Identification of growth stage molecular markers in Trichoderma sp. 'atroviride type B' and their potential application in monitoring fungal growth and development in soil

Biological Control and Plant Growth Promotion Properties of Volatile Organic Compound-Producing Antagonistic Trichoderma spp

Trichoderma is environmentally vital due to their plant growth-promoting effects (such as enhancement of nutrients supply, suppression of plant pathogens, and promotion of plant defense). Biogenic volatile organic compounds (VOCs) are diverse chemical substances emitted by Trichoderma spp. The potential role of VOCs in biological control and plant growth promotion has recently been recognized. Here, the Trichoderma-VOCs' performance for plant growth promotion and suppression of plant pathogens are evaluated. We further investigated VOC emission profiles of T. harzianum using GC-MS. The Trichoderma-VOCs exhibited significant (p < 0.05) antifungal properties against all tested pathogenic fungi. T. atroviride-VOCs showed a decisive inhibition of Alternaria panax, Botrytis cinerea, Cylindrocarpon destructans, and Sclerotinia nivalis. The germinating seeds demonstrated growth enhancement in the presence of Trichoderma-VOCs emitted by different strains. Low levels of cyclopentasiloxane, decamethyl, cyclotetrasiloxane, and octamethyl were found in T. harzianum KNU1 strain whereas cyclopentasiloxane, decamethyl, cyclotetrasiloxane, and octamethyl showed higher emission levels as Si-containing compounds. The results reveal the potentiality of VOCs as a biocontrol resource against deleterious rhizosphere microorganisms and underline the importance of Trichoderma-VOCs emissions in regulating plant growth and development.

TBRG-1 a Ras-like protein in Trichoderma virens involved in conidiation, development, secondary metabolism, mycoparasitism, and biocontrol unveils a new family of Ras-GTPases

Ras-GTPases are nucleotide hydrolases involved in key cellular processes. In fungi, Ras-GTPases regulate conidiation, development, virulence, and interactions with other fungi or plants. Trichoderma spp. are filamentous saprophytic fungi, widely distributed along all latitudes, characterized by their rapid growth and metabolic diversity. Many species of this genus interact with other fungi, animals or plants. Furthermore, these fungi are used as biocontrol agents due to their ability to antagonize phytopathogenic fungi and oomycetes, through competence, antibiosis, and parasitism. However, the genetic and molecular regulation of these processes is scarcely described in these fungi. In this work, we investigated the role of the gene tbrg-1 product (GenBank accession number XP_013956100; JGI ID: Tv_70852) of T. virens during its interaction with other fungi and plants. Sequence analyses predicted that TBRG-1 bears the characteristic domains of Ras-GTPases; however, its size (1011 aa) is 3- to 4-times bigger compared with classical GTPases. Interestingly, phylogenetic analyses grouped the TBRG-1 protein with hypothetical proteins of similar sizes, sharing conserved regions; whereas other known Ras-GTPases were perfectly grouped with their respective families. These facts led us to classify TBRG-1 into a new family of Ras-GTPases, the Big Ras-GTPases (BRG). Therefore, the gene was named tbrg-1 (TrichodermaBigRas-GTPase-1). Quantification of conidia and scanning electron microscopy showed that the mutants-lacking tbrg-1 produced less conidia, as well as a delayed conidiophore development compared to the wild-type (wt). Moreover, a deregulation of conidiation-related genes (con-10, con-13, and stuA) was observed in tbrg-1-lacking strains, which indicates that TBRG-1 is necessary for proper conidiophore and conidia development. Furthermore, the lack of tbrg-1 affected positively the antagonistic capability of T. virens against the phytopathogens Rhizoctonia solani, Sclerotium rolfsii, and Fusarium oxysporum, which was consistent with the expression patterns of mycoparasitism-related genes, sp1 and cht1, that code for a protease and for a chitinase, respectively. Furthermore, the antibiosis effect of mycelium-free culture filtrates of Δtbrg-1 against R. solani was considerably enhanced. The expression of secondary metabolism-related genes, particularly gliP, showed an upregulation in Δtbrg-1, which paralleled an increase in gliotoxin production as compared to the wt. These results indicate that TBRG-1 plays a negative role in secondary metabolism and antagonism. Unexpectedly, the biocontrol activity of Δtbrg-1 was ineffective to protect the tomato seeds and seedlings against R. solani. On the contrary, Δtbrg-1 behaved like a plant pathogen, indicating that TBRG-1 is probably implicated in the recognition process for establishing a beneficial relationship with plants.

