Identification of effector-like proteins in Trichoderma spp. and role of a hydrophobin in the plant-fungus interaction and mycoparasitism

The fungal NADPH oxidase is an essential element for the molecular dialog between Trichoderma and Arabidopsis

Members of the fungal genus Trichoderma stimulate growth and reinforce plant immunity. Nevertheless, how fungal signaling elements mediate the establishment of a successful Trichoderma-plant interaction is largely unknown. In this work, we analyzed growth, root architecture and defense in an Arabidopsis-Trichoderma co-cultivation system, including the wild-type (WT) strain of the fungus and mutants affected in NADPH oxidase. Global gene expression profiles were assessed in both the plant and the fungus during the establishment of the interaction. Trichoderma atroviride WT improved root branching and growth of seedling as previously reported. This effect diminished in co-cultivation with the ∆nox1, ∆nox2 and ∆noxR null mutants. The data gathered of the Arabidopsis interaction with the ∆noxR strain showed that the seedlings had a heightened immune response linked to jasmonic acid in roots and shoots. In the fungus, we observed repression of genes involved in complex carbohydrate degradation in the presence of the plant before contact. However, in the absence of NoxR, such repression was lost, apparently due to a poor ability to adequately utilize simple carbon sources such as sucrose, a typical plant exudate. Our results unveiled the critical role played by the Trichoderma NoxR in the establishment of a fine-tuned communication between the plant and the fungus even before physical contact. In this dialog, the fungus appears to respond to the plant by adjusting its metabolism, while in the plant, fungal perception determines a delicate growth-defense balance.

Effects of Dark Septate Endophytes Strain A024 on Damping-off Biocontrol, Plant Growth and the Rhizosphere Soil Enviroment of Pinus sylvestris var. mongolica Annual Seedlings

Dark septate endophytes (DSEs) exert a vital role in promoting plant growth, improving mineral absorption, biological disease control, and enhancing plant stress resistance. The effects of dark septate endophyte strain, Phialocephala bamuru A024 on damping-off biocontrol, plant development, nutrients within the rhizosphere soil, as well as bacterial communities in the annual seedlings of P. sylvestris var. Mongolica were studied. According to our findings, following P. bamuru A024 inoculation, the damping-off disease morbidity decreased significantly compared with control, some physiological indices such as β-1,3-glucanase, chitinase enzyme activity as well as a soluble protein and proline content in P. sylvestris var. mongolica were elevated under R. solani stress. After inoculation with P. bamuru A024, the biomass in seedlings, nutrients in soil, root structure index, together with activities of soil enzymes were remarkably up-regulated relative to control (p < 0.05). As suggested by the results of high-throughput sequencing, the microbial structure in the rhizosphere soil of the P. sylvestris var. mongolica showed significant differences (p < 0.05) after P. bamuru A024 inoculation compared to control treatment and the rhizosphere soil bacterial community structure after DSE A024 inoculation was positively correlated to the main soil nutrition indices.

Trichoderma: The "Secrets" of a Multitalented Biocontrol Agent

The plant-Trichoderma-pathogen triangle is a complicated web of numerous processes. Trichoderma spp. are avirulent opportunistic plant symbionts. In addition to being successful plant symbiotic organisms, Trichoderma spp. also behave as a low cost, effective and ecofriendly biocontrol agent. They can set themselves up in various patho-systems, have minimal impact on the soil equilibrium and do not impair useful organisms that contribute to the control of pathogens. This symbiotic association in plants leads to the acquisition of plant resistance to pathogens, improves developmental processes and yields and promotes absorption of nutrient and fertilizer use efficiency. Among other biocontrol mechanisms, antibiosis, competition and mycoparasitism are among the main features through which microorganisms, including Thrichoderma, react to the presence of other competitive pathogenic organisms, thereby preventing or obstructing their development. Stimulation of every process involves the biosynthesis of targeted metabolites like plant growth regulators, enzymes, siderophores, antibiotics, etc. This review summarizes the biological control activity exerted by Trichoderma spp. and sheds light on the recent progress in pinpointing the ecological significance of Trichoderma at the biochemical and molecular level in the rhizosphere as well as the benefits of symbiosis to the plant host in terms of physiological and biochemical mechanisms. From an applicative point of view, the evidence provided herein strongly supports the possibility to use Trichoderma as a safe, ecofriendly and effective biocontrol agent for different crop species.

