Primary phloem-specific expression of a Zinnia elegans homeobox gene.

Some plant homeobox genes are expressed specifically in vascular cells and are assumed to function in the differentiation of specific types of vascular cells. However, homeobox genes exhibiting primary phloem-specific expression have not been reported. To elucidate the molecular mechanisms of vascular development, we undertook to isolate from Zinnia elegans primary phloem-specific homeobox genes that may function in phloem development. An HD-Zip type homeobox gene, ZeHB3, was isolated. This gene encodes a class I HD-Zip protein, and constitutes a gene subfamily with the Daucus carota gene CHB6, and Arabidopsis thaliana genes Athb-5, Athb-6, and Athb-16. In situ hybridization of 1-, 14- and 50-day-old plants demonstrated that ZeHB3 mRNA accumulation is restricted to a few cells destined to differentiate into phloem cells and to the immature phloem cells surrounding the sieve elements and companion cells. ZeHB3 protein was also localized to immature phloem cells. These findings clearly indicate that ZeHB3 is a novel homeobox gene that marks, and may function in, the early stages of phloem differentiation.     

The Arabidopsis Athb-8, -9 and genes are members of a small gene family coding for highly related HD-ZIP proteins

We report the isolation and characterization of two Arabidopsis homeobox genes highly related to the Athb-8 gene. The full-length cDNAs encode proteins of 841 and 852 amino acids which we have designated Athb-9 and -14, respectively. Athb-8, -9 and -14 are members of a small family of HD-Zip proteins (HD-ZIP III) characterized by a HD-Zip motif confined to the N-terminus of the polypeptide. The spatial organization of the HD-Zip domain of Athb-8, -9 and -14 is different from that of the Athb-1 (a member of the HD-ZIP I family) and Athb-2 (a member of the HD-ZIP II family) HD-Zip domains. DNA binding analysis performed with random-sequence DNA templates showed that the Athb-9 HD-Zip (HD-Zip-9) domain, but not the Athb-9 HD alone, binds to DNA. The HD-Zip-9 domain recognizes a 11 bp pseudopalindromic sequence (GTAAT(G/C)ATTAC), as determined by selecting high-affinity binding sites from random-sequence DNA. Moreover, gel retardation assays demonstrated that the HD-Zip-9 domain binds to DNA as a dimer. These data support the notion that the HD-ZIP III domain interacts with DNA recognition elements in a fashion similar to the HD-ZIP I and II domains.     

Roles for Class III HD-Zip and KANADI genes in Arabidopsis root development

Meristems within the plant body differ in their structure and the patterns and identities of organs they produce. Despite these differences, it is becoming apparent that shoot and root apical and vascular meristems share significant gene expression patterns. Class III HD-Zip genes are required for the formation of a functional shoot apical meristem. In addition, Class III HD-Zip and KANADI genes function in patterning lateral organs and vascular bundles produced from the shoot apical and vascular meristems, respectively. We utilize both gain- and loss-of-function mutants and gene expression patterns to analyze the function of Class III HD-Zip and KANADI genes in Arabidopsis roots. Here we show that both Class III HD-Zip and KANADI genes play roles in the ontogeny of lateral roots and suggest that Class III HD-Zip gene activity is required for meristematic activity in the pericycle analogous to its requirement in the shoot apical meristem.     

HD-Zip III转录因子家族与植物细胞分化

细胞分化是生物生长发育的重要过程, 受到一系列信号的精确调控。植物特有的转录因子HD-Zip III在细胞分化中发挥了重要作用。该文对HD-Zip III基因类型和结构特点进行了简要介绍, 重点论述了HD-Zip III在胚胎形态发生、顶端分生组织形成、叶极性建立和维管组织分化等发育过程中的作用, 系统总结了HD-Zip III基因在不同层次受到的调控, 探讨了该家族基因与陆生维管植物进化的关系。     

Developmental role and auxin responsiveness of Class III homeodomain leucine zipper gene family members in rice

Members of the Class III homeodomain leucine zipper (Class III HD-Zip) gene family are central regulators of crucial aspects of plant development. To better understand the roles of five Class III HD-Zip genes in rice (Oryza sativa) development, we investigated their expression patterns, ectopic expression phenotypes, and auxin responsiveness. Four genes, OSHB1 to OSHB4, were expressed in a localized domain of the shoot apical meristem (SAM), the adaxial cells of leaf primordia, the leaf margins, and the xylem tissue of vascular bundles. In contrast, expression of OSHB5 was observed only in phloem tissue. Plants ectopically expressing microRNA166-resistant versions of the OSHB3 gene exhibited severe defects, including the ectopic production of leaf margins, shoots, and radialized leaves. The treatment of seedlings with auxin quickly induced ectopic OSHB3 expression in the entire region of the SAM, but not in other tissues. Furthermore, this ectopic expression of OSHB3 was correlated with leaf initiation defects. Our findings suggest that rice Class III HD-Zip genes have conserved functions with their homologs in Arabidopsis (Arabidopsis thaliana), but have also acquired specific developmental roles in grasses or monocots. In addition, some Class III HD-Zip genes may regulate the leaf initiation process in the SAM in an auxin-dependent manner.     

