Multifaceted impact of trichothecene metabolites on plant-microbe interactions and human health

New Insights into the Mechanism of Trichoderma virens-Induced Developmental Effects on Agrostis stolonifera Disease Resistance against Dollar Spot Infection

Trichoderma is internationally recognized as a biocontrol fungus for its broad-spectrum antimicrobial activity. Intriguingly, the crosstalk mechanism between the plant and Trichoderma is dynamic, depending on the Trichoderma strains and the plant species. In our previous study, the Trichoderma virens 192-45 strain showed better pathogen inhibition through the secretive non-volatile and volatile substrates. Therefore, we studied transcriptional and metabolic responses altered in creeping bentgrass (Agrostis stolonifera L.) with T. virens colonization prior to a challenge with Clarireedia homoeocarpa. This fungal pathogen causes dollar spot on various turfgrasses. When the pathogen is deficient, the importance of T. virens to the enhancement of plant growth can be seen in hormonal production and microbe signaling, such as indole-3-acrylic acid. Therefore, these substrates secreted by T. virens and induced genes related to plant growth can be the ‘pre-defense’ for ensuing pathogen attacks. During C. homoeocarpa infection, the Trichoderma–plant interaction activates defense responses through the SA- and/or JA-dependent pathway, induced by T. virens and its respective exudates, such as oleic, citric, and stearic acid. Thus, we will anticipate a combination of genetic engineering and exogenous application targeting these genes and metabolites, which could make creeping bentgrass more resistant to dollar spot and other pathogens.

Antifungal Secondary Metabolites Produced by the Fungal Endophytes: Chemical Diversity and Potential Use in the Development of Biopesticides

Plant diseases caused by phytopathogenic fungi can lead to huge losses in the agricultural fields and therefore remain a continuous threat to the global food security. Chemical-based fungicides contributed significantly in securing crop production. However, indiscriminate application of fungicides has led to increased chemical resistance and potential risks to human health and environment. Thus, there is an urgent need for searching for new bioactive natural products and developing them into new biopesticides. Fungal endophytes, microorganisms that reside in the fresh tissues of living plants, are regarded as untapped sources of novel natural products for exploitation in agriculture and/or medicine. Chemical examination of endophytic fungi has yielded enormous antifungal natural products with potential use in the development of biopesticides. This review summarizes a total of 132 antifungal metabolites isolated from fungal endophytes in the past two decades. The emphasis is on the unique chemical diversity of these metabolic products, together with their relevant antifungal properties. Moreover, some "star molecules," such as griseofulvin and trichothecene, as well as their synthetic derivatives that possess high potential as candidates of new natural fungicides, are also presented herein.

Melatonin enhances cotton immunity to Verticillium wilt via manipulating lignin and gossypol biosynthesis

Plants endure challenging environments in which they are constantly threatened by diverse pathogens. The soil-borne fungus Verticillium dahliae is a devastating pathogen affecting many plant species including cotton, in which it significantly reduces crop yield and fiber quality. Melatonin involvement in plant immunity to pathogens has been reported, but the mechanisms of melatonin-induced plant resistance are unclear. In this study, the role of melatonin in enhancing cotton resistance to V. dahliae was investigated. At the transcriptome level, exogenous melatonin increased the expression of genes in phenylpropanoid, mevalonate (MVA), and gossypol pathways after V. dahliae inoculation. As a result, lignin and gossypol, the products of these metabolic pathways, significantly increased. Silencing the serotonin N-acetyltransferase 1 (GhSNAT1) and caffeic acid O-methyltransferase (GhCOMT) melatonin biosynthesis genes compromised cotton resistance, with reduced lignin and gossypol levels after V. dahliae inoculation. Exogenous melatonin pre-treatment prior to V. dahliae inoculation restored the level of cotton resistance reduced by the above gene silencing effects. Melatonin levels were higher in resistant cotton cultivars than in susceptible cultivars after V. dahliae inoculation. The findings indicate that melatonin affects lignin and gossypol synthesis genes in phenylpropanoid, MVA, and gossypol pathways, thereby enhancing cotton resistance to V. dahliae.

Evolution of catalytic microenvironment governs substrate and product diversity in trichodiene synthase and other terpene fold enzymes

Trichodiene synthase, a terpene fold enzyme catalyzes the first reaction of trichodermin biosynthesis that is an economically important secondary metabolite. Sequence search analysis revealed that the proteins containing terpene fold are present in bacteria, fungi and plants. Terpene fold protein from Selaginella moellendorffii, a lycophyte, appeared at the interface of the microbes and plants in the evolutionary scale. Amino acid residues present around the catalytic pocket determines the size of the substrate as well as product molecules. It has been observed that the overall molecular evolution of the catalytic pockets dictates the choice of substrates/products of the proteins. It was further observed that N-terminus of multi-domain terpene fold proteins may assist in the interactions with the pyrophosphate part of the substrates. The phylogenetic analysis of these proteins further revealed that the enzymes are clustered into groups based on the domains present additional to the catalytic domains. We have also observed inter-domain 'puckering forceps' type motions in the multi-domains using normal mode analysis which were further correlated with their functions. The evolutionary clustering of these proteins was also influenced by the presence/absence of cofactor interacting motifs. These results may be used to modify/enhance the functions of these enzymes using protein engineering methods.

Structural basis of transport function in major facilitator superfamily protein from Trichoderma harzianum

Trichothecenes are the sesquiterpenes secreted by Trichoderma spp. residing in the rhizosphere. These compounds have been reported to act as plant growth promoters and bio-control agents. The structural knowledge for the transporter proteins of their efflux remained limited. In this study, three-dimensional structure of Thmfs1 protein, a trichothecene transporter from Trichoderma harzianum, was homology modelled and further Molecular Dynamics (MD) simulations were used to decipher its mechanism. Fourteen transmembrane helices of Thmfs1 protein are observed contributing to an inward-open conformation. The transport channel and ligand binding sites in Thmfs1 are identified based on heuristic, iterative algorithm and structural alignment with homologous proteins. MD simulations were performed to reveal the differential structural behaviour occurring in the ligand free and ligand bound forms. We found that two discrete trichothecene binding sites are located on either side of the central transport tunnel running from the cytoplasmic side to the extracellular side across the Thmfs1 protein. Detailed analysis of the MD trajectories showed an alternative access mechanism between N and C-terminal domains contributing to its function. These results also demonstrate that the transport of trichodermin occurs via hopping mechanism in which the substrate molecule jumps from one binding site to another lining the transport tunnel.