The homeodomain transcription factor TaHDZipI-2 from wheat regulates frost tolerance, flowering time and spike development in transgenic barley

Genome-wide investigation of HD-ZIP gene family and functional characterization of BnaHDZ149 and BnaHDZ22 in salt and drought response in Brassica napus L

HD-ZIP proteins comprise a plant-specific transcription factor family, which play pivotal roles in plant development and adaptation to ever-changing environment. Although HD-ZIP family members have been identified in some plant species, so far our knowledge about HD-ZIP genes in rapeseed is still limited. In this study, 178 Brassica napus HD-ZIP (BnaHDZ) family members were identified in the rapeseed genome. The phylogenetic relationship, chromosomal locations, intron-exon structures, motif composition, and expression patterns of the BnaHDZ members were analyzed. The BnaHDZ family can be phylogenetically divided into four categories (Ⅰ, Ⅱ, Ⅲ and Ⅳ). Genome-wide transcriptome analysis revealed that most of the HD-ZIP I members respond to at least one abiotic stress. Two closely homologous stress-responsive HD-ZIP Ⅰ genes, BnaHDZ22 and BnaHDZ149, were identified to be involved in drought and salt responses, and selected for further functional characterization. Overexpressing BnaHDZ149 in rapeseed increased salt sensitivity of the transgenic plants, whereas overexpressing BnaHDZ22 increased sensitivity of the transgenic plants to polyethylene glycol (PEG)-simulated drought stress. This research provides not only a comprehensive landscape of BnaHDZ genes, but also a theoretical basis for elucidating the molecular mechanism of the abiotic stress responses of the HD-ZIP family in rapeseed.

KIN10-mediated HB16 protein phosphorylation and self-association improve cassava disease resistance by transcriptional activation of lignin biosynthesis genes

Cassava bacterial blight significantly affects cassava yield worldwide, while major cassava cultivars are susceptible to this disease. Therefore, it is crucial to identify cassava disease resistance gene networks and defence molecules for the genetic improvement of cassava cultivars. In this study, we found that MeHB16 transcription factor as a differentially expressed gene in cassava cultivars with contrasting disease resistance, positively modulated disease resistance by modulating defence molecule lignin accumulation. Further investigation showed that MeHB16 physically interacted with itself via the leucine-Zippe domain (L-Zip), which was necessary for the transcriptional activation of downstream lignin biosynthesis genes. In addition, protein kinase MeKIN10 directly interacted with MeHB16 to promote its phosphorylation at Ser6, which in turn enhanced MeHB16 self-association and downstream lignin biosynthesis. In summary, this study revealed the molecular network of MeKIN10-mediated MeHB16 protein phosphorylation improved cassava bacterial blight resistance by fine-tuning lignin biosynthesis and provides candidate genes and the defence molecule for improving cassava disease resistance.

The roles of HD-ZIP proteins in plant abiotic stress tolerance

Homeodomain leucine zipper (HD-ZIP) proteins are plant-specific transcription factors that contain a homeodomain (HD) and a leucine zipper (LZ) domain. The highly conserved HD binds specifically to DNA and the LZ mediates homodimer or heterodimer formation. HD-ZIP transcription factors control plant growth, development, and responses to abiotic stress by regulating downstream target genes and hormone regulatory pathways. HD-ZIP proteins are divided into four subclasses (I-IV) according to their sequence conservation and function. The genome-wide identification and expression profile analysis of HD-ZIP proteins in model plants such as Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa) have improved our understanding of the functions of the different subclasses. In this review, we mainly summarize and discuss the roles of HD-ZIP proteins in plant response to abiotic stresses such as drought, salinity, low temperature, and harmful metals. HD-ZIP proteins mainly mediate plant stress tolerance by regulating the expression of downstream stress-related genes through abscisic acid (ABA) mediated signaling pathways, and also by regulating plant growth and development. This review provides a basis for understanding the roles of HD-ZIP proteins and potential targets for breeding abiotic stress tolerance in plants.