Whole Genome Sequencing Reveals Major Deletions in the Genome of M7, a Gamma Ray-Induced Mutant of Trichoderma virens That Is Repressed in Conidiation, Secondary Metabolism, and Mycoparasitism

Trichoderma virens is a commercial biofungicide used in agriculture. We have earlier isolated a mutant of T. virens using gamma ray-induced mutagenesis. This mutant, designated as M7, is defective in morphogenesis, secondary metabolism, and mycoparasitism. The mutant does not produce conidia, and the colony is hydrophilic. M7 cannot utilize cellulose and chitin as a sole carbon source and is unable to parasitize the plant pathogens Rhizoctonia solani and Pythium aphanidermatum in confrontation assay. Several volatile (germacrenes, beta-caryophyllene, alloaromadendrene, gamma-muurolene) and non-volatile (viridin, viridiol, gliovirin, heptelidic acid) metabolites are not detected in M7. In transcriptome analysis, many genes related to secondary metabolism, carbohydrate metabolism, hydrophobicity, and transportation, among others, were found to be downregulated in the mutant. Using whole genome sequencing, we identified five deletions in the mutant genome, totaling about 250 kb (encompassing 71 predicted ORFs), which was confirmed by PCR. This study provides novel insight into genetics of morphogenesis, secondary metabolism, and mycoparasitism and eventually could lead to the identification of novel regulators of beneficial traits in plant beneficial fungi Trichoderma spp. We also suggest that this mutant can be developed as a microbial cell factory for the production of secondary metabolites and proteins.

IPA-1 a Putative Chromatin Remodeler/Helicase-Related Protein of Trichoderma virens Plays Important Roles in Antibiosis Against Rhizoctonia solani and Induction of Arabidopsis Systemic Disease Resistance

Trichoderma spp. are filamentous fungi that colonize plant roots conferring beneficial effects to plants, either indirectly through the induction of their defense systems or directly through the suppression of phytopathogens in the rhizosphere. Transcriptomic analyses of Trichoderma spp. emerged as a powerful method for identifying the molecular events underlying the establishment of this beneficial relationship. Here, we focus on the transcriptomic response of Trichoderma virens during its interaction with Arabidopsis seedlings. The main response of T. virens to cocultivation with Arabidopsis was the repression of gene expression. The biological processes of transport and metabolism of carbohydrates were downregulated, including a set of cell wall-degrading enzymes putatively relevant for root colonization. Repression of such genes reached their basal levels at later times in the interaction, when genes belonging to the biological process of copper ion transport were induced, a necessary process providing copper as a cofactor for cell wall-degrading enzymes with the auxiliary activities class. RNA-Seq analyses showed the induction of a member of the SNF2 family of chromatin remodelers/helicase-related proteins, which was named IPA-1 (increased protection of Arabidopsis-1). Sequence analyses of IPA-1 showed its closest relatives to be members of the Rad5/Rad16 and SNF2 subfamilies; however, it grouped into a different clade. Although deletion of IPA-1 in T. virens did not affect its growth, the antibiotic activity of Δipa-1 culture filtrates against Rhizoctonia solani diminished but it remained unaltered against Botrytis cinerea. Triggering of the plant defense genes in plants treated with Δipa-1 was higher, showing enhanced resistance against Pseudomonas syringae but not against B. cinerea as compared with the wild type.

EffHunter: A Tool for Prediction of Effector Protein Candidates in Fungal Proteomic Databases

Pathogens are able to deliver small-secreted, cysteine-rich proteins into plant cells to enable infection. The computational prediction of effector proteins remains one of the most challenging areas in the study of plant fungi interactions. At present, there are several bioinformatic programs that can help in the identification of these proteins; however, in most cases, these programs are managed independently. Here, we present EffHunter, an easy and fast bioinformatics tool for the identification of effectors. This predictor was used to identify putative effectors in 88 proteomes using characteristics such as size, cysteine residue content, secretion signal and transmembrane domains.

Trichoderma atroviride as a promising biocontrol agent in seed coating for reducing Fusarium damping-off on maize

AIMS

The objective of this work was to identify a fungal strain showing potential biocontrol abilities against two Fusarium damping-off agents and to test it as a Biological Control Agent (BCA) in maize seed coating under field conditions.

METHODS AND RESULTS

A collection of native fungal strains associated with maize in Belgium was screened for antagonistic potential against Fusarium avenaceum and Fusarium culmorum. The strain with highest biocontrol potential was identified as an endophytic Trichoderma atroviride BC0584. In greenhouse, it significantly improves the emergence of seedlings infected by F. avenaceum or F. culmorum pathogens. In most field trials carried out during the season 2017, it significantly increased the emergence rate of infected seedlings compared to untreated seeds. One slurriable powder formulation allows BCA conidia to survive over a 6-month storage period at 4°C.

