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

A comparative cell wall analysis of Trichoderma spp. confirms a conserved polysaccharide scaffold and suggests an important role for chitosan in mycoparasitism

Fungal cell walls are dynamic extracellular matrices that enable efficient adaptation to changing environments. While the cell wall compositions of yeasts, human, and plant pathogenic fungi have been studied to some extent, the cell walls of mycoparasites remain poorly characterized. Trichoderma species comprise a diverse group of soil fungi with different survival strategies and lifestyles. The comparative study of cell wall carbohydrate-active enzymes in 13 Trichoderma spp. revealed that the types of enzymes involved in chitin and chitosan metabolism are phylogenetically distant between mycoparasitic and saprotrophic species. Here, we compare the carbohydrate composition and function of the cell wall of a saprotrophic strain Trichoderma reesei with that of the mycoparasitic, biological control agent Trichoderma atroviride. Monosaccharide and glycosidic linkage analyses as well as dual in situ interaction assays showed that the cell wall polysaccharide composition is conserved between both species, except for the amounts of chitin detected. The results suggest that the observed accumulation of chitosan during mycoparasitism may prevent host recognition. Remarkably, Trichoderma atroviride undergoes dynamic cell wall adaptations during both vegetative development and mycoparasitism, which appears to be confirmed by an evolutionarily expanded group of specialized enzymes. Overall, our analyses support the notion that habitat specialization is reflected in cell wall architecture and that plastic chitin remodeling may confer an advantage to mycoparasites, ultimately enabling the successful invasion and parasitism of plant pathogens. This information may potentially be exploited for the control of crop diseases using biological agents.

IMPORTANCE

Trichoderma species are emerging model fungi for the development of biocontrol agents and are used in industrial biotechnology as efficient enzyme producers. Fungal cell walls are complex structures that differ in carbohydrate, protein, and enzyme composition across taxa. Here, we present a chemical characterization of the cell walls of two Trichoderma spp., namely the predominantly saprotrophic Trichoderma reesei and the mycoparasite Trichoderma atroviride. Chemical profiling revealed that Trichoderma spp. remodel their cell wall to adapt to particular lifestyles, with dynamic changes during vegetative development. Importantly, we found that chitosan accumulation during mycoparasitism of a fungal host emerged as a sophisticated strategy underpinning an effective attack. These insights shed light on the molecular mechanisms that allow mycoparasites to overcome host defenses and can be exploited to improve the application of T. atroviride in biological pest control. Moreover, our results provide valuable information for targeting the fungal cell wall for therapeutic purposes.

Argonaute and Dicer are essential for communication between Trichoderma atroviride and fungal hosts during mycoparasitism

Trichoderma species are known for their mycoparasitic activity against phytopathogenic fungi that cause significant economic losses in agriculture. During mycoparasitism, Trichoderma spp. recognize molecules produced by the host fungus and release secondary metabolites and hydrolytic enzymes to kill and degrade the host's cell wall. Here, we explored the participation of the Trichoderma atroviride RNAi machinery in the interaction with six phytopathogenic fungi of economic importance. We determined that both Argonaute-3 and Dicer-2 play an essential role during mycoparasitism. Using an RNA-Seq approach, we identified that perception, detox, and cell wall degradation depend on the T. atroviride-RNAi when interacting with Alternaria alternata, Rhizoctonia solani AG2, and R. solani AG5. Furthermore, we constructed a gene co-expression network that provides evidence of two gene modules regulated by RNAi, which play crucial roles in essential processes during mycoparasitism. In addition, based on small RNA-seq, we conclude that siRNAs regulate amino acid and carbon metabolism and communication during the Trichoderma-host interaction. Interestingly, our data suggest that siRNAs might regulate allorecognition (het) and transport genes in a cross-species manner. Thus, these results reveal a fine-tuned regulation in T. atroviride dependent on siRNAs that is essential during the biocontrol of phytopathogenic fungi, showing a greater complexity of this process than previously established.IMPORTANCEThere is an increasing need for plant disease control without chemical pesticides to avoid environmental pollution and resistance, and the health risks associated with the application of pesticides are increasing. Employing Trichoderma species in agriculture to control fungal diseases is an alternative plant protection strategy that overcomes these issues without utilizing chemical fungicides. Therefore, understanding the biocontrol mechanisms used by Trichoderma species to antagonize other fungi is critical. Although there has been extensive research about the mechanisms involved in the mycoparasitic capability of Trichoderma species, there are still unsolved questions related to how Trichoderma regulates recognition, attack, and defense mechanisms during interaction with a fungal host. In this work, we report that the Argonaute and Dicer components of the RNAi machinery and the small RNAs they process are essential for gene regulation during mycoparasitism by Trichoderma atroviride.

Plant growth-promoting bacteria potentiate antifungal and plant-beneficial responses of Trichoderma atroviride by upregulating its effector functions

Trichoderma uses different molecules to establish communication during its interactions with other organisms, such as effector proteins. Effectors modulate plant physiology to colonize plant roots or improve Trichoderma's mycoparasitic capacity. In the soil, these fungi can establish relationships with plant growth-promoting bacteria (PGPBs), thus affecting their overall benefits on the plant or its fungal prey, and possibly, the role of effector proteins. The aim of this study was to determine the induction of Trichoderma atroviride gene expression coding for effector proteins during the interaction with different PGPBs, Arabidopsis or the phytopathogen Fusarium brachygibbosum, and to determine whether PGPBs potentiates the beneficial effects of T. atroviride. During the interaction with F. brachygibbosum and PGPBs, the effector coding genes epl1, tatrx2 and tacfem1 increased their expression, especially during the consortia with the bacteria. During the interaction of T. atroviride with the plant and PGPBs, the expression of epl1 and tatrx2 increased, mainly with the consortium formed with Pseudomonas fluorescens UM270, Bacillus velezensis AF12, or B. halotolerans AF23. Additionally, the consortium formed by T. atroviride and R. badensis SER3 stimulated A. thaliana PR1:GUS and LOX2:GUS for SA- and JA-mediated defence responses. Finally, the consortium of T. atroviride with SER3 was better at inhibiting pathogen growth, but the consortium of T. atroviride with UM270 was better at promoting Arabidopsis growth. These results showed that the biocontrol capacity and plant growth-promoting traits of Trichoderma spp. can be potentiated by PGPBs by stimulating its effector functions.

