Expression of the fluorescent proteins DsRed and EGFP to visualize early events of colonization of the chickpea blight fungus Ascochyta rabiei

The nuclear effector ArPEC25 from the necrotrophic fungus Ascochyta rabiei targets the chickpea transcription factor CaβLIM1a and negatively modulates lignin biosynthesis, increasing host susceptibility

Fungal pathogens deploy a barrage of secreted effectors to subvert host immunity, often by evading, disrupting, or altering key components of transcription, defense signaling, and metabolic pathways. However, the underlying mechanisms of effectors and their host targets are largely unexplored in necrotrophic fungal pathogens. Here, we describe the effector protein Ascochyta rabiei PEXEL-like Effector Candidate 25 (ArPEC25), which is secreted by the necrotroph A. rabiei, the causal agent of Ascochyta blight disease in chickpea (Cicer arietinum), and is indispensable for virulence. After entering host cells, ArPEC25 localizes to the nucleus and targets the host LIM transcription factor CaβLIM1a. CaβLIM1a is a transcriptional regulator of CaPAL1, which encodes phenylalanine ammonia lyase (PAL), the regulatory, gatekeeping enzyme of the phenylpropanoid pathway. ArPEC25 inhibits the transactivation of CaβLIM1a by interfering with its DNA-binding ability, resulting in negative regulation of the phenylpropanoid pathway and decreased levels of intermediates of lignin biosynthesis, thereby suppressing lignin production. Our findings illustrate the role of fungal effectors in enhancing virulence by targeting a key defense pathway that leads to the biosynthesis of various secondary metabolites and antifungal compounds. This study provides a template for the study of less explored necrotrophic effectors and their host target functions.

Invasion and Colonization of Pathogenic Fusarium oxysporum R1 in Crocus sativus L. during Corm Rot Disease Progression

The corm rot of saffron caused by Fusarium oxysporum (Fox) has been reported to be the most destructive fungal disease of the herb globally. The pathogen, Fusarium oxysporum R1 (Fox R1) isolated by our group from Kashmir, India, was found to be different from Fusarium oxysporum f.sp. gladioli commonly reported corm rot agent of saffron. In the present study, Fox R1 was further characterized using housekeeping genes and pathogenicity tests, as Fusarium oxysporum R1 f.sp. iridacearum race 4. Though Fox R1 invaded the saffron plant through both corm and roots, the corm was found to be the preferred site of infection. In addition, the route of pathogen movement wastracked by monitoring visual symptoms, semi-quantitative PCR, quantitative-PCR (q-PCR), real-time imaging of egfp-tagged Fusarium oxysporum R1, and Fox R1 load quantification. This study is the first study of its kind on the bidirectional pathogenesis from corm to roots and vice-versa, as the literature only reports unidirectional upward movement from roots to other parts of the plant. In addition, the colonization pattern of Fox R1 in saffron corms and roots was studied. The present study involved a systematic elucidation of the mode and mechanism of pathogenesis in the saffron Fusarium oxysporum strain R1 pathosystem.

Characterization of a GH5 endoxylanase from Penicillium funiculosum and its synergism with GH16 endo-1,3(4)-glucanase in saccharification of sugarcane bagasse

The production of second-generation fuels from lignocellulosic residues such as sugarcane bagasse (SCB) requires the synergistic interaction of key cellulose-degrading enzymes and accessory proteins for their complete deconstruction to useful monomeric sugars. Here, we recombinantly expressed and characterized unknown GH5 xylanase from P. funiculosum (PfXyn5) in Pichia pastoris, which was earlier found in our study to be highly implicated in SCB saccharification. The PfXyn5 has a molecular mass of ~ 55 kDa and showed broad activity against a range of substrates like xylan, xyloglucan, laminarin and p-nitrophenyl-β-D-xylopyranoside, with the highest specific activity of 0.7 U/mg against xylan at pH 4.5 and 50 °C. Analysis of the degradation products of xylan and SCB by PfXyn5 showed significant production of xylooligosaccharides (XOS) with a degree of polymerization (DP) ranging from two (DP₂) to six (DP₆), thus, suggesting that the PfXyn5 is an endo-acting enzyme. The enzyme synergistically improved the saccharification of SCB when combined with the crude cellulase cocktail of P. funiculosum with a degree of synergism up to 1.32. The PfXyn5 was further expressed individually and simultaneously with a notable GH16 endoglucanase (PfEgl16) in a catabolite-derepressed strain of P. funiculosum, PfMig1⁸⁸, and the saccharification efficiency of the secretomes from the resulting transformants were investigated on SCB. The secretome of PfMig1⁸⁸ overexpressing Xyn5 or Egl16 increased the saccharification of SCB by 9% or 7%, respectively, over the secretome of PfMig1⁸⁸, while the secretome of dual transformant increased SCB saccharification by ~ 15% at the same minimal protein concentration.