CONCLUSIONS

The fungal BC0584 strain is a promising BCA that could be an alternative to synthetic fungicides. It is adapted to local environmental conditions, is easily and cheaply produced and can be stored in a low-cost formulation.

SIGNIFICANCE AND IMPACT OF THE STUDY

In Belgium, this is the first study to use a T. atroviride native strain against Fusarium damping-off on maize crop. Modes of action and required conditions for ensuring high biocontrol activity in the field have still to be investigated.

Mycoparasitism illuminated by genome and transcriptome sequencing of Coniothyrium minitans, an important biocontrol fungus of the plant pathogen Sclerotinia sclerotiorum

Coniothyrium minitans is a mycoparasite of the notorious plant pathogen Sclerotinia sclerotiorum. To further understand the parasitism of C. minitans, we assembled and analysed its genome and performed transcriptome analyses. The genome of C. minitans strain ZS-1 was assembled into 350 scaffolds and had a size of 39.8 Mb. A total of 11 437 predicted genes and proteins were annotated, and 30.8 % of the blast hits matched proteins encoded by another member of the Pleosporales, Paraphaeosphaeria sporulosa, a worldwide soilborne fungus with biocontrol ability. The transcriptome of strain ZS-1 during the early interaction with S. sclerotiorum at 0, 4 and 12 h was analysed. The detected expressed genes were involved in responses to host defenses, including cell-wall-degrading enzymes, transporters, secretory proteins and secondary metabolite productions. Seventeen differentially expressed genes (DEGs) of fungal cell-wall-degrading enzymes (FCWDs) were up-regulated during parasitism, with only one down-regulated. Most of the monocarboxylate transporter genes of the major facilitator superfamily and all the detected ABC transporters, especially the heavy metal transporters, were significantly up-regulated. Approximately 8 % of the 11 437 proteins in C. minitans were predicted to be secretory proteins with catalytic activity. In the molecular function category, hydrolase activity, peptidase activity and serine hydrolase activity were enriched. Most genes involved in serine hydrolase activity were significantly up-regulated. This genomic analysis and genome-wide expression study demonstrates that the mycoparasitism process of C. minitans is complex and a broad range of proteins are deployed by C. minitans to successfully invade its host. Our study provides insights into the mechanisms of the mycoparasitism between C. minitans and S. sclerotiorum and identifies potential secondary metabolites from C. minitans for application as a biocontrol agent.

Trichoderma Counteracts the Challenge of Phytophthora nicotianae Infections on Tomato by Modulating Plant Defense Mechanisms and the Expression of Crinkler, Necrosis-Inducing Phytophthora Protein 1, and Cellulose-Binding Elicitor Lectin Pathogenic Effectors

Decoding the mechanisms of plant defense against plant pathogens in a scenario where antagonistic activity and the plant growth-promoting effects of useful organisms intervene simultaneously is a new frontier of plant pathology. Here, we demonstrated that (i) two selected strains of Trichoderma asperellum and Trichoderma atroviride promoted tomato (Solanum lycopersicum) growth and reduced the severity of disease caused by the oomycete Phytophthora nicotianae and (ii) the genetic patterns of the components of the experimental model system tomato-Trichoderma spp.-P. nicotianae were differentially expressed. The beneficial effects in both the promotion of the growth of host plant and the biological control of the pathogen by two selected strains of different Trichoderma species were tested both in planta and in vitro. In both respects, T. atroviride demonstrated to be more effective than T. asperellum. Additionally, the simultaneous transcriptional reprogramming of several plant defense-related genes, pathogen effectors, and mycoparasitism-related genes in tomato, P. nicotianae, and Trichoderma spp., respectively, was evaluated during the three-component interaction. Results support the hypothesis that Trichoderma spp. elicit the expression of plant defense-related genes. As expected, a mycoparasitism-related gene was significantly up-regulated in Trichoderma-colonizing tomato plants infected by P. nicotianae. Finally, a marked up-regulation of the genes encoding two necrosis-inducing effectors was observed in P. nicotianae infecting tomato plants colonized by Trichoderma. In conclusion, this study is a contribution toward understanding the genetic pathways related with the ability of Trichoderma spp. to counteract the challenge of P. nicotianae infections on tomato. Additionally, the experiments revealed the beneficial effects in the tomato growth promotion of a new T. atroviride strain and its good antagonistic effectiveness in the biological control of root and crown rot incited by P. nicotianae, confirming that Trichoderma spp. can be a powerful tool in integrated pest management strategies of Phytophthora diseases of horticultural crops.