Trichoderma virens Big Ras GTPase-1, a molecular switch for transforming a mutualistic fungus to plants in a deleterious microbe

Trichoderma spp. are ascomycete filamentous fungi widely distributed worldwide that establish mutualistic relationships with plants by antagonizing phytopathogens in the rhizosphere and colonizing the plant roots, hence, promoting plant growth and triggering the systemic resistance against phytopathogens. During the first stages of root colonization by Trichoderma, plants recognize the fungus as an invader by inducing the plant defense system, including the production of reactive oxygen species (ROS). Some members of the small Ras GTPases regulate NADPH oxidases and, therefore, ROS production. However, their role in mutualistic microorganisms that colonize plant roots is poorly understood. It has been demonstrated that Trichoderma virens strains lacking TBRG-1, a member of a new family of the Ras GTPases, impair their biocontrol capability and behave like a pathogen on tomato seeds and seedlings. Here, we found that TBRG-1 is essential in T. virens to maintain the mutualistic relationship with plants because a mutant-lacking tbrg-1 gen could not induce plant growth in Arabidopsis and tomatoes. Furthermore, treatment of Arabidopsis seedlings with Δtbrg-1 induced strongly PR-1a, the systemic acquired resistance (SAR) marker gene at early times of the interaction, which correlated with enhanced foliar damage by Botrytis cinerea, resembling the behavior of a biotrophic phytopathogen. Additionally, leaves of plants treated with either T. virens wild-type (wt) or Δtbrg-1 and challenged or not with Botrytis showed ROS production to a different extent, as well as differential expression of cell detoxification-related genes, CAT1, and APX1. Root colonization assays of sid-2 and jar1 mutant lines affected in SAR and induced systemic resistance (ISR) by Δtbrg-1 and the wt strain, suggest an important role of both pathways in the recognition of the fungus and that TBRG-1 plays a pivotal role in root colonization. Furthermore, we found that TBRG-1 is a negative regulator of NoxR expression, which may impact the mutualistic interaction.

In Silico Evaluation, Phylogenetic Analysis, and Structural Modeling of the Class II Hydrophobin Family from Different Fungal Phytopathogens

The class II hydrophobin group (HFBII) is an extracellular group of proteins that contain the HFBII domain and eight conserved cysteine residues. These proteins are exclusively secreted by fungi and have multiple functions with a probable role as effectors. In the present study, a total of 45 amino acid sequences of hydrophobin class II proteins from different phytopathogenic fungi were retrieved from the NCBI database. We used the integration of well-designed bioinformatic tools to characterize and predict their physicochemical parameters, novel motifs, 3D structures, multiple sequence alignment (MSA), evolution, and functions as effector proteins through molecular docking. The results revealed new features for these protein members. The ProtParam tool detected the hydrophobicity properties of all proteins except for one hydrophilic protein (KAI3335996.1). Out of 45 proteins, six of them were detected as GPI-anchored proteins by the PredGPI server. Different 3D structure templates with high pTM scores were designed by Multifold v1, AlphaFold2, and trRosetta. Most of the studied proteins were anticipated as apoplastic effectors and matched with the ghyd5 gene of Fusarium graminearum as virulence factors. A protein-protein interaction (PPI) analysis unraveled the molecular function of this group as GTP-binding proteins, while a molecular docking analysis detected a chitin-binding effector role. From the MSA analysis, it was observed that the HFBII sequences shared conserved 2 Pro (P) and 2 Gly (G) amino acids besides the known eight conserved cysteine residues. The evolutionary analysis and phylogenetic tree provided evidence of episodic diversifying selection at the branch level using the aBSREL tool. A detailed in silico analysis of this family and the present findings will provide a better understanding of the HFBII characters and evolutionary relationships, which could be very useful in future studies.

Glucose-6-phosphate 1-Epimerase CrGlu6 Contributes to Development and Biocontrol Efficiency in Clonostachys chloroleuca

Clonostachys chloroleuca (formerly classified as C. rosea) is an important mycoparasite active against various plant fungal pathogens. Mitogen-activated protein kinase (MAPK) signaling pathways are vital in mycoparasitic interactions; they participate in responses to diverse stresses and mediate fungal development. In previous studies, the MAPK-encoding gene Crmapk has been proven to be involved in mycoparasitism and the biocontrol processes of C. chloroleuca, but its regulatory mechanisms remain unclear. Aldose 1-epimerases are key enzymes in filamentous fungi that generate energy for fungal growth and development. By protein-protein interaction assays, the glucose-6-phosphate 1-epimerase CrGlu6 was found to interact with Crmapk, and expression of the CrGlu6 gene was significantly upregulated when C. chloroleuca colonized Sclerotinia sclerotiorum sclerotia. Gene deletion and complementation analyses showed that CrGlu6 deficiency caused abnormal morphology of hyphae and cells, and greatly reduced conidiation. Moreover, deletion mutants presented much lower antifungal activities and mycoparasitic ability, and control efficiency against sclerotinia stem rot was markedly decreased. When the CrGlu6 gene was reinserted, all biological characteristics and biocontrol activities were recovered. These findings provide new insight into the mechanisms of glucose-6-phosphate 1-epimerase in mycoparasitism and help to further reveal the regulation of MAPK and its interacting proteins in the biocontrol of C. chloroleuca.

Prediction of Effector Proteins from Trichoderma longibrachiatum Through Transcriptome Sequencing

Trichoderma longibrachiatum SMF2 is an important biocontrol strain isolated by our group that can promote plant growth and induce plant disease resistance. To further study its biocontrol mechanism, the effector proteins secreted by T. longibrachiatum SMF2 were analyzed through bioinformatics and transcriptome sequencing. Overall, 478 secretory proteins produced by T. longibrachiatum were identified, of which 272 were upregulated after treatment with plants. Functional annotation showed that 36 secretory proteins were homologous with different groups of effectors from pathogenic microorganisms. Moreover, the quantitative PCR results of six putative effector proteins were consistent with those of transcriptome sequencing. Taken together, these findings indicate that the secretory proteins secreted by T. longibrachiatum SMF2 may act as effectors to facilitate its own growth and colonization or to induce plant immunity response.

Molecular interaction between plants and Trichoderma species against soil-borne plant pathogens

Trichoderma spp. (Hypocreales) are used worldwide as a lucrative biocontrol agent. The interactions of Trichoderma spp. with host plants and pathogens at a molecular level are important in understanding the various mechanisms adopted by the fungus to attain a close relationship with their plant host through superior antifungal/antimicrobial activity. When working in synchrony, mycoparasitism, antibiosis, competition, and the induction of a systemic acquired resistance (SAR)-like response are considered key factors in deciding the biocontrol potential of Trichoderma. Sucrose-rich root exudates of the host plant attract Trichoderma. The soluble secretome of Trichoderma plays a significant role in attachment to and penetration and colonization of plant roots, as well as modulating the mycoparasitic and antibiosis activity of Trichoderma. This review aims to gather information on how Trichoderma interacts with host plants and its role as a biocontrol agent of soil-borne phytopathogens, and to give a comprehensive account of the diverse molecular aspects of this interaction.

