Identification and characterization of PgHZ1, a novel homeodomain leucine-zipper gene isolated from white spruce (Picea glauca) tissue

A member of the homeodomain-leucine zipper (HD-Zip) family was isolated from white spruce (Picea glauca) and designated as PgHZ1 (Gene Bank Accession No. DQ201170). The gene has an open reading frame of 1268 bp and encodes a protein of 309 amino acid residues. PgHZ1 has all the features of a HD-ZIP protein: a homeodomain composed by three α-helices involved in DNA binding and an adjacent leucine zipper motif for protein–protein interaction. Phylogenetic analyses and sequence allignments with several Arabidopsis HD-ZIP members reveal that PgHZ1 belongs to the same monophyletic group of ATHB3, 13, 20, and 23 with which it shares a respective amino acid similarities of 74%, 71%, 68%, and 61%. Expression studies during spruce somatic embryogenesis reveal that the transcript levels of PgHZ1 increase during the late phases of proliferation and remain high during the subsequent embryo growth on the ABA-containing maturation medium. Such an increase does not occur in a non-embryogenic line characterized by a developmental block. Arabidopsis plants with ectopic PgHZ1 expression show an increase sensitivity to ABA, as estimated in seed germination and root growth tests. Compared to wild type plants, plants over-expressing PgHZ1 driven by the CAMV 35S promoter show a variety of phenotypic deviations, including a reduced inflorescence growth, increased branching, small rosette leaves and a delay in flowering. Somatic embryos produced from 35S:PgHZ1 Arabidopsis plants display a heavy accumulation of storage products and remain in a developmental program even if subjected to prolonged culture. This is in contrast to wild type somatic embryos in which storage products are quickly mobilized and the germination program is initiated after only 15 days in maturation. Overall these data support the notion that PgHZ1 confers hypersensitivity to ABA and that proper expression of this gene may be required for proper embryonic growth.