Characterization of the autophagy-related gene BmATG8 in Bipolaris maydis

Phosphatidylcholine levels regulate hyphal elongation and differentiation in the filamentous fungus Aspergillus oryzae

Filamentous fungi are eukaryotic microorganisms that differentiate into diverse cellular forms. Recent research demonstrated that phospholipid homeostasis is crucial for the morphogenesis of filamentous fungi. However, phospholipids involved in the morphological regulation are yet to be systematically analyzed. In this study, we artificially controlled the amount of phosphatidylcholine (PC), a primary membrane lipid in many eukaryotes, in a filamentous fungus Aspergillus oryzae, by deleting the genes involved in PC synthesis or by repressing their expression. Under the condition where only a small amount of PC was synthesized, A. oryzae hardly formed aerial hyphae, the basic structures for asexual development. In contrast, hyphae were formed on the surface or in the interior of agar media (we collectively called substrate hyphae) under the same conditions. Furthermore, we demonstrated that supplying sufficient choline to the media led to the formation of aerial hyphae from the substrate hyphae. We suggested that acyl chains in PC were shorter in the substrate hyphae than in the aerial hyphae by utilizing the strain in which intracellular PC levels were controlled. Our findings suggested that the PC levels regulate hyphal elongation and differentiation processes in A. oryzae and that phospholipid composition varied depending on the hyphal types.

HOG1 Mitogen-Activated Protein Kinase Pathway–Related Autophagy Induced by H2O2 in Lentinula edodes Mycelia

Mycelial ageing is associated with ROS and autophagy in Lentinula edodes. However, the underlying cellular and molecular mechanisms between ROS and autophagy remain obscure. This study induced autophagy in L. edodes mycelia through exogenous H2O2 treatment. Results showed that 100 μM H2O2 treatment for 24 h significantly inhibited mycelial growth. H2O2 caused the depolarisation of MMP and accumulation of TUNEL-positive nuclei, which was similar to the ageing phenotype of L. edodes mycelia. Transcriptome analysis showed that differentially expressed genes were enriched in the mitophagic, autophagic, and MAPK pathways. LeAtg8 and LeHog1 were selected as hub genes. RNA and protein levels of LeATG8 increased in the H2O2-treated mycelia. Using fluorescent labelling, we observed for the first time the classic ring structure of autophagosomes in a mushroom, while 3D imaging suggested that these autophagosomes surrounded the nuclei to degrade them at specific growth stages. Phospho-LeHOG1 protein can translocate from the cytoplasm to the nucleus to regulate mycelial cells, resisting ROS-induced oxidative stress. Furthermore, LeATG8 expression was suppressed when LeHOG1 phosphorylation was inhibited. These results suggest that the LeATG8-dependent autophagy in L. edodes mycelial is closely associated with the activity or even phosphorylation of LeHOG1.

Distinct roles for different autophagy-associated genes in the virulence of the fungal wheat pathogen Zymoseptoria tritici

The fungal wheat pathogen Zymoseptoria tritici causes major crop losses as the causal agent of the disease Septoria tritici blotch. The infection cycle of Z. tritici displays two distinct phases, beginning with an extended symptomless phase of 1-2 weeks, before the fungus induces host cell death and tissue collapse in the leaf. Recent evidence suggests that the fungus uses little host-derived nutrition during asymptomatic colonisation, raising questions as to the sources of energy required for this initial growth phase. Autophagy is crucial for the pathogenicity of other fungal plant pathogens through its roles in supporting cellular differentiation and growth under starvation. Here we characterised the contributions of the autophagy genes ZtATG1 and ZtATG8 to the development and virulence of Z. tritici. Deletion of ZtATG1 led to inhibition of autophagy but had no impact on starvation-induced hyphal differentiation or virulence, suggesting that autophagy is not required for Z. tritici pathogenicity. Contrastingly, ZtATG8 deletion delayed the transition to necrotrophic growth, despite having no influence on filamentous growth under starvation, pointing to an autophagy-independent role of ZtATG8 during Z. tritici infection. To our knowledge, this study represents the first to find autophagy not to contribute to the virulence of a fungal plant pathogen, and reveals novel roles for different autophagy-associated proteins in Z. tritici.

