The Exocyst Regulates Hydrolytic Enzyme Secretion at Hyphal Tips and Septa in the Banana Fusarium Wilt Fungus Fusarium odoratissimum

The CHY-type zinc finger protein MoChy1 is involved in polarized growth, conidiation, autophagy and pathogenicity of Magnaporthe oryzae

Polarized growth is critical for the development of filamentous phytopathogens, and the CHY-type zinc finger protein Chy1 regulates microtubule assembly to influence polarized growth and thereby affect plant infections. However, the biological role of a Chy1 homolog MoChy1 remains unknown in Magnaporthe oryzae. We found here that the MoChy1-GFP was distributed in the cytoplasm outside the vacuole in hyphae and localized mainly to the vacuole compartments as the appressorium matured. The Mochy1 mutants showed an extremely slow growth rate, curved and branched mycelium, reduced conidiation, and a smaller size in the appressorium. Meanwhile, the Mochy1 mutants showed increased sensitivity to benomyl, damaged microtubule cytoskeleton, and mislocalized polarisome protein MoSpa2 and chitin synthase MoChs6 in hyphae. Compared to Guy11, the Mochy1 mutants exhibited increased sensitivity to H2O2, impaired ability to eliminate host-derived ROS and reduced penetration into host plants, resulting in a strong reduction in pathogenicity of Mochy1 mutants. Furthermore, the Mochy1 mutants also exhibited defects in chitin distribution, osmotic stress tolerance, and septin ring organization during appressorium differentiation and fungal development. Nonselective autophagy was negatively regulated in Mochy1 mutants compared to Guy11. In summary, MoChy1 plays multiple roles in fungal polar growth and full virulence of M. oryzae.

A Network of Sporogenesis-Responsive Genes Regulates the Growth, Asexual Sporogenesis, Pathogenesis and Fusaric Acid Production of Fusarium oxysporum f. sp. cubense

The conidia produced by Fusarium oxysporum f. sp. cubense (Foc), the causative agent of Fusarium Wilt of Banana (FWB), play central roles in the disease cycle, as the pathogen lacks a sexual reproduction process. Until now, the molecular regulation network of asexual sporogenesis has not been clearly understood in Foc. Herein, we identified and functionally characterized thirteen (13) putative sporulation-responsive genes in Foc, namely FocmedA(a), FocmedA(b), abaA-L, FocflbA, FocflbB, FocflbC, FocflbD, FocstuA, FocveA, FocvelB, wetA-L, FocfluG and Foclae1. We demonstrated that FocmedA(a), abaA-L, wetA-L, FocflbA, FocflbD, FocstuA, FocveA and Foclae1 mediate conidiophore formation, whereas FocmedA(a) and abaA-L are important for phialide formation and conidiophore formation. The expression level of abaA-L was significantly decreased in the ΔFocmedA(a) mutant, and yeast one-hybrid and ChIP-qPCR analyses further confirmed that FocMedA(a) could bind to the promoter of abaA-L during micro- and macroconidiation. Moreover, the transcript abundance of the wetA-L gene was significantly reduced in the ΔabaA-L mutant, and it not only was found to function as an activator of micro- and macroconidium formation but also served as a repressor of chlamydospore production. In addition, the deletions of FocflbB, FocflbC, FocstuA and Foclae1 resulted in increased chlamydosporulation, whereas FocflbD and FocvelB gene deletions reduced chlamydosporulation. Furthermore, FocflbC, FocflbD, Foclae1 and FocmedA(a) were found to be important regulators for pathogenicity and fusaric acid synthesis in Foc. The present study therefore advances our understanding of the regulation pathways of the asexual development and functional interdependence of sporulation-responsive genes in Foc.

Core-Shell Droplet-Based Microfluidic Screening System for Filamentous Fungi

Filamentous fungi are competitive hosts for the production of drugs, proteins, and chemicals. However, their utility is limited by screening methods and low throughput. In this work, a universal high-throughput system for optimizing protein production in filamentous fungi was described. Droplet microfluidics was used to encapsulate large mutant strain pools in biocompatible core-shell microdroplets designed to avoid mycelial punctures and thus sustain prolonged culture. The self-assembled split GFP was then used to characterize the secretory capacity of the strains and isolate strains with superior production titers according to the fluorescence signals. The platform was applied to optimize the α-amylase secretion of Aspergillus niger, resulting in the isolation of a strain with 2.02-fold higher secretion capacity. The system allows the analysis of >105 single cells per h and will facilitate ultrahigh-throughput screening experiments of filamentous fungi. This method could help identify improved hosts for the large-scale production of biotechnology-relevant proteins. This is a broadly applicable system that can be equally used in other hosts.

