Autophagy related protein MoAtg14 is involved in differentiation, development and pathogenicity in the rice blast fungus Magnaporthe oryzae 1Scientific RepoRts | 7 40018 | DOI 10 1038/srep40018 www na[.]
www.nature.com/scientificreports OPEN received: 11 April 2016 accepted: 01 December 2016 Published: 09 January 2017 Autophagy-related protein MoAtg14 is involved in differentiation, development and pathogenicity in the rice blast fungus Magnaporthe oryzae Xiao-Hong Liu1, Ya-Hui Zhao1, Xue-Ming Zhu1, Xiao-Qing Zeng4, Lu-Yao Huang1, Bo Dong5, Zhen-Zhu Su1,3, Yao Wang1, Jian-Ping Lu2 & Fu-Cheng Lin1 Autophagy is the major intracellular degradation system by which cytoplasmic materials are delivered to and degraded in the vacuole/lysosome in eukaryotic cells MoAtg14 in M oryzae, a hitherto uncharacterized protein, is the highly divergent homolog of the yeast Atg14 and the mammal BARKOR The MoATG14 deletion mutant exhibited collapse in the center of the colonies, poor conidiation and a complete loss of virulence Significantly, the ΔMoatg14 mutant showed delayed breakdown of glycogen, less lipid bodies, reduced turgor pressure in the appressorium and impaired conidial autophagic cell death The autophagic process was blocked in the ΔMoatg14 mutant, and the autophagic degradation of the marker protein GFP-MoAtg8 was interrupted GFP-MoAtg14 co-localized with mCherry-MoAtg8 in the aerial hypha In addition, a conserved coiled-coil domain was predicted in the N-terminal region of the MoAtg14 protein, a domain which could mediate the interaction between MoAtg14 and MoAtg6 The coiled-coil domain of the MoAtg14 protein is essential for its function in autophagy and pathogenicity Magnaporthe oryzae, the causal agent of rice blast, has been chosen as a model to study the interaction between fungi and plants Common to many other plant pathogenic fungi, M oryzae elaborates a signature penetration structure, the appressorium, to infect its host1–3 The whole infectious cycle of M oryzae, from surface recognition, adherence, and appressorium formation to infectious growth and pathogenicity, is closely related to signal transduction pathways and protein degradation processes The typical signal transduction pathways, including mitogen activated protein kinase (MAPK), cyclic adenosine monophosphate (cAMP), and calcium signal transduction pathways4–7, and the protein degradation processes, including autophagy8–10, ubiquitin mediated protein degradation11–13 and calpains14,15, have been confirmed to play significant roles in cell cycling, cellular differentiation and pathogenesis of M oryzae Autophagy is an intracellular degradation system that delivers cytoplasmic materials to the lysosome/vacuole during development and in response to nutrient stress in eukaryotic cells16 The autophagy process was verified as an essential catabolic process that plays important roles in cell stress management and nutrient homeostasis The differentiation, cell vitality, and infectious structures are impaired when the autophagy process is blocked Current studies have shown that the Atg proteins required for autophagy constitute the following five functional groups: (i) the Atg1 kinase complex (Atg1-13-17-29-31), (ii) the Atg9 membrane protein recycling system, (iii) the class III phosphatidylinositol 3-kinase (PI3-K) complex (Atg6-Atg14-Vps15-Vps34) (hereafter, State Key Laboratory for Rice Biology, Biotechnology Institute, Zhejiang University, Hangzhou, 310058, China College of Life Sciences, Zhejiang University, Hangzhou, 310058, China 3Agricultural Technology Extension Center, Zhejiang University, Hangzhou, 310058, China 4State Intellectual Property Office of the People’s Republic of China, Beijing, 100080, China 5State Key Laboratory of Breeding Base for Zhejiang Sustainable Pest and Disease Control, Institute of Virology and Biotechnology, Zhejiang Academy of Agricultural Sciences, Hangzhou 310021, Zhejiang Province, China Correspondence and requests for materials should be addressed to F.