Ascochyta rabiei: A threat to global chickpea production

The necrotrophic fungus Ascochyta rabiei causes Ascochyta blight (AB) disease in chickpea. A. rabiei infects all aerial parts of the plant, which results in severe yield loss. At present, AB disease occurs in most chickpea-growing countries. Globally increased incidences of A. rabiei infection and the emergence of new aggressive isolates directed the interest of researchers toward understanding the evolution of pathogenic determinants in this fungus. In this review, we summarize the molecular and genetic studies of the pathogen along with approaches that are helping in combating the disease. Possible areas of future research are also suggested.

TAXONOMY

kingdom Mycota, phylum Ascomycota, class Dothideomycetes, subclass Coelomycetes, order Pleosporales, family Didymellaceae, genus Ascochyta, species rabiei.

PRIMARY HOST

A. rabiei survives primarily on Cicer species.

DISEASE SYMPTOMS

A. rabiei infects aboveground parts of the plant including leaves, petioles, stems, pods, and seeds. The disease symptoms first appear as watersoaked lesions on the leaves and stems, which turn brown or dark brown. Early symptoms include small circular necrotic lesions visible on the leaves and oval brown lesions on the stem. At later stages of infection, the lesions may girdle the stem and the region above the girdle falls off. The disease severity increases at the reproductive stage and rounded lesions with concentric rings, due to asexual structures called pycnidia, appear on leaves, stems, and pods. The infected pod becomes blighted and often results in shrivelled and infected seeds.

DISEASE MANAGEMENT STRATEGIES

Crop failures may be avoided by judicious practices of integrated disease management based on the use of resistant or tolerant cultivars and growing chickpea in areas where conditions are least favourable for AB disease development. Use of healthy seeds free of A. rabiei, seed treatments with fungicides, and proper destruction of diseased stubbles can also reduce the fungal inoculum load. Crop rotation with nonhost crops is critical for controlling the disease. Planting moderately resistant cultivars and prudent application of fungicides is also a way to combat AB disease. However, the scarcity of AB-resistant accessions and the continuous evolution of the pathogen challenges the disease management process.

USEFUL WEBSITES

https://www.ndsu.edu/pubweb/pulse-info/resourcespdf/Ascochyta%20blight%20of%20chickpea.pdf https://saskpulse.com/files/newsletters/180531_ascochyta_in_chickpeas-compressed.pdf http://www.pulseaus.com.au/growing-pulses/bmp/chickpea/ascochyta-blight http://agriculture.vic.gov.au/agriculture/pests-diseases-and-weeds/plant-diseases/grains-pulses-and-cereals/ascochyta-blight-of-chickpea http://www.croppro.com.au/crop_disease_manual/ch05s02.php https://www.northernpulse.com/uploads/resources/722/handout-chickpeaascochyta-nov13-2011.pdf http://oar.icrisat.org/184/1/24_2010_IB_no_82_Host_Plant https://www.crop.bayer.com.au/find-crop-solutions/by-pest/diseases/ascochyta-blight.

