PaHB1 is an evolutionary conserved HD-GL2 homeobox gene expressed in the protoderm during Norway spruce embryo development

In angiosperms, the protoderm or outer cell layer is the first tissue to di fferentiate in the embryo proper. In gymnosperms, it is not known whether a protoderm is defined and similarly differentiated, Here, we report a cDNA designated PaHB1 (for Picea abies Homeobox1), which is expressed during som atic embryogenesis in Norway spruce. PaHB1 exon/intron organization and its corresponding protein are highly similar to those of the HD-GL2 angiosperm counterparts. A phylogenetic analysis reveals that PaHB1 is strongly assoc iated with one subclass consisting of protoderm/epiderm-specific genes. Mor eover, PaHB1 expression switches from a ubiquitous expression in proembryog enic masses to an outer cell layer-specific localization during somatic emb ryo development. Ectopic expression of PaHB1 in somatic embryos leads to an early developmental block. The transformed embryos lack a smooth surface. These findings show that the PaHB1 expression pattern is highly analogous t o angiosperm HD-GL2 homologues, suggesting similarities in the definition o f the outer cell layer in seed plants. (C) 2001 Wiley-Liss, Inc.