The inconspicuous gatekeeper: endophytic Serendipita vermifera acts as extended plant protection barrier in the rhizosphere

In nature, beneficial and pathogenic fungi often simultaneously colonise plants. Despite substantial efforts to understand the composition of natural plant-microbe communities, the mechanisms driving such multipartite interactions remain largely unknown. Here we address how the interaction between the beneficial root endophyte Serendipita vermifera and the pathogen Bipolaris sorokiniana affects fungal behaviour and determines barley host responses using a gnotobiotic soil-based split-root system. Fungal confrontation in soil resulted in induction of B. sorokiniana genes involved in secondary metabolism and a significant repression of genes encoding putative effectors. In S. vermifera, genes encoding hydrolytic enzymes were strongly induced. This antagonistic response was not activated during the tripartite interaction in barley roots. Instead, we observed a specific induction of S. vermifera genes involved in detoxification and redox homeostasis. Pathogen infection but not endophyte colonisation resulted in substantial host transcriptional reprogramming and activation of defence. In the presence of S. vermifera, pathogen infection and disease symptoms were significantly reduced despite no marked alterations of the plant transcriptional response. The activation of stress response genes and concomitant repression of putative effector gene expression in B. sorokiniana during confrontation with the endophyte suggest a reduction of the pathogen's virulence potential before host plant infection.

Tal6 From Trichoderma atroviride Is a LysM Effector Involved in Mycoparasitism and Plant Association

LysM effectors play a relevant role during the plant colonization by successful phytopathogenic fungi, since they enable them to avoid either the triggering of plant defense mechanisms or their attack effects. Tal6, a LysM protein from Trichoderma atroviride, is capable of binding to complex chitin. However, until now its biological function is not completely known, particularly its participation in plant–Trichoderma interactions. We obtained T. atroviride Tal6 null mutant and Tal6 overexpressing strains and determined the role played by this protein during Trichoderma-plant interaction and mycoparasitism. LysM effector Tal6 from T. atroviride protects the hyphae from chitinases by binding to chitin of the fungal cell wall, increases the fungus mycoparasitic capacity, and modulates the activation of the plant defense system. These results show that beneficial fungi also employ LysM effectors to improve their association with plants.

A Novel Seed-Dressing Formulation Based on an Improved Mutant Strain of Trichoderma virens, and Its Field Evaluation

Using gamma-ray-induced mutagenesis, we have developed a mutant (named G2) of Trichoderma virens that produced two- to three-fold excesses of secondary metabolites, including viridin, viridiol, and some yet-to-be identified compounds. Consequently, this mutant had improved antibiosis against the oomycete test pathogen Pythium aphanidermatum. A transcriptome analysis of the mutant vis-à-vis the wild-type strain showed upregulation of several secondary-metabolism-related genes. In addition, many genes predicted to be involved in mycoparasitism and plant interactions were also upregulated. We used tamarind seeds as a mass multiplication medium in solid-state fermentation and, using talcum powder as a carrier, developed a novel seed dressing formulation. A comparative evaluation of the wild type and the mutant in greenhouse under high disease pressure (using the test pathogen Sclerotium rolfsii) revealed superiority of the mutant over wild type in protecting chickpea (Cicer arietinum) seeds and seedlings from infection. We then undertook extensive field evaluation (replicated micro-plot trials, on-farm demonstration trials, and large-scale trials in farmers' fields) of our mutant-based formulation (named TrichoBARC) for management of collar rot (S. rolfsii) in chickpea and lentil (Lens culinaris) over multiple locations in India. In certain experiments, other available formulations were included for comparison. This formulation consistently, over multiple locations and years, improved seed germination, reduced seedling mortality, and improved plant growth and yield. We also noticed growth promotion, improved pod bearing, and early flowering (7-10 days) in TrichoBARC-treated chickpea and lentil plants under field conditions. In toxicological studies in animal models, this formulation exhibited no toxicity to mammals, birds, or fish.

Trichoderma/pathogen/plant interaction in pre-harvest food security

Large losses before crop harvesting are caused by plant pathogens, such as viruses, bacteria, oomycetes, fungi, and nematodes. Among these, fungi are the major cause of losses in agriculture worldwide. Plant pathogens are still controlled through application of agrochemicals, causing human disease and impacting environmental and food security. Biological control provides a safe alternative for the control of fungal plant pathogens, because of the ability of biocontrol agents to establish in the ecosystem. Some Trichoderma spp. are considered potential agents in the control of fungal plant diseases. They can interact directly with roots, increasing plant growth, resistance to diseases, and tolerance to abiotic stress. Furthermore, Trichoderma can directly kill fungal plant pathogens by antibiosis, as well as via mycoparasitism strategies. In this review, we will discuss the interactions between Trichoderma/fungal pathogens/plants during the pre-harvest of crops. In addition, we will highlight how these interactions can influence crop production and food security. Finally, we will describe the future of crop production using antimicrobial peptides, plants carrying pathogen-derived resistance, and plantibodies.

