The Trichoderma harzianum Kelch Protein ThKEL1 Plays a Key Role in Root Colonization and the Induction of Systemic Defense in Brassicaceae Plants

Trichoderma spp.-mediated mitigation of heat, drought, and their combination on the Arabidopsis thaliana holobiont: a metabolomics and metabarcoding approach

Introduction

The use of substances to increase productivity and resource use efficiency is now essential to face the challenge of feeding the rising global population with the less environmental impact on the ecosystems. Trichoderma-based products have been used as biopesticides, to inhibit pathogenic microorganisms, and as biostimulants for crop growth, nutrient uptake promotion, and resistance to abiotic stresses.

Methods

In this work, plant metabolomics combined with roots and rhizosphere bacterial metabarcoding were exploited to inspect the performance of Trichoderma spp. biostimulants on Arabidopsis thaliana under drought, heat and their combination and its impact on plant holobiont.

Results and discussion

An overall modulation of N-containing compounds, phenylpropanoids, terpenes and hormones could be pointed out by metabolomics. Moreover, metabarcoding outlined an impact on alpha and beta-diversity with an abundance of Proteobacteria, Pseudomonadales, Burkholderiales, Enterobacteriales and Azospirillales. A holobiont approach was applied as an integrated analytical strategy to resolve the coordinated and complex dynamic interactions between the plant and its rhizosphere bacteria using Arabidopsis thaliana as a model host species.

Trichoderma hamatum can act as an inter-plant communicator of foliar pathogen infections by colonizing the roots of nearby plants: A new inter-plant "wired communication"

Trichoderma is a genus of filamentous fungi widely studied and used as a biological control agent in agriculture. However, its ability to form fungal networks for inter-plant communication by means of the so-called inter-plant "wired communication" has not yet been addressed. In our study we used the model plant Arabidopsis thaliana, the fungus Trichoderma hamatum (isolated from Brassicaceae plants) and the pathogens Sclerotinia sclerotiorum and Xanthomonas campestris (necrotrophic fungus and hemibiotrophic bacteria, respectively). We performed different combinations of isolated/neighboring plants and root colonization/non-colonization by T. hamatum, as well as foliar infections with the pathogens. In this way, we were able to determine how, in the absence of T. hamatum, there is an inter-plant communication that induces systemic resistance in neighboring plants of plants infected by the pathogens. On the other hand, the plants colonized by T. hamatum roots show a greater systemic resistance against the pathogens. Regarding the role of T. hamatum as an inter-plant communicator, it is the result of an increase in foliar signaling by jasmonic acid (increased expression of LOX1 and VSP2 genes and decreased expression of ICS1 and PR-1 genes), antagonistically increasing root signaling by salicylic acid (increased expression of ICS1 and PR-1 genes and decreased expression of LOX1 and VSP2). This situation prevents root colonization by T. hamatum of the foliarly infected plant and leads to massive colonization of the neighboring plant, where jasmonic acid-mediated systemic defenses are induced.

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.

Trichoderma Species: Our Best Fungal Allies in the Biocontrol of Plant Diseases-A Review

Biocontrol agents (BCA) have been an important tool in agriculture to prevent crop losses due to plant pathogens infections and to increase plant food production globally, diminishing the necessity for chemical pesticides and fertilizers and offering a more sustainable and environmentally friendly option. Fungi from the genus Trichoderma are among the most used and studied microorganisms as BCA due to the variety of biocontrol traits, such as parasitism, antibiosis, secondary metabolites (SM) production, and plant defense system induction. Several Trichoderma species are well-known mycoparasites. However, some of those species can antagonize other organisms such as nematodes and plant pests, making this fungus a very versatile BCA. Trichoderma has been used in agriculture as part of innovative bioformulations, either just Trichoderma species or in combination with other plant-beneficial microbes, such as plant growth-promoting bacteria (PGPB). Here, we review the most recent literature regarding the biocontrol studies about six of the most used Trichoderma species, T. atroviride, T. harzianum, T. asperellum, T. virens, T. longibrachiatum, and T. viride, highlighting their biocontrol traits and the use of these fungal genera in Trichoderma-based formulations to control or prevent plant diseases, and their importance as a substitute for chemical pesticides and fertilizers.

