Age-dependent transformation frequency in elite wheat varieties

An established protocol for generating transgenic wheat for wheat functional genomics via particle bombardment

Wheat is one of the most important food crops in the world and is considered one of the top targets in crop biotechnology. With the high-quality reference genomes of wheat and its relative species and the recent burst of genomic resources in Triticeae, demands to perform gene functional studies in wheat and genetic improvement have been rapidly increasing, requiring that production of transgenic wheat should become a routine technique. While established for more than 20 years, the particle bombardment-mediated wheat transformation has not become routine yet, with only a handful of labs being proficient in this technique. This could be due to, at least partly, the low transformation efficiency and the technical difficulties. Here, we describe the current version of this method through adaptation and optimization. We report the detailed protocol of producing transgenic wheat by the particle gun, including several critical steps, from the selection of appropriate explants (i.e., immature scutella), the preparation of DNA-coated gold particles, and several established strategies of tissue culture. More importantly, with over 20 years of experience in wheat transformation in our lab, we share the many technical details and recommendations and emphasize that the particle bombardment-mediated approach has fewer limitations in genotype dependency and vector construction when compared with the Agrobacterium-mediated methods. The particle bombardment-mediated method has been successful for over 30 wheat genotypes, from the tetraploid durum wheat to the hexaploid common wheat, from modern elite varieties to landraces. In conclusion, the particle bombardment-mediated wheat transformation has demonstrated its potential and wide applications, and the full set of protocol, experience, and successful reports in many wheat genotypes described here will further its impacts, making it a routine and robust technique in crop research labs worldwide.

Low-temperature pretreatment of explants and high maltose concentration during callus culture improves particle-bombardment-mediated stable transgene expression in common wheat

Biolistic transformation systems are widely used to introduce foreign genes into common wheat (Triticum aestivum L.); however, these techniques often generate high transgene copy numbers and complex transgene integration patterns that hinder the stable expression of the transgenes. To improve the efficiency of stable transgene expression, we examined the effect of low-temperature pretreatment of wheat flower spikes and of high maltose concentration (HMC) in the medium during the subsequent callus culture. Tillers of the spring wheat cultivar Bobwhite were stored at 5°C without water for one week before the isolation of their immature scutellar tissues, and the resulting particle-bombarded explants were cultured on 15% maltose for a month. Together, these treatments significantly increased the number of recovered transgenic lines expressing the reporter gene. The low-temperature pretreatment eliminated the negative effects of HMC, and HMC improved the efficiency of stable transgene expression. Southern blot analysis revealed that transgenic lines recovered after HMC treatment integrated a lower copy number of transgenes than those cultured at normal (4%) maltose concentration. These findings suggest that the HMC-mediated reduction of the transgene copy number results from the suppression of plasmid DNA rearrangement before or during transgene integration into the wheat genome.

Choice of Explant for Plant Genetic Transformation

Particle bombardment or biolistic transformation is an efficient, versatile method. This method does not need any vector for the gene transfer and is not dependent on the cell type, species, and genotype. The success of any transformation technique depends on the starting experimental materials or the explants. Here, we describe the factors that have influenced the choice of explants in biolistic transformation. Many general factors in the selection of explants in the development of transgenic plants are presented here. Therefore, this chapter provides extensive guidelines regarding the choice of explants for researchers working on various plant genetic transformation techniques.

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.

Efficient regeneration potential is closely related to auxin exposure time and catalase metabolism during the somatic embryogenesis of immature embryos in Triticum aestivum L

Regeneration of cultured tissue is a prerequisite of Agrobacterium- and biolistic-mediated plant transformation. In this study, an efficient protocol for improving wheat (Triticum aestivum L.) immature embryo regeneration was developed. Based on the statistical analysis of embryogenic callus induction efficiency, green spot differentiation efficiency, and plant regeneration efficiency from five wheat accessions, improved culture conditions were found to be more effective for embryogenic callus production than the traditional conditions. Using semi-quantitative reverse transcription polymerase chain reaction, a candidate gene, designated as TaCAT1, which encodes a catalase was identified to have a significant correlation with high-regeneration trait of wheat immature embryos. Three amino acid substitutions were found in TaCAT1 protein between high- and low-regeneration wheat accessions. Hydrogen peroxide content in the cultured calli increased from day 5 to 15, and then decreased sharply on day 20, followed by a second peak on day 25 during regeneration stage. Furthermore, a 3,500-bp 5' flanking region upstream of the first codon ATG of TaCAT1 was isolated using inverse polymerase chain reaction. In silico, analysis revealed that the TaCAT1 promoter contained two regulatory motifs associated with responses to auxin.

