusarium graminearum is one of the causal agents of Fusarium head blight and produces the trichothecene mycotoxin, deoxynivalenol (DON). Thioredoxin reductases (TRRs) play critical roles in the recycling of oxidized thioredoxin. However, their functions are not well known in plant pathogenic fungi. In this study, we characterized a TRR orthologue FgTRR in F. graminearum. The FgTRR-GFP fusion protein localized to the cytoplasm. FgTRR gene deletion demonstrated that FgTRR is involved in hyphal growth, conidiation, sexual reproduction, DON production, and virulence. The ΔTRR mutants also exhibited a defect in pigmentation, the expression level of aurofusarin biosynthesis-related genes was significantly decreased in the FgTRR mutant. Furthermore, the ΔTRR mutants were more sensitive to oxidative stress and aggravated apoptosis-like cell death compared with the wild type strain. Taken together, these results indicate that FgTRR is important in development and pathogenicity in F. graminearum.