Evolution of the class III HD-Zip gene family in land plants

Arabidopsis class III homeodomain-leucine zipper (HD-Zip III) proteins play overlapping, distinct, and antagonistic roles in key aspects of development that have evolved during land plant evolution. To better understand this gene family's role in plant evolution and development as well as to address broader questions of how duplicated genes functionally diversify, we investigated the evolutionary history of this gene family. Phylogenetic analyses including homologs from diverse land plants indicate that a gene duplication event before the angiosperm--gymnosperm split gave rise to two gene lineages that diversified during angiosperm plant radiation. Heterologous expression of an HD-Zip III gene from the nonvascular plant moss within the Arabidopsis HD-zip III revoluta mutant modified but did not complement the phenotype. Comparison of the expression domains of flowering and nonflowering plant homologs indicate an ancestral role in vascular development and organ initiation but not in specifying organ polarity, a prominent role for angiosperm homologs.     

Evolution of class III homeodomain-leucine zipper genes in streptophytes

Land plants underwent tremendous evolutionary change following the divergence of the ancestral lineage from algal relatives. Several important developmental innovations appeared as the embryophyte clade diversified, leading to the appearance of new organs and tissue types. To understand how these changes came about, we need to identify the fundamental genetic developmental programs that are responsible for growth, patterning, and differentiation and describe how these programs were modified and elaborated through time to produce novel morphologies. Class III homeodomain-leucine zipper (class III HD-Zip) genes, identified in the model plant Arabidopsis thaliana, provide good candidates for basic land plant patterning genes. We show that these genes may have evolved in a common ancestor of land plants and their algal sister group and that the gene family has diversified as land plant lineages have diversified. Phylogenetic analysis, expression data from nonflowering lineages, and evidence from Arabidopsis and other flowering plants indicate that class III HD-Zip genes acquired new functions in sporophyte apical growth, vascular patterning and differentiation, and leaf development. Modification of expression patterns that accompanied diversification of class III HD-Zip genes likely played an important role in the evolution of land plant form.     

Gene expression patterns associated with in vitro tracheary element formation in isolated single mesophyll cells of Zinnia elegans.

Tracheary element formation from isolated Zinnia leaf mesophyll cells is an excellent system for the dissection of patterned secondary cell wall thickening and lignification. We used mRNAs from cells cultured for 48 h in the induction medium to isolate differentially regulated genes. Thirteen unique cDNA clones were isolated using a subtractive hybridization method. These clones can be divided into three distinct groups according to their characteristic gene expression in different media. The first group includes those genes whose expression is induced in the basal medium without 1-naphthaleneacetic acid (NAA) and benzyladenine; this indicates that the expression of these genes is regulated by chemical and physical factors other than these hormones. Three of these clones, p48h-229, p48h-114, and p48h-102, show significant homology to a pathogenesis-related protein II, a serine proteinase inhibitor, and a sunflower anther-specific proline-rich protein, respectively. The second group includes those genes whose expression is mainly NAA induced. One of these clones, p48h-10, shows high protein sequence homology to a barley aleurone-specific cDNA, B11E. The p48h-10-encoded protein shares some common characteristics of plant nonspecific lipid transfer proteins (low molecular weight, the secretion signal peptide, eight conserved cysteine residues, and a basic protein), although no significant protein sequence homology is found between p48-10 and other plant nonspecific lipid transfer proteins. The third group includes those genes whose expression is induced primarily in the induction medium; this indicates that the expression of these genes is closely associated with the process of tracheary element formation. Two of these clones, p48h-107 and p48h-17, show high homology to adenylate kinase and papaya proteinase I, respectively. The possible roles of these differentiation-specific genes during tracheary element formation are discussed.     

Gene family structure,expression and functional analysis of HD-Zip III genes in angiosperm and gymnosperm forest trees

Background  

Class III Homeodomain Leucine Zipper (HD-Zip III) proteins have been implicated in the regulation of cambium identity, as well as primary and secondary vascular differentiation and patterning in herbaceous plants. They have been proposed to regulate wood formation but relatively little evidence is available to validate such a role. We characterised and compared HD-Zip III gene family in an angiosperm tree, Populus spp. (poplar), and the gymnosperm Picea glauca (white spruce), representing two highly evolutionarily divergent groups.     

Tissue-specific localization of mRNA for carrot homeobox genes, CHBs, in carrot somatic embryos

We examined spatial and temporal expression patterns of four carrot HD-Zip I homeobox genes in somatic embryos. The mRNAs for CHB3, CHB4 and CHB5 were accumulated preferentially in the innermost cortical cell layers of the embryo axis in the torpedo-shaped embryo. In contrast, the accumulation of CHB6 mRNA was restricted to procambial cells of the heart- and torpedo-shaped embryos. In the embryonic cotyledons and the hypocotyl of the seedlings, all of the mRNAs for the four genes were located in the vascular tissues. These findings indicate that different HD-Zip I homeobox genes may be involved in the differentiation of specific tissues during somatic embryogenesis.