The effect of additional virulence genes on transformation efficiency, transgene integration and expression in rice plants using the pGreen/pSoup dual binary vector system

Studies on Pure Mlb® (Multiple Left Border) Technology and Its Impact on Vector Backbone Integration in Transgenic Cassava

Imperfect T-DNA processing is common during Agrobacterium-mediated transformation, which integrates vector backbone sequences into the plant genome. However, regulatory restrictions prevent such transgenic plants from being developed for commercial deployment. The binary vector pCAMBIA2300 was modified by incorporating multiple left border (Mlb^®) repeats and was tested in BY2 cells, tobacco, and cassava plants to address this issue. PCR analyses confirmed a twofold increase in the vector backbone free events in the presence of triple left borders in all three systems tested. Vector backbone read-through past the LB was reduced significantly; however, the inclusion of Mlbs^® did not effectively address the beyond right border read-through. Also, Mlbs^® increased the frequency of single-copy and vector backbone free events (clean events) twice compared to a single LB construct. Here, we briefly narrate the strength and limitations of using Mlb^® technology and reporter genes in reducing the vector backbone transfer in transgenic events.

Stable and Efficient Agrobacterium-Mediated Genetic Transformation of Larch Using Embryogenic Callus

Larix olgensis or larch is an economically important coniferous tree species with rapid growth in the early stages, strong adaptability, and a short time to harvest. The genetic improvement of larch has garnered considerable attention in recent years for reclaiming timber forests. However, traditional breeding methods are largely ineffective for achieving rapid genetic improvement of L. olgensis. Studies show that the efficiency of plant regeneration can be improved by optimizing somatic embryogenesis. On this basis, we devised a stable, fast and efficient Agrobacterium-mediated genetic transformation method using suspended embryogenic calluses as explants and β-glucuronidase as the reporter. We evaluated the effects of the Agrobacterium load, co-culture period, and addition of acetosyringone and transformant screening antibiotic on the transformation efficiency. In addition, we tested the pCAMBIA 1300-PtHCA 2-1 promoter-GUS binary expression vector, which contains the GUS gene ORF under the control of Populus trichocarpa high cambial activity PtHCA 2-1 promoter, and observed the tissue-specific expression of the GUS gene in the somatic embryos of transgenic larch. This novel technique can not only accelerate the generation of superior transgenic strains of L. olgensis but also aid in future gene functional studies.

An efficient and reproducible Agrobacterium-mediated transformation method for hexaploid wheat (Triticum aestivum L.)

BACKGROUND

Despite wheat being a worldwide staple, it is still considered the most difficult to transform out of the main cereal crops. Therefore, for the wheat research community, a freely available and effective wheat transformation system is still greatly needed.

RESULTS

We have developed and optimised a reproducible Agrobacterium-mediated transformation system for the spring wheat cv 'Fielder' that yields transformation efficiencies of up to 25%. We report on some of the important factors that influence transformation efficiencies. In particular, these include donor plant health, stage of the donor material, pre-treatment by centrifugation, vector type and selection cassette. Transgene copy number data for independent plants regenerated from the same original immature embryo suggests that multiple transgenic events arise from single immature embryos, therefore, actual efficiencies might be even higher than those reported.

CONCLUSION

We reported here a high-throughput, highly efficient and repeatable transformation system for wheat and this system has been used successfully to introduce genes of interest, for RNAi, over-expression and for CRISPR-Cas9 based genome editing.

Coexpression of octopine and succinamopine Agrobacterium virulence genes to generate high quality transgenic events in maize by reducing vector backbone integration

Agrobacterium-mediated transformation is a complex process that is widely utilized for generating transgenic plants. However, one of the major concerns of this process is the frequent presence of undesirable T-DNA vector backbone sequences in the transgenic plants. To mitigate this deficiency, a ternary strain of A. tumefaciens was modified to increase the precision of T-DNA border nicking such that the backbone transfer is minimized. This particular strain supplemented the native succinamopine VirD1/VirD2 of EHA105 with VirD1/VirD2 derived from an octopine source (pTi15955), the same source as the binary T-DNA borders tested here, residing on a ternary helper plasmid containing an extra copy of the succinamopine VirB/C/G operons and VirD1. Transformation of maize immature embryos was carried out with two different test constructs, pDAB101556 and pDAB111437, bearing the reporter YFP gene and insecticidal toxin Cry1Fa gene, respectively, contained in the VirD-supplemented and regular control ternary strains. Molecular analyses of ~ 700 transgenic events revealed a significant 2.6-fold decrease in events containing vector backbone sequences, from 35.7% with the control to 13.9% with the VirD-supplemented strain for pDAB101556 and from 24.9% with the control to 9.3% with the VirD-supplemented strain for pDAB111437, without compromising transformation efficiency. In addition, while the number of single copy events recovered was similar, there was a 24-26% increase in backbone-free events with the VirD-supplemented strain compared to the control strain. Thus, supplementing existing VirD1/VirD2 genes in Agrobacterium, to recognize diverse T-DNA borders, proved to be a useful tool to increase the number of high quality events in maize.