The Signal Peptidase FoSpc2 Is Required for Normal Growth, Conidiation, Virulence, Stress Response, and Regulation of Light Sensitivity in Fusarium odoratissimum

Signal peptidase (SPase) is responsible for cleavage of N-terminal signal peptides in most secretory precursor proteins and many membrane proteins during maturation. In this study, we identified four components of the SPase complex (FoSec11, FoSpc1, FoSpc2, and FoSpc3) in the banana wilt fungal pathogen Fusarium odoratissimum. We proved that interactions exist among the four SPase subunits by bimolecular fluorescence complementation (BiFC) and affinity purification and mass spectrometry (AP-MS) assays. Among the four SPase genes, FoSPC2 was successfully deleted. FoSPC2 deletion caused defects in vegetative growth, conidiation, and virulence. Loss of FoSPC2 also affected the secretion of some pathogenicity-related extracellular enzymes, suggesting that SPase without FoSpc2 may have a lower efficiency in managing the maturation of the extracellular enzymes in F. odoratissimum. In addition, we found that the ΔFoSPC2 mutant had increased sensitivity to light, and the colonies of the mutant grew faster under all-dark conditions than under all-light conditions. We further observed that deletion of FoSPC2 affected expression of the blue light photoreceptor gene FoWC2, leading to cytoplasmic accumulation of FoWc2 under all-light conditions. Since FoWc2 has signal peptides, FoSpc2 may regulate the expression and subcellular localization of FoWc2 indirectly. Contrary to its response to light, the ΔFoSPC2 mutant displayed a significant decreased sensitivity to osmotic stress, and culturing the mutant under osmotic stress conditions restored both the localization of FoWc2 and light sensitivity of the ΔFoSPC2, suggesting that a cross talk between osmotic stress and light response pathways in F. odoratissimum and FoSpc2 takes part in these processes. IMPORTANCE In this study, we identified four components of SPase in the banana wilt pathogen Fusarium odoratissimum and characterized the SPase FoSpc2. Loss of FoSPC2 affected the secretion of extracellular enzymes, suggesting that SPase without FoSpc2 may have a lower efficiency in managing the maturation of the extracellular enzymes in F. odoratissimum. In addition, this is the first time that we have found a relationship between the SPase and fungal light response. Deletion of FoSPC2 resulted in decreased sensitivity to the osmotic stresses but with increased sensitivity to light. Continuous light inhibited the growth rate of the ΔFoSPC2 mutant and affected the cellular localization of the blue light photoreceptor FoWc2 in this mutant, but culturing the mutant under osmotic stress both restored the localization of FoWc2 and eliminated the light sensitivity of the ΔFoSPC2 mutant, suggesting that loss of FoSPC2 may affect a cross talk between the osmotic stress and light response pathways in F. odoratissimum.

SNARE Protein Snc1 Is Essential for Vesicle Trafficking, Membrane Fusion and Protein Secretion in Fungi

Fungi are an important group of microorganisms that play crucial roles in a variety of ecological and biotechnological processes. Fungi depend on intracellular protein trafficking, which involves moving proteins from their site of synthesis to the final destination within or outside the cell. The soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE) proteins are vital components of vesicle trafficking and membrane fusion, ultimately leading to the release of cargos to the target destination. The v-SNARE (vesicle-associated SNARE) Snc1 is responsible for anterograde and retrograde vesicle trafficking between the plasma membrane (PM) and Golgi. It allows for the fusion of exocytic vesicles to the PM and the subsequent recycling of Golgi-localized proteins back to the Golgi via three distinct and parallel recycling pathways. This recycling process requires several components, including a phospholipid flippase (Drs2-Cdc50), an F-box protein (Rcy1), a sorting nexin (Snx4-Atg20), a retromer submit, and the COPI coat complex. Snc1 interacts with exocytic SNAREs (Sso1/2, Sec9) and the exocytic complex to complete the process of exocytosis. It also interacts with endocytic SNAREs (Tlg1 and Tlg2) during endocytic trafficking. Snc1 has been extensively investigated in fungi and has been found to play crucial roles in various aspects of intracellular protein trafficking. When Snc1 is overexpressed alone or in combination with some key secretory components, it results in enhanced protein production. This article will cover the role of Snc1 in the anterograde and retrograde trafficking of fungi and its interactions with other proteins for efficient cellular transportation.