Three strategies of transgenic manipulation for crop improvement

Heterologous expression of exogenous genes, overexpression of endogenous genes, and suppressed expression of undesirable genes are the three strategies of transgenic manipulation for crop improvement. Up to 2020, most (227) of the singular transgenic events (265) of crops approved for commercial release worldwide have been developed by the first strategy. Thirty-eight of them have been transformed by synthetic sequences transcribing antisense or double-stranded RNAs and three by mutated copies for suppressed expression of undesirable genes (the third strategy). By the first and the third strategies, hundreds of transgenic events and thousands of varieties with significant improvement of resistance to herbicides and pesticides, as well as nutritional quality, have been developed and approved for commercial release. Their application has significantly decreased the use of synthetic pesticides and the cost of crop production and increased the yield of crops and the benefits to farmers. However, almost all the events overexpressing endogenous genes remain at the testing stage, except one for fertility restoration and another for pyramiding herbicide tolerance. The novel functions conferred by the heterologously expressing exogenous genes under the control of constitutive promoters are usually absent in the recipient crops themselves or perform in different pathways. However, the endogenous proteins encoded by the overexpressing endogenous genes are regulated in complex networks with functionally redundant and replaceable pathways and are difficult to confer the desirable phenotypes significantly. It is concluded that heterologous expression of exogenous genes and suppressed expression by RNA interference and clustered regularly interspaced short palindromic repeats-cas (CRISPR/Cas) of undesirable genes are superior to the overexpression of endogenous genes for transgenic improvement of crops.

Overexpression of McHB7 Transcription Factor from Mesembryanthemum crystallinum Improves Plant Salt Tolerance

Mesembryanthemum crystallinum (common ice plant) is one of the facultative halophyte plants, and it serves as a model for investigating the molecular mechanisms underlying its salt stress response and tolerance. Here we cloned one of the homeobox transcription factor (TF) genes, McHB7, from the ice plant, which has 60% similarity with the Arabidopsis AtHB7. Overexpression of the McHB7 in Arabidopsis (OE) showed that the plants had significantly elevated relative water content (RWC), chlorophyll content, superoxide dismutase (SOD), and peroxidase (POD) activities after salt stress treatment. Our proteomic analysis identified 145 proteins to be significantly changed in abundance, and 66 were exclusively increased in the OE plants compared to the wild type (WT). After salt treatment, 979 and 959 metabolites were significantly increased and decreased, respectively, in the OE plants compared to the WT. The results demonstrate that the McHB7 can improve photosynthesis, increase the leaf chlorophyll content, and affect the TCA cycle by regulating metabolites (e.g., pyruvate) and proteins (e.g., citrate synthase). Moreover, McHB7 modulates the expression of stress-related proteins (e.g., superoxide dismutase, dehydroascorbate reductase, and pyrroline-5-carboxylate synthase B) to scavenge reactive oxygen species and enhance plant salt tolerance.

Identification of the yield traits related haplotype combinations of transcription factor genes TaHDZ34 in common wheat

A predominant objective in wheat breeding is improving yield-related traits. The homeodomain-leucine zipper (HD-Zip) transcription factor plays a significant role in plant growth and development. In this study, we cloned all homeologs of TaHDZ34, which is a member of the HD-Zip class IV transcription factor family in wheat (Triticum aestivum L.). Sequence polymorphism analysis showed that TaHDZ-A34, TaHDZ-B34, and TaHDZ-D34 formed five, six, and six haplotypes, respectively, and the genes were divided into two main haplotype groups. We also developed functional molecular markers. The TaHDZ34 genes were divided into eight main haplotype combinations. Association analysis and distinct population validation preliminarily indicated that TaHDZ34 genes modulate grain number per spike, effective spikelet number per spike, thousand kernel weight, and flag leaf area per plant in wheat. Hap-ABD was the most effective haplotype combination of TaHDZ34. Subcellular localization showed that TaHDZ-A34 was localized to the nucleus. The interacting proteins of TaHDZ-A34 were involved in protein synthesis/degradation, energy production and transportation, and photosynthesis. Geographic distribution and frequencies of TaHDZ34 haplotype combinations suggested that Hap-Abd and Hap-AbD were preferentially selected in Chinese wheat breeding programs. The high-yield-related haplotype combination Hap-ABD provided beneficial genetic resources for the marker-assisted selection of new wheat cultivars.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-022-01298-5.

