Biolistic transformation of wheat with centrophenoxine as a synthetic auxin

Genome-wide analysis of transmembrane 9 superfamily genes in wheat (Triticum aestivum) and their expression in the roots under nitrogen limitation and Bacillus amyloliquefaciens PDR1 treatment conditions

Introduction

Transmembrane 9 superfamily (TM9SF) proteins play significant roles in plant physiology. However, these proteins are poorly characterized in wheat (Triticum aestivum). The present study aimed at the genome-wide analysis of putative wheat TM9SF (TraesTM9SF) proteins and their potential involvement in response to nitrogen limitation and Bacillus amyloliquefaciens PDR1 treatments.

Methods

TraesTM9SF genes were retrieved from the wheat genome, and their physiochemical properties, alignment, phylogenetic, motif structure, cis-regulatory element, synteny, protein-protein interaction (PPI), and transcription factor (TF) prediction analyses were performed. Transcriptome sequencing and quantitative real-time polymerase reaction (qRT-PCR) were performed to detect gene expression in roots under single or combined treatments with nitrogen limitation and B. amyloliquefaciens PDR1.

Results and discussion

Forty-seven TraesTM9SF genes were identified in the wheat genome, highlighting the significance of these genes in wheat. TraesTM9SF genes were absent on some wheat chromosomes and were unevenly distributed on the other chromosomes, indicating that potential regulatory functions and evolutionary events may have shaped the TraesTM9SF gene family. Fifty-four cis-regulatory elements, including light-response, hormone response, biotic/abiotic stress, and development cis-regulatory elements, were present in the TraesTM9SF promoter regions. No duplication of TraesTM9SF genes in the wheat genome was recorded, and 177 TFs were predicted to target the 47 TraesTM9SF genes in a complex regulatory network. These findings offer valued data for predicting the putative functions of uncharacterized TM9SF genes. Moreover, transcriptome analysis and validation by qRT-PCR indicated that the TraesTM9SF genes are expressed in the root system of wheat and are potentially involved in the response of this plant to single or combined treatments with nitrogen limitation and B. amyloliquefaciens PDR1, suggesting their functional roles in plant growth, development, and stress responses.

Conclusion

These findings may be vital in further investigation of the function and biological applications of TM9SF genes in wheat.

Overexpression of the TaSHN1 transcription factor in bread wheat leads to leaf surface modifications, improved drought tolerance, and no yield penalty under controlled growth conditions

Transcription factors regulate multiple networks, mediating the responses of organisms to stresses, including drought. Here, we investigated the role of the wheat transcription factor TaSHN1 in crop growth and drought tolerance. TaSHN1, isolated from bread wheat, was characterized for molecular interactions and functionality. The overexpression of TaSHN1 in wheat was followed by the evaluation of T₂ and T₃ transgenic lines for drought tolerance, growth, and yield components. Leaf surface changes were analysed by light microscopy, SEM, TEM, and GC-MS/GC-FID. TaSHN1 behaves as a transcriptional activator in a yeast transactivation assay and binds stress-related DNA cis-elements, determinants of which were revealed using 3D molecular modelling. The overexpression of TaSHN1 in transgenic wheat did not result in a yield penalty under the controlled plant growth conditions of a glasshouse. Transgenic lines had significantly lower stomatal density and leaf water loss and exhibited improved recovery after severe drought, compared with control plants. The comparative analysis of cuticular waxes revealed an increased accumulation of alkanes in leaves of transgenic lines. Our data demonstrate that TaSHN1 may operate as a positive modulator of drought stress tolerance. Positive attributes could be mediated through an enhanced accumulation of alkanes and reduced stomatal density.

A biolistic method for high-throughput production of transgenic wheat plants with single gene insertions

BACKGROUND

The relatively low efficiency of biolistic transformation and subsequent integration of multiple copies of the introduced gene/s significantly complicate the genetic modification of wheat (Triticum aestivum) and other plant species. One of the key factors contributing to the reproducibility of this method is the uniformity of the DNA/gold suspension, which is dependent on the coating procedure employed. It was also shown recently that the relative frequency of single copy transgene inserts could be increased through the use of nanogram quantities of the DNA during coating.