-C.L (email: fuchenglin@zju edu.cn) Scientific Reports | 7:40018 | DOI: 10.1038/srep40018 www.nature.com/scientificreports/ PI3-K indicates the class III PI3-K), (iv) the Atg12-Atg5-Atg16 protein conjugation system, and (v) the Atg8 lipid conjugation system16,17 For the past two decades, the core proteins of every autophagy group have been verified to function in M oryzae, for example, MoAtg1 in the Atg1 kinase complex18, MoAtg9 in the Atg9 membrane protein recycling system19, MoAtg5 in the conjugation system20, and MoAtg4 and MoAtg8 in the lipid conjugation system8,21,22 Extensive research has been performed on other plant pathogenic fungi, including Fusarium23–25, Colletotrichum26–28, Ustilago29,30, and Verticillium31 However, there are few studies of the PI3-K complex in plant pathogenic fungi PI3-K is essential for both autophagy and vacuolar protein sorting in yeast and mammals32–34 Atg14 is the autophagy-specific subunit of the PI3-K complex and a key factor in determining the function of the PI3-K complex Atg14 is present on the preautophagosomal structure (PAS) and is thought to function at an early stage of autophagosome formation Initially, Atg14 was only described in yeast species35 Recently, a protein with an extremely low similarity to yeast Atg14 was identified in humans and named ATG14/ATG14L/BARKOR36,37 In M oryzae, a conventional BLAST database search failed to identify a homolog of Atg14 Using a combination of Pfam domain analysis, position specific iterated (PSI)-BLAST and the pattern hit-initiated basic local alignment search tool (PHI-BLAST), MoAtg14 was identified in the database of M oryzae The conserved coiled-coil protein MGG_03698, designated MoAtg14 in the genome of M oryzae, was shown to have very weak similarity to ScAtg14 and HsAtg14 To date, no experimental evidence has emerged to explain the functions of the homolog of Atg14 in M oryzae In our research, we found that MoAtg14 was conserved in the filamentous ascomycetes Deletion of MoAtg14 resulted in defects in conidiation, breakdown of glycogen and lipid bodies, turgor pressure of appressoria, pathogenicity, and the autophagy process Subcellular localization and microscopic examination indicated that MoAtg14 is present on the PAS and plays key roles at the stage of autophagosome formation The localization of MoAtg8 was impaired in the MoAtg14 deletion mutant The conserved coiled-coil domain of MoAtg14 plays critical roles in M oryzae Results Identification of MoAtg14 in M oryzae. Pfam domain analysis of the M oryzae proteome was used to identify the proteins The integrated module of the Pfam domain was searched with the CLC Genomics Workbench (Qiagen, Germany) using the default parameters The Pfam database used in the analysis was version 27 MGG_03698 and MGG_13375 were found to contain the conserved domain PF10186 We reanalyzed protein databases at the NCBI by position specific iterated (PSI-BLAST) and pattern hit-initiated basic local alignment search tool (PHI-BLAST) using both yeast and human Atg14 The conserved coiled-coil protein MGG_03698 in the genome of M oryzae was confirmed to have weak similarity to ScAtg14 and HsAtg14 and was designated MoAtg14 The other protein, MGG_13375, showed more similarity to mammalian UVRAG proteins (a counterpart of the mammalian Vps38)37,38, implying that MGG_13375 might represent the fungal ortholog of Vps38 Analysis of the domain of MoAtg14 showed that it contains a conserved Cys-rich motif at its N-terminus (Fig. 1A) The motif is also present in yeast and human Atg14, and it displays high levels of similarity to homologs in other filamentous ascomycetes, including Gaeumannomyces graminis (55% identity), Colletotrichum orbiculare (50% identity), Fusarium graminearum (46% identity) and Blumeria graminis (39% identity) To verify the high similarity of MoAtg14 with Atg14 in other ascomycetes, we selected F graminearum Atg14 (FgAtg14) and Trichoderma reesei Atg14 (TrAtg14) to