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In ascomycetes, 1,8-dihydroxynaphthalene (DHN) melanin plays important protective functions and its production is usually coupled with development and environmental stress responses. The regulation of melanin biosynthesis, however, remains obscure. Colletotrichum fructicola is a phytopathogen with a broad host range that produces melanized appressoria and perithecia. In this study, we annotated melanin genes in a high-quality C. fructicola genome and characterized two zinc finger transcription factors (TFs) ( cmr1 and cmr2) that form a loosely organized gene cluster with several melanin biosynthesis genes. Deleting either TF abolished melanization in both mycelia and perithecia but did not affect appressoria. The deletion mutants also showed perithecial development defects. Overexpressing cmr1 in Δ cmr2 strongly activated the expression of melanin biosynthesis genes including pks1, scd1, t4hr1, and thr1 and caused hyper-accumulation of charcoal to black pigment(s). On the other hand, overexpressing cmr2 in Δ cmr1 activated pks1, t4hr1, and thr1, but not scd1. We conclude that proper DHN melanin accumulation in C. fructicola requires the cooperative function of two in-cluster TFs that also regulate perithecial development. The study clarifies DHN melanin regulations in C. fructicola and expands the function of melanin in-cluster TFs to sex regulation.

Glomerella leaf spot (GLS), caused by Colletotrichum fructicola, is a severe disease worldwide on apple, causing defoliation, leaf and fruit spot, and substantial yield loss. However, little is known about its molecular mechanisms of pathogenesis. Previous transcriptome analysis revealed that a transcription factor, CfMcm1, was induced during leaf infection. In the present work, expression pattern analysis verified that the CfMcm1 gene was strongly expressed in conidia and early infection. Phenotypic analysis revealed that the gene deletion mutant Δ CfMcm1 lost pathogenicity to apple leaves by inhibiting conidial germination and appressorium formation. In addition to appressorium-mediated pathogenicity, Δ CfMcm1 colonization and hyphal extension in wounded apple fruit was also reduced, and conidial germination mode and conidial color were altered. Δ CfMcm1 displayed impairment of cell wall integrity and response to stress caused by oxidation, osmosis, and an acid environment. Furthermore, the deletion mutant produced fewer and smaller perithecia and no ascospores. In contrast, melanin deposition in mycelia of Δ CfMcm1 was strengthened. Further comparative transcriptome and quantitative PCR analysis revealed that CfMcm1 modulated expression of genes related to conidial development ( CfERG5A, CfERG5B, CfHik5, and CfAbaA), appressorium formation ( CfCBP1 and CfCHS7), pectin degradation ( CfPelA and CfPelB), sexual development ( CfMYB, CfFork, CfHMG, and CfMAT1-2-1), and melanin biosynthesis ( CfCmr1, CfPKS1, CfT4HR1, CfTHR1, and CfSCD1). Our results demonstrated that CfMcm1 is a pivotal regulator possessing multiple functions in pathogenicity, asexual and sexual reproduction, and melanin biosynthesis.

With the rapid development of DC technology, the low resistance and strong instantaneous impact of DC fault caused great pressure to the power grid's construction and operation. The suppression and buffering of DC fault energy has positive significance to the safe and economic operation of DC grid. Taking advantage of thyristor's advantages in voltage and current withstanding level, cost, interruption ability and reliability, and the fast interruption ability of high-speed switch, a medium voltage DC hybrid current limiter topology is proposed in this paper. The current limiting principle of hybrid current limiter is analyzed, and the ability of current limiting combined with the model analysis results of the current limiter's key components is simulated and verified. Furthermore, the key influencing factors of the current limiting characteristics are analyzed. Finally, according to the characteristics of DC system, an integrated topology scheme of hybrid DC current limiting and interruption is proposed. The results show that the proposed topology is feasible and effective in field of current limiting capability and cost application.

