SummaryThe Norway spruce (Picea abies) gene DAL2 shows distinct structural similarities to angiosperm MADS-box genes which act in the control of the development of the sexual organs of the flower. Transcription of DAL2 is restricted to the reproductive organs, the unisexual cones, of Norway spruce. In this paper we show that DAL2 in the compound female cone is exclusively expressed in the developing ovule-bearing organ, the ovuliferous scale. When expressed constitutively in transgenic Arabidopsis the gene causes developmental alterations very similar to those observed in plants ectopically expressing the Arabidopsis gene AGAMOUS and the closely related Brassica napus gene BAG1. These alterations include homeotic transformations of floral organs. On the basis of these data and analysis of the phylogeny of the plant MADSbox gene family, we propose that DAL2 acts to control reproductive organ development in spruce. We also propose that DAL2 shares a common origin with AGAMOUS and related genes from other angiosperms, in an ancestral MADS-box gene that was active in the control of ontogeny of ovule-bearing organs in the unknown last common ancestor of conifers and angiosperms.

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We have isolated a homeobox-containing gene from Arabidopsis thaliana using a degenerate oligonucleotide probe corresponding to the most conserved region of the homeodomain. This strategy has been used previously to isolate homeobox-containing genes from Caenorhabditis, and recently from A. thaliana. The Arabidopsis genes have an unusual structure in that they have a leucine zipper motif adjacent to the carboxy terminal region of the homeo domain, a feature not found in homeobox-containing genes isolated from animals. We report the isolation and primary structure of a new member of this Arabidopsis homeobox-leucine zipper gene family. This new member has the homeodomain and leucine-zipper motif similar to the two genes previously identified, but differs from these genes in the part corresponding to the carboxy terminus of the polypeptide, as well as in size and isoelectric point of the protein.

A recently discovered class of genes in Arabidopsis thaliana encode putative transcription factors which contain a homeodomain closely linked to a leucine zipper motif. We have previously reported on the cloning and cDNA sequence of one gene of this class, Athb-3. In this article we show this gene to be expressed predominantly in the cortex of the root and the stem. Using the Athb-3 clone as a probe we have isolated cDNA clones corresponding to three novel homeodomain-leucine zipper proteins. These clones, Athb-5, Athb-6 and Athb-7, hybridized to transcripts that were relatively abundant in the leaf, but also present in other vegetative organs, as well as in the flower. Only weak hybridization was observed to seed pod samples. These observations indicate that these Athb genes have major functions in the mature plant, and therefore, in contrast to homeobox genes in other eukaryotes and to the kn-1 gene in maize, are unlikely to function in the primary control of developmental processes during embryogenesis or organogenesis. The deduced amino acid sequences of Athb-5, Athb-6 and Athb-7 are highly similar to the previously isolated Athb-1, Athb-2 and Athb-3 in the homeodomain and leucine-zipper parts of the proteins, whereas the similarities to homeodomain proteins from other eukaryotes are limited. The Athb proteins thus constitute a new and well defined class of homeodomain proteins, apparently unique to plants.

Many homeobox genes control essential developmental processes in animals and plants. In this report, we describe the first cDNA corresponding to a homeobox gene isolated from a gymnosperm, the HBK1 gene from the conifer Picea abies (L.) Karst (Norway spruce). The sequence shows distinct similarities specifically to the KNOX (knotted-like homeobox) class of homeobox genes known from different angiosperm plants. The deduced amino acid sequence of HBK1 is strikingly similar within the homeodomain (84% identical) to the maize gene Knotted1 (Kn1), which acts to regulate cell differentiation in the shoot meristem. This similarity suggested that the phylogenetic association of HBK1 with the KNOX genes might be coupled to a conservation of gene function. In support of this suggestion, we have found HBK1 to be expressed in the apical meristem in the central population of nondifferentiated stem cells, but not in organ primordia developing at the f lanks of the meristem. This pattern of expression is similar to that of Kn1 in the maize meristem. We show further that HBK1, when expressed ectopically in transgenic Arabidopsis plants, causes aberrations in leaf development that are similar to the effects of ectopic expression of angiosperm KNOX genes on Arabidopsis development. Taken together, these data suggest that HBK1 has a role, similar to the KNOX genes in angiosperms, in the control of cellular differentiation in the apical meristem of spruce. The data also indicate that KNOX-gene regulation of vegetative development is an ancient feature of seed plants that was present in the last common ancestor of conifers and angiosperms.