Summary Somatic embryogenesis of Norway spruce (Picea

abies L.) is a versatile model system to study molecular mecha-nisms regulating embryo development because it proceeds through defined developmental stages corresponding to spe-cific culture treatments. Normal embryonic development in-volves early differentiation of proembryogenic masses (PEMs) into somatic embryos, followed by early and late embryogeny leading to the formation of mature cotyledonary embryos. In some cell lines there is a developmental arrest at the PEM–so-matic embryo transition. To learn more about the molecular mechanisms regulating embryogenesis, we compared the tran-script profiles of two normal lines and one developmentally ar-rested line. Ribonucleic acid, extracted from these cell lines at successive developmental stages, was analyzed on DNA microarrays containing 2178 expressed sequence tags (ESTs) (corresponding to 2110 unique cDNAs) from loblolly pine (Pinus taeda L.). Hybridization between spruce and pine spe-cies on microarrays has been shown to be effective (van Zyl et al. 2002, Stasolla et al. 2003). In contrast to the developmen-tally arrested line, the early phases of normal embryo develop-ment are characterized by a precise pattern of gene expression, i.e., repression followed by induction. Comparison of tran-script levels between successive stages of embryogenesis al-lowed us to identify several genes that showed unique expres-sion responses during normal development. Several of these genes encode proteins involved in detoxification processes, methionine synthesis and utilization, and carbohydrate metab-olism. The potential role of these genes in embryo develop-ment is discussed