Taugt17b1 Overexpression in Trichoderma atroviride Enhances Its Ability to Colonize Roots and Induce Systemic Defense of Plants

Trichoderma atroviride, a soil fungus, has important applications in the biocontrol of plant diseases. Glycosyltransferases enhance the root colonization ability of Trichoderma spp. This study aimed to functionally characterize glycosyltransferase Taugt17b1 in T. atroviride. We investigated the effect of Taugt17b1 overexpression in T. atroviride H18-1-1 on its biocontrol properties, especially its ability to colonize roots. Our results demonstrated that the overexpression of the Taugt17b1 increases the T. atroviride colony growth rate, improves its root colonization ability, promotes the growth and activity of the defensive enzymatic system of plants, and prevents plant diseases. This study put forth a new role of T. atroviride glycosyltransferase and furthered the understanding of the mechanisms by which fungal biocontrol agents exert their effect.

Plant Defense Elicitation by the Hydrophobin Cerato-Ulmin and Correlation with Its Structural Features

Cerato-ulmin (CU) is a 75-amino-acid-long protein that belongs to the hydrophobin family. It self-assembles at hydrophobic-hydrophilic interfaces, forming films that reverse the wettability properties of the bound surface: a capability that may confer selective advantages to the fungus in colonizing and infecting elm trees. Here, we show for the first time that CU can elicit a defense reaction (induction of phytoalexin synthesis and ROS production) in non-host plants (Arabidopsis) and exerts its eliciting capacity more efficiently when in its soluble monomeric form. We identified two hydrophobic clusters on the protein's loops endowed with dynamical and physical properties compatible with the possibility of reversibly interconverting between a disordered conformation and a β-strand-rich conformation when interacting with hydrophilic or hydrophobic surfaces. We propose that the plasticity of those loops may be part of the molecular mechanism that governs the protein defense elicitation capability.

Insights into the ecological generalist lifestyle of Clonostachys fungi through analysis of their predicted secretomes

Introduction

The fungal secretome comprise diverse proteins that are involved in various aspects of fungal lifestyles, including adaptation to ecological niches and environmental interactions. The aim of this study was to investigate the composition and activity of fungal secretomes in mycoparasitic and beneficial fungal-plant interactions.

Methods

We used six Clonostachys spp. that exhibit saprotrophic, mycotrophic and plant endophytic lifestyles. Genome-wide analyses was performed to investigate the composition, diversity, evolution and gene expression of Clonostachys secretomes in relation to their potential role in mycoparasitic and endophytic lifestyles.

Results and discussion

Our analyses showed that the predicted secretomes of the analyzed species comprised between 7 and 8% of the respective proteomes. Mining of transcriptome data collected during previous studies showed that 18% of the genes encoding predicted secreted proteins were upregulated during the interactions with the mycohosts Fusarium graminearum and Helminthosporium solani. Functional annotation of the predicted secretomes revealed that the most represented protease family was subclass S8A (11-14% of the total), which include members that are shown to be involved in the response to nematodes and mycohosts. Conversely, the most numerous lipases and carbohydrate-active enzyme (CAZyme) groups appeared to be potentially involved in eliciting defense responses in the plants. For example, analysis of gene family evolution identified nine CAZyme orthogroups evolving for gene gains (p ≤ 0.05), predicted to be involved in hemicellulose degradation, potentially producing plant defense-inducing oligomers. Moreover, 8-10% of the secretomes was composed of cysteine-enriched proteins, including hydrophobins, important for root colonization. Effectors were more numerous, comprising 35-37% of the secretomes, where certain members belonged to seven orthogroups evolving for gene gains and were induced during the C. rosea response to F. graminearum or H. solani. Furthermore, the considered Clonostachys spp. possessed high numbers of proteins containing Common in Fungal Extracellular Membranes (CFEM) modules, known for their role in fungal virulence. Overall, this study improves our understanding of Clonostachys spp. adaptation to diverse ecological niches and establishes a basis for future investigation aiming at sustainable biocontrol of plant diseases.

Trichoderma and its role in biological control of plant fungal and nematode disease

Trichoderma is mainly used to control soil-borne diseases as well as some leaf and panicle diseases of various plants. Trichoderma can not only prevent diseases but also promotes plant growth, improves nutrient utilization efficiency, enhances plant resistance, and improves agrochemical pollution environment. Trichoderma spp. also behaves as a safe, low-cost, effective, eco-friendly biocontrol agent for different crop species. In this study, we introduced the biological control mechanism of Trichoderma in plant fungal and nematode disease, including competition, antibiosis, antagonism, and mycoparasitism, as well as the mechanism of promoting plant growth and inducing plant systemic resistance between Trichoderma and plants, and expounded on the application and control effects of Trichoderma in the control of various plant fungal and nematode diseases. From an applicative point of view, establishing a diversified application technology for Trichoderma is an important development direction for its role in the sustainable development of agriculture.

Fungal Effectoromics: A World in Constant Evolution

Effectors are small, secreted molecules that mediate the establishment of interactions in nature. While some concepts of effector biology have stood the test of time, this area of study is ever-evolving as new effectors and associated characteristics are being revealed. In the present review, the different characteristics that underly effector classifications are discussed, contrasting past and present knowledge regarding these molecules to foster a more comprehensive understanding of effectors for the reader. Research gaps in effector identification and perspectives for effector application in plant disease management are also presented, with a focus on fungal effectors in the plant-microbe interaction and interactions beyond the plant host. In summary, the review provides an amenable yet thorough introduction to fungal effector biology, presenting noteworthy examples of effectors and effector studies that have shaped our present understanding of the field.

Microbial Effectors: Key Determinants in Plant Health and Disease

Effectors are small, secreted molecules that alter host cell structure and function, thereby facilitating infection or triggering a defense response. Effectoromics studies have focused on effectors in plant-pathogen interactions, where their contributions to virulence are determined in the plant host, i.e., whether the effector induces resistance or susceptibility to plant disease. Effector molecules from plant pathogenic microorganisms such as fungi, oomycetes and bacteria are major disease determinants. Interestingly, the effectors of non-pathogenic plant organisms such as endophytes display similar functions but have different outcomes for plant health. Endophyte effectors commonly aid in the establishment of mutualistic interactions with the plant and contribute to plant health through the induction of systemic resistance against pathogens, while pathogenic effectors mainly debilitate the plant's immune response, resulting in the establishment of disease. Effectors of plant pathogens as well as plant endophytes are tools to be considered in effectoromics for the development of novel strategies for disease management. This review aims to present effectors in their roles as promotors of health or disease for the plant host.

