Abstract
Plants emit volatile organic compounds (VOCs) that induce metabolomic, transcriptomic, and behavioral reactions in receiver organisms, including insect pollinators and herbivores. VOCs’ composition and concentration may influence plant-insect or plant-plant interactions and affect soil microbes that may interfere in plant-plant communication. Many Trichoderma fungi act as biocontrol agents of phytopathogens and plant growth promoters. Moreover, they can stimulate plant defense mechanisms against insect pests. This study evaluated VOCs’ emission by olive trees (Olea europaea L.) when selected Trichoderma fungi or metabolites were used as soil treatments. Trichoderma harzianum strains M10, T22, and TH1, T. asperellum strain KV906, T. virens strain GV41, and their secondary metabolites harzianic acid (HA), and 6-pentyl-α-pyrone (6PP) were applied to olive trees. Charcoal cartridges were employed to adsorb olive VOCs, and gas chromatography mass spectrometry (GC-MS) analysis allowed their identification and quantification. A total of 45 volatile compounds were detected, and among these, twenty-five represented environmental pollutants and nineteen compounds were related to olive plant emission. Trichoderma strains and metabolites differentially enhanced VOCs production, affecting three biosynthetic pathways: methylerythritol 1-phosphate (MEP), lipid-signaling, and shikimate pathways. Multivariate analysis models showed a characteristic fingerprint of each plant-fungus/metabolite relationship, reflecting a different emission of VOCs by the treated plants. Specifically, strain M10 and the metabolites 6PP and HA enhanced the monoterpene syntheses by controlling the MEP pathway. Strains GV41, KV906, and the metabolite HA stimulated the hydrocarbon aldehyde formation (nonanal) by regulating the lipid-signaling pathway. Finally, Trichoderma strains GV41, M10, T22, TH1, and the metabolites HA and 6PP improve aromatic syntheses at different steps of the shikimate pathway.
Highlights
Synthetic pesticides are employed in agriculture to control pests, avoid crop yield losses and product damage
Olive trees were treated with Trichoderma spore suspensions (T. harzianum strains M10, TH1, T22; T. asperellum strain KV906, T. virens strain GV41) or their metabolite solutions (HA, 6PP) once per month from April to September
In this study olive trees were treated with Trichoderma spore suspensions or their metabolite solutions, and Radiello® technology was used to trap the volatile organic compounds (VOCs) [35,36]
Summary
Synthetic pesticides are employed in agriculture to control pests, avoid crop yield losses and product damage. Microbes of the genus Trichoderma are the most studied and marketed fungal BCAs used as active ingredients of various bioformulations in agriculture [4] They exert a direct activity against pathogens using different mechanisms: antibiosis, parasitism, and competition for nutrients and space [5]. Trichoderma spp. have shown rhizosphere competence, may improve plant health and growth, enhance nutrient availability and uptake, induce host resistance, and modify the plant metabolome [6,7]. These positive effects are associated with the production of effector metabolites that selected Trichoderma strains can release during the multicomponent interactions with the plant, pathogen, and other microbes [8,9]. Trichoderma determines host-induced plant volatile alteration after root colonization in response to the inoculation of different microbial symbioses [10]
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