The formation of the ectomycorrhizaimplies an alteration in gene expression of both theplant and fungal partners, a process which starts beforethe formation of any symbiotic interface. However, littleis known on the regulation pattern occurring indiVerent parts of the root system. Our experimentalsystem consisting of a micropropagated oak with ahierarchical root system was shown to exhibit symbiosisfunctional traits prior to any mycorrhizal tissuediVerentiation after the inoculation with the basidiomycetePiloderma croceum. Using a cDNA array, theplant gene regulation was analyzed in the pre-mycorrhizalphase. Seventy-Wve transcripts showed diVerentialexpression in pre-mycorrhizal lateral and principalroots, and both root types exhibited diVerent sets ofresponsive genes. For transcripts selected according toa statistical analysis, the alteration in gene expressionwas conWrmed by RT-PCR and quantitative real-timePCR. Genes regulated in pre-mycorrhizal lateral rootsdisplayed an almost identical expression in mycorrhizas.In contrast, genes regulated in pre-mycorrhizalprincipal roots were often regulated diVerently in ectomycorrhizas.Down-regulation aVected most of the regulatedgenes involved in metabolism, whereas most ofthe regulated genes related to cell rescue functions,water regulation and defence response were up-regulated.Regulation of such genes could explain theincrease of global resistance observed in mycorrhizalplants.