Trichoderma - genomes and genomics as treasure troves for research towards biology, biotechnology and agriculture

The genus Trichoderma is among the best studied groups of filamentous fungi, largely because of its high relevance in applications from agriculture to enzyme biosynthesis to biofuel production. However, the physiological competences of these fungi, that led to these beneficial applications are intriguing also from a scientific and ecological point of view. This review therefore summarizes recent developments in studies of fungal genomes, updates on previously started genome annotation efforts and novel discoveries as well as efforts towards bioprospecting for enzymes and bioactive compounds such as cellulases, enzymes degrading xenobiotics and metabolites with potential pharmaceutical value. Thereby insights are provided into genomes, mitochondrial genomes and genomes of mycoviruses of Trichoderma strains relevant for enzyme production, biocontrol and mycoremediation. In several cases, production of bioactive compounds could be associated with responsible genes or clusters and bioremediation capabilities could be supported or predicted using genome information. Insights into evolution of the genus Trichoderma revealed large scale horizontal gene transfer, predominantly of CAZyme genes, but also secondary metabolite clusters. Investigation of sexual development showed that Trichoderma species are competent of repeat induced point mutation (RIP) and in some cases, segmental aneuploidy was observed. Some random mutants finally gave away their crucial mutations like T. reesei QM9978 and QM9136 and the fertility defect of QM6a was traced back to its gene defect. The Trichoderma core genome was narrowed down to 7000 genes and gene clustering was investigated in the genomes of multiple species. Finally, recent developments in application of CRISPR/Cas9 in Trichoderma, cloning and expression strategies for the workhorse T. reesei as well as the use genome mining tools for bioprospecting Trichoderma are highlighted. The intriguing new findings on evolution, genomics and physiology highlight emerging trends and illustrate worthwhile perspectives in diverse fields of research with Trichoderma.

Microbiota manipulation through the secretion of effector proteins is fundamental to the wealth of lifestyles in the fungal kingdom

Fungi are well-known decomposers of organic matter that thrive in virtually any environment on Earth where they encounter wealths of other microbes. Some fungi evolved symbiotic lifestyles, including pathogens and mutualists, that have mostly been studied in binary interactions with their hosts. However, we now appreciate that such interactions are greatly influenced by the ecological context in which they take place. While establishing their symbioses, fungi not only interact with their hosts but also with the host-associated microbiota. Thus, they target the host and its associated microbiota as a single holobiont. Recent studies have shown that fungal pathogens manipulate the host microbiota by means of secreted effector proteins with selective antimicrobial activity to stimulate disease development. In this review, we discuss the ecological contexts in which such effector-mediated microbiota manipulation is relevant for the fungal lifestyle and argue that this is not only relevant for pathogens of plants and animals but also beneficial in virtually any niche where fungi occur. Moreover, we reason that effector-mediated microbiota manipulation likely evolved already in fungal ancestors that encountered microbial competition long before symbiosis with land plants and mammalian animals evolved. Thus, we claim that effector-mediated microbiota manipulation is fundamental to fungal biology.

Aspergillus Hydrophobins: Physicochemical Properties, Biochemical Properties, and Functions in Solid Polymer Degradation

Hydrophobins are small amphipathic proteins conserved in filamentous fungi. In this review, the properties and functions of Aspergillus hydrophobins are comprehensively discussed on the basis of recent findings. Multiple Aspergillus hydrophobins have been identified and categorized in conventional class I and two non-conventional classes. Some Aspergillus hydrophobins can be purified in a water phase without organic solvents. Class I hydrophobins of Aspergilli self-assemble to form amphipathic membranes. At the air–liquid interface, RolA of Aspergillus oryzae self-assembles via four stages, and its self-assembled films consist of two layers, a rodlet membrane facing air and rod-like structures facing liquid. The self-assembly depends mainly on hydrophobin conformation and solution pH. Cys4–Cys5 and Cys7–Cys8 loops, disulfide bonds, and conserved Cys residues of RodA-like hydrophobins are necessary for self-assembly at the interface and for adsorption to solid surfaces. AfRodA helps Aspergillus fumigatus to evade recognition by the host immune system. RodA-like hydrophobins recruit cutinases to promote the hydrolysis of aliphatic polyesters. This mechanism appears to be conserved in Aspergillus and other filamentous fungi, and may be beneficial for their growth. Aspergilli produce various small secreted proteins (SSPs) including hydrophobins, hydrophobic surface–binding proteins, and effector proteins. Aspergilli may use a wide variety of SSPs to decompose solid polymers.

Trichoderma spp. Genes Involved in the Biocontrol Activity Against Rhizoctonia solani

Rhizoctonia solani is a pathogen that causes considerable harm to plants worldwide. In the absence of hosts, R. solani survives in the soil by forming sclerotia, and management methods, such as cultivar breeding, crop rotations, and fungicide sprays, are insufficient and/or inefficient in controlling R. solani. One of the most challenging problems facing agriculture in the twenty-first century besides with the impact of global warming. Environmentally friendly techniques of crop production and improved agricultural practices are essential for long-term food security. Trichoderma spp. could serve as an excellent example of a model fungus to enhance crop productivity in a sustainable way. Among biocontrol mechanisms, mycoparasitism, competition, and antibiosis are the fundamental mechanisms by which Trichoderma spp. defend against R. solani, thereby preventing or obstructing its proliferation. Additionally, Trichoderma spp. induce a mixed induced systemic resistance (ISR) or systemic acquired resistance (SAR) in plants against R. solani, known as Trichoderma-ISR. Stimulation of every biocontrol mechanism involves Trichoderma spp. genes responsible for encoding secondary metabolites, siderophores, signaling molecules, enzymes for cell wall degradation, and plant growth regulators. Rhizoctonia solani biological control through genes of Trichoderma spp. is summarized in this paper. It also gives information on the Trichoderma-ISR in plants against R. solani. Nonetheless, fast-paced current research on Trichoderma spp. is required to properly utilize their true potential against diseases caused by R. solani.

Isolation and identification of Arbutus unedo L. fungi endophytes and biological control of Phytophthora cinnamomi in vitro

Strawberry tree (Arbutus unedo, Ericaceae) is an evergreen tree with a circum-Mediterranean distribution. It has a great ecological and economic importance as a source of bioactive compounds with industrial applications and for fruit production. This study aims to characterize the fungi microbiome of this forestry species in order to develop biological control strategies in the increasing orchard production area. For this purpose, fungi endophytes were isolated from wild strawberry tree plants, and a molecular identification was carried out. In vitro assays were carried out to evaluate and characterize the antagonism of some endophytes. Among the several fungi endophytes isolated from strawberry tree (a total of 53 from 20 genera), a Trichoderma atroviride strain proved to have antagonism effect against several phytopathogens, including Alternaria alternata, Botrytis cinerea, Glomerella cingulata, and Mycosphaerella aurantia. This antagonism was particularly effective against Phytophthora cinnamomi, causing a reduction in growth of about 80% on this invasive oomycete. An enzymatic assay revealed the production of several enzymes by T. atroviride, such as cellulases, chitinases, glucosidases, alkaline phosphatases, and proteases, which is one of the several mechanisms known to be involved on Trichoderma biological control ability. The enzymatic activity, in particular that of cell wall-degrading enzymes, was accentuated when in a dual culture with P. cinnamomi. The production of serine proteases, aspartyl proteases, metalloproteases, and cysteine proteases was also detected in an experiment carried out in liquid medium, suggesting the involvement of these proteases on Trichoderma mycoparasitism mechanisms. Finally, in a three-way interaction with in vitro strawberry tree plants, the T. atroviride strain identified on this study (Au50) was able to protect the plants against P. cinnamomi, thus proving its potential as a biological control agent.