Glomerella leaf spot (GLS), caused by Colletotrichum fructicola, is a rapidly emerging disease leading to defoliation, fruit spot, and storage fruit rot on apple in China. Little is known about the mechanisms of GLS pathogenesis. Early transcriptome analysis revealed that expression of the zinc finger transcription factor Ste12 gene in C. fructicola (CfSte12) was upregulated in appressoria and leaf infection. To investigate functions of CfSte12 during pathogenesis, we constructed gene deletion mutants (ΔCfSte12) by homologous recombination. Phenotypic analysis revealed that CfSte12 was involved in pathogenesis of nonwounded apple fruit and leaf, as well as wounded apple fruit. Subsequent histological studies revealed that loss of pathogenicity by ΔCfSte12 on apple leaf was expressed as defects of conidium germination, appressorium development, and appressorium-mediated penetration. Further RNA sequencing-based transcriptome comparison revealed that CfSte12 modulates the expression of genes related to appressorium function (e.g., genes for the tetraspanin PLS1, Gas1-like proteins, cutinases, and melanin biosynthesis) and candidate effectors likely involved in plant interaction. In sum, our results demonstrated that CfSte12 is a key regulator of early apple GLS pathogenesis in C. fructicola. In addition, CfSte12 is also needed for sexual development of perithecia and ascospores. IMPORTANCE Glomerella leaf spot (GLS) is an emerging fungal disease of apple that causes huge economic losses in Asia, North America, and South America. The damage inflicted by GLS manifests in rapid necrosis of leaves, severe defoliation, and necrotic spot on the fruit surface. However, few studies have addressed mechanisms of GLS pathogenesis. In this study, we identified and characterized a key pathogenicity-related transcription factor, CfSte12, of Colletotrichum fructicola that contributes to GLS pathogenesis. We provide evidence that the CfSte12 protein regulates many important pathogenic processes of GLS, including conidium germination, appressorium formation, appressorium-mediated penetration, and colonization. CfSte12 also impacts development of structures needed for sexual reproduction which are vital for the GLS disease cycle. These results reveal a key pathogenicity-related transcription factor, CfSte12, in C. fructicola that causes GLS.

Voltage Source Converter based High Voltage Direct Current (VSC-HVDC) transmission system is an important equipment for building smart power grid, as well as an important support for offshore wind power, isolated island power supply and grid integration of photovoltaic power generation. As a key equipment in VSC-HVDC, DC circuit breaker has attracted more and more attention. With the analysis of existing current DC circuit breaker, this paper proposed a technical solution of the hybrid DC circuit breaker based on cascaded Insulated Gate Bipolar Translator (IGBT) full bridge sub modules, introduced the topology of breaking principle, deduced the parameter design method of each component of circuit breaker, and analyzed the influence of several factors such as the number of cascaded modules of IGBT full bridge on the breaking performance of circuit breaker. In this solution, in normal flow state, the main branch with low on-state loss is composed of fast mechanical switch and fewer semiconductor devices. When the current is broken off, the current is transferred to the transfer branch through the cooperation of the main branch and the transfer branch semiconductor device. Then the capacitor is charged by the system voltage in order to achieve second transfer. Finally, the system energy is absorbed and dissipated by the energy dissipation branch. The technical solution achieved the fast break ability with short-circuit current peak value of 15kA under DC voltage of 10kV, and the breaking time is less than 3ms. The DC circuit breaker solution: 1) Adopt modular cascade design with good expansibility. 2) Adopt Bridge structure which can realize two-way breaking, and there is no need to adjust trigger time and device parameters for different short-circuit current. 3) Low on-state loss, externally manifested as low impedance. In summary, the proposed breaking solution of hybrid DC circuit breaker can quickly break off short-circuit current. It is one of the effective solutions for the future development of networking VSC-HVDC distribution system.

As the essential protection component in DC distribution system, medium voltage DC circuit breaker that needs to have the characteristics of breaking DC fault current quickly and reliably is the guarantee of the system safe operation. In this paper, taking advantage of IGCT's advantages of low on-state loss, fast turn-off speed and high reliability, a topology of medium voltage hybrid DC circuit breaker with high breaking current, fast breaking speed and high reliability is proposed, which combines the quick breaking performance of high-speed switch. The optimal parameters of the topology scheme are determined by studying the influence of arc voltage, the charging voltage of capacitor, capacitance and transfer reactor on the breaking characteristics and current transfer characteristics of hybrid circuit breaker. The circuit breaker proposed in this paper can realize the current breaking from 10 A to 20 kA under the conditions, the clearing time is less than 2 ms. The research results show that the topology of Hybrid DC circuit breaker proposed in this paper has good applicability and feasibility.