Optimizing the Agrobacterium tumifaciens mediated transformation conditions in Colletotrichum lindemuthianum: A step forward to unravel the functions of pathogenicity arsenals

Colletotrichum lindemuthianum is a hemibiotrophic fungal pathogen that causes bean anthracnose and it is rated among the top 10 important diseases infecting beans. Currently our knowledge on molecular mechanisms underlying C. lindemuthianum pathogenesis is limited. About five pathogenicity genes have been identified in C. lindemuthianum using Restricted Enzyme Mediated Integration (REMI) and the transformation using Agroinfection has not been optimized. In this study, a series of experiments were conducted to optimize the key parameters affecting the Agrobacterium tumefaciens- mediated transformation (ATMT) for C. lindemuthianum. The transformation efficiency increased with increase in spore concentration and co-cultivation time. However, the optimum conditions that yielded significant number of transformants were 10⁶ ml^-1 spore concentration, co-cultivation time of 72 h, incubation at 25ºC and using a cellulose membrane filter for the co-cultivation. The optimized protocol resulted in establishment of large mutant library (2400). A few mutants were melanin deficient and a few were unable to produce conidia. To determine the altered pathogenicity, two new approaches such as detached leaf and twig techniques proved reliable and require fewer resources to screen the large mutant libraries in a short time. Among the 1200 transformants tested for virulence, 90% transformants were pathogenically similar to wild type (race 2047), 96 and 24 were reduced and impaired, respectively. The altered avirulent transformants can prove vital for understanding the missing link between growth and developmental stages of pathogen with virulence. This platform will help to develop strategies to determine the potential pathogenicity genes and to decipher molecular mechanisms of host-pathogen interactions in more detail.

Modulation of fungal virulence through CRZ1 regulated F-BAR-dependent actin remodeling and endocytosis in chickpea infecting phytopathogen Ascochyta rabiei

Polarized hyphal growth of filamentous pathogenic fungi is an essential event for host penetration and colonization. The long-range early endosomal trafficking during hyphal growth is crucial for nutrient uptake, sensing of host-specific cues, and regulation of effector production. Bin1/Amphiphysin/Rvs167 (BAR) domain-containing proteins mediate fundamental cellular processes, including membrane remodeling and endocytosis. Here, we identified a F-BAR domain protein (ArF-BAR) in the necrotrophic fungus Ascochyta rabiei and demonstrate its involvement in endosome-dependent fungal virulence on the host plant Cicer arietinum. We show that ArF-BAR regulates endocytosis at the hyphal tip, localizes to the early endosomes, and is involved in actin dynamics. Functional studies involving gene knockout and complementation experiments reveal that ArF-BAR is necessary for virulence. The loss-of-function of ArF-BAR gene results in delayed formation of apical septum in fungal cells near growing hyphal tip that is crucial for host penetration, and impaired secretion of a candidate effector having secretory signal peptide for translocation across the endoplasmic reticulum membrane. The mRNA transcripts of ArF-BAR were induced in response to oxidative stress and infection. We also show that ArF-BAR is able to tubulate synthetic liposomes, suggesting the functional role of F-BAR domain in membrane tubule formation in vivo. Further, our studies identified a stress-induced transcription factor, ArCRZ1 (Calcineurin-responsive zinc finger 1), as key transcriptional regulator of ArF-BAR expression. We propose a model in which ArCRZ1 functions upstream of ArF-BAR to regulate A. rabiei virulence through a mechanism that involves endocytosis, effector secretion, and actin cytoskeleton regulation.

Blocking drug efflux mechanisms facilitate genome engineering process in hypercellulolytic fungus, Penicillium funiculosum NCIM1228

BACKGROUND

Penicillium funiculosum NCIM1228 is a non-model filamentous fungus that produces high-quality secretome for lignocellulosic biomass saccharification. Despite having desirable traits to be an industrial workhorse, P. funiculosum has been underestimated due to a lack of reliable genetic engineering tools. Tolerance towards common fungal antibiotics had been one of the major hindrances towards development of reliable transformation tools against the non-model fungi. In this study, we sought to understand the mechanism of drug tolerance of P. funiculosum and the provision to counter it. We then attempted to identify a robust method of transformation for genome engineering of this fungus.

