The production of plant growth enhancers by Trichoderma species

[摘要] English: The world population is growing at an exponential rate, placing extreme pressure on already scarce natural resources. This leads to changes in our environment with consequences such as climate change. It is predicted that South Africa will develop a more semi-arid and dry climate, which will endanger agricultural sustainability and place even more pressure on the already insufficient food supply. The need to increase agricultural productivity and quality has consequently led to an excessive use of chemical fertilisers, generating severe environmental pollution and, with global climate change, places increased pressure on world food production. It has become imperative that alternative means of crop protection and productivity enhancers are investigated. The use of Trichoderma and Trichoderma-based products as biofertilisers, biopesticides, and growth stimulators are alternatives to sustaining high production with low ecological impact.Trichoderma (teleomorph Hypocrea) is a fungal genus with its members found in ecosystems around the world. Trichoderma species can reduce the severity of plant diseases by inhibiting phytopathogens through antagonistic and mycoparasitic capabilities. In recent decades, it has been established that some Trichoderma strains can interact directly with roots, increasing plant growth, enhancing resistance to disease and increasing tolerance to abiotic stresses. This makes species of Trichoderma potential model organisms for sustained improvement of crop yield and health in agriculture.Trichoderma species are prolific producers of secondary metabolites. More than 120 structurally distinct Trichoderma secondary metabolites have been determined and described analytically, with more being discovered continually. Secondary metabolites are natural products that are biologically active. The vast majority of these compounds, however, have obscure or unknown functions, but they do play an important role in the functioning of the producing fungus, and are crucial to their diversity.The close association of Trichoderma and other microbes with plant roots has been well established. These microbes have direct and indirect influence on the growth of host plants, of which secondary metabolites are key instigators. Growth influences include increased germination rate, more rapid and increased flowering, increased height and weight of plants with more developed root systems, and increased yield of both grain and fruit crops. This positive symbiosis between Trichoderma and the host plant is dependent on a plethora of environmental factors. If these factors are not suitable for the fungus the benefits they convey will not be as noticeable and may not be present at all. Metabolites of Trichoderma species could be used as new biopesticides and biofertilisers based on their active compounds rather than on the living culture. This could have a significant beneficial impact on increasing crop yields and improving crop health.In this study, Trichoderma species diversity was investigated in Southern Africa in order to identify possible candidates which may provide plant growth enhancing benefits due to secondary metabolite stimulation. ITS gene sequencing was conducted to identify 54 Trichoderma species isolates accessible from the CGJM culture collection at the University of the Free State and from a variety of regions and host substrates. A phylogenetic tree was constructed to determine relatedness among species. In total, 54 isolates were identified using molecular techniques representing 11 species and three of the four taxonomic sections of Trichoderma. Trichoderma atroviride was the most prominent isolate with 24 strains and Trichoderma harzianum constituted 13 of the total isolates with preference to agricultural niches. Other species identified were T. asperellum, T. gamsii, T. citrinoviride, T. viride, T. longibrachiatum, T. reesei, T. spirale and T. hamatum. This study revealed a great diversity despite the limited number of samples represented in the CGJM culture collection. This indicates that South Africa has a richness of Trichoderma species, the full extent of which will only be realised through a comprehensive study.Trichoderma harzianum (CGJM 2295) and Trichoderma virens (CGJM 2307) were selected for metabolite screening as literature indicated higher possibilities for the production of the desired metabolites. Trichoderma species were cultivated in broth culture to produce possible plant growth promoting secondary metabolites. The culture broth was analysed for secondary metabolites as indicated in literature and T22azaphilone, 1-, 8-dihydroxy-3-methyl-anthraquinone, T39butenolide, and a metabolite similar to gliotoxin were qualitatively detected and identified using liquid mass spectrometry with transition monitoring. All of these metabolites have previously been shown to have growth promoting benefits on plants, indicating that Trichoderma species from natural environments in South Africa do possess the ability to produce these metabolites.Following the identification of secondary metabolites produced by Trichoderma harzianum (CGJM 2295), the goal was to determine the plant growth promoting benefits of these metabolites in the absence of the living organism, using only the cell-free culture broth containing the secondary metabolites. Wheat and maize seedlings were tested for increased length and weight after 10 days of growth on metabolite infused water agar. Wheat shoot length, root mass and root length were most affected with a significant increase of 36.8% for shoot length, 45.1% for root mass and 25.3% for root length when compared to untreated plants. In maize plants there was a significant increase of biomass in root length with an increase of 16.8% when compared to untreated plants, root mass increase of 20.4% and a shoot length increase of 13.4%.The results of this study indicated the possible use of Trichoderma secondary metabolites as novel plant growth enhancers. These results also hold promise for future studies with regards to field and glasshouse trials to establish whether increased growth in the first 10 days could ultimately correlate to increased yield. Trichoderma biotechnology is a field of study with endless possibilities and applications, and may be a contributing factor to overcoming challenges in the production of sufficient food stores, improving crop protection and enhancing plant yield in agriculture in South Africa.

[发布机构] University of the Free State