MAPK Cascades Mediating Biocontrol Activity of Trichoderma brevicrassum Strain TC967

Trichoderma brevicrassum strain TC967 is a novel biological control agent (BCA) against the plant pathogen Rhizoctonia solani and promotes plant growth. Mitogen-activated protein kinase (MAPK) cascades are involved in a variety of physiological functions of Trichoderma, but functions of each MAPK in regulating biocontrol have not been characterized in a single Trichoderma strain. In this study, we assembled and annotated the genome of strain TC967 and identified its three MAPK gene sequences. Functions of Fus3-, Slt2-, and Hog1-MAPK in strain TC967 were dissected. All three MAPKs were involved in hyphal growth. Hog1-MAPK was essential for conidiation and tolerance to saline/osmotic stress. Both Fus3- and Slt2-MAPK deletion mutants reduced cell-wall integrity (CWI) and increased the activities of chitinase and protease. The growth of Rhizoctoniasolani was further inhibited by volatile organic compounds (VOCs) and secondary metabolites produced by Fus3- and Slt2-MAPK deletion mutants, respectively. Biocontrol assays demonstrated that Fus3- and Slt2-MAPK deletion mutants were considerably more effective in disease control than the wild-type strain. RNA-seq analysis revealed that MAPK collectively played a major role in regulating biocontrol-related gene expressions, including the genes in charge of secondary metabolism, fungal cell wall-degrading enzymes (FCWDEs), and small secreted cysteine-rich proteins (SSCPs).

Microbial interaction mediated programmed cell death in plants

Food demand of growing population can only be met by finding solutions for sustaining the crop yield. The understanding of basic mechanisms employed by microorganisms for the establishment of parasitic relationship with plants is a complex phenomenon. Symbionts and biotrophs are dependent on living hosts for completing their life cycle, whereas necrotrophs utilize dead cells for their growth and establishment. Hemibiotrophs as compared to other microbes associate themselves with plants in two phase's, viz. early bio-phase and later necro-phase. Plants and microbes interact with each other using receptors present on host cell surface and elicitors (PAMPs and effectors) produced by microbes. Plant-microbe interaction either leads to compatible or incompatible reaction. In response to various biotic and abiotic stress factors, plant undergoes programmed cell death which restricts the growth of biotrophs or hemibiotrophs while necrotrophs as an opportunist starts growing on dead tissue for their own benefit. PCD regulation is an outcome of plant-microbe crosstalk which entirely depends on various biochemical events like generation of reactive oxygen species, nitric oxide, ionic efflux/influx, CLPs, biosynthesis of phytohormones, phytoalexins, polyamines and certain pathogenesis-related proteins. This phenomenon mostly occurs in resistant and non-host plants during invasion of pathogenic microbes. The compatible or incompatible host-pathogen interaction depends upon the presence or absence of host plant resistance and pathogenic race. In addition to host-pathogen interaction, the defense induction by beneficial microbes must also be explored and used to the best of its potential. This review highlights the mechanism of microbe- or symbiont-mediated PCD along with defense induction in plants towards symbionts, biotrophs, necrotrophs and hemibiotrophs. Here we have also discussed the possible use of beneficial microbes in inducing systemic resistance in plants against pathogenic microbes.

Comparative Secretome Analyses of Trichoderma/Arabidopsis Co-cultures Identify Proteins for Salt Stress, Plant Growth Promotion, and Root Colonization

Numerous Trichoderma strains are beneficial for plants, promote their growth, and confer stress tolerance. A recently described novel Trichoderma strain strongly promotes the growth of Arabidopsis thaliana seedlings on media with 50 mM NaCl, while 150 mM NaCl strongly stimulated root colonization and induced salt-stress tolerance in the host without growth promotion. To understand the dynamics of plant-fungus interaction, we examined the secretome from both sides and revealed a substantial change under different salt regimes, and during co-cultivation. Stress-related proteins, such as a fungal cysteine-rich Kp4 domain-containing protein which inhibits plant cell growth, fungal WSC- and CFEM-domain-containing proteins, the plant calreticulin, and cell-wall modifying enzymes, disappear when the two symbionts are co-cultured under high salt concentrations. In contrast, the number of lytic polysaccharide monooxygenases increases, which indicates that the fungus degrades more plant lignocellulose under salt stress and its lifestyle becomes more saprophytic. Several plant proteins involved in plant and fungal cell wall modifications and root colonization are only found in the co-cultures under salt stress, while the number of plant antioxidant proteins decreased. We identified symbiosis- and salt concentration-specific proteins for both partners. The Arabidopsis PYK10 and a fungal prenylcysteine lyase are only found in the co-culture which promoted plant growth. The comparative analysis of the secretomes supports antioxidant enzyme assays and suggests that both partners profit from the interaction under salt stress but have to invest more in balancing the symbiosis. We discuss the role of the identified stage- and symbiosis-specific fungal and plant proteins for salt stress, and conditions promoting root colonization and plant growth.

Target highlights in CASP14: Analysis of models by structure providers

The biological and functional significance of selected Critical Assessment of Techniques for Protein Structure Prediction 14 (CASP14) targets are described by the authors of the structures. The authors highlight the most relevant features of the target proteins and discuss how well these features were reproduced in the respective submitted predictions. The overall ability to predict three-dimensional structures of proteins has improved remarkably in CASP14, and many difficult targets were modeled with impressive accuracy. For the first time in the history of CASP, the experimentalists not only highlighted that computational models can accurately reproduce the most critical structural features observed in their targets, but also envisaged that models could serve as a guidance for further studies of biologically-relevant properties of proteins.