Trichoderma as a Model to Study Effector-Like Molecules

Plants are capable of perceiving microorganisms by coordinating processes to establish different forms of plant–microbe relationships. Plant colonization is governed in fungal and bacterial systems by secreted effector molecules, suppressing plant defense responses and modulating plant physiology to promote either virulence or compatibility. Proteins, secondary metabolites, and small RNAs have been described as effector molecules that use different mechanisms to establish the interaction. Effector molecules have been studied in more detail due to their involvement in harmful interactions, leading to a negative impact on agriculture. Recently, research groups have started to study the effectors in symbiotic interactions. Interestingly, most symbiotic effectors are members of the same families present in phytopathogens. Nevertheless, the quantity and ratio of secreted effectors depends on the microorganism and the host, suggesting a complex mechanism of recognition between the plant and their associated microorganisms. Fungi belonging to Trichoderma genus interact with plants by inducing their defense system and promoting plant growth. Research suggests that some of these effects are associated with effector molecules that Trichoderma delivers during the association with the plant. In this review, we will focus on the main findings concerning the effector molecules reported in Trichoderma spp. and their role during the interaction with plants, mainly in the molecular dialogue that takes place between them.

A hydrophobin gene, Hyd9, plays an important role in the formation of aerial hyphae and primordia in Flammulina filiformis

Flammulina filiformis is an edible fungus that is largely cultivated and widely consumed around the world. The quantity and quality of the primordia, which gives rise to the fruiting body, affects its production efficiency. Hydrophobins are involved in the formation of the fruiting body of macrofungi. However, functional verification of the hydrophobin genes is limited to date. In this study, we used gene silencing and overexpression analyses to investigate the function of one F. filiformis hydrophobin gene (Hyd9) during the development of the fruiting body. The Hyd9-silenced transformants exhibited sparse aerial hyphae, resulting in fewer primordia and fruiting bodies. In contrast, the Hyd9 overexpression strain displayed denser aerial hyphae and more primordia. The phenotypes of these transgenic lines strongly suggested that Hyd9 plays an important role in the formation of aerial hyphal knots (the primary stage of primordia) and primordia in F. filiformis. These results will be beneficial for developing more efficient methods to induce primordia formation in F. filiformis and other commercially valuable mushrooms.

Hydrophobin HFBII-4 from Trichoderma asperellum induces antifungal resistance in poplar

Herein, the class II hydrophobin gene HFBII-4 was cloned from the biocontrol agent Trichoderma asperellum ACCC30536 and recombinant rHFBII-4 was expressed in Pichia pastoris GS115. Treatment of Populus davidiana × P. alba var. pyramidalis (PdPap poplar) with rHFBII-4 altered the expression levels of genes in the auxin, salicylic acid (SA), and jasmonic acid (JA) signal transduction pathways. Polyphenol oxidase (PPO) and phenylalanine ammonia lyase (PAL) enzyme activities were induced with rHFBII-4. Evans Blue and nitro blue tetrazolium (NBT) staining indicated that cell membrane permeability and reactive oxygen species were lower in the leaves of plants treated with rHFBII-4. The chlorophyll content was higher than that of control at 2-5 days after treatment. Furthermore, poplar seedlings were inoculated with Alternaria alternata, disease symptoms were observed. The diseased area was smaller in leaves induced with rHFBII-4 compared with control. In summary, rHFBII-4 enhances resistance to A. alternata.

Trichoderma Histone Deacetylase HDA-2 Modulates Multiple Responses in Arabidopsis