Molecular Tools for Monitoring Trichoderma in Agricultural Environments

Various Trichoderma species possess significance in agricultural systems as biofertilizers or biocontrol agents (BCAs). Besides these beneficial features, certain Trichoderma species can also act as agricultural pests, causing the green mold disease of cultivated mushrooms. This double-faced nature of the genus in agricultural environments points at the importance of proper monitoring tools, which can be used to follow the presence and performance of candidate as well as patented and/or registered biocontrol strains, to assess the possible risks arising from their application, but also to track harmful, unwanted Trichoderma species like the green molds in mushroom growing facilities. The objective of this review is to discuss the molecular tools available for the species- and strain-specific monitoring of Trichoderma, ranging from immunological approaches and fingerprinting tools to exogenous markers, specific primers used in polymerase chain reaction (PCR) as well as "omics" approaches.

Environmental Growth Conditions of Trichoderma spp. Affects Indole Acetic Acid Derivatives, Volatile Organic Compounds, and Plant Growth Promotion

Trichoderma species are soil-borne filamentous fungi widely utilized for their many plant health benefits, such as conferring improved growth, disease resistance and abiotic stress tolerance to their hosts. Many Trichoderma species are able to produce the auxin phytohormone indole-3-acetic acid (IAA), and its production has been suggested to promote root growth. Here we show that the production of IAA is strain dependent and diverse external stimuli are associated with its production. In in vitro assays, Arabidopsis primary root length was negatively affected by the interaction with some Trichoderma strains. In soil experiments, a continuum effect on plant growth was shown and this was also strain dependent. In plate assays, some strains of Trichoderma spp. inhibited the expression of the auxin reporter gene DR5 in Arabidopsis primary roots but not secondary roots. When Trichoderma spp. and A. thaliana were physically separated, enhancement of both shoot and root biomass, increased root production and chlorophyll content were observed, which strongly suggested that volatile production by the fungus influenced the parameters analyzed. Trichoderma strains T. virens Gv29.8, T. atroviride IMI206040, T. sp. "atroviride B" LU132, and T. asperellum LU1370 were demonstrated to promote plant growth through volatile production. However, contrasting differences were observed with LU1370 which had a negative effect on plant growth in soil but a positive effect in plate assays. Altogether our results suggest that the mechanisms and molecules involved in plant growth promotion by Trichoderma spp. are multivariable and are affected by the environmental conditions.

Volatile organic compounds emitted by Trichoderma species mediate plant growth

Background

Many Trichoderma species are applied as biofungicides and biofertilizers to agricultural soils to enhance crop growth. These filamentous fungi have the ability to reduce plant diseases and promote plant growth and productivity through overlapping modes of action including induced systemic resistance, antibiosis, enhanced nutrient efficiency, and myco-parasitism. Trichoderma species are prolific producers of many small metabolites with antifungal, antibacterial, and anticancer properties. Volatile metabolites of Trichoderma also have the ability to induce resistance to plant pathogens leading to improved plant health. In this study, Arabidopsis plants were exposed to mixtures of volatile organic compounds (VOCs) emitted by growing cultures of Trichoderma from 20 strains, representing 11 different Trichoderma species.

Results

We identified nine Trichoderma strains that produced plant growth promoting VOCs. Exposure to mixtures of VOCs emitted by these strains increased plant biomass (37.1–41.6 %) and chlorophyll content (82.5–89.3 %). Trichoderma volatile-mediated changes in plant growth were strain- and species-specific. VOCs emitted by T. pseudokoningii (CBS 130756) were associated with the greatest Arabidopsis growth promotion. One strain, T. atroviride (CBS 01-209), in our screen decreased growth (50.5 %) and chlorophyll production (13.1 %). Similarly, tomatoes exposed to VOCs from T. viride (BBA 70239) showed a significant increase in plant biomass (>99 %), larger plant size, and significant development of lateral roots. We also observed that the tomato plant growths were dependent on the duration of the volatile exposure. A GC–MS analysis of VOCs from Trichoderma strains identified more than 141 unique compounds including several unknown sesquiterpenes, diterpenes, and tetraterpenes.

Conclusions

Plants grown in the presence of fungal VOCs emitted by different species and strains of Trichoderma exhibited a range of effects. This study demonstrates that the blend of volatiles produced by actively growing fungi and volatile exposure time in plant development both influence the outcome of volatile-mediated interactions. Only some of our growth promoting strains produced microbial VOCs known to enhance plant growth. Compounds such as 6-pentyl-2H-pyran-2-one were not common to all promoting strains. We found that biostimulatory strains tended to have a larger number of complex terpenes which may explain the variation in growth induced by different Trichoderma strains.