A potential role of salicylic acid in the evolutionary behavior of Trichoderma as a plant pathogen: from Marchantia polymorpha to Arabidopsis thaliana

Recognition of the interaction of Trichoderma during the evolution of land plants plays a potential key role in the development of the salicylic acid defense pathway and the establishment of a mutualistic relationship. Marchantia polymorpha is a common liverwort considered in recent years as a model plant for evolutionary studies on plant-microorganism interactions. Despite the lack of research, remarkable results have been reported regarding the understanding of metabolic and evolutionary processes of beneficial and/or harmful interactions, owing to a better understanding of the origin and evolution of different plant defense pathways. In this study, we have carried out work on the direct and indirect interactions (exudates and volatiles) of M. polymorpha with different species of the fungal genus Trichoderma. These interactions showed different outcomes, including resistance or even growth promotion and disease. We have analyzed the level of tissue colonization and defense-related gene expression. Furthermore, we have used the pteridophyte Dryopteris affinis and the angiosperm Arabidopsis thaliana, as subsequent steps in plant evolution, together with the plant pathogen Rhizoctonia solani as a control of plant pathogenicity. Trichoderma virens, T. brevicompactum and T. hamatum are pathogens of M. polymorpha, while exudates of T. asperellum are harmful to the plant. The analysis of the expression of several defense genes in M. polymorpha and A. thaliana showed that there is a correlation of the transcriptional activation of SA-related genes with resistance or susceptibility of M. polymorpha to Trichoderma. Moreover, exogenous SA provides resistance to the virulent Trichoderma species. This beneficial fungus may have had an evolutionary period of interaction with plants in which it behaved as a plant pathogen until plants developed a defense system to limit its colonization through a defense response mediated by SA.

Endophytic fungi from kale (Brassica oleracea var. acephala) modify roots-glucosinolate profile and promote plant growth in cultivated Brassica species. First description of Pyrenophora gallaeciana

Endophytic fungi of crops can promote plant growth through various mechanisms of action (i.e., improve nutrient uptake and nutrient use efficiency, and produce and modulate plant hormones). The genus Brassica includes important horticultural crops, which have been little studied in their interaction with endophytic fungi. Previously, four endophytic fungi were isolated from kale roots (Brassica oleracea var. acephala), with different benefits for their host, including plant growth promotion, cold tolerance, and induction of resistance to pathogens (Xanthomonas campestris) and pests (Mamestra brassicae). In the present work, the molecular and morphological identification of the four different isolates were carried out, describing them as the species Acrocalymma vagum, Setophoma terrestris, Fusarium oxysporum, and the new species Pyrenophora gallaeciana. In addition, using a representative crop of each Brassica U’s triangle species and various in vitro biochemical tests, the ability of these fungi to promote plant growth was described. In this sense, the four fungi used promoted the growth of B. rapa, B. napus, B. nigra, B. juncea, and B. carinata, possibly due to the production of auxins, siderophores, P solubilization or cellulase, xylanase or amylase activity. Finally, the differences in root colonization between the four endophytic fungi and two pathogens (Leptosphaeria maculans and Sclerotinia sclerotiorum) and the root glucosinolate profile were studied, at different times. In this way, how the presence of progoitrin in the roots reduces their colonization by endophytic and pathogenic fungi was determined, while the possible hydrolysis of sinigrin to fungicidal products controls the colonization of endophytic fungi, but not of pathogens.

Interactions of fungi with non-isothiocyanate products of the plant glucosinolate pathway: A review on product formation, antifungal activity, mode of action and biotransformation

The glucosinolate pathway, which is present in the order Brassicales, is one of the most researched defensive natural product biosynthesis pathways. Its core molecules, the glucosinolates are broken down upon pathogen challenge or tissue damage to yield an array of natural products that may help plants defend against the stressor. Though the most widely known glucosinolate decomposition products are the antimicrobial isothiocyanates, there is a wide range of other volatile and non-volatile natural products that arise from this biosynthetic pathway. This review summarizes our current knowledge on the interaction of these much less examined, non-isothiocyanate products with fungi. It deals with compounds including (1) glucosinolates and their biosynthesis precursors; (2) glucosinolate-derived nitriles (e.g. derivatives of 1H-indole-3-acetonitrile), thiocyanates, epithionitriles and oxazolidine-2-thiones; (3) putative isothiocyanate downstream products such as raphanusamic acid, 1H-indole-3-methanol (= indole-3-carbinol) and its oligomers, 1H-indol-3-ylmethanamine and ascorbigen; (4) 1H-indole-3-acetonitrile downstream products such as 1H-indole-3-carbaldehyde (indole-3-carboxaldehyde), 1H-indole-3-carboxylic acid and their derivatives; and (5) indole phytoalexins including brassinin, cyclobrassinin and brassilexin. Herein, a literature review on the following aspects is provided: their direct antifungal activity and the proposed mechanisms of antifungal action, increased biosynthesis after fungal challenge, as well as data on their biotransformation/detoxification by fungi, including but not limited to fungal myrosinase activity.