An Insight into T-DNA Integration Events in Medicago sativa

The molecular mechanisms of transferred DNA (T-DNA) integration into the plant genome are still not completely understood. A large number of integration events have been analyzed in different species, shedding light on the molecular mechanisms involved, and on the frequent transfer of vector sequences outside the T-DNA borders, the so-called vector backbone (VB) sequences. In this work, we characterized 46 transgenic alfalfa (Medicago sativa L.) plants (events), generated in previous works, for the presence of VB tracts, and sequenced several T-DNA/genomic DNA (gDNA) junctions. We observed that about 29% of the transgenic events contained VB sequences, within the range reported in other species. Sequence analysis of the T-DNA/gDNA junctions evidenced larger deletions at LBs compared to RBs and insertions probably originated by different integration mechanisms. Overall, our findings in alfalfa are consistent with those in other plant species. This work extends the knowledge on the molecular events of T-DNA integration and can help to design better transformation protocols for alfalfa.

Generation of Marker- and/or Backbone-Free Transgenic Wheat Plants via Agrobacterium-Mediated Transformation

Horizontal transfer of antibiotic resistance genes to animals and vertical transfer of herbicide resistance genes to the weedy relatives are perceived as major biosafety concerns in genetically modified (GM) crops. In this study, five novel vectors which used gusA and bar as a reporter gene and a selection marker gene, respectively, were constructed based on the pCLEAN dual binary vector system. Among these vectors, 1G7B and 5G7B carried two T-DNAs located on two respective plasmids with 5G7B possessing an additional virGwt gene. 5LBTG154 and 5TGTB154 carried two T-DNAs in the target plasmid with either one or double right borders, and 5BTG154 carried the selectable marker gene on the backbone outside of the T-DNA left border in the target plasmid. In addition, 5BTG154, 5LBTG154, and 5TGTB154 used pAL154 as a helper plasmid which contains Komari fragment to facilitate transformation. These five dual binary vector combinations were transformed into Agrobacterium strain AGL1 and used to transform durum wheat cv Stewart 63. Evaluation of the co-transformation efficiencies, the frequencies of marker-free transgenic plants, and integration of backbone sequences in the obtained transgenic lines indicated that two vectors (5G7B and 5TGTB154) were more efficient in generating marker-free transgenic wheat plants with no or minimal integration of backbone sequences in the wheat genome. The vector series developed in this study for generation of marker- and/or backbone-free transgenic wheat plants via Agrobacterium-mediated transformation will be useful to facilitate the creation of "clean" GM wheat containing only the foreign genes of agronomic importance.

Overexpression of the rice AKT1 potassium channel affects potassium nutrition and rice drought tolerance

Potassium (K(+)) is the most important cationic nutrient for all living organisms and has roles in most aspects of plant physiology. To assess the impact of one of the main K(+) uptake components, the K(+) inward rectifying channel AKT1, we characterized both loss of function and overexpression of OsAKT1 in rice. In many conditions, AKT1 expression correlated with K(+) uptake and tissue K(+) levels. No salinity-related growth phenotype was observed for either loss or gain of function mutants. However, a correlation between AKT1 expression and root Na(+) when the external Na/K ratio was high suggests that there may be a role for AKT1 in Na(+) uptake in such conditions. In contrast to findings with Arabidopsis thaliana, we did not detect any change in growth of AKT1 loss of function mutants in the presence of NH4 (+) Nevertheless, NH4 (+)-dependent inhibition was detected during K(+) uptake assays in loss of function and wild type plants, depending on pre-growth conditions. The most prominent result of OsAKT1 overexpression was a reduction in sensitivity to osmotic/drought stress in transgenic plants: the data suggest that AKT1 overexpression improved rice osmotic and drought stress tolerance by increasing tissue levels of K(+), especially in the root.