RESULTS

Penicillium funiculosum showed a high degree of drug tolerance towards hygromycin, zeocin and nourseothricin, thereby hindering their use as selectable markers to obtain recombinant transformants. Transcriptome analysis suggested a high level expression of efflux pumps belonging to ABC and MFS family, especially when complex carbon was used in growth media. Antibiotic selection medium was optimized using a combination of efflux pump inhibitors and suitable carbon source to prevent drug tolerability. Protoplast-mediated and Agrobacterium-mediated transformation were attempted for identifying efficiencies of linear and circular DNA in performing genetic manipulation. After finding Ti-plasmid-based Agrobacterium-mediated transformation more suitable for P. funiculosum, we improvised the system to achieve random and homologous recombination-based gene integration and deletion, respectively. We found single-copy random integration of the T-DNA cassette and could achieve 60% efficiency in homologous recombination-based gene deletions. A faster, plasmid-free, and protoplast-based CRISPR/Cas9 gene-editing system was also developed for P. funiculosum. To show its utility in P. funiculosum, we deleted the gene coding for the most abundant cellulase Cellobiohydrolase I (CBH1) using a pair of sgRNA directed towards both ends of cbh1 open reading frame. Functional analysis of ∆cbh1 strain revealed its essentiality for the cellulolytic trait of P. funiculosum secretome.

CONCLUSIONS

In this study, we addressed drug tolerability of P. funiculosum and developed an optimized toolkit for its genome modification. Hence, we set the foundation for gene function analysis and further genetic improvements of P. funiculosum using both traditional and advanced methods.

Synergistic Action of a Lytic Polysaccharide Monooxygenase and a Cellobiohydrolase from Penicillium funiculosum in Cellulose Saccharification under High-Level Substrate Loading

Lytic polysaccharide monooxygenases (LPMOs) are crucial industrial enzymes required in the biorefinery industry as well as in the natural carbon cycle. These enzymes, known to catalyze the oxidative cleavage of glycosidic bonds, are produced by numerous bacterial and fungal species to assist in the degradation of cellulosic biomass. In this study, we annotated and performed structural analysis of an uncharacterized LPMO from Penicillium funiculosum (PfLPMO9) based on computational methods in an attempt to understand the behavior of this enzyme in biomass degradation. PfLPMO9 exhibited 75% and 36% sequence identities with LPMOs from Thermoascus aurantiacus (TaLPMO9A) and Lentinus similis (LsLPMO9A), respectively. Furthermore, multiple fungal genetic manipulation tools were employed to simultaneously overexpress LPMO and cellobiohydrolase I (CBH1) in a catabolite-derepressed strain of Penicillium funiculosum, PfMig1⁸⁸ (an engineered variant of P. funiculosum), to improve its saccharification performance toward acid-pretreated wheat straw (PWS) at 20% substrate loading. The resulting transformants showed improved LPMO and CBH1 expression at both the transcriptional and translational levels, with ∼200% and ∼66% increases in ascorbate-induced LPMO and Avicelase activities, respectively. While the secretome of PfMig⁸⁸ overexpressing LPMO or CBH1 increased the saccharification of PWS by 6% or 13%, respectively, over the secretome of PfMig1⁸⁸ at the same protein concentration, the simultaneous overexpression of these two genes led to a 20% increase in saccharification efficiency over that observed with PfMig1⁸⁸, which accounted for 82% saccharification of PWS under 20% substrate loading.IMPORTANCE The enzymatic hydrolysis of cellulosic biomass by cellulases continues to be a significant bottleneck in the development of second-generation biobased industries. While increasing efforts are being made to obtain indigenous cellulases for biomass hydrolysis, the high production cost of this enzyme remains a crucial challenge affecting its wide availability for the efficient utilization of cellulosic materials. This is because it is challenging to obtain an enzymatic cocktail with balanced activity from a single host. This report describes the annotation and structural analysis of an uncharacterized lytic polysaccharide monooxygenase (LPMO) gene in Penicillium funiculosum and its impact on biomass deconstruction upon overexpression in a catabolite-derepressed strain of P. funiculosum Cellobiohydrolase I (CBH1), which is the most important enzyme produced by many cellulolytic fungi for the saccharification of crystalline cellulose, was further overexpressed simultaneously with LPMO. The resulting secretome was analyzed for enhanced LPMO and exocellulase activities and the corresponding improvement in saccharification performance (by ∼20%) under high-level substrate loading using a minimal amount of protein.

