Summary A full-length cDNA clone named QRCPE

(Quercus robur crown preferentially expressed) that is differentially expressed during in vitro culture of mature and juvenile-like shoots of Quercus robur L. was identified by differential display. The deduced amino acid sequence showed that the encoded protein is small, contains a predicted N-terminal hydrophobic signal peptide that targets the protein to the cell wall, and is rich in glycine and histidine residues. Accumulation of QRCPE mRNA was higher in oak microshoots derived from crown branch shoot cultures than in oak microshoots derived from basal shoot cultures at the end of the multiplication and rooting period. Among organs, the highest accumulation of QRCPE transcripts was detected in roots, followed by stems and leaves, with preferential accumulation in specific organs of ontogenetically older shoots. Although QRCPE mRNA was abundant in oak zygotic and somatic embryos, almost no QRCPE mRNA accumulation was detected in nodular callus cells, suggesting a possible role of this gene in embryonic development. In proliferating shoot cultures of two chestnut (Castanea sativa Mill.) clones, the QRCPE homolog was preferentially expressed in crown-derived shoots. On the other hand, expression analysis of QRCPE in juvenile and mature material from soil-grown oak plants indicated that this gene is expressed from the embryonic to mature phases, but is progressively down-regulated during plant maturation. In vitro culture conditions induced changes in QRCPE transcript abundance in both basal and crown shoots in a phase-dependent manner. We conclude that QRCPE expression in oak is correlated with the ontogenetic stage of shoots, and thus this gene may be useful as a potential molecular marker for maturation-related characteristics.