Overexpression of the potassium channel TPKb in small vacuoles confers osmotic and drought tolerance to rice

Potassium (K(+) ) is the most important cationic nutrient for all living organisms. Vacuolar two-pore K(+) (TPK) channels are important players in the regulation of cellular levels of K(+) but have not been characterised in rice. In order to assess the role of OsTPKb, a K(+) selective ion channel predominantly expressed in the tonoplast of small vacuoles, we generated overexpressing (OX) lines using a constitutive promoter and compared their phenotypes with control plants. Relative to control plants, OX lines showed better growth when exposed to low-K(+) or water stress conditions. K(+) uptake was greater in OX lines which may be driven by increased AKT1 and HAK1 activity. The enhanced K(+) uptake led to tissue K(+) levels that were raised in roots and shoots. Furthermore, energy dispersive X-ray (EDX) analyses showed a higher cytoplasm: vacuole K(+) ratio which is likely to contribute to the increased stress tolerance. In all, the data suggest that TPKb can alter the K(+) status of small vacuoles, which is important for general cellular K(+) homeostasis which, in turn, affects stress tolerance.

In vivo protein-protein interaction studies with BiFC: conditions, cautions, and caveats

Bimolecular fluorescence complementation (BiFC), performed with suitable controls and the right conditions, can be a straightforward and simple method to assess protein-protein interactions accessible to anyone with basic confocal microscopy skills. It is of course not without its own potential pitfalls and requires specific controls. Here we describe its use to study the interactions between pathogen effector proteins and host proteins inside plant cells.

Identification of the domains of cauliflower mosaic virus protein P6 responsible for suppression of RNA silencing and salicylic acid signalling

Cauliflower mosaic virus (CaMV) encodes a 520 aa polypeptide, P6, which participates in several essential activities in the virus life cycle including suppressing RNA silencing and salicylic acid-responsive defence signalling. We infected Arabidopsis with CaMV mutants containing short in-frame deletions within the P6 ORF. A deletion in the distal end of domain D-I (the N-terminal 112 aa) of P6 did not affect virus replication but compromised symptom development and curtailed the ability to restore GFP fluorescence in a GFP-silenced transgenic Arabidopsis line. A deletion in the minimum transactivator domain was defective in virus replication but retained the capacity to suppress RNA silencing locally. Symptom expression in CaMV-infected plants is apparently linked to the ability to suppress RNA silencing. When transiently co-expressed with tomato bushy stunt virus P19, an elicitor of programmed cell death in Nicotiana tabacum, WT P6 suppressed the hypersensitive response, but three mutants, two with deletions within the distal end of domain D-I and one involving the N-terminal nuclear export signal (NES), were unable to do so. Deleting the N-terminal 20 aa also abolished the suppression of pathogen-associated molecular pattern-dependent PR1a expression following agroinfiltration. However, the two other deletions in domain D-I retained this activity, evidence that the mechanisms underlying these functions are not identical. The D-I domain of P6 when expressed alone failed to suppress either cell death or PR1a expression and is therefore necessary but not sufficient for all three defence suppression activities. Consequently, concerns about the biosafety of genetically modified crops carrying truncated ORFVI sequences appear unfounded.

Advances and remaining challenges in the transformation of barley and wheat

Highly efficient and cost-effective transformation technologies are essential for studying gene function in the major cereal crops, wheat and barley. Demand for efficient transformation systems to allow over-expression, or RNAi-mediated silencing of target genes, is greatly increasing. This is due to technology advances, such as rapid genome sequencing, enhancing the rate of gene discovery and thus leading to a large number of genes requiring functional analysis through transformation pipelines. Barley can be transformed at very high efficiency but the methods are genotype-dependent. Wheat is more difficult to transform, however, recent advances are also allowing the development of high-throughput transformation systems in wheat. For many gene function studies, barley can be used as a model for wheat due to its highly efficient transformation rates and smaller, less complex genome. An ideal transformation system needs to be extremely efficient, simple to perform, inexpensive, genotype-independent, and give the required expression of the transgene. Considerable progress has been made in enhancing transformation efficiencies, controlling transgene expression and in understanding and manipulating transgene insertion. However, a number of challenges still remain, one of the key ones being the development of genotype-independent transformation systems for wheat and barley.