Resistance Induction in Olive Tree (Olea europaea) Against Verticillium Wilt by Two Beneficial Microorganisms and a Copper Phosphite Fertilizer

Enhancement of the natural defenses of a plant by beneficial microorganisms, i.e., endophytic bacteria and fungi or fertilizers such as copper phosphonates, could result in a potential alternative strategy against verticillium wilt of olive tree (Olea europaea). In this study, two beneficial microorganisms (the fungus Aureobasidium pullulans AP08 and the bacterium Bacillus amyloliquefaciens PAB-024) and a phosphonate salt copper phosphite (CuPh) were evaluated for their effectiveness as host resistance inducers against Verticillium dahliae in olive. To this end, 6-month-old healthy olive plants of the susceptible cultivar Picual were treated by foliar or root applications by spraying 15 ml per plant or by irrigation with 350 ml per plant of the dilutions of each product (CuPh: 3 or 10 ml l^-1, respectively; PAB-024: 10⁸ UFC ml^-1; AP08: 10⁶ UFC ml^-1). Treatments were conducted weekly from 2 weeks before inoculation to 10 days after inoculation. A cornmeal-water-sand mixture (1:2:9; w:v:w) colonized by V. dahliae was used for plant inoculation. Additionally, treated and noninoculated, nontreated and inoculated, and nontreated and noninoculated plants were included for comparative purposes. Disease severity progress and shoot fresh weight were assessed. Parameters involved in plant resistance were monitored through determination and quantification of reactive oxygen species (ROS) response (H₂O₂), and evaluation of hormones was done by gene expression analysis. Aureobasidium pullulans and CuPh were the most effective in disease reduction in planta by foliar or root application, respectively. Plants treated with CuPh showed significantly higher shoot fresh weight compared to the other treatments. ROS was significantly enhanced in plants treated with B. amyloliquefaciens PAB-024 compared to the rest of treatments and control. With regard to the evaluation of hormones, high levels of salicylic acid were detected on leaves from all treatment combinations, but without significant enhancements compared to the nontreated control. Regarding the gene expression related to salicylic acid, only the WRKY5 gene has shown a strong enhancement in the treatment with B. amyloliquefaciens. On the other hand, a strong accumulation of jasmonic acid and jasmonic acid-isoleucine in plants treated with A. pullulans was observed in all the tissues analyzed and also in the roots of plants treated with B. amyloliquefaciens and CuPh.

Trichoderma hamatum Increases Productivity, Glucosinolate Content and Antioxidant Potential of Different Leafy Brassica Vegetables

Brassica crops include important vegetables known as "superfoods" due to the content of phytochemicals of great interest to human health, such as glucosinolates (GSLs) and antioxidant compounds. On the other hand, Trichoderma is a genus of filamentous fungi that includes several species described as biostimulants and/or biological control agents in agriculture. In a previous work, an endophytic strain of Trichoderma hamatum was isolated from kale roots (Brassica oleracea var. acephala), describing its ability to induce systemic resistance in its host plant. In the present work, some of the main leafy Brassica crops (kale, cabbage, leaf rape and turnip greens) have been root-inoculated with T. hamatum, having the aim to verify the possible capacity of the fungus as a biostimulant in productivity as well as the foliar content of GSLs and its antioxidant potential, in order to improve these "superfoods". The results reported, for the first time, an increase in the productivity of kale (55%), cabbage (36%) and turnip greens (46%) by T. hamatum root inoculation. Furthermore, fungal inoculation reported a significant increase in the content of total GSLs in cabbage and turnip greens, mainly of the GSLs sinigrin and gluconapin, respectively, along with an increase in their antioxidant capacity. Therefore, T. hamatum could be a good agricultural biostimulant in leafy Brassica crops, increasing the content of GSLs and antioxidant potential of great food and health interest.