Meiotic Silencing in Dothideomycetous Bipolaris maydis

The filamentous ascomycete Bipolaris maydis is a plant pathogen that causes corn leaf blight and has been used in cytological studies of sexual reproduction. In this fungus, when null mutants of each septin are crossed with the wild-type strain, all ascospores derived from the same asci show abnormal morphology. The phenomenon was remarkably similar to the event known as "ascus dominance" in Neurospora crassa, which is known to be caused by MSUD (meiotic silencing by unpaired DNA). However, it is not clear whether B. maydis possesses functional MSUD. The object of this study is to elucidate whether this fungus carries a functional MSUD system that causes ascus dominance in the crosses of septin mutants and the wild-type strain. The results of homozygous and heterozygous crossing tests with mutants, having the insertional CDC10-septin gene sequence into the genome, suggested that the ascus dominance in B. maydis is triggered by the unpaired DNA as in N. crassa. To investigate whether MSUD is caused by the same mechanism as in N. crassa, an RNA-dependent RNA polymerase, one of the essential factors in MSUD, was identified and disrupted (Δrdr1) in B. maydis. When the Δrdr1 strain was crossed with each mutant of the septins, ascus dominance did not occur in all crosses. These results suggest that this ascus dominance is caused by RNA silencing triggered by an unpaired gene, as in N. crassa, and septin genes were affected by this silencing. To date, although MSUD has been found only in Fusarium graminearum and N. crassa, which are classified as Sordariomycetes, this study showed that MSUD is also functional in B. maydis, which is classified as a Dothideomycete. These results showed the possibility that this posttranscriptional regulation is extensively conserved among filamentous ascomycetes.

SsATG8 and SsNBR1 mediated-autophagy is required for fungal development, proteasomal stress response and virulence in Sclerotinia sclerotiorum

Autophagy plays vital roles in the interaction between the necrotrophic fungal pathogen Sclerotinia sclerotiorum and its hosts. However, so far, only little is known about the impacts of autophagy machinery in S. sclerotiorum per se on the fungal morphogenesis and pathogenesis. Here, through functional genomic approaches, we showed that SsATG8, one of the core components of the autophagy machinery, and its interactor SsNBR1, an autophagy cargo receptor, are important for vegetative growth, sclerotial formation, oxalic acid (OA) production, compound appressoria development, and virulence of S. sclerotiorum. Complementation assays with chimeric fusion constructs revealed that both LDS [AIM (ATG8 interacting motif) / LIR (LC3-interacting region) docking site] and UDS [UIM (ubiquitin-interacting motif) docking site] sites of the SsATG8 are required for its functions in autophagy and pathogenesis. Importantly, ΔSsatg8 and ΔSsnbr1 mutants showed enhanced sensitivity to the exogenous treatment with the proteasome inhibitors bortezomib and carfilzomib, and ΔSsnbr1 mutant had decreased expression of SsATG8 under the proteasomal stress conditions, suggesting that a cross-talk exists between ubiquitin-proteasome and selective autophagy pathways, which enables downstream protein degradation to proceed properly during diverse biological processes. Collectively, our data indicate that SsATG8- and SsNBR1-mediated autophagy is crucial for S. sclerotiorum development, proteasomal stress response and virulence.

An exocyst component, Sec5, is essential for ascospore formation in Bipolaris maydis

In this study, we identified Sec5 in Bipolaris maydis, a homologue of Sec5 in Saccharomyces cerevisiae and a possible exocyst component of the fungus. To examine how Sec5 affects the life cycle of B. maydis, we generated null mutant strains of the gene (Δsec5). The Δsec5 strains showed a strong reduction in hyphal growth and a slight reduction in pathogenicity. In sexual reproduction, they possessed the ability to develop pseudothecia. However, all ascospores were aborted in any of the asci obtained from crosses between Δsec5 and the wild-type. Our cytological study revealed that the abortion was caused by impairments of the post-meiotic stages in ascospore development, where ascospore delimitation and young spore elongation occur.

An arabinose-induced enhancement of asexual reproduction and concomitant changes in metabolic state in the filamentous fungus Bipolaris maydis