Genome-Wide Characterization and Expression Analysis of HD-ZIP Gene Family in Dendrobium officinale

The homeodomain-leucine zipper (HD-ZIP) gene family, as one of the plant-specific transcription factor families, plays an important role in regulating plant growth and development as well as in response to diverse stresses. Although it has been extensively characterized in many plants, the HD-ZIP family is not well-studied in Dendrobium officinale, a valuable ornamental and traditional Chinese medicinal herb. In this study, 37 HD-ZIP genes were identified in Dendrobium officinale (Dohdzs) through the in silico genome search method, and they were classified into four subfamilies based on phylogenetic analysis. Exon–intron structure and conserved protein domain analyses further supported the prediction with the same group sharing similar gene and protein structures. Furthermore, their expression patterns were investigated in nine various tissues and under cold stress based on RNA-seq datasets to obtain the tissue-specific and cold-responsive candidates. Finally, Dohdz5, Dohdz9, and Dohdz12 were selected to validate their expression through qRT-PCR analysis, and they displayed significantly differential expression under sudden chilling stress, suggesting they might be the key candidates underlying cold stress response. These findings will contribute to better understanding of the regulatory roles of the HD-ZIP family playing in cold stress and also will provide the vital targets for further functional studies of HD-ZIP genes in D. officinale.

HD-ZIP Gene Family: Potential Roles in Improving Plant Growth and Regulating Stress-Responsive Mechanisms in Plants

Exploring the molecular foundation of the gene-regulatory systems underlying agronomic parameters or/and plant responses to both abiotic and biotic stresses is crucial for crop improvement. Thus, transcription factors, which alone or in combination directly regulated the targeted gene expression levels, are appropriate players for enlightening agronomic parameters through genetic engineering. In this regard, homeodomain leucine zipper (HD-ZIP) genes family concerned with enlightening plant growth and tolerance to environmental stresses are considered key players for crop improvement. This gene family containing HD and LZ domain belongs to the homeobox superfamily. It is further classified into four subfamilies, namely HD-ZIP I, HD-ZIP II, HD-ZIP III, and HD-ZIP IV. The first HD domain-containing gene was discovered in maize cells almost three decades ago. Since then, with advanced technologies, these genes were functionally characterized for their distinct roles in overall plant growth and development under adverse environmental conditions. This review summarized the different functions of HD-ZIP genes in plant growth and physiological-related activities from germination to fruit development. Additionally, the HD-ZIP genes also respond to various abiotic and biotic environmental stimuli by regulating defense response of plants. This review, therefore, highlighted the various significant aspects of this important gene family based on the recent findings. The practical application of HD-ZIP biomolecules in developing bioengineered plants will not only mitigate the negative effects of environmental stresses but also increase the overall production of crop plants.

