The zinc finger protein StMR1 affects the pathogenicity and melanin synthesis of Setosphaeria turcica and directly regulates the expression of DHN melanin synthesis pathway genes

Positive selection and functional diversification of transcription factor Cmr1 homologs in Alternaria

Transcription factor Cmr1 (Colletotrichum melanin regulation 1) and its homologs in several plant fungal pathogens are the regulators of the 1,8-dihydroxynaphthalene (DHN)-melanin biosynthesis pathway and have evolved functional diversification in morphology and pathogenicity. The fungal genus Alternaria comprises the group of "black fungi" that are rich in DHN-melanin in the primary cell wall and septa of the conidia. Some Alternaria species cause many economically important plant diseases worldwide. However, the evolution and function of Cmr1 homologs in Alternaria remain poorly understood. Here, we identified a total of forty-two Cmr1 homologs from forty-two Alternaria spp. and all contained one additional diverse fungal specific transcription factor motif. Phylogenetic analysis indicated the division of these homologs into five major clades and three branches. Dated phylogeny showed the A and D clades diverged latest and earliest, respectively. Molecular evolutionary analyses revealed that three amino acid sites of Cmr1 homologs in Alternaria were the targets of positive selection. Asmr1, the homolog of Cmr1 in the potato early blight pathogen, Alternaria solani was amplified and displayed the sequence conservation at the amino acid level in different A. solani isolates. Asmr1 was further confirmed to have the transcriptional activation activity and was upregulated during the early stage of potato infection. Deletion of asmr1 led to the decreased melanin content and pathogenicity, deformed conidial morphology, and responses to cell wall and fungicide stresses in A. solani. These results suggest positive selection and functional divergence have played a role in the evolution of Cmr1 homologs in Alternaria. KEY POINTS: • Cmr1 homologs were under positive selection in Alternaria species • Asmr1 is a functional transcription factor, involved in spore development, melanin biosynthesis, pathogenicity, and responses to cell wall and fungicide stresses in A. solani • Cmr1 might be used as a potential taxonomic marker of the genus Alternaria.

The catalytic subunit of type 2A protein phosphatase negatively regulates conidiation and melanin biosynthesis in Setosphaeria turcica

Northern corn leaf blight caused by Setosphaeria turcica is a major fungal disease responsible for significant reductions in maize yield worldwide. Eukaryotic type 2A protein phosphatase (PP2A) influences growth and virulence in a number of pathogenic fungi, but little is known about its roles in S. turcica. Here, we functionally characterized S. turcica StPP2A-C, which encodes the catalytic C subunit of StPP2A. StPP2A-C deletion slowed colony growth, conidial germination, and appressorium formation but increased conidiation, melanin biosynthesis, glycerol content, and disease lesion size on maize. These effects were associated with expression changes in genes related to calcium signaling, conidiation, laccase activity, and melanin and glycerol biosynthesis, as well as changes in intra- and extracellular laccase activity. A pull-down screen for candidate StPP2A-c interactors revealed an interaction between StPP2A-c and StLac1. Theoretical modeling and yeast two-hybrid experiments confirmed that StPP2A-c interacted specifically with the copper ion binding domain of StLac1 and that Cys267 of StPP2A-c was required for this interaction. StPP2A-C expression thus appears to promote hyphal growth and reduce pathogenicity in S. turcica, at least in part by altering melanin synthesis and laccase activity; these insights may ultimately support the development of novel strategies for biological management of S. turcica.

Two zinc finger proteins, VdZFP1 and VdZFP2, interact with VdCmr1 to promote melanized microsclerotia development and stress tolerance in Verticillium dahliae

BACKGROUND

Melanin plays important roles in morphological development, survival, host-pathogen interactions and in the virulence of phytopathogenic fungi. In Verticillum dahliae, increases in melanin are recognized as markers of maturation of microsclerotia which ensures the long-term survival and stress tolerance, while decreases in melanin are correlated with increased hyphal growth in the host. The conserved upstream components of the VdCmr1-regulated pathway controlling melanin production in V. dahliae have been extensively identified, but the direct activators of this pathway are still unclear.

