Role of Bacillus spp. in antagonism between Pleurotus ostreatus and Trichoderma harzianum in heat-treated wheat-straw substrates.

This study aimed to identify bacteria involved in Trichodermaharzianum inhibition while promoting Pleurotus ostreatus defences in order to favour cultivation-substrate selectivity for mushroom production. PCR-DGGE profiles of total DNA from wheat-straw substrate showed weak differences between bacterial communities from substrate inoculated with P. ostreatus with or without T. harzianum. The major cultivable bacteria were isolated from three batches of wheat-straw-based cultivation substrates showing an efficient selectivity. They were screened for their ability to inhibit T.harzianum. By using specific media for bacterial isolation and by sequencing certain 16S-rDNA, we observed that Bacillus spp. were the main inhibitors. Among them, a dominant species was identified as Paenibacillus polymyxa. This species was co-cultivated on agar media with P. ostreatus. The measurement of laccase activities from culture plugs indicated that P. polymyxa induced increases in enzyme activities. Bacillus spp. and specifically P. polymyxa from cultivation substrates are implicated in their selectivity by both inhibiting the growth of T.harzianum and stimulating defences of the mushroom P. ostreatus through the induction of laccases. The management of microbial communities during P.ostreatus cultivation-substrate preparation in order to favour P. polymyxa and other Bacillus spp. growth, can be a way to optimize the development of P. ostreatus for mushroom production or other environmental uses of this fungus.

Interspecific interactions with Trichoderma longibrachiatum induce Pleurotus ostreatus defence reactions based on the production of laccase isozymes.

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.