An autophagy gene, TrATG5, affects conidiospore differentiation in Trichoderma reesei

The LAMMER Kinase MoKns1 Regulates Growth, Conidiation and Pathogenicity in Magnaporthe oryzae

Magnaporthe oryzae is an important pathogen that causes a devastating disease in rice. It has been reported that the dual-specificity LAMMER kinase is conserved from yeast to animal species and has a variety of functions. However, the functions of the LAMMER kinase have not been reported in M. oryzae. In this study, we identified the unique LAMMER kinase MoKns1 and analyzed its function in M. oryzae. We found that in a MoKNS1 deletion mutant, growth and conidiation were primarily decreased, and pathogenicity was almost completely lost. Furthermore, our results found that MoKns1 is involved in autophagy. The ΔMokns1 mutant was sensitive to rapamycin, and MoKns1 interacted with the autophagy-related protein MoAtg18. Compared with the wild-type strain 70−15, autophagy was significantly enhanced in the ΔMokns1 mutant. In addition, we also found that MoKns1 regulated DNA damage stress pathways, and the ΔMokns1 mutant was more sensitive to hydroxyurea (HU) and methyl methanesulfonate (MMS) compared to the wild-type strain 70−15. The expression of genes related to DNA damage stress pathways in the ΔMokns1 mutant was significantly different from that in the wild-type strain. Our results demonstrate that MoKns1 is an important pathogenic factor in M. oryzae involved in regulating autophagy and DNA damage response pathways, thus affecting virulence. This research on M. oryzae pathogenesis lays a foundation for the prevention and control of M. oryzae.

AoATG5 plays pleiotropic roles in vegetative growth, cell nucleus development, conidiation, and virulence in the nematode-trapping fungus Arthrobotrys oligospora

Autophagy is an evolutionarily conserved process in eukaryotes, which is regulated by autophagy-related genes (ATGs). Arthrobotrys oligospora is a representative species of nematode-trapping (NT) fungi that can produce special traps for nematode predation. To elucidate the biological roles of autophagy in NT fungi, we characterized an orthologous Atg protein, AoAtg5, in A. oligospora. We found that AoATG5 deletion causes a significant reduction in vegetative growth and conidiation, and that the transcript levels of several sporulation-related genes were significantly downregulated during sporulation stage. In addition, the cell nuclei were significantly reduced in the ΔAoATG5 mutant, and the transcripts of several genes involved in DNA biosynthesis, repair, and ligation were significantly upregulated. In ΔAoATG5 mutants, the autophagic process was significantly impaired, and trap formation and nematocidal activity were significantly decreased. Comparative transcriptome analysis results showed that AoAtg5 is involved in the regulation of multiple cellular processes, such as autophagy, nitrogen metabolism, DNA biosynthesis and repair, and vesicular transport. In summary, our results suggest that AoAtg5 is essential for autophagy and significantly contributes to vegetative growth, cell nucleus development, sporulation, trap formation, and pathogenicity in A. oligospora, thus providing a basis for future studies focusing on related mechanisms of autophagy in NT fungi.

The FpPPR1 Gene Encodes a Pentatricopeptide Repeat Protein That Is Essential for Asexual Development, Sporulation, and Pathogenesis in Fusarium pseudograminearum

Fusarium crown rot (FCR) and Fusarium head blight (FHB) are caused by Fusarium pseudograminearum and are newly emerging diseases of wheat in China. In this study, we characterized FpPPR1, a gene that encodes a protein with 12 pentatricopeptide repeat (PPR) motifs. The radial growth rate of the ΔFpppr1 deletion mutant was significantly slower than the wild type strain WZ-8A on potato dextrose agar plates and exhibited significantly smaller colonies with sector mutations. The aerial mycelium of the mutant was almost absent in culture tubes. The ΔFpppr1 mutant was able to produce spores, but spores of abnormal size and altered conidium septum shape were produced with a significant reduction in sporulation compared to wild type. ΔFpppr1 failed to cause disease on wheat coleoptiles and barley leaves using mycelia plugs or spore suspensions. The mutant phenotypes were successfully restored to the wild type levels in complemented strains. FpPpr1-GFP signals in spores and mycelia predominantly overlapped with Mito-tracker signals, which substantiated the mitochondria targeting signal prediction of FpPpr1. RNAseq revealed significant transcriptional changes in the ΔFpppr1 mutant with 1,367 genes down-regulated and 1,333 genes up-regulated. NAD-binding proteins, thioredoxin, 2Fe-2S iron-sulfur cluster binding domain proteins, and cytochrome P450 genes were significantly down-regulated in ΔFpppr1, implying the dysfunction of mitochondria-mediated reductase redox stress in the mutant. The mating type idiomorphic alleles MAT1-1-1, MAT1-1-2, and MAT1-1-3 in F. pseudograminearum were also down-regulated after deletion of FpPPR1 and validated by real-time quantitative PCR. Additionally, 21 genes encoding putative heterokaryon incompatibility proteins were down-regulated. The yellow pigmentation of the mutant was correlated with reduced expression of PKS12 cluster genes. Taken together, our findings on FpPpr1 indicate that this PPR protein has multiple functions in fungal asexual development, regulation of heterokaryon formation, mating-type, and pathogenesis in F. pseudograminearum.