Plant Transcription Factors Involved in Drought and Associated Stresses

Transcription factors (TFs) play a significant role in signal transduction networks spanning the perception of a stress signal and the expression of corresponding stress-responsive genes. TFs are multi-functional proteins that may simultaneously control numerous pathways during stresses in plants-this makes them powerful tools for the manipulation of regulatory and stress-responsive pathways. In recent years, the structure-function relationships of numerous plant TFs involved in drought and associated stresses have been defined, which prompted devising practical strategies for engineering plants with enhanced stress tolerance. Vast data have emerged on purposely basic leucine zipper (bZIP), WRKY, homeodomain-leucine zipper (HD-Zip), myeloblastoma (MYB), drought-response elements binding proteins/C-repeat binding factor (DREB/CBF), shine (SHN), and wax production-like (WXPL) TFs that reflect the understanding of their 3D structure and how the structure relates to function. Consequently, this information is useful in the tailored design of variant TFs that enhances our understanding of their functional states, such as oligomerization, post-translational modification patterns, protein-protein interactions, and their abilities to recognize downstream target DNA sequences. Here, we report on the progress of TFs based on their interaction pathway participation in stress-responsive networks, and pinpoint strategies and applications for crops and the impact of these strategies for improving plant stress tolerance.

Altering Plant Architecture to Improve Performance and Resistance

High-stress resistance and yield are major goals in crop cultivation, which can be addressed by modifying plant architecture. Significant progress has been made in recent years to understand how plant architecture is controlled under various growth conditions, recognizing the central role phytohormones play in response to environmental stresses. miRNAs, transcription factors, and other associated proteins regulate plant architecture, mainly via the modulation of hormone homeostasis and signaling. To generate crop plants of ideal architecture, we propose simultaneous editing of multiple genes involved in the regulatory networks associated with plant architecture as a feasible strategy. This strategy can help to address the need to increase grain yield and/or stress resistance under the pressures of the ever-increasing world population and climate change.

Characterization of wheat homeodomain-leucine zipper family genes and functional analysis of TaHDZ5-6A in drought tolerance in transgenic Arabidopsis

BACKGROUND

Many studies in Arabidopsis and rice have demonstrated that HD-Zip transcription factors play important roles in plant development and responses to abiotic stresses. Although common wheat (Triticum aestivum L.) is one of the most widely cultivated and consumed food crops in the world, the function of the HD-Zip proteins in wheat is still largely unknown.

RESULTS

To explore the potential biological functions of HD-Zip genes in wheat, we performed a bioinformatics and gene expression analysis of the HD-Zip family. We identified 113 HD-Zip members from wheat and classified them into four subfamilies (I-IV) based on phylogenic analysis against proteins from Arabidopsis, rice, and maize. Most HD-Zip genes are represented by two to three homeoalleles in wheat, which are named as TaHDZX_ZA, TaHDZX_ZB, or TaHDZX_ZD, where X denotes the gene number and Z the wheat chromosome on which it is located. TaHDZs in the same subfamily have similar protein motifs and intron/exon structures. The expression profiles of TaHDZ genes were analysed in different tissues, at different stages of vegetative growth, during seed development, and under drought stress. We found that most TaHDZ genes, especially those in subfamilies I and II, were induced by drought stress, suggesting the potential importance of subfamily I and II TaHDZ members in the responses to abiotic stress. Compared with wild-type (WT) plants, transgenic Arabidopsis plants overexpressing TaHDZ5-6A displayed enhanced drought tolerance, lower water loss rates, higher survival rates, and higher proline content under drought conditions. Additionally, the transcriptome analysis identified a number of differentially expressed genes between 35S::TaHDZ5-6A transgenic and wild-type plants, many of which are involved in stress response.

CONCLUSIONS

Our results will facilitate further functional analysis of wheat HD-Zip genes, and also indicate that TaHDZ5-6A may participate in regulating the plant response to drought stress. Our experiments show that TaHDZ5-6A holds great potential for genetic improvement of abiotic stress tolerance in crops.

The role of HD-Zip class I transcription factors in plant response to abiotic stresses

Abiotic stresses usually affect plant growth and development, indirectly or directly causing crop production reduction and even plant death. To survive, plants utilize different mechanisms to adapt themselves to continuously changing surrounding environmental stresses. Homeodomain-leucine zipper (HD-Zip) transcription factors are unique to the plant kingdom and divided into four different subfamilies (HD-Zip I∼IV). Many HD-Zip I members have been shown to play critical roles in the regulation of plant developmental processes, signaling networks and responses to environmental stresses. This review focuses on the role of HD-Zip I transcription factors in plant responses to various abiotic stresses, including abscisic acid-mediated stress, drought and cold stress, oxidative stress, helping to identify the potential regulatory mechanisms that alleviate abiotic stress in plants.