Fetal genes in mother's blood: a novel mechanism for telegony?

Telegony is a discredited genetic phenomenon that a previous male may influence the characteristics of offspring subsequently borne by the same female to another male. Although its reality was acknowledged by such authorities as Charles Darwin and Herbert Spencer, it has been met with skepticism because of a lack of understanding of the theoretical basis for telegony. With the discovery of fetal genes in mother's blood, the penetration of somatic cells by sperm, and the ability of RNA to program genome rearrangement, mechanisms might exist for telegony.

Agrobacterium tumefaciens-mediated genetic transformation of cereals using immature embryos

A critical step in the development of a robust Agrobacterium tumefaciens-mediated transformation -system for cereal crop plants is the establishment of optimal conditions for efficient T-DNA delivery into target tissue, from which plants can be regenerated. Although, Agrobacterium-mediated transformation of cereals is an important method that has been widely used by many laboratories around the world, routine protocols have been established only in specific cultivars within a species and with specific tissues of high regeneration potential. Cocultivation of highly embryogenic callus tissue or healthy immature embryos with A. tumefaciens is considered one of the critical factors in successful genetic transformation of crop plants. Immature embryos collected only from vigorously growing healthy and green plants grown in the field or in the well-conditioned greenhouse are the ideal target for genetic transformation of recalcitrant crop species. Here, we describe an Agrobacterium-mediated transformation method that uses immature embryos as the starting material for inoculation with Agrobacterium. The aim of this chapter is to provide the key steps/components involved in Agrobacterium-mediated transformation of cereal crops. However, these steps or components often vary between protocols and from laboratory to laboratory, and can be optimized or modified based on the requirement of a specific cultivar or species.

Food security: increasing yield and improving resource use efficiency

Food production and security will be a major issue for supplying an increasing world population. The problem will almost certainly be exacerbated by climate change. There is a projected need to double food production by 2050. In recent times, the trend has been for incremental modest yield increases for most crops. There is an urgent need to develop integrated and sustainable approaches that will significantly increase both production per unit land area and the resource use efficiency of crops. This review considers some key processes involved in plant growth and development with some examples of ways in which molecular technology, plant breeding and genetics may increase the yield and resource use efficiency of wheat. The successful application of biotechnology to breeding is essential to provide the major increases in production required. However, each crop and each specific agricultural situation presents specific requirements and targets for optimisation. Some increases in production will come about as new varieties are developed which are able to produce satisfactory crops on marginal land presently not considered appropriate for arable crops. Other new varieties will be developed to increase both yield and resource use efficiency on the best land.

Agrobacterium-mediated transformation of durum wheat (Triticum turgidum L. var. durum cv Stewart) with improved efficiency

An efficient Agrobacterium-mediated durum wheat transformation system has been developed for the production of 121 independent transgenic lines. This improved system used Agrobacterium strain AGL1 containing the superbinary pGreen/pSoup vector system and durum wheat cv Stewart as the recipient plant. Acetosyringone at 400 microM was added to both the inoculation and cultivation medium, and picloram at 10 mg l(-1) and 2 mg l(-1) was used in the cultivation and induction medium, respectively. Compared with 200 microM in the inoculation and cultivation media, the increased acetosyringone concentration led to significantly higher GUS (beta-glucuronidase) transient expression and T-DNA delivery efficiency. However, no evident effects of acetosyringone concentration on regeneration frequency were observed. The higher acetosyringone concentration led to an improvement in average final transformation efficiency from 4.7% to 6.3%. Furthermore, the concentration of picloram in the co-cultivation medium had significant effects on callus induction and regeneration. Compared with 2 mg l(-1) picloram in the co-cultivation medium, increasing the concentration to 10 mg l(-1) picloram resulted in improved final transformation frequency from 2.8% to 6.3%, with the highest frequency of 12.3% reached in one particular experiment, although statistical analysis showed that this difference in final transformation efficiency had a low level of significance. Stable integration of foreign genes, their expression, and inheritance were confirmed by Southern blot analyses, GUS assay, and genetic analysis. Analysis of T(1) progeny showed that, of the 31 transgenic lines randomly selected, nearly one-third had a segregation ratio of 3:1, while the remainder had ratios typical of two or three independently segregating loci.

