Regulation of conidiation and antagonistic properties of the soil-borne plant beneficial fungus Trichoderma virens by a novel proline-, glycine-, tyrosine-rich protein and a GPI-anchored cell wall protein

Genome and transcriptome sequencing of Trichoderma harzianum T4, an important biocontrol fungus of Rhizoctonia solani, reveals genes related to mycoparasitism

Trichoderma harzianum is a well-known biological control strain and a mycoparasite of Rhizoctonia solani. To explore the mechanisms of mycoparasitism, the genome and transcriptome of T. harzianum T4 were both assembled and analyzed in this study. The genome of T. harzianum T4 was assembled into 106 scaffolds, sized 41.25 Mb, and annotated with a total of 8118 predicted genes. We analyzed the transcriptome of T. harzianum T4 against R. solani in a dual culture in three culture periods: before contact (BC), during contact (C), and after contact (AC). Transcriptome sequencing identified 1092, 1222, and 2046 differentially expressed genes (DEGs), respectively. These DEGs, which are involved in pathogen recognition and signal transduction, hydrolase, transporters, antibiosis, and defense-related functional genes, are significantly upregulated in the mycoparasitism process. The results of genome and transcriptome analysis indicated that the mycoparasitism process of T. harzianum T4 was very complex. T. harzianum successfully recognizes and invades host cells and kills plant pathogens by regulating various DEGs at different culture periods. The relative expression levels of the 26 upregulated DEGs were confirmed by RT-qPCR to validate the reliability of the transcriptome data. The results provide insight into the molecular mechanisms underlying T. harzianum T4's mycoparasitic processes, and they provide a potential molecular target for the biological control mechanism of T. harzianum T4.

Comparative Small RNA and Degradome Sequencing Provides Insights into Antagonistic Interactions in the Biocontrol Fungus Clonostachys rosea

Necrotrophic mycoparasitism is an intricate process involving recognition, physical mycelial contact, and killing of host fungi (mycohosts). During such interactions, mycoparasites undergo a complex developmental process involving massive regulatory changes of gene expression to produce a range of chemical compounds and proteins that contribute to the parasitism of the mycohosts. Small RNAs (sRNAs) are vital components of posttranscriptional gene regulation, although their role in gene expression regulation during mycoparasitisms remain understudied. Here, we investigated the role of sRNA-mediated gene regulation in mycoparasitism by performing sRNA and degradome tag sequencing of the mycoparasitic fungus Clonostachys rosea interacting with the plant-pathogenic mycohosts Botrytis cinerea and Fusarium graminearum at two time points. The majority of differentially expressed sRNAs were downregulated during the interactions with the mycohosts compared to a C. rosea self-interaction control, thus allowing desuppression (upregulation) of mycohost-responsive genes. Degradome analysis showed a positive correlation between high degradome counts and antisense sRNA mapping and led to the identification of 201 sRNA-mediated potential gene targets for 282 differentially expressed sRNAs. Analysis of sRNA potential gene targets revealed that the regulation of genes coding for membrane proteins was a common response against both mycohosts. The regulation of genes involved in oxidative stress tolerance and cellular metabolic and biosynthetic processes was exclusive against F. graminearum, highlighting common and mycohost-specific gene regulation of C. rosea. By combining these results with transcriptome data collected during a previous study, we expand the understanding of the role of sRNA in regulating interspecific fungal interactions and mycoparasitism.

IMPORTANCE Small RNAs (sRNAs) are emerging as key players in pathogenic and mutualistic fungus-plant interactions; however, their role in fungus-fungus interactions remains elusive. In this study, we employed the necrotrophic mycoparasite Clonostachys rosea and the plant-pathogenic mycohosts Botrytis cinerea and Fusarium graminearum and investigated the sRNA-mediated gene regulation in mycoparasitic interactions. The combined approach of sRNA and degradome tag sequencing identified 201 sRNA-mediated putative gene targets for 282 differentially expressed sRNAs, highlighting the role of sRNA-mediated regulation of mycoparasitism in C. rosea. We also identified 36 known and 13 novel microRNAs (miRNAs) and their potential gene targets at the endogenous level and at a cross-species level in B. cinerea and F. graminearum, indicating a role of cross-species RNA interference (RNAi) in mycoparasitism, representing a novel mechanism in biocontrol interactions. Furthermore, we showed that C. rosea adapts its transcriptional response, and thereby its interaction mechanisms, based on the interaction stages and identity of the mycohost.