Genome-Wide Identification and Expression Analysis of HD-ZIP I Gene Subfamily in Nicotiana tabacum

The homeodomain-leucine zipper (HD-Zip) gene family, whose members play vital roles in plant growth and development, and participate in responding to various stresses, is an important class of transcription factors currently only found in plants. Although the HD-Zip gene family, especially the HD-Zip I subfamily, has been extensively studied in many plant species, the systematic report on HD-Zip I subfamily in cultivated tobacco (Nicotiana tabacum) is lacking. In this study, 39 HD-Zip I genes were systematically identified in N. tabacum (Nt). Interestingly, that 64.5% of the 31 genes with definite chromosome location information were found to originate from N. tomentosoformis, one of the two ancestral species of allotetraploid N. tabacum. Phylogenetic analysis divided the NtHD-Zip I subfamily into eight clades. Analysis of gene structures showed that NtHD-Zip I proteins contained conserved homeodomain and leucine-zipper domains. Three-dimensional structure analysis revealed that most NtHD-Zip I proteins in each clade, except for those in clade η, share a similar structure to their counterparts in Arabidopsis. Prediction of cis-regulatory elements showed that a number of elements responding to abscisic acid and different abiotic stresses, including low temperature, drought, and salinity, existed in the promoter region of NtHD-Zip I genes. The prediction of Arabidopsis ortholog-based protein-protein interaction network implied that NtHD-Zip I proteins have complex connections. The expression profile of these genes showed that different NtHD-Zip I genes were highly expressed in different tissues and could respond to abscisic acid and low-temperature treatments. Our study provides insights into the evolution and expression patterns of NtHD-Zip I genes in N. tabacum and will be useful for further functional characterization of NtHD-Zip I genes in the future.

Genome-wide identification and expression profile of HD-ZIP genes in physic nut and functional analysis of the JcHDZ16 gene in transgenic rice

BACKGROUND

Homeodomain-leucine zipper (HD-ZIP) transcription factors play important roles in the growth, development and stress responses of plants, including (presumably) physic nut (Jatropha curcas), which has high drought and salinity tolerance. However, although physic nut's genome has been released, there is little knowledge of the functions, expression profiles and evolutionary histories of the species' HD-ZIP genes.

RESULTS

In this study, 32 HD-ZIP genes were identified in the physic nut genome (JcHDZs) and divided into four groups (I-IV) based on phylogenetic analysis with homologs from rice, maize and Arabidopsis. The analysis also showed that most of the JcHDZ genes were closer to members from Arabidopsis than to members from rice and maize. Of the 32 JcHDZ genes, most showed differential expression patterns among four tissues (root, stem cortex, leaf, and seed). Expression profile analysis based on RNA-seq data indicated that 15 of the JcHDZ genes respond to at least one abiotic stressor (drought and/or salinity) in leaves at least at one time point. Transient expression of a JcHDZ16-YFP fusion protein in Arabidopsis protoplasts cells showed that JcHDZ16 is localized in the nucleus. In addition, rice seedlings transgenically expressing JcHDZ16 had lower proline contents and activities of antioxidant enzymes (catalase and superoxide dismutase) together with higher relative electrolyte leakage and malondialdehyde contents under salt stress conditions (indicating higher sensitivity) than wild-type plants. The transgenic seedlings also showed increased sensitivity to exogenous ABA, and increases in the transcriptional abundance of several salt stress-responsive genes were impaired in their responses to salt stress. Further data on JcHDZ16-overexpressing plants subjected to salt stress treatment verified the putative role of JcHDZ genes in salt stress responses.

