Characterization of a symbiosis- and auxin-regulated glutathione-S-transferase from Eucalyptus globulus roots

The formation of ectomycorrhizal symbiotic tissues between tree roots and fungal hyphae is accompanied by large morphological and molecular modifications. In the Eucalyptus globulus-Pisolithus microcarpus ectomycorrhiza, it has been shown that many plant genes are up-regulated during the onset of the interaction. Among them, EgHypar showed a high degree of homology with plant auxin-induced glutathione-S-transferases (GST). Its expression was up-regulated not only in the ectomycorrhiza, but also by exogenous application of auxins and hypaphorine, the tryptophane betaine excreted by P. microcarpus during ectomycorrhiza formation. In this study, a recombinant EgHYPAR polypeptide was produced in Escherichia coli. The purified polypeptide showed an in vitro GST activity, which was inhibited by indole-3-acetic acid but not by synthetic auxins or hypaphorine. Antibodies were raised against the recombinant EgHYPAR polypeptide and western blot analyses showed that the accumulation of the EgHYPAR polypeptide was enhanced in ectomycorrhizas. Immunolocalization studies demonstrated a preferential cytosolic localization of EgHYPAR close to the plasma membrane and the cell wall in cells of both un-colonized and colonized roots. The putative role of the EGHYPAR GST in mycorrhiza development will be discussed.