Thyrostroma carpophilum insertional mutagenesis: A step towards understanding its pathogenicity mechanism

Thyrostroma carpophilum, a causal agent of shot hole disease of stone fruits, cause severe loss in economically important fruit crops of Kashmir. Understanding its pathogenesis at molecular level will aid in devising a better management strategy. In this study, we optimized Agrobacterium tumefaciens mediated transformation (ATMT) conditions for T. carpophilum using PBIF2-EGFP construct. Using this protocol, we obtained 328 positive transformants per 10⁴ spores and subsequent sub-culturing of transformants on selective and non-selective media resulted in stable T-DNA integration. Southern blot analysis revealed that most of the transformants embodied single T-DNA integration. Using this method, we obtained a small-scale transformant library (2050 transformants). Among this pool, we tested 1005 transformants for their pathogenicity; out of which 185 showed complete pathogenicity loss, 35 displayed reduced virulence and 785 were pathogenically similar to wild type. Out of this experimental stock, three transformants from each category were randomly selected to dissect the infection assay. The findings deciphered that transformants with complete pathogenicity loss failed to penetrate the host tissue and a few transformants failed to sporulate in laboratory. Transformants from reduced category could not form appressorium and occasionally sporulated. Transformants similar to wild type were morphologically and pathogenically similar to wild type because of un-alteration in their modus operandi. Our work provides a new platform to understand the pathogenicity mechanism of T. carpophilum. The optimized ATMT protocol will help in developing large transformant library that can help to identify the virulence arsenals necessary for the pathogen to cause disease.

Transcript profiling reveals potential regulators for oxidative stress response of a necrotrophic chickpea pathogen Ascochyta rabiei

Necrotrophic pathogens experience host-generated oxidative stress during pathogenesis. They overcome such hostile environment by intricate mechanisms which are largely understudied. In this article, reference-based transcriptome analysis of a devastating Ascochyta Blight (AB) disease causing chickpea pathogen Ascochyta rabiei was explored to get insights into survival mechanisms under oxidative stress. Here, expression profiling of mock-treated and menadione-treated fungus was carried out by RNA-Seq approach. A significant number of genes in response to oxidative stress were overrepresented, suggestive of a robust and coordinated defense system of A. rabiei. A total 73 differentially expressed genes were filtered out from both the transcriptomes, among them 64 were up-regulated and 9 were found down-regulated. The gene ontology and KEGG mapping were conducted to comprehend the possible regulatory roles of differentially expressed genes in metabolic networks and biosynthetic pathways. Transcript profiling, KEGG pathway and gene ontology-based enrichment analysis revealed 12 (16.43%) stress responsive factors, 25 (34.24%) virulence associated genes, 10 (13.69%) putative effectors and 28 (38.35%) important interacting proteins associated with various metabolic pathways. In addition, genes with differential expression were further explored for underlying putative pathogenicity factors. We identified five genes ST47_g10291, ST47_g9396, ST47_g10294, ST47_g4395, and ST47_g7191 that were common to stress and fungal pathogenicity. The factors recognized in this work can be used to establish molecular tools to explain the regulatory gene networks engaged in stress response of fungal pathogens and disease management.

Generation of Epichloë Strains Expressing Fluorescent Proteins Suitable for Studying Host-Endophyte Interactions and Characterisation of a T-DNA Integration Event

Methods for the identification and localisation of endophytic fungi are required to study the establishment, development, and progression of host-symbiont interactions, as visible reactions or disease symptoms are generally absent from host plants. Fluorescent proteins have proved valuable as reporter gene products, allowing non-invasive detection in living cells. This study reports the introduction of genes for two fluorescent proteins, green fluorescent protein (GFP) and red fluorescent protein, DsRed, into the genomes of two distinct perennial ryegrass (Lolium perenne L.)-associated Epichloë endophyte strains using A. tumefaciens-mediated transformation. Comprehensive characterisation of reporter gene-containing endophyte strains was performed using molecular genetic, phenotypic, and bioinformatic tools. A combination of long read and short read sequencing of a selected transformant identified a single complex T-DNA insert of 35,530 bp containing multiple T-DNAs linked together. This approach allowed for comprehensive characterisation of T-DNA integration to single-base resolution, while revealing the unanticipated nature of T-DNA integration in the transformant analysed. These reporter gene endophyte strains were able to establish and maintain stable symbiotum with the host. In addition, the same endophyte strain labelled with two different fluorescent proteins were able to cohabit the same plant. This knowledge can be used to provide the basis to develop strategies to gain new insights into the host-endophyte interaction through independent and simultaneous monitoring in planta throughout its life cycle in greater detail.