CONCLUSION

Our results may provide foundations for further investigation of functions of JcHDZ genes in responses to abiotic stress, and promote application of JcHDZ genes in physic nut breeding.

Development of Drought-Tolerant Transgenic Wheat: Achievements and Limitations

Crop yield improvement is necessary to keep pace with increasing demand for food. Due to climatic variability, the incidence of drought stress at crop growth stages is becoming a major hindering factor to yield improvement. New techniques are required to increase drought tolerance along with improved yield. Genetic modification for increasing drought tolerance is highly desirable, and genetic engineering for drought tolerance requires the expression of certain stress-related genes. Genes have been identified which confer drought tolerance and improve plant growth and survival in transgenic wheat. However, less research has been conducted for the development of transgenic wheat as compared to rice, maize, and other staple food. Furthermore, enhanced tolerance to drought without any yield penalty is a major task of genetic engineering. In this review, we have focused on the progress in the development of transgenic wheat cultivars for improving drought tolerance and discussed the physiological mechanisms and testing of their tolerance in response to inserted genes under control or field conditions.

Field-grown transgenic wheat expressing the sunflower gene HaHB4 significantly outyields the wild type

HaHB4 is a sunflower transcription factor belonging to the homeodomain-leucine zipper I family whose ectopic expression in Arabidopsis triggers drought tolerance. The use of PCR to clone the HaHB4 coding sequence for wheat transformation caused unprogrammed mutations producing subtle differences in its activation ability in yeast. Transgenic wheat plants carrying a mutated version of HaHB4 were tested in 37 field experiments. A selected transgenic line yielded 6% more (P<0.001) and had 9.4% larger water use efficiency (P<0.02) than its control across the evaluated environments. Differences in grain yield between cultivars were explained by the 8% improvement in grain number per square meter (P<0.0001), and were more pronounced in stress (16% benefit) than in non-stress conditions (3% benefit), reaching a maximum of 97% in one of the driest environments. Increased grain number per square meter of transgenic plants was accompanied by positive trends in spikelet numbers per spike, tillers per plant, and fertile florets per plant. The gene transcripts associated with abiotic stress showed that HaHB4's action was not dependent on the response triggered either by RD19 or by DREB1a, traditional candidates related to water deficit responses. HaHB4 enabled wheat to show some of the benefits of a species highly adapted to water scarcity, especially in marginal regions characterized by frequent droughts.

Genome-wide analysis and stress-responsive expression of CCCH zinc finger family genes in Brassica rapa

BACKGROUND

Ubiquitous CCCH nucleic acid-binding motif is found in a wide-variety of organisms. CCCH genes are involved in plant developmental processes and biotic and abiotic stress responses. Brassica rapa is a vital economic crop and classical model plant of polyploidy evolution, but the functions of CCCH genes in B. rapa are unclear.

RESULTS

In this study, 103 CCCH genes in B. rapa were identified. A comparative analysis of the chromosomal position, gene structure, domain organization and duplication event between B. rapa and Arabidopsis thaliana were performed. Results showed that CCCH genes could be divided into 18 subfamilies, and segmental duplication might mainly contribute to this family expansion. C-X_7/8-C-X₅-C₃-H was the most commonly found motif, but some novel CCCH motifs were also found, along with some loses of typical CCCH motifs widespread in other plant species. The multifarious gene structures and domain organizations implicated functional diversity of CCCH genes in B. rapa. Evidence also suggested functional redundancy in at least one subfamily due to high conservation between members. Finally, the expression profiles of subfamily-IX genes indicated that they are likely involved in various stress responses.

CONCLUSION

This study provides the first genome-wide characterization of the CCCH genes in B. rapa. The results suggest that B. rapa CCCH genes are likely functionally divergent, but mostly involved in plant development and stress response. These results are expected to facilitate future functional characterization of this potential RNA-binding protein family in Brassica crops.