Raising yield potential in wheat

Recent advances in crop research have the potential to accelerate genetic gains in wheat, especially if co-ordinated with a breeding perspective. For example, improving photosynthesis by exploiting natural variation in Rubisco's catalytic rate or adopting C(4) metabolism could raise the baseline for yield potential by 50% or more. However, spike fertility must also be improved to permit full utilization of photosynthetic capacity throughout the crop life cycle and this has several components. While larger radiation use efficiency will increase the total assimilates available for spike growth, thereby increasing the potential for grain number, an optimized phenological pattern will permit the maximum partitioning of the available assimilates to the spikes. Evidence for underutilized photosynthetic capacity during grain filling in elite material suggests unnecessary floret abortion. Therefore, a better understanding of its physiological and genetic basis, including possible signalling in response to photoperiod or growth-limiting resources, may permit floret abortion to be minimized for a more optimal source:sink balance. However, trade-offs in terms of the partitioning of assimilates to competing sinks during spike growth, to improve root anchorage and stem strength, may be necessary to prevent yield losses as a result of lodging. Breeding technologies that can be used to complement conventional approaches include wide crossing with members of the Triticeae tribe to broaden the wheat genepool, and physiological and molecular breeding strategically to combine complementary traits and to identify elite progeny more efficiently.

Biolistics transformation of wheat

We present a complete, step-by-step guide to the production of transformed wheat plants using a particle bombardment device to deliver plasmid DNA into immature embryos and the regeneration of transgenic plants via somatic embryogenesis. Currently, this is the most commonly used method for transforming wheat and it offers some advantages. However, it will be interesting to see whether this position is challenged as facile methods are developed for delivering DNA by Agrobacterium tumefaciens or by the production of transformants via a germ-line process (see other chapters in this book).

Selection of transformed plants

The low frequency and randomness of transgene integration into host cells, combined with the significant challenges of recovering whole plants from those rare events, makes the use of selectable marker genes routine in plant transformation experiments. For research applications that are unlikely to be grown in the field, strong herbicide- or antibiotic resistance is commonly used. Here we use genes conferring resistance to glufosinate herbicides as an example of a selectable marker in wheat transformation by either Agrobacterium or biolistics.

Stable transformation of plants

This chapter provides an overview of the main steps in the process to produce stably transformed plants. Most transformation methods use tissue culture to recover adult plants from regenerable explants and can be divided into three stages: (1) choice and preparation of explant tissue, (2) deoxyribonucleic acid (DNA) delivery, (3) callus induction/regeneration and selection. Each of these stages is introduced from a general perspective and a detailed protocol for our exemplar species, wheat, is given. We focus here on DNA delivery by particle bombardment as Agrobacterium-mediated transformation methods for wheat are reported elsewhere.

The maize Activator/Dissociation system is functional in hexaploid wheat through successive generations

The aim of the present study was to provide useful background information and evidence of the functionality of the maize Activator/Dissociation (Ac/Ds) system in hexaploid wheat. Two transgenic parental wheat lines, one harbouring the immobilised Ac element (iAc) and the other the Ds element (pUbi[Ds-uidA]bar), were crossed. Transient GUS assays confirmed that the iAc transposase is active in hexaploid wheat. Selected F1 and F2 lines were analysed by PCR using primers specific to Ac, uidA and bar genes. The primer pair Ubi/bar-tag was used to detect excision of the Ds-uidA sequence, which occurred at a frequency of 39% in the F1 generation. Lines free of Ac and showing evidence of Ds excision were subject to Southern analysis, which indicated that at least one transposition event might have occurred in these lines. Although more evidence is required to unequivocally support the reintegration of the Ds element in the wheat genome, the evidence presented here nevertheless demonstrates the effectiveness and potential value of using this system to tag genes in wheat.