complement the ΔMoatg14 mutant Reintroduction of FgAtg14 or TrAtg14 to the mutant, the defects of the ΔMoatg14 mutant could be recovered completely (Figure S1) It has been reported that three predicted coiled-coil domains exist in the N-terminal half of yeast Atg14 These coiled-coil domains are sufficient to support the autophagic ability as revealed by deletion analysis of yeast Atg14 The second coiled-coil domain of yeast Atg14 interacted with Atg635,39 However, only one coiled-coil domain exists in the N-terminus of MoAtg14 in M oryzae as predicted by COILS (http://www.ch.embnet.org/software/ COILS_form.html) (Figs 1B and S2) Our research revealed the detailed functions of MoAtg14, as described below In addition, MoVps38 contains a coiled-coil domain (Figure S3) Unfortunately, we were not able to isolate a null mutant of MoVps38 To determine the expression profiles of the MoATG14 gene during development (in vegetative hyphae, conidia, and appressoria), pathogenicity (in infective hyphae) and starvation stress (in nitrogen starved hyphae), expression was evaluated using qRT-PCR assays (Fig. 1C) Compared with the expression level of MoATG14 in vegetative hyphae, in the nitrogen starved hyphae, the expression level was more than 3-fold higher In addition, the expression level of MoATG14 was more than 2-fold higher in 4 h -appressoria and invasive hyphae than in vegetative hyphae Deletion of MoATG14 in M oryzae. To determine the biological functions of MoATG14 in M oryzae, we constructed a deletion mutant by targeted gene replacement using ATMT (Fig. 2A) Southern blot assays were performed to confirm single-copy genomic integration and exclude additional ectopic integrations An approximately 3.2 kb band was detected in ΔMoatg14 mutants, in contrast to an approximately 6.5 kb band in the wild-type strain Guy11 (Fig. 2B) Two ΔMoatg14 mutants showed comparable phenotypes, and ΔMoatg14-1 was chosen for further studies Complementation assays of ΔMoatg14-1 were carried out, and the transformant Moatg14c, which contained a full-length gene copy of MoATG14, was selected for further studies MoAtg14 is required for hyphal development, conidiogenesis and pathogenicity. On CM plates, the ΔMoatg14 mutant showed vegetative growth similar to that of the wild-type strain Guy11, and the complemented strain Moatg14c However, the ΔMoatg14 mutant showed sparse hyphae with necrotic centrality, especially on the V8 and OMA media, in contrast to the dense hyphae of Guy11 and Moatg14c (Fig. 3A) On a 10-day-old CM plate, the number of conidia produced by the ΔMoatg14 mutant was only 1/50 of the number Scientific Reports | 7:40018 | DOI: 10.1038/srep40018 www.nature.com/scientificreports/ Figure 1. (A) The amino acid sequence of the N-terminal motif containing the conserved cysteine residues in the ascomycetes fungi The conserved cysteine residues are in the box The green line indicates the start of the conserved coiled-coil region GgAtg14, accession No XP_009224438; CgAtg14, accession No EQB48915; CoAtg14, accession No ENH80301; TvAtg14, accession No XP_013959553; TrAtg14, accession No XP_006966865; FoAtg14, accession No EMT61395; FgAtg14, accession No XP_011316371; BgAtg14, accession No EPQ63265; AoAtg14, accession No BAE65502; AfAtg14, accession No XP_747209; PrAtg14, accession No CDM36188 (B) The domains of the yeast ScAtg14 and M oryzae MoAtg14 Boxes in grey indicate the coiledcoil domains (C) The expression profiles of the MoATG14 gene in development, pathogenicity and starvation stress qRT-PCR assays were carried out with RNA samples obtained from different stages of the wild-type strain Guy11, including vegetative hyphae, conidia (CO), appressoria, invasive hyphae (IH) and nitrogen starved hyphae (MM-N) Gene expression levels were normalized using the β-tubulin gene as an internal standard Data are representative of at least two independent experiments with similar results, and the error bars represent the standard deviations of three replicates (P