RESULTS

A simplified DNA/gold coating method was developed to produce fertile transgenic plants, via microprojectile bombardment of callus cultures induced from immature embryos. In this method, polyethyleneglycol (PEG) and magnesium salt solutions were utilized in place of the spermidine and calcium chloride of the standard coating method, to precipitate the DNA onto gold microparticles. The prepared microparticles were used to generate transgenics from callus cultures of commercial bread wheat cv. Gladius resulting in an average transformation frequency of 9.9%. To increase the occurrence of low transgene copy number events, nanogram amounts of the minimal expression cassettes containing the gene of interest and the hpt gene were used for co-transformation. A total of 1538 transgenic wheat events were generated from 15,496 embryos across 19 independent experiments. The variation of single copy insert frequencies ranged from 16.1 to 73.5% in the transgenic wheat plants, which compares favourably to published results.

CONCLUSIONS

The DNA/gold coating procedure presented here allows efficient, large scale transformation of wheat. The use of nanogram amounts of vector DNA improves the frequency of single copy transgene inserts in transgenic wheat plants.

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.

The wheat TabZIP2 transcription factor is activated by the nutrient starvation-responsive SnRK3/CIPK protein kinase

The understanding of roles of bZIP factors in biological processes during plant development and under abiotic stresses requires the detailed mechanistic knowledge of behaviour of TFs. Basic leucine zipper (bZIP) transcription factors (TFs) play key roles in the regulation of grain development and plant responses to abiotic stresses. We investigated the role and molecular mechanisms of function of the TabZIP2 gene isolated from drought-stressed wheat plants. Molecular characterisation of TabZIP2 and derived protein included analyses of gene expression and its target promoter, and the influence of interacting partners on the target promoter activation. Two interacting partners of TabZIP2, the 14-3-3 protein, TaWIN1 and the bZIP transcription factor TaABI5L, were identified in a Y2H screen. We established that under elevated ABA levels the activity of TabZIP2 was negatively regulated by the TaWIN1 protein and positively regulated by the SnRK3/CIPK protein kinase WPK4, reported previously to be responsive to nutrient starvation. The physical interaction between the TaWIN1 and the WPK4 was detected. We also compared the influence of homo- and hetero-dimerisation of TabZIP2 and TaABI5L on DNA binding. TabZIP2 gene functional analyses were performed using drought-inducible overexpression of TabZIP2 in transgenic wheat. Transgenic plants grown under moderate drought during flowering, were smaller than control plants, and had fewer spikes and seeds per plant. However, a single seed weight was increased compared to single seed weights of control plants in three of four evaluated transgenic lines. The observed phenotypes of transgenic plants and the regulation of TabZIP2 activity by nutrient starvation-responsive WPK4, suggest that the TabZIP2 could be the part of a signalling pathway, which controls the rearrangement of carbohydrate and nutrient flows in plant organs in response to drought.

Characterization of auxin transporter PIN6 plasma membrane targeting reveals a function for PIN6 in plant bolting

Auxin gradients are sustained by series of influx and efflux carriers whose subcellular localization is sensitive to both exogenous and endogenous factors. Recently the localization of the Arabidopsis thaliana auxin efflux carrier PIN-FORMED (PIN) 6 was reported to be tissue-specific and regulated through unknown mechanisms. Here, we used genetic, molecular and pharmacological approaches to characterize the molecular mechanism(s) controlling the subcellular localization of PIN6. PIN6 localizes to endomembrane domains in tissues with low PIN6 expression levels such as roots, but localizes at the plasma membrane (PM) in tissues with increased PIN6 expression such as the inflorescence stem and nectary glands. We provide evidence that this dual localization is controlled by PIN6 phosphorylation and demonstrate that PIN6 is phosphorylated by mitogen-activated protein kinases (MAPKs) MPK4 and MPK6. The analysis of transgenic plants expressing PIN6 at PM or in endomembrane domains reveals that PIN6 subcellular localization is critical for Arabidopsis inflorescence stem elongation post-flowering (bolting). In line with a role for PIN6 in plant bolting, inflorescence stems elongate faster in pin6 mutant plants than in wild-type plants. We propose that PIN6 subcellular localization is under the control of developmental signals acting on tissue-specific determinants controlling PIN6-expression levels and PIN6 phosphorylation.