Roles of autophagy-related genes in conidiogenesis and blastospore formation, virulence, and stress response of Beauveria bassiana

Autophagy is a conserved intracellular recycling mechanism, in which autophagy-related genes 12 and 16 (ATG12 and ATG16) function in a complex controlling the ubiquitin-like conjugation system. In the insect-pathogenic fungus Beauveria bassiana, ATG12 and ATG16 were functionally characterized. Disruption of BbATG12 or BbATG16 resulted in the absence of autophagic bodies under starvation stress. ΔBbATG12 and ΔBbATG16 mutant strains displayed similar defects in asexual development (conidiation and blastospore formation) and tolerance to oxidative stress. ΔBbATG16 strain exhibited the impaired growth on the media with gelatin or chitin as a single nitrogen source, and ΔBbATG12 displayed decreased growth on the media with sucrose, fructose or maltose as a single carbon source. Both BbATG12 and BbATG16 were required for fungal virulence. BbATG16 mutation had more effects on fungal virulence than BbATG12 in topical infection assay, although both genes had similar contributions to fungal virulence in intrahemocoel injection assay. This study indicates that BbATG12 and BbATG16 mediate diverse biological functions in addition to their convergent roles in autophagy.

Autophagy in Paracoccidioides brasiliensis under normal mycelia to yeast transition and under selective nutrient deprivation

Paracoccidioides spp. is a thermally dimorphic fungus endemic to Latin America and the etiological agent of paracoccidioidomycosis (PCM), a granulomatous disease acquired through fungal propagule inhalation by its mammalian host. The infection is established after successful mycelia to yeast transition in the host pulmonary alveoli. The challenging environment inside the host exposes the fungus to the need of adaptation in order to circumvent nutritional, thermal, oxidative, immunological and other stresses that can directly affect their survival. Considering that autophagy is a response to abrupt environmental changes and is induced by stress conditions, this study hypothesizes that this process might be crucially involved in the adaptation of Paracoccidioides spp. to the host and, therefore, it is essential for the proper establishment of the disease. By labelling autophagous vesicles with monodansylcadaverine, autophagy was observed as an early event in cells during the normal mycelium to yeast transition, as well as in yeast cells of P. brasiliensis under glucose deprivation, and under either rapamycin or 3-methyladenine (3-MA). Findings in this study demonstrated that autophagy is triggered in P. brasiliensis during the thermal-induced mycelium to yeast transition and by glucose-limited conditions in yeasts, both of which modulated by rapamycin or 3-MA. Certainly, further genetic and in vivo analyses are needed in order to finally address the contribution of autophagy for adaptation. Yet, our data propose that autophagy possibly plays an important role in Paracoccidioides brasiliensis virulence and pathogenicity.

An autophagy gene, HoATG5, is involved in sporulation, cell wall integrity and infection of wounded barley leaves

The endophytic fungus Harpophora oryzae is a beneficial endosymbiont isolated from wild rice. H. oryzae can not only promote rice growth and biomass accumulation but also protect rice roots from invasion by its close relative Magnaporthe oryzae. Autophagy is a highly evolutionary conserved process from lower to higher eukaryotic organisms, and is involved in the maintenance of normal cell differentiation and development. In this study, we isolated a gene (HoATG5) which encodes an essential protein required for autophagy from the beneficial endophyte fungus H. oryzae. Using targeted gene replacement, a ΔHoATG5 mutant was generated and used to investigate the biological functions of autophagy in H. oryzae. We found that the autophagic process was blocked in the HoATG5 deletion mutant. The mutant showed increased vegetative growth and sporulation, and was sensitive to nutrient starvation. The ΔHoATG5 mutant lost its ability to penetrate and infect the wounded barley leaves. These results provide new knowledge to elaborate the molecular machinery of autophagy in endophytic fungi.