Benefits to Plant Health and Productivity From Enhancing Plant Microbial Symbionts

Plants exist in close association with uncountable numbers of microorganisms around, on, and within them. Some of these endophytically colonize plant roots. The colonization of roots by certain symbiotic strains of plant-associated bacteria and fungi results in these plants performing better than plants whose roots are colonized by only the wild populations of microbes. We consider here crop plants whose roots are inhabited by introduced organisms, referring to them as Enhanced Plant Holobionts (EPHs). EPHs frequently exhibit resistance to specific plant diseases and pests (biotic stresses); resistance to abiotic stresses such as drought, cold, salinity, and flooding; enhanced nutrient acquisition and nutrient use efficiency; increased photosynthetic capability; and enhanced ability to maintain efficient internal cellular functioning. The microbes described here generate effects in part through their production of Symbiont-Associated Molecular Patterns (SAMPs) that interact with receptors in plant cell membranes. Such interaction results in the transduction of systemic signals that cause plant-wide changes in the plants' gene expression and physiology. EPH effects arise not only from plant-microbe interactions, but also from microbe-microbe interactions like competition, mycoparasitism, and antibiotic production. When root and shoot growth are enhanced as a consequence of these root endophytes, this increases the yield from EPH plants. An additional benefit from growing larger root systems and having greater photosynthetic capability is greater sequestration of atmospheric CO2. This is transferred to roots where sequestered C, through exudation or root decomposition, becomes part of the total soil carbon, which reduces global warming potential in the atmosphere. Forming EPHs requires selection and introduction of appropriate strains of microorganisms, with EPH performance affected also by the delivery and management practices.

New species and records of Trichoderma isolated as mycoparasites and endophytes from cultivated and wild coffee in Africa

A survey for species of the genus Trichoderma occurring as endophytes of Coffea, and as mycoparasites of coffee rusts (Hemileia), was undertaken in Africa; concentrating on Cameroon and Ethiopia. Ninety-four isolates of Trichoderma were obtained during this study: 76 as endophytes of healthy leaves, stems and berries and, 18 directly from colonized rust pustules. A phylogenetic analysis of all isolates used a combination of three genes: translation elongation factor-1α (tef1), rpb2 and cal for selected isolates. GCPSR criteria were used for the recognition of species; supported by morphological and cultural characters. The results reveal a previously unrecorded diversity of Trichoderma species endophytic in both wild and cultivated Coffea, and mycoparasitic on Hemileia rusts. Sixteen species were delimited, including four novel taxa which are described herein: T. botryosum, T. caeruloviride, T. lentissimum and T. pseudopyramidale. Two of these new species, T. botryosum and T. pseudopyramidale, constituted over 60% of the total isolations, predominantly from wild C. arabica in Ethiopian cloud forest. In sharp contrast, not a single isolate of Trichoderma was obtained using the same isolation protocol during a survey of coffee in four Brazilian states, suggesting the existence of a 'Trichoderma void' in the endophyte mycobiota of coffee outside of Africa. The potential use of these African Trichoderma isolates in classical biological control, either as endophytic bodyguards-to protect coffee plants from Hemileia vastatrix, the fungus causing coffee leaf rust (CLR)-or to reduce its impact through mycoparasitism, is discussed, with reference to the on-going CLR crisis in Central America.

Current Status of the Disease-Resistant Gene(s)/QTLs, and Strategies for Improvement in Brassica juncea