l-Arabinose, a major constituent pentose of plant cell-wall polysaccharides, has been suggested to be a less preferred carbon source for fungi but to be a potential signalling molecule that can cause distinct genome-wide transcriptional changes in fungal cells. Here, we explore the possibility that this unique pentose influences the morphological characteristics of the phytopathogenic fungus Bipolaris maydis strain HITO7711. When grown on plate media under different sugar conditions, the mycelial dry weight of cultures on l-arabinose was as low as that with no sugar, suggesting that l-arabinose does not substantially contribute to vegetative growth. However, the intensity of conidiation on l-arabinose was comparable to or even higher than that on d-glucose and on d-xylose, in contrast to the poor conidiation under the no-sugar condition. To explore the physiological basis of the passive growth and active conidiation on l-arabinose, we next investigated cellular responses of the fungus to these sugar conditions. Transcriptional analysis of genes related to carbohydrate metabolism showed that l-arabinose stimulates carbohydrate utilization through the hexose monophosphate shunt (HMP shunt), a catabolic pathway parallel to glycolysis and which participates in the generation of the reducing agent NADPH (the reduced form of nicotinamide adenine dinucleotide phosphate). Then, the HMP shunt was impaired by disrupting the related gene BmZwf1, which encodes glucose-6-phosphate dehydrogenase in this fungus. The resulting mutants on l-arabinose showed remarkably decreased conidiation, but a conversely increased mycelial dry weight compared with the wild-type. Our study demonstrates that l-arabinose acts to enhance resource allocation to asexual reproduction in B. maydis HITO7711 at the cost of vegetative growth, and suggests that this is mediated by the concomitant stimulation of the HMP shunt.

UvAtg8-Mediated Autophagy Regulates Fungal Growth, Stress Responses, Conidiation, and Pathogenesis in Ustilaginoidea virens

BACKGROUND

Ustilaginoidea virens has become one of the most devastating rice pathogens in China, as well as other rice-growing areas. Autophagy is an important process in normal cell differentiation and development among various organisms. To date, there has been no optimized experimental system introduced for the study of autophagy in U. virens. In addition, the function of autophagy in pathogenesis remains unknown in U. virens. Therefore, the functional analyses of UvAtg8 may potentially shed some light on the regulatory mechanism and function of autophagy in U. virens.

RESULTS

In this study, we characterized the functions of UvAtg8, which is a homolog of Saccharomyces cerevisiae ScAtg8, in the rice false smut fungus U. virens. The results showed that UvATG8 is essential for autophagy in U. virens. Also, the GFP-UvATG8 strain, which could serve as an appropriate marker for monitoring autophagy in U. virens, was generated. Furthermore, this study found that the ΔUvatg8 mutant was defective in the vegetative growth, conidiation, adaption to oxidative, hyperosmotic, cell wall stresses, and production of toxic compounds. Pathogenicity assays indicated that deletion of UvATG8 resulted in significant reduction in virulence of U. virens. Further microscopic examinations of the infection processes revealed that the severe virulence defects in the ∆Uvatg8 were mainly caused by the highly reduced conidiation and secondary spore formation.

CONCLUSIONS

Our results indicated that the UvAtg8 is necessary for the fungal growth, stresses responses, conidiation, secondary spore formation, and pathogenicity of U. virens. Moreover, our research finding will potentially assist in further clarifying the molecular mechanism of U. virens infection, as well as provide a good marker for autophagy in U. virens and a good reference value for the further development of effective fungicides based on gene targeting.

Biological functions of the autophagy-related proteins Atg4 and Atg8 in Cryptococcus neoformans

Autophagy is a mechanism responsible for intracellular degradation and recycling of macromolecules and organelles, essential for cell survival in adverse conditions. More than 40 autophagy-related (ATG) genes have been identified and characterized in fungi, among them ATG4 and ATG8. ATG4 encodes a cysteine protease (Atg4) that plays an important role in autophagy by initially processing Atg8 at its C-terminus region. Atg8 is a ubiquitin-like protein essential for the synthesis of the double-layer membrane that constitutes the autophagosome vesicle, responsible for delivering the cargo from the cytoplasm to the vacuole lumen. The contributions of Atg-related proteins in the pathogenic yeast in the genus Cryptococcus remain to be explored, to elucidate the molecular basis of the autophagy pathway. In this context, we aimed to investigate the role of autophagy-related proteins 4 and 8 (Atg4 and Atg8) during autophagy induction and their contribution with non-autophagic events in C. neoformans. We found that Atg4 and Atg8 are conserved proteins and that they interact physically with each other. ATG gene deletions resulted in cells sensitive to nitrogen starvation. ATG4 gene disruption affects Atg8 degradation and its translocation to the vacuole lumen, after autophagy induction. Both atg4 and atg8 mutants are more resistant to oxidative stress, have an impaired growth in the presence of the cell wall-perturbing agent Congo Red, and are sensitive to the proteasome inhibitor bortezomib (BTZ). By that, we conclude that in C. neoformans the autophagy-related proteins Atg4 and Atg8 play an important role in the autophagy pathway; which are required for autophagy regulation, maintenance of amino acid levels and cell adaptation to stressful conditions.