Overexpression of the class I homeodomain transcription factor TaHDZipI-5 increases drought and frost tolerance in transgenic wheat

Characterization of the function of stress-related genes helps to understand the mechanisms of plant responses to environmental conditions. The findings of this work defined the role of the wheat TaHDZipI-5 gene, encoding a stress-responsive homeodomain-leucine zipper class I (HD-Zip I) transcription factor, during the development of plant tolerance to frost and drought. Strong induction of TaHDZipI-5 expression by low temperatures, and the elevated TaHDZipI-5 levels of expression in flowers and early developing grains in the absence of stress, suggests that TaHDZipI-5 is involved in the regulation of frost tolerance at flowering. The TaHDZipI-5 protein behaved as an activator in a yeast transactivation assay, and the TaHDZipI-5 activation domain was localized to its C-terminus. The TaHDZipI-5 protein homo- and hetero-dimerizes with related TaHDZipI-3, and differences between DNA interactions in both dimers were specified at 3D molecular levels. The constitutive overexpression of TaHDZipI-5 in bread wheat significantly enhanced frost and drought tolerance of transgenic wheat lines with the appearance of undesired phenotypic features, which included a reduced plant size and biomass, delayed flowering and a grain yield decrease. An attempt to improve the phenotype of transgenic wheat by the application of stress-inducible promoters with contrasting properties did not lead to the elimination of undesired phenotype, apparently due to strict spatial requirements for TaHDZipI-5 overexpression.

Genome-Wide Identification and Expression Analysis of the HD-Zip Gene Family in Wheat (Triticum aestivum L.)

The homeodomain-leucine zipper (HD-Zip) gene family, as plant-specific transcription factors, plays an important role in plant development and growth as well as in the response to diverse stresses. Although HD-Zip genes have been extensively studied in many plants, they had not yet been studied in wheat, especially those involved in response to abiotic stresses. In this study, 46 wheat HD-Zip genes were identified using a genome-wide search method. Phylogenetic analysis classified these genes into four groups, numbered 4, 5, 17 and 20 respectively. In total, only three genes with A, B and D homoeologous copies were identified. Furthermore, the gene interaction networks found that the TaHDZ genes played a critical role in the regulatory pathway of organ development and osmotic stress. Finally, the expression profiles of the wheat HD-Zips in different tissues and under various abiotic stresses were investigated using the available RNA sequencing (RNA-Seq) data and then validated by quantitative real-time polymerase chain reaction (qRT-PCR) to obtain the tissue-specific and stress-responsive candidates. This study systematically identifies the HD-Zip gene family in wheat at the genome-wide level, providing important candidates for further functional analysis and contributing to the better understanding of the molecular basis of development and stress tolerance in wheat.

CaHDZ27, a Homeodomain-Leucine Zipper I Protein, Positively Regulates the Resistance to Ralstonia solanacearum Infection in Pepper

Homeodomain-leucine zipper class I (HD-Zip I) transcription factors have been functionally characterized in plant responses to abiotic stresses, but their roles in plant immunity are poorly understood. Here, a HD-Zip I gene, CaHZ27, was isolated from pepper (Capsicum annum) and characterized for its role in pepper immunity. Quantitative real-time polymerase chain reaction showed that CaHDZ27 was transcriptionally induced by Ralstonia solanacearum inoculation and exogenous application of methyl jasmonate, salicylic acid, or ethephon. The CaHDZ27-green fluorescent protein fused protein was targeted exclusively to the nucleus. Chromatin immunoprecipitation demonstrated that CaHDZ27 bound to the 9-bp pseudopalindromic element (CAATAATTG) and triggered β-glucuronidase expression in a CAATAATTG-dependent manner. Virus-induced gene silencing of CaHDZ27 significantly attenuated the resistance of pepper plants against R. solanacearum and downregulated defense-related marker genes, including CaHIR1, CaACO1, CaPR1, CaPR4, CaPO2, and CaBPR1. By contrast, transient overexpression of CaHDZ27 triggered strong cell death mediated by the hypersensitive response and upregulated the tested immunity-associated marker genes. Ectopic CaHDZ27 expression in tobacco enhances its resistance against R. solanacearum. These results collectively suggest that CaHDZ27 functions as a positive regulator in pepper resistance against R. solanacearum. Bimolecular fluorescence complementation and coimmunoprecipitation assays indicate that CaHDZ27 monomers bind with each other, and this binding is enhanced significantly by R. solanacearum inoculation. We speculate that homodimerization of CaHZ27 might play a role in pepper response to R. solanacearum, further direct evidence is required to confirm it.