Extracellular Vesicles from Fusarium graminearum Contain Protein Effectors Expressed during Infection of Corn

Fusarium graminearum (Fgr) is a devastating filamentous fungal pathogen that causes diseases in cereals, while producing mycotoxins that are toxic for humans and animals, and render grains unusable. Low efficiency in managing Fgr poses a constant need for identifying novel control mechanisms. Evidence that fungal extracellular vesicles (EVs) from pathogenic yeast have a role in human disease led us to question whether this is also true for fungal plant pathogens. We separated EVs from Fgr and performed a proteomic analysis to determine if EVs carry proteins with potential roles in pathogenesis. We revealed that protein effectors, which are crucial for fungal virulence, were detected in EV preparations and some of them did not contain predicted secretion signals. Furthermore, a transcriptomic analysis of corn (Zea mays) plants infected by Fgr revealed that the genes of some of the effectors were highly expressed in vivo, suggesting that the Fgr EVs are a mechanism for the unconventional secretion of effectors and virulence factors. Our results expand the knowledge on fungal EVs in plant pathogenesis and cross-kingdom communication, and may contribute to the discovery of new antifungals.

Bioprospecting Trichoderma: A Systematic Roadmap to Screen Genomes and Natural Products for Biocontrol Applications

Natural products derived from microbes are crucial innovations that would help in reaching sustainability development goals worldwide while achieving bioeconomic growth. Trichoderma species are well-studied model fungal organisms used for their biocontrol properties with great potential to alleviate the use of agrochemicals in agriculture. However, identifying and characterizing effective natural products in novel species or strains as biological control products remains a meticulous process with many known challenges to be navigated. Integration of recent advancements in various "omics" technologies, next generation biodesign, machine learning, and artificial intelligence approaches could greatly advance bioprospecting goals. Herein, we propose a roadmap for assessing the potential impact of already known or newly discovered Trichoderma species for biocontrol applications. By screening publicly available Trichoderma genome sequences, we first highlight the prevalence of putative biosynthetic gene clusters and antimicrobial peptides among genomes as an initial step toward predicting which organisms could increase the diversity of natural products. Next, we discuss high-throughput methods for screening organisms to discover and characterize natural products and how these findings impact both fundamental and applied research fields.

Effects on Capsicum annuum Plants Colonized with Trichoderma atroviride P. Karst Strains Genetically Modified in Taswo1, a Gene Coding for a Protein with Expansin-like Activity

Here, we analyzed the effects on Capsicum annuum plants of Trichoderma atroviride P. Karst strains altered in the expression of SWOLLENIN (SWO1), a protein with amorphogenic activity on plant cell wall components. Strains of T. atroviride that overexpressed the Taswo1 gene were constructed as well as deletion mutants. A novel, cheap and accurate method for assessing root colonization was developed. Colonization assays showed that the Taswo1 overexpressing strains invaded the host root better than the WT, resulting in a stronger plant growth-promoting effect. The expression of plant defense marker genes for both the systemic acquired resistance and induced systemic resistance pathways was enhanced in plants inoculated with Taswo1 overexpressing strains, while inoculation with deletion mutant strains resulted in a similar level of expression to that observed upon inoculation with the wild-type strain. Response to pathogen infection was also enhanced in the plants inoculated with the Taswo1 overexpressing strains, and surprisingly, an intermediate level of protection was achieved with the mutant strains. Tolerance to abiotic stresses was also higher in plants inoculated with the Taswo1 overexpressing strains but was similar in plants inoculated with the wild-type or the mutant strains. Compatible osmolyte production in drought conditions was studied. This study may contribute to improving Trichoderma biocontrol and biofertilization abilities.

Early Transcriptome Response of Trichoderma virens to Colonization of Maize Roots

Trichoderma virens is a well-known mycoparasitic fungal symbiont that is valued for its biocontrol capabilities. T. virens initiates a symbiotic relationship with a plant host through the colonization of its roots. To achieve colonization, the fungus must communicate with the host and evade its innate defenses. In this study, we explored the genes involved with the host communication and colonization process through transcriptomic profiling of the wild-type fungus and selected deletion mutants as they colonized maize roots. Transcriptome profiles of the T. virens colonization of maize roots over time revealed that 24 h post inoculation appeared to be a key time for plant-microbe communication, with many key gene categories, including signal transduction mechanisms and carbohydrate transport and metabolism, peaking in expression at this early colonization time point. The transcriptomic profiles of Sm1 and Sir1 deletion mutants in the presence of plants demonstrated that Sir1, rather than Sm1, appears to be the key regulator of the fungal response to maize, with 64% more unique differentially expressed genes compared to Sm1. Additionally, we developed a novel algorithm utilizing gene clustering and coexpression network analyses to select potential colonization-related gene targets for characterization. About 40% of the genes identified by the algorithm would have been missed using previous methods for selecting gene targets.

Gene Editing of the Decoy Receptor LeEIX1 Increases Host Receptivity to Trichoderma Bio-Control

Fungal and bacterial pathogens generate devastating diseases and cause significant tomato crop losses worldwide. Due to chemical pesticides harming the environment and human health, alternative disease control strategies, including microorganismal bio-control agents (BCAs), are increasingly sought-after in agriculture. Bio-control microorganisms such as Trichoderma spp. have been shown to activate induced systemic resistance (ISR) in the host. However, examples of highly active bio-control microorganisms in agricultural settings are still lacking, due primarily to inconsistency in bio-control efficacy, often leading to widespread disease prior to the required ISR induction in the host. As part of its plant colonization strategy, Trichoderma spp. can secrete various compounds and molecules, which can effect host priming/ISR. One of these molecules synthesized and secreted from several species of Trichoderma is the family 11 xylanase enzyme known as ethylene inducing xylanase, EIX. EIX acts as an ISR elicitor in specific plant species and varieties. The response to EIX in tobacco and tomato cultivars is controlled by a single dominant locus, termed LeEIX, which contains two receptors, LeEIX1 and LeEIX2, both belonging to a class of leucine-rich repeat cell-surface glycoproteins. Both receptors are able to bind EIX, however, while LeEIX2 mediates plant defense responses, LeEIX1 acts as a decoy receptor and attenuates EIX induced immune signaling of the LeEIX2 receptor. By mutating LeEIX1 using CRISPR/Cas9, here, we report an enhancement of receptivity to T. harzianum mediated ISR and disease bio-control in tomato.

Biocontrol Agents: Toolbox for the Screening of Weapons against Mycotoxigenic Fusarium

The aim of this study was to develop a set of experiments to screen and decipher the mechanisms of biocontrol agents (BCAs), isolated from commercial formulation, against two major mycotoxigenic fungi in cereals, Fusarium graminearum and Fusarium verticillioides. These two phytopathogens produce mycotoxins harmful to human and animal health and are responsible for the massive use of pesticides, for the protection of cereals. It is therefore essential to better understand the mechanisms of action of alternative control strategies such as the use of BCAs in order to optimize their applications. The early and late stages of interaction between BCAs and pathogens were investigated from germination of spores to the effects on perithecia (survival form of pathogen). The analysis of antagonist activities of BCAs revealed different strategies of biocontrol where chronological, process combination and specialization aspects of interactions are discussed. Streptomyces griseoviridis main strategy is based on antibiosis with the secretion of several compounds with anti-fungal and anti-germination activity, but also a mixture of hydrolytic enzymes to attack pathogens, which compensates for an important deficit in terms of spatial colonization capacity. It has good abilities in terms of nutritional competition. Trichoderma asperellum is capable of activating a very wide range of defenses and attacks combining the synthesis of various antifungal compounds (metabolite, enzymes, VOCs), with different targets (spores, mycelium, mycotoxins), and direct action by mycoparasitism and mycophagy. Concerning Pythium oligandrum, its efficiency is mainly due to its strong capacity to colonize the environment, with a direct action via microbial predation, stimulation of its reproduction at the contact of pathogens and the reduction of perithecia formation.