Trichoderma spp. are a rich source of secondary metabolites and volatile organic compounds (VOCs), which may induce plant defenses and modulate plant growth. In filamentous fungi, chromatin modifications regulate secondary metabolism. In this study we investigated how the absence of histone deacetylase HDA-2 in the Trichoderma atroviride strain Δhda-2 impacts its effect on a host, Arabidopsis (Arabidopsis thaliana). The production of VOCs and their impact on plant growth and development were assessed as well. The Δhda-2 strain was impaired in its ability to colonize Arabidopsis roots, thus affecting the promotion of plant growth and modulation of plant defenses against foliar pathogens Botrytis cinerea and Pseudomonas syringae, which normally result from interaction with T. atroviride Furthermore, Δhda-2 VOCs were incapable of triggering plant defenses to counterattack foliar pathogens. The Δhda-2 overproduced the VOC 6-pentyl-2H-pyran-2-one (6-PP), which resulted in enhanced root branching and differentially regulated phytohormone-related genes. Analysis of ten VOCs (including 6-PP) revealed that three of them positively regulated plant growth, whereas six had the opposite effect. Assessment of secondary metabolites, detoxification, and communication with plant-related genes showed a dual role for HDA-2 in T. atroviride gene expression regulation during its interaction with plants. Chromatin immunoprecipitation of acetylated histone H3 on the promoters of plant-responsive genes in Δhda-2 showed, in the presence of Arabidopsis, low levels of epl-1 and abc-2 compared with that in the wild type; whereas ctf-1 presented high constitutive levels, supporting a dual role of HDA-2 in gene regulation. This work highlights the importance of HDA-2 as a global regulator in Trichoderma to modulate multiple responses in Arabidopsis.

Trichoderma as a Model to Study Effector-Like Molecules

Plants are capable of perceiving microorganisms by coordinating processes to establish different forms of plant-microbe relationships. Plant colonization is governed in fungal and bacterial systems by secreted effector molecules, suppressing plant defense responses and modulating plant physiology to promote either virulence or compatibility. Proteins, secondary metabolites, and small RNAs have been described as effector molecules that use different mechanisms to establish the interaction. Effector molecules have been studied in more detail due to their involvement in harmful interactions, leading to a negative impact on agriculture. Recently, research groups have started to study the effectors in symbiotic interactions. Interestingly, most symbiotic effectors are members of the same families present in phytopathogens. Nevertheless, the quantity and ratio of secreted effectors depends on the microorganism and the host, suggesting a complex mechanism of recognition between the plant and their associated microorganisms. Fungi belonging to Trichoderma genus interact with plants by inducing their defense system and promoting plant growth. Research suggests that some of these effects are associated with effector molecules that Trichoderma delivers during the association with the plant. In this review, we will focus on the main findings concerning the effector molecules reported in Trichoderma spp. and their role during the interaction with plants, mainly in the molecular dialogue that takes place between them.

Trichoderma Species: Versatile Plant Symbionts

Because of the need to provide food for the growing population, agricultural activity is faced with the huge challenge of counteracting the negative effects generated by adverse environmental factors and diseases caused by pathogens on crops, while avoiding environmental pollution due to the excessive use of agrochemicals. The exploitation of biological systems that naturally increase plant vigor, preparing them against biotic and abiotic stressors that also promote their growth and productivity represents a useful and viable strategy to help face these challenges. Fungi from the genus Trichoderma have been widely used in agriculture as biocontrol agents because of their mycoparasitic capacity and ability to improve plant health and protection against phytopathogens, which makes it an excellent plant symbiont. The mechanisms employed by Trichoderma include secretion of effector molecules and secondary metabolites that mediate the beneficial interaction of Trichoderma with plants, providing tolerance to biotic and abiotic stresses. Here we discuss the most recent advances in understanding the mechanisms employed by this opportunistic plant symbiont as biocontrol agent and plant growth promoter. In addition, through genome mining we approached a less explored factor that Trichoderma could be using to become successful plant symbionts, the production of phytohormones-auxins, cytokinins, abscisic acid, gibberellins, among others. This approach allowed us to detect sets of genes encoding proteins potentially involved in phytohormone biosynthesis and signaling. We discuss the implications of these findings in the physiology of the fungus and in the establishment of its interaction with plants.

The Hydrophobin HYTLO1 Secreted by the Biocontrol Fungus Trichoderma longibrachiatum Triggers a NAADP-Mediated Calcium Signalling Pathway in Lotus japonicus

Trichoderma filamentous fungi are increasingly used as biocontrol agents and plant biostimulants. Growing evidence indicates that part of the beneficial effects is mediated by the activity of fungal metabolites on the plant host. We have investigated the mechanism of plant perception of HYTLO1, a hydrophobin abundantly secreted by Trichoderma longibrachiatum, which may play an important role in the early stages of the plant-fungus interaction. Aequorin-expressing Lotus japonicus suspension cell cultures responded to HYTLO1 with a rapid cytosolic Ca2+ increase that dissipated within 30 min, followed by the activation of the defence-related genes MPK3, WRK33, and CP450. The Ca2+-dependence of these gene expression was demonstrated by using the extracellular Ca2+ chelator EGTA and Ned-19, a potent inhibitor of the nicotinic acid adenine dinucleotide phosphate (NAADP) receptor in animal cells, which effectively blocked the HYTLO1-induced Ca2+ elevation. Immunocytochemical analyses showed the localization of the fungal hydrophobin at the plant cell surface, where it forms a protein film covering the plant cell wall. Our data demonstrate the Ca2+-mediated perception by plant cells of a key metabolite secreted by a biocontrol fungus, and provide the first evidence of the involvement of NAADP-gated Ca2+ release in a signalling pathway triggered by a biotic stimulus.