Agrobacterium tumefaciens-mediated transformation and expression of GFP in Ascochyta lentis to characterize ascochyta blight disease progression in lentil

The plant immune system is made up of a complex response network that involves several lines of defense to fight invading pathogens. Fungal plant pathogens on the other hand, have evolved a range of ways to infect their host. The interaction between Ascochyta lentis and two lentil genotypes was explored to investigate the progression of ascochyta blight (AB) in lentils. In this study, we developed an Agrobacterium tumefaciens-mediated transformation system for A. lentis by constructing a new binary vector, pATMT-GpdGFP, for the constitutive expression of green fluorescent protein (EGFP). Green fluorescence was used as a highly efficient vital marker to study the developmental changes in A. lentis during AB disease progression on the susceptible and resistant lentil accessions, ILL6002 and ILL7537, respectively. The initial infection stages were similar in both the resistant and susceptible accessions where A. lentis uses infection structures such as germ tubes and appressoria to gain entry into the host while the host uses defense mechanisms to prevent pathogen entry. Penetration was observed at the junctions between neighbouring epidermal cells and occasionally, through the stomata. The pathogen attempted to penetrate and colonize ILL7537, but further fungal advancement appeared to be halted, and A. lentis did not enter the mesophyll. Successful entry and colonization of ILL6002 coincided with structural changes in A. lentis and the onset of necrotic lesions 5–7 days post inoculation. Once inside the leaf, A. lentis continued to grow, colonizing all parts of the leaf followed by plant cell collapse. Pycnidia-bearing spores appeared 14 days post inoculation, which marks the completion of the infection cycle. The use of fluorescent proteins in plant pathogenic fungi together with confocal laser scanning microscopy, provide a valuable tool to study the intracellular dynamics, colonization strategy and infection mechanisms during plant-pathogen interaction.

Genetic manipulation of Fonsecaea pedrosoi using particles bombardment and Agrobacterium mediated transformation

Fonsecaea pedrosoi, a melanized fungal pathogen that causes Chromoblastomycosis, a human disease with a worldwide distribution. Biolistic is a widely used technique for direct delivery of genetic material into intact cells by particles bombardment. Another well-established transformation method is Agrobacterium-mediated transformation (ATMT), which involves the transfer of a T-DNA from the bacterium to the target cells. In F. pedrosoi there are no reports of established protocols for genetic transformation, which require optimization of physical and biological parameters. In this work, intact conidia of F. pedrosoi were particle bombarded and subjected to ATMT. In addition, we proposed hygromycin B, nourseothricin and neomycin as dominant selective markers for F. pedrosoi and vectors were constructed. We tested two parameters for biolistic: the distance of the particles to the target cells and time of cells recovery in nonselective medium. The biolistic efficiency was 37 transformants/μg of pFpHYG, and 45 transformants/μg of pAN7.1. Transformants expressing GFP were successfully obtained by biolistic. A co-culture ratio of 10: 1 (bacterium: conidia) and co-incubation time of 72 h yielded the largest number of transformants after ATMT. Southern blot analysis showed the number of foreign DNA insertion into the genome is dependent upon the plasmid used to generate the mutants. This work describes for the first time two efficient methods for genetic modification of Fonsecaea and these results open new avenues to better understand the biology and pathogenicity of the main causal agent of this neglected disease.

A highly efficient Agrobacterium tumefaciens-mediated transformation system for the postharvest pathogen Penicillium digitatum using DsRed and GFP to visualize citrus host colonization

Penicillium digitatum is a major postharvest pathogen of citrus crops. This fungus broadly spreads worldwide and causes green mold disease, which results in severe losses for citrus production. Understanding of the citrus infection by P. digitatum may help develop effective strategies for controlling this pathogen. In this study, we have characterized a virulent strain of P. digitatum isolated in Vietnam and established a highly efficient Agrobacterium tumefaciens-mediated transformation (ATMT) system for this fungal strain with two newly constructed binary vectors. These binary vectors harbor dominant selectable markers for hygromycin or nourseothricin resistance, and expression cassettes for the red fluorescent protein (DsRed) or the green fluorescent protein (GFP), respectively. Using the established ATMT system, the transformation efficiency of the Vietnamese strain could reach a very high yield of 1240±165 transformants per 10⁶ spores. Interestingly, we found that GFP is much better than DsRed for in situ visualization of citrus fruit colonization by the fungus. Additionally, we showed that the transformation system can also be used to generate T-DNA insertion mutants for screening non-pathogenic or less virulent strains. Our work provides a new platform including a virulent tropical strain of P. digitatum, an optimized ATMT method and two newly constructed binary vectors for investigation of the postharvest pathogen. This platform will help develop strategies to dissect molecular mechanisms of host-pathogen interactions in more detail as well as to identify potential genes of pathogenicity by either insertional mutagenesis or gene disruption in this important pathogenic fungus.

