Trends in Plant Science
Volume 12, Issue 9, September 2007, Pages 419-426
Journal home page for Trends in Plant Science

Review
The true story of the HD-Zip family

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The HD-Zip family of transcription factors is unique to the plant kingdom. These proteins exhibit the singular combination of a homeodomain with a leucine zipper acting as a dimerization motif. They can be classified into four subfamilies, according to a set of distinctive features that include DNA-binding specificities, gene structures, additional common motifs and physiological functions. Some HD-Zip proteins participate in organ and vascular development or meristem maintenance. Others mediate the action of hormones or are involved in responses to environmental conditions. Here, we review recent data for this family of transcription factors from a wide variety of plant species to unravel their crucial role in plant development.

Section snippets

Homeodomains and the homeotic phenomena

A homeobox (HB) encodes a protein domain, the homeodomain (HD), which is a conserved 60-amino acid motif present in transcription factors found in all the eukaryotic organisms. This 60-amino acid sequence folds into a characteristic three-helix structure that is able to interact specifically with DNA. Most HDs are able to bind DNA as monomers with high affinity, through interactions made by helix III (the so-called recognition helix) and a disordered N-terminal arm located beyond helix I. The

The HD-Zip family

Members of the HD-Zip family have a leucine zipper motif (LZ) immediately downstream of the HD. The two motifs are present in transcription factors found in species belonging to other eukaryotic kingdoms, but their association in a single protein is unique to plants [6]. The HD is responsible for the specific binding to DNA, whereas LZ acts as a dimerization motif. HD-Zip proteins bind to DNA as dimers, and the absence of LZ absolutely abolishes their binding ability, which indicates that the

Subfamily I

In Arabidopsis, subfamily I (HD-Zip I) is composed of seventeen members (ATHB1/HAT5, ATHB3/HAT7, ATHB5– ATHB7, ATHB12, ATHB13, ATHB16, ATHB20– ATHB23, ATHB40, ATHB51– ATHB54). It is worth noting that the names ATHB21–ATHB23 (At2g18550, At2g36610 and At1g26960) were also given to three genes belonging to the ZF-HD family (At2g02540, At4g24660 and At5g39760) [8], which can lead to confusion. The genes of HD-Zip I have a common intron/exon distribution in agreement with their phylogenetic

Subfamily I

The expression of these genes is regulated by external factors such as drought, extreme temperatures, osmotic stresses and illumination conditions, and is specific to different tissues and organs of the plant. Their role as transcription factors is related to developmental events in response to such environmental conditions, particularly those in which abiotic factors generate stress 23, 24, 25, 26, 27, 28.

Different subsets of Arabidopsis HD-Zip I genes that bear a close phylogenetic

Evolutionary history of the HD-Zip family

HD-Zip-encoding genes have been isolated from a wide variety of plants, such as Solanum lycopersicum, Craterostigma plantagineum, Zea mays, Pisum sativum, Glycine max, Daucus carota, Heliantus annuus, Nicotiana sylvestris, Silene latifolia, Picea excelsa, Zinnia elegans, Lotus japonicus, Medicago truncatula, Brassica napus and Physcomitrella patens (a moss), which includes monocots and dicots, C3 and C4 plants. Many of the identified proteins have been well studied and seem to bear not only

Concluding remarks and future perspectives

HD-Zip proteins have been well studied in the past few years. Functional and DNA-binding studies have demonstrated that each of the HD-Zip proteins studied participates in alternative signal transduction pathways and mediates a cross-talk between some of the pathways. Nonetheless, many questions are still unanswered. How can these proteins that have similar binding specificity and that show overlapping expression patterns, function in different pathways? What are the functions of these

Acknowledgements

Our work is supported by grants from CONICET (PIP N°6983), ANPCyT (PAV 137/2/2; PICTO-UNL 108–13204, CABBIO 2004 N°3), and Universidad Nacional del Litoral. RLC is a member of CONICET. FDA and CAD are fellows of the same Institution. PAM is a fellow of ANPCyT.

Glossary

Orthologous genes
genes from different organisms that diverged after a speciation event and exhibit a high homology.
Paralogous genes
genes from the same organism that have arisen by a recent duplication event of a common ancestral gene.

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