Trichoderma asperelloides ethanolic extracts efficiently inhibit Staphylococcus growth and biofilm formation

Fungi from the widely distributed genus Trichoderma are of great biotechnological interest, being currently used in a vast range of applications. Here, we report that high-molecular weight fraction (HWF) derived from Trichoderma asperelloides ethanolic extract exhibits antibiotic activity against staphylococcal biofilms. The antibacterial and anti-biofilm properties of T. asperelloides extracts were evaluated by well-established assays in Staphylococcus aureus ATCC strains (29213 and 6538) and in one clinical isolate from bovine mastitis. The HWF from T. asperelloides eradicated S. aureus by causing substantial matrix de-structuring and biomass reduction (p < 10-5) at concentrations as low as 2.3 μg mL-1. Additionally, we present ultra-structure analysis by the use of scanning electron microscopy as well as transmission microscopy, which showed that T. asperelloides killed cells through cell wall and membrane disturbance. Remarkably, the HWF from T. asperelloides killed S. aureus and eradicated its biofilms in a greater performance than gentamicin (p < 10-5), a known potent antibiotic against S. aureus. Our results indicate that extract from T. asperelloides may represent a promising candidate for the development of new antibiotics against gram-positive bacteria.

CfPDIP1, a novel secreted protein of Colletotrichum falcatum, elicits defense responses in sugarcane and triggers hypersensitive response in tobacco

Colletotrichum falcatum, a hemibiotrophic fungal pathogen, causes one of the major devastating diseases of sugarcane-red rot. C. falcatum secretes a plethora of molecular signatures that might play a crucial role during its interaction with sugarcane. Here, we report the purification and characterization of a novel secreted protein of C. falcatum that elicits defense responses in sugarcane and triggers hypersensitive response (HR) in tobacco. The novel protein purified from the culture filtrate of C. falcatum was identified by MALDI TOF/TOF MS and designated as C. falcatum plant defense-inducing protein 1 (CfPDIP1). Temporal transcriptional profiling showed that the level of CfPDIP1 expression was greater in incompatible interaction than the compatible interaction until 120 h post-inoculation (hpi). EffectorP, an in silico tool, has predicted CfPDIP1 as a potential effector. Functional characterization of full length and two other domain deletional variants (CfPDIP1ΔN1-21 and CfPDIP1ΔN1-45) of recombinant CfPDIP1 proteins has indicated that CfPDIP1ΔN1-21 variant elicited rapid alkalinization and induced a relatively higher production of hydrogen peroxide (H2O2) in sugarcane suspension culture. However, in Nicotiana tabacum, all the three forms of recombinant CfPDIP1 proteins triggered HR along with the induction of H2O2 production and callose deposition. Further characterization using detached leaf bioassay in sugarcane revealed that foliar priming with CfPDIP1∆1-21 has suppressed the extent of lesion development, even though the co-infiltration of CfPDIP1∆1-21 with C. falcatum on unprimed leaves increased the extent of lesion development than control. Besides, the foliar priming has induced systemic expression of major defense-related genes with the concomitant reduction of pathogen biomass and thereby suppression of red rot severity in sugarcane. Comprehensively, the results have suggested that the novel protein, CfPDIP1, has the potential to trigger a multitude of defense responses in sugarcane and tobacco upon priming and might play a potential role during plant-pathogen interactions.

Geosmithia-Ophiostoma: a New Fungus-Fungus Association

In Europe as in North America, elms are devastated by Dutch elm disease (DED), caused by the alien ascomycete Ophiostoma novo-ulmi. Pathogen dispersal and transmission are ensured by local species of bark beetles, which established a novel association with the fungus. Elm bark beetles also transport the Geosmithia fungi genus that is found in scolytids' galleries colonized by O. novo-ulmi. Widespread horizontal gene transfer between O. novo-ulmi and Geosmithia was recently observed. In order to define the relation between these two fungi in the DED pathosystem, O. novo-ulmi and Geosmithia species from elm, including a GFP-tagged strain, were grown in dual culture and mycelial interactions were observed by light and fluorescence microscopy. Growth and sporulation of O. novo-ulmi in the absence or presence of Geosmithia were compared. The impact of Geosmithia on DED severity was tested in vivo by co-inoculating Geosmithia and O. novo-ulmi in elms. A close and stable relation was observed between the two fungi, which may be classified as mycoparasitism by Geosmithia on O. novo-ulmi. These results prove the existence of a new component in the complex of organisms involved in DED, which might be capable of reducing the disease impact.

