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Laccases are phenoloxidases involved in aromatic compound transformation but also in stress response towards antagonist species such as Trichoderma sp. In this study intracellular isoforms of laccases produced by Pleurotus ostreatus in liquid cultures with or without Trichoderma longibrachiatum showed five isoforms with various intensities depending on the culture conditions suggesting a basal expression of these enzymes, which can be induced by interspecific interactions. A first attempt to analyse the induction of P. ostreatus laccase-gene expression by a biotic factor was realized using semi-quantitative RT-PCR. We showed that the transcription of a laccase gene of P. ostreatus can be modified by a biotic stress such as T. longibrachiatum.

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Trichoderma spp., soil filamentous fungi, are antagonists that can cause great losses in mushroom production. We have investigated the influence of T. longibrachiatum on the production of lignocellulolytic enzymes by Pleurotus ostreatus during its vegetative growth on a straw-based cultivation substrate that either had been sterilized, pasteurized or not heat treated. The variations in the lignocellulolytic activities and the electrophoretic patterns in single and dual cultures were used as a tool for perturbation assessment. The various heat treatments of the wheat straw before inoculation affected both the bacterial populations and the abilities of T. longibrachiatum and P. ostreatus to colonize the substrate and to produce extracellar lignocellulolytic enzymes. Interactions between T. longibrachiatum and the microflora of the substrate led to a great decrease of hydrolytic activities due to reduced colonization of the substrate. Pleurotus ostreatus also was affected but it was less sensitive than T. longibrachiatum. As a consequence, in dual cultures with P. ostreatus, the competitive ability of T. longibrachiatum was reduced by bacteria in the substrates. The presence of total microflora or thermotolerant microflora increased the production of phenoloxidase activities by P. ostreatus, despite reduced colonization of the substrate. This contributed to the improvement of the competitive ability of P. ostreatus in the pasteurized substrate. Furthermore, a direct effect of bacteria on T. longibrachiatum also was observed. In sterilized substrate, both laccase and Mn-peroxydase activities were increased dramatically in dual cultures due to a faster production of a laccase isoform, which was stimulated by T. longibrachiatum.