Phloem-specific Expression of a Plant Homeobox Gene During Secondary Phases of Vascular Development

This paper reports the isolation and characterization of a homeobox gene (VAHOX1) from Lycopersicon esculentum encoding a homeodomain protein that contains a leucine zipper motif. The bipartite homeodomain-leucine zipper (HD-Zip) motif has only been found in homeobox genes from dicotyledonous plants, indicating that this type of transcription factor regulates particular developmental processes in these plant species. Here, the genomic organization, sequence comparison and expression analysis of VAHOX1 are described. Transcriptional fusion of the 5' promoter region of VAHOX1 with the reporter GUS gene (VAHOX1-GUS) and expression analysis of this construct in transgenic plants indicates that VAHOX1 is specifically expressed in the phloem during phases of secondary growth. Unlike other plant homeobox genes, VAHOX1 does not appear to be expressed in meristems, and the function of VAHOX1 is considered a likely candidate molecule that may participate in the regulation of the identity and/or activity of phloem tissues during secondary phases of vascular development.