CpATG8, a Homolog of Yeast Autophagy Protein ATG8, Is Required for Pathogenesis and Hypovirus Accumulation in the Chest Blight Fungus

Autophagy is a degradation system in the cell, involved in the turnover of cellular components, development, differentiation, immune responses, protection against pathogens, and cell death. Autophagy is induced by nutrient starvation, in which cytoplasmic components and organelles are digested via vacuoles/lysosomes. In this study, by using electron microscopy, we observed that hypovirus CHV1-EP713 infection of Cryphonectria parasitica, the causative agent of chestnut blight disease, caused proliferation of autophagic-like vesicles. This phenomenon could be mimicked by treating the wild-type strain of the fungus EP155 with the autophagy induction drug rapamycin. Some of the hypovirulence-associated traits, including reduced pigmentation and conidiation, were also observed in the rapamycin-treated EP155. Quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) revealed that genes involved in autophagy were up-regulated in expression. Deletion of cpatg8, a gene encoding a homolog of ATG8 in Saccharomyces cerevisiae, resulted in attenuation of virulence and reduction in sporulation, as well as accumulation of the double-stranded viral RNA. Furthermore, virus-encoded p29 protein was found to co-localize with CpATG8, implying that the viral protein may interfere with the function of CpATG8. Taken together, these findings show that cpatg8 can be regulated by the hypovirus and is required for virulence and development of the fungus and accumulation of viral dsRNA in chestnut blight fungus.

Baseline sensitivity of Bipolaris maydis to the novel succinate dehydrogenase inhibitor benzovindiflupyr and its efficacy

Benzovindiflupyr is a novel member of succinate dehydrogenase inhibitor (SDHI) fungicides. The filamentous fungus Bipolaris maydis Nisik. et Miyake was the causal agent of southern corn leaf blight (SCLB). Here, baseline sensitivity of B. maydis to benzovindiflupyr was established by mycelial growth and conidium germination methods using 96 B. maydis isolates collected from various places of Jiangsu Province of China, and EC₅₀ values ranged from 0.0321 to 0.9149 μg/ml with the mean value of 0.3446 (±0.2248) μg/ml for mycelial growth, and 0.1864 to 0.964 μg/ml with the mean value of 0.5060 (±0.2094) μg/ml for conidium germination respectively. Treated with benzovindiflupyr, the distribution of nuclei and septum of hyphae did not change, but hyphae of offshoot and conidial production of B. maydis decreased significantly, the cell membrane permeability increased. The result of transmission electron microscope showed that the cross section of hypha was out of shape, the cell wall became thin and sparse, the cell membrane were distinctly damaged, organelles dissolved and vacuolated, and the cell nearly broke up. The results suggested that benzovindiflupyr had strong activity against mycelial growth and conidial production of B. maydis by damaging cell wall, membrane and organelles. The protective and curative activity assays for benzovindiflupyr indicated that benzovindiflupyr exhibited excellent suppression of B. maydis development on detached corn leaves. In protective activity assay with application of benzovindiflupyr at 10 μg/ml, the control efficacy reached to 100%. In curative activity assay with application of benzovindiflupyr at 50 μg/ml, the control efficacy reached to 90.72%. This is the first report of baseline sensitivity of B. maydis to benzovindiflupyr and its biological activity against B. maydis. It is recommended that benzovindiflupyr is a excellent candidate for controlling SCLB.

The Maize NBS-LRR Gene ZmNBS25 Enhances Disease Resistance in Rice and Arabidopsis

Nucleotide-binding site-leucine-rich repeat (NBS-LRR) domain proteins are immune sensors and play critical roles in plant disease resistance. In this study, we cloned and characterized a novel NBS-LRR gene ZmNBS25 in maize. We found that ZmNBS25 could response to pathogen inoculation and salicylic acid (SA) treatment in maize, and transient overexpression of ZmNBS25 induced a hypersensitive response in tobacco. High-performance liquid chromatography (HPLC) analysis showed that, compared to control plants, ZmNBS25 overexpression (ZmNBS25-OE) in Arabidopsis and rice resulted in higher SA levels. By triggering the expression of certain defense-responsive genes, ZmNBS25-OE enhanced the resistance of Arabidopsis and rice to Pseudomonas syringae pv. tomato DC3000 and sheath blight disease, respectively. Moreover, we found little change of grain size and 1000-grain weight between ZmNBS25-OE rice lines and controls. Together, our results suggest that ZmNBS25 can function as a disease resistance gene across different species, being a valuable candidate for engineering resistance in breeding programs.