The aim of this study was to test the effect of acetosyringone (AS) on the accumulation of the histidine kinase coding mRNA, using in vitro propagated Pisolithus tinctorius. In the precontact phase of the ectomycorrhizal symbiosis, it is hypothesized that the plants releases chemicals, which are sensed by the fungal mycelium and in turn trigger the start of the symbiosis. The nature of such molecules is largely unknown; however, plant metabolite and their structural analogues have been widely used to induce infection in different microbe-plant interactions. A histidine kinase in Agrobacterium tumefaciens was activated by AS in the first step during the infection of dicotyledonous plants. A conserved gene fragment from the ectomycorrhizal fungus Pisolithus tinctorius was cloned using degenerate primers of conserved regions characteristic of the histidine kinase gene family. Then, the accumulation of this transcript in fungi incubated with AS was analyzed. RT-PCR and in situ hybridization suggest that this mRNA is synthesized in the presence of this plant analog in AS-treated mycelia. The findings presented here suggest a role of a histidine kinase involved in the early stages of ectomycorrhizal differentiation.