The repulsive mechanism has the advantages of short response time, fast opening speed and simple structure, and fully matches the short circuit breaking of the DC system to quickly establish the insulation gap, widely used in medium voltage DC circuit breakers. In this paper, a bistable permanent magnet repulsive mechanism of 10kV DC circuit breaker is proposed. The magnetostatic research is carried out for the permanent magnet holding mechanism and the repulsive mechanism, and based on the research results, the experimental prototype is built and the prototype experiment is completed. The reliability of the function of bistable permanent magnet repulsion mechanism is verified by the experiment. According to the breaking demand of 10kV DC circuit breaker, the overall design scheme of a bistable permanent magnet repulsive mechanism is proposed. According to the theory of electromagnetic field, the magnetostatic model and dynamic model of the permanent magnet mechanism are built. The influence of key structural parameters on the static retention characteristics is studied, and the structural parameters of the permanent magnet mechanism are determined. Based on the parameters calculated by the simulation, an experimental prototype of the bistable permanent magnet repulsive mechanism is built. The experimental scheme of the rated opening test and the rapid opening test is designed, and the prototype rated breaking curve, the quick opening response time curve and the quick opening characteristic curve are obtained. Comparing the experimental results with the requirements of the circuit breaker, the breaking characteristics of the repulsive mechanism can meet the two breaking requirements of the rated breaking and short-circuit breaking of the circuit breaker, and verify the feasibility of the prototype of the repulsive mechanism. The simulation results and experimental results verify the accuracy and practicability of the simulation model within an acceptable error range.

Due to its environment-friendless and excellent insulation performance, C 4 F 7 N and its mixtures with CO 2 have received extensive attention as SF 6 substitute gas in recent years. However, the experimental test on arc distinguish performance of C 4 F 7 N and its mixtures is quite expensive and time consuming. Therefore, it is very important to investigate the arc plasma behavior in C 4 F 7 N and to make a comparation with arc plasma in SF 6 by means of simulation. In this thesis, a 2D axial symmetry nozzle model based on the magneto-hydrodynamics theory has been built and calculated. The basic arc parameters are described by the coupled equations of fluid field and electromagnetic field. The temperature distribution of arc plasma in C 4 F 7 N, CO 2 and SF 6 has been calculated. The C 4 F 7 N arc has a similar arc radius to SF 6 arc in steady state. And the turbulence effect may need to be consider in the C 4 F 7 N arc simulation.

The Ascomycetes fungus Colletotrichum fructicola causes severe diseases on a wide range of crops, fruits, and vegetables. Its pathogenic mechanisms, however, remain poorly understood. Mitogen-activated protein kinases (MAPKs) are conserved regulators of fungal development and pathogenesis. In this study, a Fus3/Kss1-related MAPK from C. fructicola was functionally characterized via gene deletion. On potato dextrose agar (PDA) and oatmeal agar media, the CfPMK1 gene deletion mutants (ΔCfPMK1) were slightly reduced in radial growth rate, severely limited in aerial hyphal differentiation and hyphal melanization, and formed deformed perithecia that were smaller in size and more compactly organized relative to wild type. When artificially inoculated on plants, conidia of these mutants failed to differentiate appressoria or penetrate cuticle, and their pathogenicity defect could not be rescued by wounding plant tissue prior to inoculation. On PDA, ΔCfPMK1 mutants were hypersensitive to osmotic stresses, but were more tolerant to membrane and cell wall stresses. Genetic complementation rescued all phenotypic changes associated with CfPMK1 gene deletion. Based on GFP fusion expression, CfPMK1 protein accumulation was detected at all life stages, and the accumulation level was higher in nascent appressoria relative to conidia. Overall, this study identified CfPMK1 as a key regulator of appressorium and sexual development, pathogenesis, and stress tolerance in C. fructicola.

