Variation in transformation frequencies among six common wheat cultivars through particle bombardment of scutellar tissues

Protocol for efficient regulation of in vitro morphogenesis in einkorn (Triticum monococcum L.), a recalcitrant diploid wheat species

Einkorn (Triticum monococcum L.) is A-genome diploid wheat that has a potential to become a useful model for understanding the biology and genomics in Triticeae. Unfortunately, the application of modern technologies such as genetic engineering, RNAi-based gene silencing and genome editing is not available for einkorn as there is no efficient in vitro tissue culture and plant regeneration system. In the present study an efficient and simple protocol for plant regeneration via direct or indirect somatic embryogenesis and organogenesis has been developed. Various auxins used as sole inductors in einkorn displayed low effect for morphogenesis (0–8%) and plant regeneration (1–2 shoots per explant). The addition of Daminozide, the inhibitor of biosynthesis of gibberellins, together with auxin significantly improved the formation of morphogenic structures, especially when Dicamba (51.4%) and Picloram (56.6%) were used for combination; furthermore, the simultaneous addition of cytokinin into induction medium significantly promoted in vitro performance. Among the tested cytokinins, the urea-type substances, such as TDZ and CPPU were more effective than the adenine type ones, BA and Zeatin, for the regulation of morphogenesis; especially, TDZ was more effective than CPPU for shoot formation (11.73 vs. 7.04 per regenerating callus). The highest morphogenic response of 90.2% with the production of more than 10 shoots per initial explant was observed when 3.0 mg/L Dicamba, 50.0 mg/L Daminozide and 0.25 mg/L TDZ were combined together. Along with the identification of appropriate induction medium, the optimal developmental stage for einkorn was found as partially transparent immature embryo in size of around 1.0 mm. Although in the present study the critical balance between plant growth regulators was established for einkorn only, we assume that further the proposed strategy could be successfully applied to other recalcitrant cereal species and genotypes.

Potassium chloride and rare earth elements improve plant growth and increase the frequency of the Agrobacterium tumefaciens-mediated plant transformation

Plant transformation efficiency depends on the ability of the transgene to successfully interact with plant host factors. Our previous work and the work of others showed that manipulation of the activity of host factors allows for increased frequency of transformation. Recently we reported that exposure of tobacco plants to increased concentrations of ammonium nitrate increases the frequency of both homologous recombination and plant transgenesis. Here we tested the influence of KCl and salts of rare earth elements, Ce and La on the efficiency of Agrobacterium-mediated plant transformation. We found that exposure to KCl, CeCl(3) and LaCl(3) leads to an increase in recombination frequency in Arabidopsis and tobacco. Plants grown in the presence of CeCl(3) and LaCl(3) had higher biomass, longer roots and greater root number. Analysis of transformation efficiency showed that exposure of tobacco plants to 50 mM KCl resulted in ~6.0-fold increase in the number of regenerated calli and transgenic plants as compared to control plants. Exposure to various concentrations of CeCl(3) showed a maximum increase of ~3.0-fold in both the number of calli and transgenic plants. Segregation analysis showed that exposure to KCl and cerium (III) chloride leads to more frequent integrations of the transgene at a single locus. Analysis of transgene intactness showed better preservation of right T-DNA border during transgene integration. Our data suggest that KCl and CeCl(3) can be effectively used to improve quantity and quality of transgene integrations.

Characterization and functional analysis of three wheat genes with homology to the CONSTANS flowering time gene in transgenic rice

The CONSTANS (CO) gene of Arabidopsis plays a key role in the photoperiodic flowering pathway. To investigate photoperiod responses in cereals in more detail, we isolated three kinds of CO/Hd1 (rice ortholog of CO) homolog from hexaploid wheat, derived from the A, B, and D genomes and designated as wheat ortholog of CO from A genome (TaHd1-1), TaHd1-2, and TaHd1-3, respectively. They were highly similar to each other and to Hd1, and in addition harbored two conserved regions: two zinc finger motifs and CONSTANS, CONSTANS-LIKE and TIMING OF CAB EXPRESSION 1 (CCT) domain like CO/Hd1. They were located on the long arm of the homoeologous chromosome 6. TaHd1-2 harbored a 63-bp deletion at the promoter region containing the GATA-1 box, and consequently, we detected no subsequent transcript. The TaHd1-1 genomic clone was introduced to a rice line deficient in Hd1 function. Transgenic plants complemented the functions of rice Hd1: they promoted heading under short-day (SD) conditions and delayed it under long-day (LD)/natural conditions, indicating that Hd1 proteins from SD and LD plants share common structures and functions.

Age-dependent transformation frequency in elite wheat varieties

Wheat is a major world crop and as such is a primary target for improvement of agronomic characteristics via genetic engineering. Optimization of transformation is essential in order to overcome the relatively low transformation frequencies encountered with wheat. Transformation of elite wheat varieties is not always successful due to variability in regeneration and transformation frequencies between varieties. In this work, two elite wheat varieties with a relatively high embryogenic capacity were transformed by particle bombardment. A strong correlation between transformation frequency and the age of wheat donor plants was observed in both varieties. The mean transformation frequency rose from 0.7% to 5% when using immature embryos from old and young donor plants, respectively. This was observed in both varieties, the best bombardments achieving up to 7.3% frequency. Using explants at an optimal developmental stage from donor plants grown under environmentally-controlled conditions has improved the reproducibility of transformation efficiency of elite wheat varieties and leads to the production of apparently phenotypically normal, fertile, transgenic plants.

Procedures allowing the transformation of a range of European elite wheat (Triticum aestivum L.) varieties via particle bombardment

Ten current European wheat varieties were transformed at efficiencies ranging from 1-17% (mean 4% across varieties) following modifications in particle bombardment and tissue culture procedures. All plants surviving phosphinothricin selection were screened for uidA and bar gene activity, and for the presence of marker gene sequences by PCR analysis. A minimum of 35% plant 'escape' frequency was achieved with selection on 4 mg l(-1) gluphosinate ammonium after shoot initiation. Mean co-transformation frequency with various genes-of-interest was 66%. The estimated number of insertions of the uidA gene in 25 lines were; 1-2 in 32%, 3-5 in 52%, and 6-8 in 16% of lines. In T(1) progenies, marker genes segregated in a Mendelian fashion in 50% of 39 lines analysed, as determined by transgene activity assays. Based on PCR analysis, it appeared that in some lines the occurrence of distorted segregation was due to poor transmission of the transgenes.

Advances in cereal gene transfer

Over the past five years, transgenic strains of various major cereals have been produced, with transformation of rice and maize being most common. A majority of the cereal transformants obtained to date has been generated by the particle bombardment technique, but Agrobacterium-mediated transformation is rapidly becoming the method of choice. Rice, the plant in which transformation-related technology is most advanced, appears to be the model monocotyledon for basic and applied studies.