Agrobacterium-mediated transformation of the temperate grass Brachypodium distachyon (genotype Bd21) for T-DNA insertional mutagenesis

Brachypodium distachyon is a promising model system for the structural and functional genomics of temperate grasses because of its physical, genetic and genome attributes. The sequencing of the inbred line Bd21 (http://www.brachypodium.org) started in 2007. However, a transformation method remains to be developed for the community standard line Bd21. In this article, a facile, efficient and rapid transformation system for Bd21 is described using Agrobacterium-mediated transformation of compact embryogenic calli (CEC) derived from immature embryos. Key features of this system include: (i) the use of the green fluorescent protein (GFP) associated with hygromycin selection for rapid identification of transgenic calli and plants; (ii) the desiccation of CEC after inoculation with Agrobacterium; (iii) the utilization of Bd21 plants regenerated from tissue culture as a source of immature embryos; (iv) the control of the duration of the selection process; and (v) the supplementation of culture media with CuSO4 prior to and during the regeneration of transgenic plants. Approximately 17% of CEC produced transgenic plants, enabling the generation of hundreds of T-DNA insertion lines per experiment. GFP expression was observed in primary transformed Bd21 plants (T0) and their progeny (T1). The Mendelian inheritance of the transgenes was confirmed. An adaptor-anchor strategy was developed for efficient retrieval of flanking sequence tags (FSTs) of T-DNA inserts, and the resulting sequences are available in public databases. The production of T-DNA insertion lines and the retrieval of associated FSTs reported here for the reference inbred line Bd21 will facilitate large-scale functional genomics research in this model system.

The pCLEAN dual binary vector system for Agrobacterium-mediated plant transformation

The development of novel transformation vectors is essential to the improvement of plant transformation technologies. Here, we report the construction and testing of a new multifunctional dual binary vector system, pCLEAN, for Agrobacterium-mediated plant transformation. The pCLEAN vectors are based on the widely used pGreen/pSoup system and the pCLEAN-G/pCLEAN-S plasmids are fully compatible with the existing pGreen/pSoup vectors. A single Agrobacterium can harbor (1) pCLEAN-G and pSoup, (2) pGreen and pCLEAN-S, or (3) pCLEAN-G and pCLEAN-S vector combination. pCLEAN vectors have been designed to enable the delivery of multiple transgenes from distinct T-DNAs and/or vector backbone sequences while minimizing the insertion of superfluous DNA sequences into the plant nuclear genome as well as facilitating the production of marker-free plants. pCLEAN vectors contain a minimal T-DNA (102 nucleotides) consisting of direct border repeats surrounding a 52-nucleotide-long multiple cloning site, an optimized left-border sequence, a double left-border sequence, restriction sites outside the borders, and two independent T-DNAs. In addition, selectable and/or reporter genes have been inserted into the vector backbone sequence to allow either the counter-screening of backbone transfer or its exploitation for the production of marker-free plants. The efficiency of the different pCLEAN vectors has been assessed using transient and stable transformation assays in Nicotiana benthamiana and/or Oryza sativa.

Diploid potato (Solanum tuberosum L.) as a model crop to study transgene expression

This paper presents a method of Agrobacterium-mediated transformation for two diploid breeding lines of potato, and gives a detailed analysis of reporter gene expression. In our lab, these lines were also used to obtain tetraploid somatic hybrids. We tested four newly prepared constructs based on the pGreen vector system containing the selection gene nptII or bar under the 35S or nos promoter. All these vectors carried gus under 35S. We also tested the pDM805 vector, with the bar and gus genes respectively under the Ubi1 and Act1 promoters, which are strong for monocots. The selection efficiency (about 17%) was highest in the stem and leaf explants after transformation with pGreen where nptII was under 35S. About half of the selected plants were confirmed via PCR and Southern blot analysis to be transgenic and, depending on the combination, 0 to 100% showed GUS expression. GUS expression was strongest in multi-copy transgenic plants where gus was under Act1. The same potato lines carrying multi-copy bar under Ubi1 were also highly resistant to the herbicide Basta. The suggestion of using Agrobacterium-mediated transformation of diploid lines of potato as a model crop is discussed herein.

Insights into recognition of the T-DNA border repeats as termination sites for T-strand synthesis by Agrobacterium tumefaciens

The recognition of the T-DNA left border (LB) repeat is affected by its surrounding sequences. Here, the LB regions were further characterized by molecular analysis of transgenic plants, obtained after Agrobacterium tumefaciens-mediated transformation with T-DNA vectors that had been modified in this LB region. At least the 24-bp LB repeat by itself was insufficient to terminate the T-strand synthesis. Addition of the natural inner and/or outer border regions to at least the LB repeat, even when present at a distance, enhanced the correct recognition of the LB repeat, reducing the number of plants containing vector backbone sequences. In tandem occurrence of both the octopine and nopaline LB regions with their repeats terminated the T-strand synthesis most efficiently at the LB, yielding a reproducibly high number of plants containing only the T-DNA. Furthermore, T-strand synthesis did not terminate efficiently at the right border (RB) repeat, which might indicate that signals in the outer RB region inhibit the termination of T-strand synthesis at the RB repeat.