Functional analysis of diacylglycerol O-acyl transferase 2 gene to decipher its role in virulence of Botrytis cinerea

Gray mold disease inflicted by Botrytis cinerea is a serious menace responsible for significant economic loss worldwide. Due to its polyphagous nature, the pathogen has enthused inquisitiveness in researchers to unravel its complexity. Agrobacterium tumefaciens-mediated transformation was used to generate insertional mutants of Botrytis cinerea. A mutant (BCM-55) with disruption in a gene (BcDGAT2) that encodes for diacylglycerol O-acyl transferase 2 (DGAT2), showed enervated virulence on various hosts' tissues. Enzyme DGAT2 is crucial in the final step of synthesis of triacylglycerol (TAG) that plays an important role in homeostasis of membrane and cellular processes. However, the role of DGAT2 has never been reported in a phytopathogenic fungus. In this study, BCM-55 was characterized to ascertain the role of DGAT2 in virulence of B. cinerea. The insertional mutant was defective in spore production and lacked sclerotia formation as a consequence of lower accumulation of TAG. A significant delay in spore germination in BCM-55 was accompanied with a low penetration potential. Hyphae of the mutant formed swollen endings with considerable impairment in penetration. Deletion of BcDGAT2 also led to increased sensitivity towards cell wall and membrane-disturbing agents. Furthermore, BCM-55 was deficient in the production of oxalic acid and showed lower activity of a cell wall-degrading enzyme, polygalacturonase. The role of BcDGAT2 in virulence was further confirmed by targeted deletion and complementation of the gene. The results insinuate a crucial role of BcDGAT2 in penetration and consequently virulence of B. cinerea. The study provides novel insights into plant-pathogen interactions that can be exploited to develop suitable disease management strategies.

Transcription Factor Repertoire of Necrotrophic Fungal Phytopathogen Ascochyta rabiei: Predominance of MYB Transcription Factors As Potential Regulators of Secretome

Transcription factors (TFs) are the key players in gene expression and their study is highly significant for shedding light on the molecular mechanisms and evolutionary history of organisms. During host-pathogen interaction, extensive reprogramming of gene expression facilitated by TFs is likely to occur in both host and pathogen. To date, the knowledge about TF repertoire in filamentous fungi is in infancy. The necrotrophic fungus Ascochyta rabiei, that causes destructive Ascochyta blight (AB) disease of chickpea (Cicer arietinum), demands more comprehensive study for better understanding of Ascochyta-legume pathosystem. In the present study, we performed the genome-wide identification and analysis of TFs in A. rabiei. Taking advantage of A. rabiei genome sequence, we used a bioinformatic approach to predict the TF repertoire of A. rabiei. For identification and classification of A. rabiei TFs, we designed a comprehensive pipeline using a combination of BLAST and InterProScan software. A total of 381 A. rabiei TFs were predicted and divided into 32 fungal specific families of TFs. The gene structure, domain organization and phylogenetic analysis of abundant families of A. rabiei TFs were also carried out. Comparative study of A. rabiei TFs with that of other necrotrophic, biotrophic, hemibiotrophic, symbiotic, and saprotrophic fungi was performed. It suggested presence of both conserved as well as unique features among them. Moreover, cis-acting elements on promoter sequences of earlier predicted A. rabiei secretome were also identified. With the help of published A. rabiei transcriptome data, the differential expression of TF and secretory protein coding genes was analyzed. Furthermore, comprehensive expression analysis of few selected A. rabiei TFs using quantitative real-time polymerase chain reaction revealed variety of expression patterns during host colonization. These genes were expressed in at least one of the time points tested post infection. Overall, this study illustrates the first genome-wide identification and analysis of TF repertoire of A. rabiei. This work would provide the basis for further studies to dissect role of TFs in the molecular mechanisms during A. rabiei-chickpea interactions.