Phenotyping of plants in competitive but controlled environments: a study of drought response in transgenic wheat

In recent years, the interest in new technologies for wheat improvement has increased greatly. To screen genetically modified germplasm in conditions more realistic for a field situation we developed a phenotyping platform where transgenic wheat and barley are grown in competition. In this study, we used the platform to (1) test selected promoter and gene combinations for their capacity to increase drought tolerance, (2) test the function and power of our platform to screen the performance of transgenic plants growing in competition, and (3) develop and test an imaging and analysis process as a means of obtaining additional, non-destructive data on plant growth throughout the whole growth cycle instead of relying solely on destructive sampling at the end of the season. The results showed that several transgenic lines under well watered conditions had higher biomass and/or grain weight than the wild-type control but the advantage was significant in one case only. None of the transgenics seemed to show any grain weight advantage under drought stress and only two lines had a substantially but not significantly higher biomass weight than the wild type. However, their evaluation under drought stress was disadvantaged by their delayed flowering date, which increased the drought stress they experienced in comparison to the wild type. Continuous imaging during the season provided additional and non-destructive phenotyping information on the canopy development of mini-plots in our phenotyping platform. A correlation analysis of daily canopy coverage data with harvest metrics showed that the best predictive value from canopy coverage data for harvest metrics was achieved with observations from around heading/flowering to early ripening whereas early season observations had only a limited diagnostic value. The result that the biomass/leaf development in the early growth phase has little correlation with biomass or grain yield data questions imaging approaches concentrating only on the early development stage.

The TaDREB3 transgene transferred by conventional crossings to different genetic backgrounds of bread wheat improves drought tolerance

Drought tolerance of the wheat cultivar Bobwhite was previously enhanced by transformation with a construct containing the wheat DREB3 gene driven by the stress-inducible maize Rab17 promoter. Progeny of a single T2 transgenic line were used as pollinators in crosses with four elite bread wheat cultivars from Western Australia: Bonnie Rock, IGW-2971, Magenta and Wyalkatchem, with the aim of evaluating transgene performance in different genetic backgrounds. The selected pollinator line, BW8-9-10-3, contained multiple transgene copies, had significantly improved drought tolerance compared with wild-type plants and showed no growth and development penalties or abnormalities. A single hybrid plant was selected from each cross-combination for three rounds of backcrossing with the corresponding maternal wheat cultivar. The transgene was detected in all four F1 BC3 combinations, but stress-inducible transgene expression was found in only three of the four combinations. Under well-watered conditions, the phenotypes and grain yield components of the F2 BC3 transgene-expressing lines were similar to those of corresponding recurrent parents and null-segregants. Under severe drought conditions, the backcross lines demonstrated 12-18% higher survival rates than the corresponding control plants. Two from four F3 BC3 transgenic lines showed significantly higher yield (18.9% and 21.5%) than control plants under limited water conditions. There was no induction of transgene expression under cold stress, and therefore, no improvement of frost tolerance observed in the progenies of drought-tolerant F3 BC3 lines.

Constitutive overexpression of the TaNF-YB4 gene in transgenic wheat significantly improves grain yield

Heterotrimeric nuclear factors Y (NF-Ys) are involved in regulation of various vital functions in all eukaryotic organisms. Although a number of NF-Y subunits have been characterized in model plants, only a few have been functionally evaluated in crops. In this work, a number of genes encoding NF-YB and NF-YC subunits were isolated from drought-tolerant wheat (Triticum aestivum L. cv. RAC875), and the impact of the overexpression of TaNF-YB4 in the Australian wheat cultivar Gladius was investigated. TaNF-YB4 was isolated as a result of two consecutive yeast two-hybrid (Y2H) screens, where ZmNF-YB2a was used as a starting bait. A new NF-YC subunit, designated TaNF-YC15, was isolated in the first Y2H screen and used as bait in a second screen, which identified two wheat NF-YB subunits, TaNF-YB2 and TaNF-YB4. Three-dimensional modelling of a TaNF-YB2/TaNF-YC15 dimer revealed structural determinants that may underlie interaction selectivity. The TaNF-YB4 gene was placed under the control of the strong constitutive polyubiquitin promoter from maize and introduced into wheat by biolistic bombardment. The growth and yield components of several independent transgenic lines with up-regulated levels of TaNF-YB4 were evaluated under well-watered conditions (T1-T3 generations) and under mild drought (T2 generation). Analysis of T2 plants was performed in large deep containers in conditions close to field trials. Under optimal watering conditions, transgenic wheat plants produced significantly more spikes but other yield components did not change. This resulted in a 20-30% increased grain yield compared with untransformed control plants. Under water-limited conditions transgenic lines maintained parity in yield performance.