Brassica juncea is a major oilseed crop in tropical and subtropical countries, especially in south-east Asia like India, China, Bangladesh, and Pakistan. The widespread cultivation of genetically similar varieties tends to attract fungal pathogens which cause heavy yield losses in the absence of resistant sources. The conventional disease management techniques are often expensive, have limited efficacy, and cause additional harm to the environment. A substantial approach is to identify and use of resistance sources within the Brassica hosts and other non-hosts to ensure sustainable oilseed crop production. In the present review, we discuss six major fungal pathogens of B. juncea: Sclerotinia stem rot (Sclerotinia sclerotiorum), Alternaria blight (Alternaria brassicae), White rust (Albugo candida), Downy mildew (Hyaloperonospora parasitica), Powdery mildew (Erysiphe cruciferarum), and Blackleg (Leptoshaeria maculans). From discussing studies on pathogen prevalence in B. juncea, the review then focuses on highlighting the resistance sources and quantitative trait loci/gene identified so far from Brassicaceae and non-filial sources against these fungal pathogens. The problems in the identification of resistance sources for B. juncea concerning genome complexity in host subpopulation and pathotypes were addressed. Emphasis has been laid on more elaborate and coordinated research to identify and deploy R genes, robust techniques, and research materials. Examples of fully characterized genes conferring resistance have been discussed that can be transformed into B. juncea using advanced genomics tools. Lastly, effective strategies for B. juncea improvement through introgression of novel R genes, development of pre-breeding resistant lines, characterization of pathotypes, and defense-related secondary metabolites have been provided suggesting the plan for the development of resistant B. juncea.

Development of Transgenic Brassica Crops Against Biotic Stresses Caused by Pathogens and Arthropod Pests

The Brassica genus includes one of the 10 most agronomically and economically important plant groups in the world. Within this group, we can find examples such as broccoli, cabbage, cauliflower, kale, Brussels sprouts, turnip or rapeseed. Their cultivation and postharvest are continually threatened by significant stresses of biotic origin, such as pathogens and pests. In recent years, numerous research groups around the world have developed transgenic lines within the Brassica genus that are capable of defending themselves effectively against these enemies. The present work compiles all the existing studies to date on this matter, focusing in a special way on those of greater relevance in recent years, the choice of the gene of interest and the mechanisms involved in improving plant defenses. Some of the main transgenic lines developed include coding genes for chitinases, glucanases or cry proteins, which show effective results against pathogens such as Alternaria brassicae, Leptosphaeria maculans or Sclerotinia sclerotiorum, or pests such as Lipaphis erysimi or Plutella xylostella.

Brassica oleracea var. acephala (kale) improvement by biological activity of root endophytic fungi

Brassica oleracea var. acephala (kale) is a cruciferous vegetable widely cultivated for its leaves and flower buds in Atlantic Europe and the Mediterranean area, being a food of great interest as a "superfood" today. Little has been studied about the diversity of endophytic fungi in the Brassica genus, and there are no studies regarding kale. In this study, we made a survey of the diversity of endophytic fungi present in the roots of six different Galician kale local populations. In addition, we investigated whether the presence of endophytes in the roots was beneficial to the plants in terms of growth, cold tolerance, or resistance to bacteria and insects. The fungal isolates obtained belonged to 33 different taxa. Among those, a Fusarium sp. and Pleosporales sp. A between Setophoma and Edenia (called as Setophoma/Edenia) were present in many plants of all five local populations, being possible components of a core kale microbiome. For the first time, several interactions between endophytic fungus and Brassica plants are described and is proved how different interactions are beneficial for the plant. Fusarium sp. and Pleosporales sp. B close to Pyrenophora (called as Pyrenophora) promoted plant growth and increased cold tolerance. On the other hand, isolates of Trichoderma sp., Pleosporales sp. C close to Phialocephala (called as Phialocephala), Fusarium sp., Curvularia sp., Setophoma/Edenia and Acrocalymma sp. were able to activate plant systemic resistance against the bacterial pathogen Xanthomonas campestris. We also observed that Fusarium sp., Curvularia sp. and Setophoma/Edenia confered resistance against Mamestra brassicae larvae.

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.

Trichoderma parareesei Favors the Tolerance of Rapeseed (Brassica napus L.) to Salinity and Drought Due to a Chorismate Mutase

Both drought and salinity represent the greatest plant abiotic stresses in crops. Increasing plant tolerance against these environmental conditions must be a key strategy in the development of future agriculture. The genus of Trichoderma filament fungi includes several species widely used as biocontrol agents for plant diseases but also some with the ability to increase plant tolerance against abiotic stresses. In this sense, using the species T. parareesei and T. harzianum, we have verified the differences between the two after their application in rapeseed (Brassica napus) root inoculation, with T. parareesei being a more efficient alternative to increase rapeseed productivity under drought or salinity conditions. In addition, we have determined the role that T. parareesei chorismate mutase plays in its ability to promote tolerance to salinity and drought in plants by increasing the expression of genes related to the hormonal pathways of abscisic acid (ABA) under drought stress, and ethylene (ET) under salt stress.