Identification of a homeobox-containing gene with enhanced expression during soybean (Glycine max L.) somatic embryo development.

Homeotic gene are key 'switches' that control developmental processes. Homeotic genes containing the consensus 'homeobox' domain have been identified from a number of organisms including Drosophila melanogaster, Caenorhabditis elegans, Homo sapiens, and Zea mays. Although homeotic genes have been demonstrated to be important in embryo development of some insects, amphibians, and mammals, there are no reports of their involvement in plant embryogenesis. Here, we report the isolation and characterization of a cDNA clone for a homeobox-containing gene expressed in somatic embryos of soybean. The cDNA (Sbh1 for soybean homeobox-containing gene) was isolated using maize Knotted-1 (Kn1) cDNA as a heterologous probe. The Sbh1 cDNA clone is 1515 bp long which is the approximate size of its transcript. Within the homeodomain, the amino acid sequence of a helix-turn-helix structure, and invariant and conserved residues were identified. The deduced SBH1 protein shares a high amino acid identity with KN1 protein (47.0% overall and 87.5% for the homeodomain). Southern hybridization analysis indicated that Sbh1 is a member of a small gene family. The expression of Sbh1 is development- and tissue-specific. The transcript of Sbh1 was present in early-stage somatic embryos, increased prior to cotyledon formation and decreased thereafter. Sbh1 was weakly expressed in soybean stems and hypocotyl but was not detected in other plant tissues and nonembryogenic materials. The enhanced expression during embryogenesis, the homology with maize Kn1 gene, and the regulatory nature of homeodomain proteins suggest that the SBH1 protein plays an important role in plant embryo development.