Role of homeodomain leucine zipper (HD-Zip) iv transcription factors in plant development and plant protection from deleterious environmental factors

Homeobox genes comprise an important group of genes that are responsible for regulation of developmental processes. These genes determine cell differentiation and cell fate in all eukaryotic organisms, starting from the early stages of embryo development. Homeodomain leucine zipper (HD-Zip) transcription factors are unique to the plant kingdom. Members of the HD-Zip IV subfamily have a complex domain topology and can bind several cis-elements with overlapping sequences. Many of the reported HD-Zip IV genes were shown to be specifically or preferentially expressed in plant epidermal or sub-epidermal cells. HD-Zip IV TFs were found to be associated with differentiation and maintenance of outer cell layers, and regulation of lipid biosynthesis and transport. Insights about the role of these proteins in plant cuticle formation, and hence their possible involvement in plant protection from pathogens and abiotic stresses has just started to emerge. These roles make HD-Zip IV proteins an attractive tool for genetic engineering of crop plants. To this end, there is a need for in-depth studies to further clarify the function of each HD-Zip IV subfamily member in commercially important plant species.William Chew, Maria Hrmova and Sergiy Lopat

Systematic Analysis of Sequences and Expression Patterns of Drought-Responsive Members of the HD-Zip Gene Family in Maize

Background: Members of the homeodomain-leucine zipper (HD-Zip) gene family encode transcription factors that are unique to plants and have diverse functions in plant growth and development such as various stress responses, organ formation and vascular development. Although systematic characterization of this family has been carried out in Arabidopsis and rice, little is known about HD-Zip genes in maize (Zea mays L.). Methods and Findings: In this study, we described the identification and structural characterization of HD-Zip genes in the maize genome. A complete set of 55 HD-Zip genes (Zmhdz1-55) were identified in the maize genome using Blast search tools and categorized into four classes (HD-Zip I-IV) based on phylogeny. Chromosomal location of these genes revealed that they are distributed unevenly across all 10 chromosomes. Segmental duplication contributed largely to the expansion of the maize HD-ZIP gene family, while tandem duplication was only responsible for the amplification of the HD-Zip II genes. Furthermore, most of the maize HD-Zip I genes were found to contain an overabundance of stress-related ciselements in their promoter sequences. The expression levels of the 17 HD-Zip I genes under drought stress were also investigated by quantitative real-time PCR (qRT-PCR). All of the 17 maize HD-ZIP I genes were found to be regulated by drought stress, and the duplicated genes within a sister pair exhibited the similar expression patterns, suggesting their conserved functions during the process of evolution

Transcriptomics at maize embryo/endosperm interfaces identify a novel transcriptionally distinct endosperm sub-domain adjacent to the embryo scutellum (EAS)

Seeds are complex biological systems comprising three genetically distinct tissues nested one inside another (embryo, endosperm and maternal tissues). However, the complexity of the kernel makes it difficult to understand inter compartment interactions without access to spatially accurate information. Here we took advantage of the large size of the maize kernel to characterize genome-wide expression profiles of tissues at embryo/endosperm interfaces. Our analysis identifies specific transcriptomic signatures in two interface tissues compared to whole seed compartments: The scutellar aleurone layer (SAL), and the newly named endosperm adjacent to scutellum (EAS). The EAS, which appears around 9 days after pollination and persists for around 11 days, is confined to one to three endosperm cell layers adjacent to the embryonic scutellum. Its transcriptome is enriched in genes encoding transporters. The absence of the embryo in an embryo specific (emb) mutant can alter the expression pattern of EAS marker genes. The detection of cell death in some EAS cells together with an accumulation of crushed cell walls suggests that the EAS is a dynamic zone from which cell layers in contact with the embryo are regularly eliminated, and to which additional endosperm cells are recruited as the embryo grows

The scutellar vascular bundle-specific promoter of the wheat HD-Zip IV transcription factor shows similar spatial and temporal activity in transgenic wheat, barley and rice

An HD-Zip IV gene from wheat, TaGL9, was isolated using a Y1H screen of a cDNA library prepared from developing wheat grain. TaGL9 has an amino acid sequence distinct from other reported members of the HD-Zip IV family. The 3￠ untranslated region of TaGL9 was used as a probe to isolate a genomic clone of the TaGL9 homologue from a BAC library prepared from Triticum durum L. cv. Langdon. The full-length gene containing a 3-kb-long promoter region was designated TdGL9H1. Spatial and temporal activity of TdGL9H1 was examined using promoter-GUS fusion constructs in transgenic wheat, barley and rice plants. Whole-mount and histochemical GUS staining patterns revealed grain-specific expression of TdGL9H1. GUS expression was initially observed between 3 and 8 days after pollination (DAP) in embryos at the globular stage and adjacent to the embryo fraction of the endosperm. Expression was strongest in the outer cell layer of the embryo. In developed wheat and barley embryos, strong activity of the promoter was only detected in the main vascular bundle of the scutellum, which is known to be responsible for the uptake of nutrients from the endosperm during germination and the endosperm-dependent phase of seedling development. Furthermore, this pattern of GUS staining was observed in dry seeds several weeks after harvesting but quickly disappeared during imbibition. The promoter of this gene could be a useful tool for engineering of early seedling vigour and protecting the endosperm to embryo axis pathway from pathogens during grain desiccation and storage.Nataliya Kovalchuk, Wei Wu, Omid Eini, Natalia Bazanova, Margaret Pallotta, Neil Shirley, Rohan Singh, Ainur Ismagul, Serik Eliby, Alexander Johnson, Peter Langridge and Sergiy Lopat