Vesicle trafficking, organelle functions, and unconventional secretion in fungal physiology and pathogenicity

Specific localization of appropriate sets of proteins and lipids is central to functions and integrity of organelles, which in turn underlie cellular activities of eukaryotes. Vesicle trafficking is a conserved mechanism of intracellular transport, which ensures such a specific localization to a subset of organelles. In this review article, we summarize recent advances in our understanding of how vesicle trafficking and related organelles support physiology and pathogenicity of filamentous fungi. Examples include a link between Golgi organization and polarity maintenance during hyphal tip growth, a new role of early endosomes in transport of translational machinery, involvement of endosomal/vacuolar compartments in secondary metabolite synthesis, and functions of vacuoles and autophagy in fungal development, nutrient recycling and allocation. Accumulating evidence showing the importance of unconventional secretion in fungal pathogenicity is also summarized.

Self-eating to grow and kill: autophagy in filamentous ascomycetes

Autophagy is a tightly controlled degradation process in which eukaryotic cells digest their own cytoplasm containing protein complexes and organelles in the vacuole or lysosome. Two types of autophagy have been described: macroautophagy and microautophagy. Both types can be further divided into nonselective and selective processes. Molecular analysis of autophagy over the last two decades has mostly used the unicellular ascomycetes Saccharomyces cerevisiae and Pichia pastoris. Genetic analysis in these yeasts has identified 36 autophagy-related (atg) genes; many are conserved in all eukaryotes, including filamentous ascomycetes. However, the autophagic machinery also evolved significant differences in fungi, as a consequence of adaptation to diverse fungal lifestyles. Intensive studies on autophagy in the last few years have shown that autophagy in filamentous fungi is not only involved in nutrient homeostasis but in other cellular processes such as cell differentiation, pathogenicity and secondary metabolite production. This mini-review focuses on the specific roles of autophagy in filamentous fungi.

Disruption and molecular characterization of calpains-related (MoCAPN1, MoCAPN3 and MoCAPN4) genes in Magnaporthe oryzae

Calpains are intracellular, cysteine proteases found in plants, animals and fungi functioning as signal transduction components in different cellular pathways including sporulation and alkaline adaptation in fungi. Calpains-related MoCAPN1 (MGG_14872), MoCAPN3 (MGG_15810) and MoCAPN4 (MGG_04818) genes from Magnaporthe oryzae genome which are 2604, 3513 and 771-bp in length and encoding identical proteins of 867, 1170 and 256 amino acids were functionally characterized for different phenotypes through gene disruption method. All the mutants except those for MoCAPN1 showed normal phenotypes. In pathogenicity test, the mutants did not lead to any visible changes in phenotypes causing similar blast lesions on blast susceptible rice and barley leaves as those of the Guy-11 strain suggesting no major role in pathogenicity. Germ tubes formation, appressorium formation, mycelium radial growth and mating with 2539 strain were indistinguishable among the mutants and Guy-11 strains. Cell wall integrity (congo red) test, stress response under chemical pressure (ZnSO4, CuSO4 and CdCl2), osmotic and oxidative (NaCl and H2O2) stress response, growth response on glucose and nitrogen deficient media resulted in similar results in the mutants and Guy-11 strains. However, mutants for ΔMoCAPN1 gene produced reduced (0.57±0.15B and 0.54±0.05B) conidia compared to that (1.69±0.13A) of the Guy-11 strain showing its involvement in conidiation.

Autophagy vitalizes the pathogenicity of pathogenic fungi

Plant pathogenic fungi utilize a series of complex infection structures, in particular the appressorium, to gain entry to and colonize plant tissue. As a consequence of the accumulation of huge quantities of glycerol in the cell the appressorium generates immense intracellular turgor pressure allowing the penetration peg of the appressorium to penetrate the leaf cuticle. Autophagic processes are ubiquitous in eukaryotic cells and facilitate the bulk degradation of macromolecules and organelles. The study of autophagic processes has been extended from the model yeast Saccharomyces cerevisiae to pathogenic fungi such as the rice blast fungus Magnaporthe oryzae. Significantly, null mutants for the expression of M. oryzae autophagy gene homologs lose their pathogenicity for infection of host plants. Clarification of the functions and network of interactions between the proteins expressed by M. oryzae autophagy genes will lead to a better understanding of the role of autophagy in fungal pathogenesis and help in the development of new strategies for disease control.

Role of macroautophagy in nutrient homeostasis during fungal development and pathogenesis

Macroautophagy is a non-selective, bulk degradation process conserved in eukaryotes. Response to starvation stress and/or regulation of nutrient breakdown/utilization is the major intracellular function of macroautophagy. Recent studies have revealed requirement for autophagy in diverse functions such as nutrient homeostasis, organelle degradation and programmed cell death in filamentous fungal pathogens, for proper morphogenesis and differentiation during critical steps of infection. In this review, we aim to summarize the physiological functions of autophagy in fungal virulence, with an emphasis on nutrient homeostasis in opportunistic human fungal pathogens and in the rice-blast fungus, Magnaporthe oryzae. We briefly summarize the role of autophagy on the host side: for resistance to, or subversion by, the pathogens.