RESULTS

We identified two genes encoding conserved C2H2-type zinc finger proteins VdZFP1 and VdZFP2 adjacent to VdPKS9, a gene encoding a negative regulator of both melanin biosynthesis and microsclerotia formation in V. dahliae. Both VdZFP1 and VdZFP2 were induced during microsclerotia development and were involved in melanin deposition. Their localization changed from cytoplasmic to nuclear in response to osmotic pressure. VdZFP1 and VdZFP2 act as modulators of microsclerotia melanization in V. dahliae, as confirmed by melanin biosynthesis inhibition and supplementation with the melanin pathway intermediate scytalone in albino strains. The results indicate that VdZFP1 and VdZFP2 participate in melanin biosynthesis by positively regulating VdCmr1. Based on the results obtained with yeast one- and two-hybrid (Y1H and Y2H) and bimolecular fluorescence complementation (BiFC) systems, we determined the melanin biosynthesis relies on the direct interactions among VdZFP1, VdZFP2 and VdCmr1, and these interactions occur on the cell walls of microsclerotia. Additionally, VdZFP1 and/or VdZFP2 mutants displayed increased sensitivity to stress factors rather than alterations in pathogenicity, reflecting the importance of melanin in stress tolerance of V. dahliae.

CONCLUSIONS

Our results revealed that VdZFP1 and VdZFP2 positively regulate VdCmr1 to promote melanin deposition during microsclerotia development, providing novel insight into the regulation of melanin biosynthesis in V. dahliae.

Cmcrf1, a Putative Zn2Cys6 Fungal Transcription Factor, Is Involved in Conidiation, Carotenoid Production, and Fruiting Body Development in Cordyceps militaris

Cordyceps militaris is a high-value medicinal and edible fungus that produces many bioactive compounds, including carotenoid, and thus, improving the carotenoid productivity of C. militaris will increase its commercial value. However, little is known about the genetic regulatory mechanism of carotenoid biosynthesis in C. militaris. To further understanding the regulatory mechanism of carotenoid biosynthesis, we performed a large-scale screen of T-DNA insertional mutant library and identified a defective mutant, denoted T111, whose colonies did not change color from white to yellow upon exposure to light. Mutation analysis confirmed that a single T-DNA insertion occurred in the gene encoding a 695-amino-acid putative fungal-specific transcription factor with a predicted Zn2Cys6 binuclear cluster DNA-binding domain found uniquely in fungi. Targeted deletion of this gene, denoted C. militaris carotenogenesis regulatory factor 1 (Cmcrf1), generated the ΔCmcrf1 mutant that exhibited drastically reduced carotenoid biosynthesis and failed to generate fruiting bodies. In addition, the ΔCmcrf1 mutant showed significantly increased conidiation and increased hypersensitivity to cell-wall-perturbing agents compared with the wild-type strain. However, the Cmcrf1 gene did not have an impact on the mycelia growth of C. militaris. These results show that Cmcrf1 is involved in carotenoid biosynthesis and is required for conidiation and fruiting body formation in C. militaris.

The Secreted Ribonuclease SRE1 Contributes to Setosphaeria turcica Virulence and Activates Plant Immunity

During the plant infection process, pathogens can secrete several effectors. Some of the effectors are well-known for their roles in regulating plant immunity and promoting successful pathogen colonization. However, there are few studies on the ribonuclease (RNase) effectors secreted by fungi. In the present study, we discovered a secretable RNase (SRE1) in the secretome of Setosphaeria turcica that was significantly upregulated during the early stages of S. turcica infection in maize. Knockdown of SRE1 significantly reduced the virulence of S. turcica. SRE1 can induce cell death in maize and Nicotiana benthamiana. However, unlike the conventional hypersensitive response (HR) caused by other effectors, SRE1 is not dependent on its signal peptide (SP) or plant receptor kinases (such as BAK1 and SOBIR1). SRE1-induced cell death depends upon its enzymatic activity and the N-terminal β-hairpin structure. SRE1 relies on its N-terminal β-hairpin structure to enter cells, and then degrades plant's RNA through its catalytic activity causing cytotoxic effects. Additionally, SRE1 enhances N. benthamiana's resistance to pathogenic fungi and oomycetes. In summary, SRE1 promotes the virulence of S. turcica, inducing plant cell death and activating plant immune responses.