The Ascomycetes fungus Colletotrichum fructicola causes severe diseases on a wide range of crops, fruits, and vegetables. Its pathogenic mechanisms, however, remain poorly understood. Mitogen-activated protein kinases (MAPKs) are conserved regulators of fungal development and pathogenesis. In this study, a Fus3/Kss1-related MAPK from C. fructicola was functionally characterized via gene deletion. On potato dextrose agar (PDA) and oatmeal agar media, the CfPMK1 gene deletion mutants (ΔCfPMK1) were slightly reduced in radial growth rate, severely limited in aerial hyphal differentiation and hyphal melanization, and formed deformed perithecia that were smaller in size and more compactly organized relative to wild type. When artificially inoculated on plants, conidia of these mutants failed to differentiate appressoria or penetrate cuticle, and their pathogenicity defect could not be rescued by wounding plant tissue prior to inoculation. On PDA, ΔCfPMK1 mutants were hypersensitive to osmotic stresses, but were more tolerant to membrane and cell wall stresses. Genetic complementation rescued all phenotypic changes associated with CfPMK1 gene deletion. Based on GFP fusion expression, CfPMK1 protein accumulation was detected at all life stages, and the accumulation level was higher in nascent appressoria relative to conidia. Overall, this study identified CfPMK1 as a key regulator of appressorium and sexual development, pathogenesis, and stress tolerance in C. fructicola.

Fifty percent majority rule consensus Bayesian trees based on different gene concatenation schemes. (PDF 475 kb)

DNA polymorphism sites in the GS (glutamine synthetase) and SOD2 (manganese-superoxide dismutase) alignments of C. siamense s.l. isolates. (PDF 464 kb)

Primer design strategies for PCR-based identification of C. gloeosporioides s.l.. (PDF 394 kb)

SQ01 (C. siamense s.l.) mitogenome gene features and its sequence comparison with that of YT02 (C. siamense s.l.). (PDF 362 kb)

PCR amplification results of primers tested for differentiating C. gloeosporioides s.l. from C. actutatum s.l.. (PDF 1260 kb)

Maximum likelihood phylogenetic trees based on concatenated mitochondrial proteins and concatenated coding DNAs. (PDF 322 kb)

DNA sequence alignment of C. gloeosporioides s.l. mitogenomes at the short cox3 gene coding region (76â 218Â bp) where ZH03 (C. siamense s.l.) showed strong lineage-specific sequence divergence. (PDF 847 kb)

Background The fungal species complex Colletotrichum gloeosporioides sensu lato contains over 20 plant-interacting species. These species exhibit different life styles (e.g., endophytes, foliar and fruit pathogens) and show considerable variation in host and tissue adaptation strategies. Accurate species delimitation in C. gloeosporioides s.l. is very challenging due to nascent lineage boundaries and phenotypic plasticity, which strongly impedes studies of the complex’s host-interaction biology. In this study, we first sequenced and compared nine mitogenomes belonging to four C. gloeosporioides s.l. species lineages (C. gloeosporioides, C. fructicola, C. aenigma, and C. siamense s.l.), and evaluated the usefulness of mitogenome sequence in complementing prevailing nuclear markers for species delimitation. Results The C. gloeosporioides s.l. mitogenomes ranged between 52,671 and 58,666 bp in size, and each contained an identical set of genes transcribed in the same direction. Compared with previously reported Colletotrichum mitogenomes, these mitogenomes were uniquely featured by: (1) significantly larger genome size due to richer intron content and longer intergenic region; (2) striking GC content elevation at the intergenic region; and (3) considerable intron content variation among different species lineages. Compared with nuclear DNA markers commonly used in phylogeny, the mitogenome nucleotide diversity was extremely low, yet the mitogenome alignment contained the highest number of parsimony informative sites, which allowed the generation of a high-resolution phylogeny recognizing all taxonomic lineages, including ones belonging to the very nascent C. siamense s.l. complex. The tree topology was highly congruent with the phylogeny based on nuclear marker concatenation except for lineages within C. siamense s.l. Further comparative phylogenetic analysis indicated that lineage-specific rapid divergence of GS and SOD2 markers confounded concatenation-based species relationship inference. Conclusions This study sheds light on the evolution of C. gloeosporioides s.l. mitogenomes and demonstrates that mitogenome sequence can complement prevailing nuclear markers in improving species delimitation accuracy. The mitogenome sequences reported will be valuable resources for further genetic studies with C. gloeosporioides s.l. and other Colletotrichum species. Electronic supplementary material The online version of this article (doi:10.1186/s12864-016-3480-x) contains supplementary material, which is available to authorized users.