Visualisation of plastids in endosperm, pollen and roots of transgenic wheat expressing modified GFP fused to transit peptides from wheat SSU RubisCO, rice FtsZ and maize ferredoxin III proteins

The ability to target marker proteins to specific subcellular compartments is a powerful research tool to study the structure and development of organelles. Here transit sequences from nuclear-encoded, plastid proteins, namely rice FtsZ, maize non-photosynthetic ferredoxin III (FdIII) and the small subunit of RubisCO were used to target a modified synthetic GFP (S65G, S72A) to plastids. The localisations of the fusion proteins expressed in transgenic wheat plants and under the control of the rice actin promoter were compared to an untargeted GFP control. GFP fluorescence was localised to non-green plastids in pollen, roots and seed endosperm and detected in isolated leaf chloroplasts using a GFP-specific antibody. Transit peptides appeared to influence the relative fluorescence intensities of plastids in different tissues. This is consistent with differential targeting and/or turnover of GFP fusion proteins in different plastid types. Replacement of GFP sequences with alternative coding regions enables immediate applications of our vectors for academic research and commercial applications.

Expression of fission yeast cdc25 driven by the wheat ADP-glucose pyrophosphorylase large subunit promoter reduces pollen viability and prevents transmission of the transgene in wheat

Cell number was to be measured in wheat (Triticum aestivum) endosperm expressing Spcdc25 (a fission yeast cell-cycle regulator) controlled by a supposedly endosperm-specific promoter, AGP2 (from the large subunit of ADP glucose pyrophosphorylase). Wheat was transformed by biolistics either with AGP2::GUS or AGP2::Spcdc25. PCR and RT-PCR checked integration and expression of the transgene, respectively. In cv. Chinese Spring, AGP2::GUS was unexpectedly expressed in carpels and pollen, as well as endosperm. In cv. Cadenza, three AGP2::Spcdc25 plants, AGP2::Spcdc25.1, .2 and .3, were generated. Spcdc25 expression was detected in mature leaves of AGP2::Spcdc25.1/.3 which exhibited abnormal spikes, 50% pollen viability and low seed set per plant; both were small compared with the nonexpressing and normal AGP2::Spcdc25.2. Spcdc25 was not transmitted to the T(1) in AGP2::Spcdc25.1 or .3, which developed normally. Spcdc25 was PCR-positive in AGP2::Spcdc25.2, using primers for a central portion, but not with primers for the 5' end, of the ORF, indicating a rearrangement; Spcdc25 was not expressed in either T(0) or T(1). The AGP2 promoter is not tissue-specific and Spcdc25 expression disrupted reproduction.

Factors influencing successful Agrobacterium-mediated genetic transformation of wheat

The development of a robust Agrobacterium-mediated transformation protocol for a recalcitrant species like bread wheat requires the identification and optimisation of the factors affecting T-DNA delivery and plant regeneration. We have used immature embryos from range of wheat varieties and the Agrobacterium strain AGL1 harbouring the pGreen-based plasmid pAL156, which contains a T-DNA incorporating the bar gene and a modified uidA (beta-glucuronidase) gene, to investigate and optimise major T-DNA delivery and tissue culture variables. Factors that produced significant differences in T-DNA delivery and regeneration included embryo size, duration of pre-culture, inoculation and co-cultivation, and the presence of acetosyringone and Silwet-L77 in the media. We fully describe a protocol that allowed efficient T-DNA delivery and gave rise to 44 morphologically normal, and fully fertile, stable transgenic plants in two wheat varieties. The transformation frequency ranged from 0.3% to 3.3%. Marker-gene expression and molecular analysis demonstrated that transgenes were integrated into the wheat genome and subsequently transmitted into progeny at Mendelian ratios.

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.