N-terminal zinc fingers of MaNCP1 contribute to growth, stress tolerance, and virulence in Metarhizium acridum

C2H2 zinc finger proteins (ZFPs) are a class of important transcriptional regulators in eukaryotes involved in multiple biological regulation processes. Here, MaNCP1, a C2H2 ZFP, was functionally characterized in the model entomopathogenic fungus Metarhizium acridum. Deletion of MaNCP1 delayed conidial germination and hyphal growth, decreased the conidial yield and reduced the tolerances to UV-B irradiation and heat-shock. The N-terminal zinc fingers (ZFs) of MaNCP1 made the main contributions to these traits. In addition, disruption of MaNCP1 altered the conidial surface structure and decreased the conidial hydrophobicity. Bioassays showed that the virulence of the MaNCP1-disruption strain (ΔMaNCP1) was reduced in topical inoculation compared to the WT or the mutant complemented strain (CP), and the N-terminal C2H2 ZFs made a major contribution to virulence. Furthermore, the ΔMaNCP1 and C2H2 ZFs deletion mutants (MaNCP1^∆N and MaNCP1^∆N+C) impaired cuticular penetration. RNA-seq showed that several cuticle-degrading genes were down-regulated in the ΔMaNCP1 background, suggesting that MaNCP1 plays vital roles in regulating insect cuticle penetration. In summary, MaNCP1 affected the growth, stress tolerances and virulence of M. acridum, and the N-terminal C2H2 ZFs played indispensable roles in these important biocontrol traits. These results provide further insights into the functions of C2H2 ZFPs in entomopathogenic fungi.

A polyketide synthase from Verticillium dahliae modulates melanin biosynthesis and hyphal growth to promote virulence

Background

During the disease cycle, plant pathogenic fungi exhibit a morphological transition between hyphal growth (the phase of active infection) and the production of long-term survival structures that remain dormant during “overwintering.” Verticillium dahliae is a major plant pathogen that produces heavily melanized microsclerotia (MS) that survive in the soil for 14 or more years. These MS are multicellular structures produced during the necrotrophic phase of the disease cycle. Polyketide synthases (PKSs) are responsible for catalyzing production of many secondary metabolites including melanin. While MS contribute to long-term survival, hyphal growth is key for infection and virulence, but the signaling mechanisms by which the pathogen maintains hyphal growth are unclear.

Results

We analyzed the VdPKSs that contain at least one conserved domain potentially involved in secondary metabolism (SM), and screened the effect of VdPKS deletions in the virulent strain AT13. Among the five VdPKSs whose deletion affected virulence on cotton, we found that VdPKS9 acted epistatically to the VdPKS1-associated melanin pathway to promote hyphal growth. The decreased hyphal growth in VdPKS9 mutants was accompanied by the up-regulation of melanin biosynthesis and MS formation. Overexpression of VdPKS9 transformed melanized hyphal-type (MH-type) into the albinistic hyaline hyphal-type (AH-type), and VdPKS9 was upregulated in the AH-type population, which also exhibited higher virulence than the MH-type.

Conclusions

We show that VdPKS9 is a powerful negative regulator of both melanin biosynthesis and MS formation in V. dahliae. These findings provide insight into the mechanism of how plant pathogens promote their virulence by the maintenance of vegetative hyphal growth during infection and colonization of plant hosts, and may provide novel targets for the control of melanin-producing filamentous fungi.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01330-2.