BACKGROUND: Soil microorganisms are key determinants of soil fertility and plant health. Soil phytopathogenic fungi are one of the most important causes of crop losses worldwide. Microbial biocontrol agents have been extensively studied as alternatives for controlling phytopathogenic soil microorganisms, but molecular interactions between them have mainly been characterised in dual cultures, without taking into account the soil microbial community. We used an RNA sequencing approach to elucidate the molecular interplay of a soil microbial community in response to a plant pathogen and its biocontrol agent, in order to examine the molecular patterns activated by the microorganisms. RESULTS: A simplified soil microcosm containing 11 soil microorganisms was incubated with a plant root pathogen (Armillaria mellea) and its biocontrol agent (Trichoderma atroviride) for 24 h under controlled conditions. More than 46 million paired-end reads were obtained for each replicate and 28,309 differentially expressed genes were identified in total. Pathway analysis revealed complex adaptations of soil microorganisms to the harsh conditions of the soil matrix and to reciprocal microbial competition/cooperation relationships. Both the phytopathogen and its biocontrol agent were specifically recognised by the simplified soil microcosm: defence reaction mechanisms and neutral adaptation processes were activated in response to competitive (T. atroviride) or non-competitive (A. mellea) microorganisms, respectively. Moreover, activation of resistance mechanisms dominated in the simplified soil microcosm in the presence of both A. mellea and T. atroviride. Biocontrol processes of T. atroviride were already activated during incubation in the simplified soil microcosm, possibly to occupy niches in a competitive ecosystem, and they were not further enhanced by the introduction of A. mellea. CONCLUSIONS: This work represents an additional step towards understanding molecular interactions between plant pathogens and biocontrol agents within a soil ecosystem. Global transcriptional analysis of the simplified soil microcosm revealed complex metabolic adaptation in the soil environment and specific responses to antagonistic or neutral intruders.

Department of Plant Systems Biology VIB 9052 Ghent Belgium Department of Plant Biotechnology and Bioinformatics Ghent University 9052 Ghent Belgium Bioinformatics Institute Ghent Ghent University 9000 Ghent Belgium

Department of Plant Systems Biology VIB 9052 Ghent Belgium Department of Plant Biotechnology and Bioinformatics Ghent University 9052 Ghent Belgium Bioinformatics Institute Ghent Ghent University 9000 Ghent Belgium Department of Genetics Genomics Research Institute University of Pretoria Hatfield Campus 0028 Pretoria South Africa

Department of Sustainable Ecosystems and Bioresources Research and Innovation Centre Fondazione Edmund Mach Via E Mach 1 38010 S Michele all'Adige Italy

Department of Sustainable Ecosystems and Bioresources Research and Innovation Centre Fondazione Edmund Mach Via E Mach 1 38010 S Michele all'Adige Italy Present Address Institute of Entomology Biology Centre The Czech Academy of Sciences Branišovská 31 1160 České Budějovice 37005 Czech Republic

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