Role of Trichoderma as a biocontrol agent (BCA) of phytoparasitic nematodes and plant growth inducer

Nematodes as plant pathogens adversely affect food, fiber, and biofuels production by causing plant diseases. A variety of chemical nematicides are being applied to soil, seeds, or foliage with a goal of disease prevention. Despite the proven efficacy of these chemicals against plant-parasitic nematodes, factors like prolonged residual toxicity to human health, environmental pollution, and the risk of resistance development can't be neglected. Due to these reasons, many chemicals are being banned continuously or delimited in the crop production system. Alternatively, the need for long-term strategies and integrative approaches to control plant diseases is inevitable. Trichoderma spp. are widely used in agriculture as biological control agents (BCA). To our knowledge, either very little or no information available on the most recent developments regarding Trichoderma-mediated biological control of plant-parasitic nematodes. This review summarizes the recent advances in using Trichoderma as BCA and plant growth regulator with a special focus on plant-parasitic nematodes.

Proteomic Analysis Demonstrates a Molecular Dialog Between Trichoderma guizhouense NJAU 4742 and Cucumber (Cucumis sativus L.) Roots: Role in Promoting Plant Growth

Trichoderma is a genus of filamentous fungi that play notable roles in stimulating plant growth after colonizing the root surface. However, the key proteins and molecular mechanisms governing this stimulation have not been completely elucidated. In this study, Trichoderma guizhouense NJAU 4742 was investigated in a hydroponic culture system after interacting with cucumber roots. The total proteins of the fungus were characterized, and the key metabolic pathways along with related genes were analyzed through proteomic and transcriptomic analyses. The roles played by the regulated proteins during the interaction between plants and NJAU 4742 were further examined. The intracellular or extracellular proteins from NJAU 4742 and extracellular proteins from cucumber were quantified, and the high-abundance proteins were determined which were primarily involved in the shikimate pathway (tryptophan, tyrosine, and phenylalanine metabolism, auxin biosynthesis, and secondary metabolite synthesis). Moreover, ¹⁵N-KNO₃ labeling analysis indicated that NJAU 4742 had a strong ability to convert nitrogenous amino acids, nitrate, nitrile, and amines into ammonia. The auxin synthesis and ammonification metabolism pathways of NJAU 4742 significantly contributed to plant growth. The results of this study demonstrated the crucial metabolic pathways involved in the interactions between Trichoderma spp. and plants.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Research Progress on Phytopathogenic Fungi and Their Role as Biocontrol Agents

Phytopathogenic fungi decrease crop yield and quality and cause huge losses in agricultural production. To prevent the occurrence of crop diseases and insect pests, farmers have to use many synthetic chemical pesticides. The extensive use of these pesticides has resulted in a series of environmental and ecological problems, such as the increase in resistant weed populations, soil compaction, and water pollution, which seriously affect the sustainable development of agriculture. This review discusses the main advances in research on plant-pathogenic fungi in terms of their pathogenic factors such as cell wall-degrading enzymes, toxins, growth regulators, effector proteins, and fungal viruses, as well as their application as biocontrol agents for plant pests, diseases, and weeds. Finally, further studies on plant-pathogenic fungal resources with better biocontrol effects can help find new beneficial microbial resources that can control diseases.

Comparative Study of Structural Changes of Polylactide and Poly(ethylene terephthalate) in the Presence of Trichoderma viride

Plastic pollution is one of the crucial global challenges nowadays, and biodegradation is a promising approach to manage plastic waste in an environment-friendly and cost-effective way. In this study we identified the strain of fungus Trichoderma viride GZ1, which was characterized by particularly high pectinolytic activity. Using differential scanning calorimetry, Fourier-transform infrared spectroscopy techniques, and viscosity measurements we showed that three-month incubation of polylactide and polyethylene terephthalate in the presence of the fungus lead to significant changes of the surface of polylactide. Further, to gain insight into molecular mechanisms underneath the biodegradation process, western blot hybridization was used to show that in the presence of poly(ethylene terephthalate) (PET) in laboratory conditions the fungus produced hydrophobin proteins. The mycelium adhered to the plastic surface, which was confirmed by scanning electron microscopy, possibly due to the presence of hydrophobins. Further, using atomic force microscopy we demonstrated for the first time the formation of hydrophobin film on the surface of aliphatic polylactide (PLA) and PET by T. viride GZ1. This is the first stage of research that will be continued under environmental conditions, potentially leading to a practical application.

At least three families of hyphosphere small secreted cysteine-rich proteins can optimize surface properties to a moderately hydrophilic state suitable for fungal attachment

The secretomes of filamentous fungi contain a diversity of small secreted cysteine‐rich proteins (SSCPs) that have a variety of properties ranging from toxicity to surface activity. Some SSCPs are recognized by other organisms as indicators of fungal presence, but their function in fungi is not fully understood. We detected a new family of fungal surface‐active SSCPs (saSSCPs), here named hyphosphere proteins (HFSs). An evolutionary analysis of the HFSs in Pezizomycotina revealed a unique pattern of eight single cysteine residues (C‐CXXXC‐C‐C‐C‐C‐C) and a long evolutionary history of multiple gene duplications and ancient interfungal lateral gene transfers, suggesting their functional significance for fungi with different lifestyles. Interestingly, recombinantly produced saSSCPs from three families (HFSs, hydrophobins and cerato‐platanins) showed convergent surface‐modulating activity on glass and on poly(ethylene‐terephthalate), transforming their surfaces to a moderately hydrophilic state, which significantly favoured subsequent hyphal attachment. The addition of purified saSSCPs to the tomato rhizosphere had mixed effects on hyphal attachment to roots, while all tested saSSCPs had an adverse effect on plant growth in vitro. We propose that the exceptionally high diversity of saSSCPs in Trichoderma and other fungi evolved to efficiently condition various surfaces in the hyphosphere to a fungal‐beneficial state.