MoRts1, a regulatory subunit of PP2A, is required for fungal development and pathogenicity of Magnaporthe oryzae

Protein phosphatase 2 A (PP2A) is a major heterotrimeric serine/threonine protein phosphatase comprised of three subunits, including structural subunits (A), regulatory subunits (B), and catalytic subunits (C). PP2A has been widely shown to involve in a series of cell signal transduction processes such as cell metabolism, cell cycle regulation, DNA replication, gene transcription and protein translation in yeast and mammalian. However, the roles of PP2A in pathogenic fungi Magnaporthe oryzae still remain unclear. We here found that MoRts1, a gene encoding B regulatory subunit of PP2A homologous to Saccharomyces cerevisiae Rts1, showed up-regulated transcription during conidia and initially infectious stage. Subcellular localization revealed that MoRts1-eGFP was localized to the cytoplasm and septum. Targeted disruption of MoRts1 leads to a reduction of mycelial growth and sporulation, as well as the defects of hydrophobicity, melanin pigmentation and cell wall integrity (CWI). The MoRts1 mutants were less pathogenic to the host plants, compared to the Ku80 strain, and the transcriptional levels of several pathogenicity-related Rho GTPase genes, including MoCdc42, MoRho2, MoRho3, MoRho4, MoRhoX and MoRac1, were significantly decreased in the MoRts1 mutants. Besides, two splicing variants of MoRts1 with unique functions of regulating the growth and pathogenicity were identified, and the B56 domain is vital for determining the sporulation and pathogenicity of M. oryzae. Furthermore, MoRts1 was identified to interact with PP2A catalytic subunit MoPPG1 in vivo in M. oryzae. In summary, our results showed that MoRts1 is an important regulator contributing to the fungal development, and pathogenicity of M. oryzae.

Antifungal effects and biocontrol potential of lipopeptide-producing Streptomyces against banana Fusarium wilt fungus Fusarium oxysporum f. sp. cubense

Fusarium wilt of banana (FWB), caused by Fusarium oxysporum f. sp. cubense (Foc), especially tropical race 4 (TR4), presents the foremost menace to the global banana production. Extensive efforts have been made to search for efficient biological control agents for disease management. Our previous study showed that Streptomyces sp. XY006 exhibited a strong inhibitory activity against several phytopathogenic fungi, including F. oxysporum. Here, the corresponding antifungal metabolites were purified and determined to be two cyclic lipopeptide homologs, lipopeptin A and lipopeptin B. Combined treatment with lipopeptin complex antagonized Foc TR4 by inhibiting mycelial growth and conidial sporulation, suppressing the synthesis of ergosterol and fatty acids and lowering the production of fusaric acid. Electron microscopy observation showed that lipopeptide treatment induced a severe disruption of the plasma membrane, leading to cell leakage. Lipopeptin A displayed a more pronounced antifungal activity against Foc TR4 than lipopeptin B. In pot experiments, strain XY006 successfully colonized banana plantlets and suppressed the incidence of FWB, with a biocontrol efficacy of up to 87.7%. Additionally, XY006 fermentation culture application improved plant growth parameters and induced peroxidase activity in treated plantlets, suggesting a possible role in induced resistance. Our findings highlight the potential of strain XY006 as a biological agent for FWB, and further research is needed to enhance its efficacy and mode of action in planta.

Small GTPase FoSec4-Mediated Protein Secretion Is Important for Polarized Growth, Reproduction and Pathogenicity in the Banana Fusarium Wilt Fungus Fusarium odoratissimum

Apical secretion at hyphal tips is important for the growth and development of filamentous fungi. In this study, we analyzed the role of the Rab GTPases FoSec4 involved in the secretion of the banana wilt fungal pathogen Fusarium odoratissimum. We found that the deletion of FoSEC4 affects the activity of extracellular hydrolases and protein secretion, indicating that FoSec4 plays an important role in the regulation of protein secretion in F. odoratissimum. As a typical Rab GTPase, Sec4 participates in the Rab cycle through the conversion between the active GTP-bound state and the inactive GDP-bound state, which is regulated by guanine nucleate exchange factors (GEFs) and GTPase-activating proteins (GAPs). We further found that FoSec2 can interact with dominant-negative FoSec4 (GDP-bound and nucleotide-free form, FoSec4DN), and that FoGyp5 can interact with dominant active FoSec4 (GTP-bound and constitutively active form, FoSec4CA). We evaluated the biofunctions of FoSec4, FoSec2 and FoGyp5, and found that FoSec4 is involved in the regulation of vegetative growth, reproduction, pathogenicity and the environmental stress response of F. odoratissimum, and that FocSec2 and FoGyp5 perform biofunctions consistent with FoSec4, indicating that FoSec2 and FoGyp5 may work as the GEF and the GAP, respectively, of FoSec4 in F. odoratissimum. We further found that the amino-terminal region and Sec2 domain are essential for the biological functions of FoSec2, while the carboxyl-terminal region and Tre-2/Bub2/Cdc16 (TBC) domain are essential for the biological functions of FoGyp5. In addition, FoSec4 mainly accumulated at the hyphal tips and partially colocalized with Spitzenkörper; however, FoGyp5 accumulated at the periphery of Spitzenkörper, suggesting that FoGyp5 may recognize and inactivate FoSec4 at a specific location in hyphal tips.