Evolution and expression analysis reveal the potential role of the HD-Zip gene family in regulation of embryo abortion in grapes (Vitis vinifera L.)

BACKGROUND

The HD-Zip family has a diversity of functions during plant development. In this study, we identify 33 HD-Zip transcription factors in grape and detect their expressions in ovules and somatic embryos, as well as in various vegetative organs.

RESULTS

A genome-wide survey for HD-Zip transcription factors in Vitis was conducted based on the 12 X grape genome (V. vinifera L.). A total of 33 members were identified and classified into four subfamilies (I-IV) based on phylogeny analysis with Arabidopsis, rice and maize. VvHDZs in the same subfamily have similar protein motifs and intron/exon structures. An evaluation of duplication events suggests several HD-Zip genes arose before the divergence of the grape and Arabidopsis lineages. The 33 members of HD-Zip were differentially expressed in ovules of the stenospermic grape, Thompson Seedless and of the seeded grape, Pinot noir. Most have higher expressions during ovule abortion in Thompson Seedless. In addition, transcripts of the HD-Zip family were also detected in somatic embryogenesis of Thompson Seedless and in different vegetative organs of Thompson Seedless at varying levels. Additionally, VvHDZ28 is located in the nucleus and had transcriptional activity consistent with the typical features of the HD-Zip family. Our results provide a foundation for future grape HD-Zip gene function research.

CONCLUSIONS

The identification and expression profiles of the HD-Zip transcription factors in grape, reveal their diverse roles during ovule abortion and organ development. Our results lay a foundation for functional analysis of grape HDZ genes.

MdHB1 down-regulation activates anthocyanin biosynthesis in the white-fleshed apple cultivar 'Granny Smith'

Coloration in apple (Malus×domestica) flesh is mainly caused by the accumulation of anthocyanin. Anthocyanin is biosynthesized through the flavonoid pathway and regulated by MYB, bHLH, and WD40 transcription factors (TFs). Here, we report that the HD-Zip I TF MdHB1 was also involved in the regulation of anthocyanin accumulation. MdHB1 silencing caused the accumulation of anthocyanin in 'Granny Smith' flesh, whereas its overexpression reduced the flesh content of anthocyanin in 'Ballerina' (red-fleshed apple). Moreover, flowers of transgenic tobacco (Nicotiana tabacum 'NC89') overexpressing MdHB1 showed a remarkable reduction in pigmentation. Transient promoter activation assays and yeast one-hybrid results indicated that MdHB1 indirectly inhibited expression of the anthocyanin biosynthetic genes encoding dihydroflavonol-4-reductase (DFR) and UDP-glucose:flavonoid 3-O-glycosyltransferase (UFGT). Yeast two-hybrid and bimolecular fluorescence complementation determined that MdHB1 acted as a homodimer and could interact with MYB, bHLH, and WD40 in the cytoplasm, consistent with its cytoplasmic localization by green fluorescent protein fluorescence observations. Together, these results suggest that MdHB1 constrains MdMYB10, MdbHLH3, and MdTTG1 to the cytoplasm, and then represses the transcription of MdDFR and MdUFGT indirectly. When MdHB1 is silenced, these TFs are released to activate the expression of MdDFR and MdUFGT and also anthocyanin biosynthesis, resulting in red flesh in 'Granny Smith'.