Secretion-Based Modes of Action of Biocontrol Agents with a Focus on Pseudozyma aphidis

Plant pathogens challenge our efforts to maximize crop production due to their ability to rapidly develop resistance to pesticides. Fungal biocontrol agents have become an important alternative to chemical fungicides, due to environmental concerns related to the latter. Here we review the complex modes of action of biocontrol agents in general and epiphytic yeasts belonging to the genus Pseudozyma specifically and P. aphidis in particular. Biocontrol agents act through multiple direct and indirect mechanisms, which are mainly based on their secretions. We discuss the direct modes of action, such as antibiosis, reactive oxygen species-producing, and cell wall-degrading enzyme secretions which can also play a role in mycoparasitism. In addition, we discuss indirect modes of action, such as hyperbiotrophy, induced resistance and growth promotion based on the secretion of effectors and elicitors from the biocontrol agent. Due to their unique characteristics, epiphytic yeasts hold great potential for use as biocontrol agents, which may be more environmentally friendly than conventional pesticides and provide a way to reduce our dependency on fungicides based on increasingly expensive fossil fuels. No less important, the complex mode of action of Pseudozyma-based biocontrol agents can also reduce the frequency of resistance developed by pathogens to these agents.

Deciphering Trichoderma-Plant-Pathogen Interactions for Better Development of Biocontrol Applications

Members of the fungal genus Trichoderma (Ascomycota, Hypocreales, Hypocreaceae) are ubiquitous and commonly encountered as soil inhabitants, plant symbionts, saprotrophs, and mycoparasites. Certain species have been used to control diverse plant diseases and mitigate negative growth conditions. The versatility of Trichoderma’s interactions mainly relies on their ability to engage in inter- and cross-kingdom interactions. Although Trichoderma is by far the most extensively studied fungal biocontrol agent (BCA), with a few species already having been commercialized as bio-pesticides or bio-fertilizers, their wide application has been hampered by an unpredictable efficacy under field conditions. Deciphering the dialogues within and across Trichoderma ecological interactions by identification of involved effectors and their underlying effect is of great value in order to be able to eventually harness Trichoderma’s full potential for plant growth promotion and protection. In this review, we focus on the nature of Trichoderma interactions with plants and pathogens. Better understanding how Trichoderma interacts with plants, other microorganisms, and the environment is essential for developing and deploying Trichoderma-based strategies that increase crop production and protection.

Leaping into the Unknown World of Sporisorium scitamineum Candidate Effectors

Sporisorium scitamineum is a biotrophic fungus causing sugarcane smut disease. In this study, we set up a pipeline and used genomic and dual transcriptomic data previously obtained by our group to identify candidate effectors of S. scitamineum and their expression profiles in infected smut-resistant and susceptible sugarcane plants. The expression profile of different genes after infection in contrasting sugarcane genotypes assessed by RT-qPCR depended on the plant genotypes and disease progression. Three candidate effector genes expressed earlier only in resistant plants, four expressed in both genotypes, and three later in susceptible plants. Ten genes were cloned and transiently expressed in N. benthamiana leaves to determine their subcellular location, while four localized in more than one compartment. Two candidates, g3890 having a nucleoplasmic and mitochondrial location and g5159 targeting the plant cell wall, were selected to obtain their possible corresponding host targets using co-immunoprecipitation (CoIP) experiments and mass spectrometry. Various potential interactors were identified, including subunits of the protein phosphatase 2A and an endochitinase. We investigated the presence of orthologs in sugarcane and using transcriptome data present their expression profiles. Orthologs of sugarcane shared around 70% similarity. Identifying a set of putative fungal effectors and their plant targets provides a valuable resource for functional characterization of the molecular events leading to smut resistance in sugarcane plants and uncovers further opportunities for investigation.

Characterization of Three Fusarium graminearum Effectors and Their Roles During Fusarium Head Blight

Fusarium graminearum causes Fusarium head blight (FHB) on wheat, barley, and other grains. During infection, F. graminearum produces deoxynivalenol (DON), which contaminates grain and functions as a virulence factor to promote FHB spread throughout the wheat head. F. graminearum secretes hundreds of putative effectors, which can interfere with plant immunity to promote disease development. However, the function of most of these putative effectors remains unknown. In this study, we investigated the expression profiles of 23 F. graminearum effector-coding genes during the early stage of wheat head infection. Gene expression analyses revealed that three effectors, FGSG_01831, FGSG_03599, and FGSG_12160, respectively, were highly induced in both a FHB susceptible and a moderately resistant variety. We generated deletion mutants for these effector genes and performed FHB virulence assays on wheat head using point and dip inoculations to evaluate FHB spread and initial infection. No statistically significant difference in FHB spread was observed in the deletion mutants. However, deletion mutants Δ01831 displayed a significant reduction in initial infection, and thus resulted in less DON contamination. To investigate the potential mechanisms involved, these three effectors were transiently expressed in Nicotiana benthamiana leaves. N. benthamiana leaves expressing these individual effectors had significantly reduced production of reactive oxygen species induced by chitin, but not by flg22. Furthermore, FGSG_01831 and FGSG_03599 markedly suppressed Bax-induced cell death when co-expressed with Bax in N. benthamiana leaves. Our study provides new insights into the functions of these effectors and suggests they play collective or redundant roles that likely ensure the successful plant infection.

Evolutionary compromises in fungal fitness: hydrophobins can hinder the adverse dispersal of conidiospores and challenge their survival

Fungal evolutionary biology is impeded by the scarcity of fossils, irregular life cycles, immortality, and frequent asexual reproduction. Simple and diminutive bodies of fungi develop inside a substrate and have exceptional metabolic and ecological plasticity, which hinders species delimitation. However, the unique fungal traits can shed light on evolutionary forces that shape the environmental adaptations of these taxa. Higher filamentous fungi that disperse through aerial spores produce amphiphilic and highly surface-active proteins called hydrophobins (HFBs), which coat spores and mediate environmental interactions. We exploited a library of HFB-deficient mutants for two cryptic species of mycoparasitic and saprotrophic fungi from the genus Trichoderma (Hypocreales) and estimated fungal development, reproductive potential, and stress resistance. HFB4 and HFB10 were found to be relevant for Trichoderma fitness because they could impact the spore-mediated dispersal processes and control other fitness traits. An analysis in silico revealed purifying selection for all cases except for HFB4 from T. harzianum, which evolved under strong positive selection pressure. Interestingly, the deletion of the hfb4 gene in T. harzianum considerably increased its fitness-related traits. Conversely, the deletion of hfb4 in T. guizhouense led to the characteristic phenotypes associated with relatively low fitness. The net contribution of the hfb4 gene to fitness was found to result from evolutionary tradeoffs between individual traits. Our analysis of HFB-dependent fitness traits has provided an evolutionary snapshot of the selective pressures and speciation process in closely related fungal species.

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.