Abstract
HD-Zip proteins are unique to plants, and contain a homeodomain closely linked to a leucine zipper motif, which are involved in dimerization and DNA binding. Based on homology in the HD-Zip domain, gene structure and the presence of additional motifs, HD-Zips are divided into four families, HD-Zip I–IV. Phylogenetic analysis of HD-Zip genes using transcriptomic and genomic datasets from a wide range of plant species indicate that the HD-Zip protein class was already present in green algae. Later, HD-Zips experienced multiple duplication events that promoted neo- and sub-functionalizations. HD-Zip proteins are known to control key developmental and environmental responses, and a growing body of evidence indicates a strict link between members of the HD-Zip II and III families and the auxin machineries. Interactions of HD-Zip proteins with other hormones such as brassinolide and cytokinin have also been described. More recent data indicate that members of different HD-Zip families are directly involved in the regulation of abscisic acid (ABA) homeostasis and signaling. Considering the fundamental role of specific HD-Zip proteins in the control of key developmental pathways and in the cross-talk between auxin and cytokinin, a relevant role of these factors in adjusting plant growth and development to changing environment is emerging.
Highlights
The homeodomain-leucine zipper (HD-Zip) class of proteins appears to be present exclusively in the plant kingdom and is characterized by the presence of a homeodomain closely linked to a leucine zipper motif [1]
HD-Zip proteins are unique to plants, and contain a homeodomain closely linked to a leucine zipper motif, which are involved in dimerization and DNA binding
Genome-wide expression studies revealed that several Arabidopsis HD-Zip I genes show transcriptional changes in response to treatments with abscisic acid (ABA) [4], and there is evidence that members of clades I and II of the HD-Zip I family have roles related to drought stress and ABA signaling in different plant species [35,36,37,38,39,40,41]
Summary
The homeodomain-leucine zipper (HD-Zip) class of proteins appears to be present exclusively in the plant kingdom and is characterized by the presence of a homeodomain closely linked to a leucine zipper motif [1]. The Arabidopsis genome codes for 48 HD-Zip proteins that, on the basis of sequence homology in the HD-Zip domain, the presence of additional conserved motifs, and specific intron and exon positions, have been grouped into four families: HD-Zip I (17 members), HD-Zip II (10 members), HD-Zip III (5 members) and HD-Zip IV (16 members) [2,3,4,5,6,7]. Experimental work has demonstrated that the HD-Zip domain, but not the HD by itself, interacts with DNA [12], and it has been shown that a correct spatial relationship between the HD and the leucine zipper motif is crucial for DNA binding [12]. We report recent advances on the understanding of the complex interactions of HD-Zip transcription factors between themselves and with hormone signaling networks, including those involved in abiotic and biotic stress responses
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