Functional Characterization of TvCyt2, a Member of the p450 Monooxygenases From Trichoderma virens Relevant During the Association With Plants and Mycoparasitism

Secondary metabolites are crucial for the establishment of interactions between plants and microbes, as in the case of Trichoderma-plant interactions. In the biosynthetic pathway of secondary metabolites, specific enzymes participate in the formation of hydroxyl and epoxy groups, belonging to the p450 monooxygenases family. Here, we show that the product of the gene TvCyt2 from Trichoderma virens encodes a new protein homologous to the cytochrome p450, which is down-regulated at the beginning of Trichoderma-Arabidopsis interaction. To investigate its role in the interactions established by Trichoderma spp., we analyzed the metabolic profile obtained from the overexpressing (OETvCyt2) and null mutant (Δtvcyt2) strains, observing that the OETvCyt2 strains produce a higher concentration of some metabolites than the wild-type (WT) strain. Δtvcyt2 strains showed a decreased antagonistic activity against Rhizoctonia solani in antibiosis assays. Arabidopsis plants cocultivated with the OETvCyt2 strains showed stronger induction of systemic acquired resistance than plants cocultivated with the WT strain, as well as increases in biomass and fitness. Our data suggest that the product of the TvCyt2 gene is involved in secondary metabolite biosynthesis, which can increase antagonistic activity with phytopathogenic fungi and the capacity to promote plant growth.

Transcriptome Analysis of Tomato Leaf Spot Pathogen Fusarium proliferatum: De novo Assembly, Expression Profiling, and Identification of Candidate Effectors

Leaf spot disease caused by the fungus Fusarium proliferatum (Matsushima) Nirenberg is a destructive disease of tomato plants in China. Typical symptoms of infected tomato plants are softened and wilted stems and leaves, leading to the eventual death of the entire plant. In this study, we resorted to transcriptional profile analysis to gain insight into the repertoire of effectors involved in F. proliferatum–tomato interactions. A total of 61,544,598 clean reads were de novo assembled to provide a F. proliferatum reference transcriptome. From these, 75,044 unigenes were obtained, with 19.46% of the unigenes being assigned to 276 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, with 22.3% having a homology with genes from F. fujikuroi. A total of 18,075 differentially expressed genes (DEGs) were identified, 720 of which were found to code for secreted proteins. Of these, 184 were identified as candidate effectors, while 79.89% had an upregulated expression. Moreover, 17 genes that were differentially expressed in RNA-seq studies were randomly selected for validation by quantitative real-time polymerase chain reaction (qRT–PCR). The study demonstrates that transcriptome analysis could be an effective method for identifying the repertoire of candidate effectors and may provide an invaluable resource for future functional analyses of F. proliferatum pathogenicity in F. proliferatum and tomato plant–host interactions.

Effect of Trichoderma harzianum on maize rhizosphere microbiome and biocontrol of Fusarium Stalk rot

Fusarium stalk rot (FSR) caused by Fusarium graminearum (FG) significantly affects the productivity of maize grain crops. Application of agrochemicals to control the disease is harmful to environment. In this regard, use of biocontrol agent (BCA) is an alternative to agrochemicals. Although Trichoderma species are known as BCA, the selection of host-pathogen specific Trichoderma is essential for the successful field application. Hence, we screened a total of 100 Trichoderma isolates against FG, selected Trichoderma harzianum (CCTCC-RW0024) for greenhouse experiments and studied its effect on changes of maize rhizosphere microbiome and biocontrol of FSR. The strain CCTCC-RW0024 displayed high antagonistic activity (96.30%), disease reduction (86.66%), biocontrol-related enzyme and gene expression. The root colonization of the strain was confirmed by eGFP tagging and qRT-PCR analysis. Pyrosequencing revealed that exogenous inoculation of the strain in maize rhizosphere increased the plant growth promoting acidobacteria (18.4%), decreased 66% of FG, and also increased the plant growth. In addition, metabolites of this strain could interact with pathogenicity related transcriptional cofactor FgSWi6, thereby contributing to its inhibition. It is concluded that T. harzianum strain CCTCC-RW0024 is a potential BCA against FSR.