What is the role of the nitrate reductase (euknr) gene in fungi that live in nitrate-free environments? A targeted gene knock-out study in Ampelomyces mycoparasites

Mycoparasitic fungi can be utilized as biocontrol agents (BCAs) of many plant pathogens. Deciphering the molecular mechanisms of mycoparasitism may improve biocontrol efficiency. This work reports the first functional genetic studies in Ampelomyces, widespread mycoparasites and BCAs of powdery mildew fungi, and a molecular genetic toolbox for future works. The nitrate reductase (euknr) gene was targeted to reveal the biological function of nitrate assimilation in Ampelomyces. These mycoparasites live in an apparently nitrate-free environment, i.e. inside the hyphae of powdery mildew fungi that lack any nitrate uptake and assimilation system. Homologous recombination-based gene knock-out (KO) was applied to eliminate the euknr gene using Agrobacterium tumefaciens-mediated transformation. Efficient KO of euknr was confirmed by PCR, and visible phenotype caused by loss of euknr was detected on media with different nitrogen sources. Mycoparasitic ability was not affected by knocking out euknr as a tested transformant readily parasitized Blumeria graminis and Podosphaera xanthii colonies on barley and cucumber, respectively, and the rate of mycoparasitism did not differ from the wild type. These results indicate that euknr is not involved in mycoparasitism. Dissimilatory processes, involvement in nitric oxide metabolism, or other, yet undiscovered processes may explain why a functional euknr is maintained in Ampelomyces.

Biological Control of Fungal Diseases by Trichoderma aggressivum f. europaeum and Its Compatibility with Fungicides

Our purpose was to evaluate the ability of Trichoderma aggressivum f. europaeum as a biological control agent against diseases from fungal phytopathogens. Twelve isolates of T. aggressivum f. europaeum were obtained from several substrates used for Agaricus bisporus cultivation from farms in Castilla-La Mancha (Spain). Growth rates of the 12 isolates were determined, and their antagonistic activity was analysed in vitro against Botrytis cinerea, Sclerotinia sclerotiorum, Fusarium solani f. cucurbitae, Pythium aphanidermatum, Rhizoctonia solani, and Mycosphaerella melonis, and all isolates had high growth rates. T. aggressivum f. europaeum showed high antagonistic activity for different phytopathogens, greater than 80%, except for P. aphanidermatum at approximately 65%. The most effective isolate, T. aggressivum f. europaeum TAET1, inhibited B. cinerea, S. sclerotiorum, and M. melonis growth by 100% in detached leaves assay and inhibited germination of S. sclerotiorum sclerotia. Disease incidence and severity in plant assays for pathosystems ranged from 22% for F. solani to 80% for M. melonis. This isolate reduced the incidence of Podosphaera xanthii in zucchini leaves by 66.78%. The high compatibility by this isolate with fungicides could allow its use in combination with different pest management strategies. Based on the results, T. aggressivum f. europaeum TAET1 should be considered for studies in commercial greenhouses as a biological control agent.

Inhibitory Mechanism of Trichoderma virens ZT05 on Rhizoctonia solani

Trichoderma is a filamentous fungus that is widely distributed in nature. As a biological control agent of agricultural pests, Trichoderma species have been widely studied in recent years. This study aimed to understand the inhibitory mechanism of Trichoderma virens ZT05 on Rhizoctonia solani through the side-by-side culture of T. virens ZT05 and R. solani. To this end, we investigated the effect of volatile and nonvolatile metabolites of T. virens ZT05 on the mycelium growth and enzyme activity of R. solani and analyzed transcriptome data collected from side-by-side culture. T. virens ZT05 has a significant antagonistic effect against R. solani. The mycelium of T. virens ZT05 spirally wraps around and penetrates the mycelium of R. solani and inhibits the growth of R. solani. The volatile and nonvolatile metabolites of T. virens ZT05 have significant inhibitory effects on the growth of R. solani. The nonvolatile metabolites of T. virens ZT05 significantly affect the mycelium proteins of R. solani, including catalase (CAT), superoxide dismutase (SOD), peroxidase (POD), selenium-dependent glutathione peroxidase (GSH-Px), soluble proteins, and malondialdehyde (MDA). Twenty genes associated with hyperparasitism, including extracellular proteases, oligopeptide transporters, G-protein coupled receptors (GPCRs), chitinases, glucanases, and proteases were found to be upregulated during the antagonistic process between T. virens ZT05 and R. solani. Thirty genes related to antibiosis function, including tetracycline resistance proteins, reductases, the heat shock response, the oxidative stress response, ATP-binding cassette (ABC) efflux transporters, and multidrug resistance transporters, were found to be upregulated during the side-by-side culture of T. virens ZT05 and R. solani. T. virens ZT05 has a significant inhibitory effect on R. solani, and its mechanism of action is associated with hyperparasitism and antibiosis.

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.