Variation in Growth, Colonization of Maize, and Metabolic Parameters of GFP- and DsRed-Labeled Fusarium verticillioides Strains

Autofluorescent proteins are frequently applied as visual markers in the labeling of filamentous fungi. Genes gfp and DsRed were transformed into the genome of Fusarium verticillioides via the Agrobacterium tumefaciens-mediated transformation method. The selected transformants displayed a bright green or red fluorescence in all the organelles of the growing fungal mycelia and spores (except for the vacuoles) both in cultures and in the maize (Zea mays) roots they colonized. The results of gene-specific polymerase chain reaction (PCR) analysis and the thermal asymmetrical interlaced (TAIL)-PCR analysis demonstrated that gfp and DsRed were integrated on different chromosomes of the fungus. Reductions in the colony growth on the plates at pH 4.0 and 5.5 was observed for the green fluorescent protein (GFP)-transformant G3 and the DsRed-transformant R4, but transformants G4 and R1 grew as well as the wild-type strain at pH 4.0. The speed of growth of all the transformants was similar to the wild-type strain at pH ≥ 7. The insertion of gfp and DsRed did not alter the production of extracellular enzymes and fumonisin B by F. verticillioides. The transformants expressing GFP and DsRed proteins were able to colonize maize roots. However, the four transformants examined produced fewer CFU in the root samples than the wild-type strain during a sampling period of 7 to 28 days after inoculation.

Draft genome sequencing and secretome analysis of fungal phytopathogen Ascochyta rabiei provides insight into the necrotrophic effector repertoire

Constant evolutionary pressure acting on pathogens refines their molecular strategies to attain successful pathogenesis. Recent studies have shown that pathogenicity mechanisms of necrotrophic fungi are far more intricate than earlier evaluated. However, only a few studies have explored necrotrophic fungal pathogens. Ascochyta rabiei is a necrotrophic fungus that causes devastating blight disease of chickpea (Cicer arietinum). Here, we report a 34.6 megabase draft genome assembly of A. rabiei. The genome assembly covered more than 99% of the gene space and 4,259 simple sequence repeats were identified in the assembly. A total of 10,596 high confidence protein-coding genes were predicted which includes a large and diverse inventory of secretory proteins, transporters and primary and secondary metabolism enzymes reflecting the necrotrophic lifestyle of A. rabiei. A wide range of genes encoding carbohydrate-active enzymes capable for degradation of complex polysaccharides were also identified. Comprehensive analysis predicted a set of 758 secretory proteins including both classical and non-classical secreted proteins. Several of these predicted secretory proteins showed high cysteine content and numerous tandem repeats. Together, our analyses would broadly expand our knowledge and offer insights into the pathogenesis and necrotrophic lifestyle of fungal phytopathogens.

Comparative transcriptome analysis of the necrotrophic fungus Ascochyta rabiei during oxidative stress: insight for fungal survival in the host plant

Localized cell death, known as the hypersensitive response (HR), is an important defense mechanism for neutralizing phytopathogens. The hallmark of the HR is an oxidative burst produced by the host plant. We aimed to identify genes of the necrotrophic chickpea blight fungus Ascochyta rabiei that are involved in counteracting oxidative stress. A subtractive cDNA library was constructed after menadione treatment, which resulted in the isolation of 128 unigenes. A reverse northern blot was used to compare transcript profiles after H(2)O(2), menadione and sodium nitroprusside treatments. A total of 70 unigenes were found to be upregulated by more than two-fold following menadione treatment at different time intervals. A large number of genes not previously associated with oxidative stress were identified, along with many stress-responsive genes. Differential expression patterns of several genes were validated by quantitative real-time PCR (qRT-PCR) and northern blotting. In planta qRT-PCR of several selected genes also showed differential expression patterns during infection and disease progression. These data shed light on the molecular responses of the phytopathogen A. rabiei to overcome oxidative and nitrosative stresses and advance the understanding of necrotrophic fungal pathogen survival mechanisms.