TRAMETES VERSICOLOR EXTRACELLULAR GLUCAN OLIGOMERS ARE ABLE TO INHIBIT AFLATOXINS BIOSYNTHESIS IN ASPERGILLUS FLAVUS

Aflatoxins are toxic secondary metabolites produced by Aspergillus flavus. It has been largely shown that the synthesis of aflatoxins is closely related with the oxidative stress inside and outside the cell. Trametes versicolor is a basidiomycete known for its therapeutic effects mainly due to production of several glycoproteins and exoglucans which display anti-tumoral actions and anti-oxidative effects. The addition of lyophilised filtrate (LF) of T. versicolor, in a concentration range of 0.01-2%, in the the culture medium of A. flavus strongly inhibits the biosynthesis of aflatoxin and reduces mycelium growth. If LF is previously dialysed (membrane cutoff 1000) it induces a lowering (~55-60%) of toxin inhibition ability and a similar decrease is observed when LF is digested with trypsin or with glucanases. GC-MS and 1HNMR analysis show the presence of highly branched polysaccharides (α and _-glucans) with mol. wt between 20 and 6 kDa. These glucans are probably digested by T. versicolor glucanase to biologically active smaller fragments. The presence is therefore assumed of enzymatic and oligosaccharidic components, which act in a complementary way to inhibit aflatoxins. How can extracellular glucan oligomers of T. versicolor inhibit aflatoxin biosynthesis in A. flavus? A putative receptor for glucans was identified in A. flavus genome which is similar to a plant receptor related to the onset of plant defence responses. It can be hypothesised that these fungal oligomers can act as non self signals able to modulate secondary metabolism in mycotoxigenic fungi.