Sexual transmission of transposed activator elements in transgenic tomatoes

Efficient generation of marker-free transgenic rice plants using an improved transposon-mediated transgene reintegration strategy

Marker-free transgenic plants can be developed through transposon-mediated transgene reintegration, which allows intact transgene insertion with defined boundaries and requires only a few primary transformants. In this study, we improved the selection strategy and validated that the maize (Zea mays) Activator/Dissociation (Ds) transposable element can be routinely used to generate marker-free transgenic plants. A Ds-based gene of interest was linked to green fluorescent protein in transfer DNA (T-DNA), and a green fluorescent protein-aided counterselection against T-DNA was used together with polymerase chain reaction (PCR)-based positive selection for the gene of interest to screen marker-free progeny. To test the efficacy of this strategy, we cloned the Bacillus thuringiensis (Bt) δ-endotoxin gene into the Ds elements and transformed transposon vectors into rice (Oryza sativa) cultivars via Agrobacterium tumefaciens. PCR assays of the transposon empty donor site exhibited transposition in somatic cells in 60.5% to 100% of the rice transformants. Marker-free (T-DNA-free) transgenic rice plants derived from unlinked germinal transposition were obtained from the T1 generation of 26.1% of the primary transformants. Individual marker-free transgenic rice lines were subjected to thermal asymmetric interlaced-PCR to determine Ds(Bt) reintegration positions, reverse transcription-PCR and enzyme-linked immunosorbent assay to detect Bt expression levels, and bioassays to confirm resistance against the striped stem borer Chilo suppressalis. Overall, we efficiently generated marker-free transgenic plants with optimized transgene insertion and expression. The transposon-mediated marker-free platform established in this study can be used in rice and possibly in other important crops.

Precise marker excision system using an animal-derived piggyBac transposon in plants

Accurate and effective positive marker excision is indispensable for the introduction of desired mutations into the plant genome via gene targeting (GT) using a positive/negative counter selection system. In mammals, the moth-derived piggyBac transposon system has been exploited successfully to eliminate a selectable marker from a GT locus without leaving a footprint. Here, we present evidence that the piggyBac transposon also functions in plant cells. To demonstrate the use of the piggyBac transposon for effective marker excision in plants, we designed a transposition assay system that allows the piggyBac transposition to be visualized as emerald luciferase (Eluc) luminescence in rice cells. The Eluc signal derived from piggyBac excision was observed in hyperactive piggyBac transposase-expressing rice calli. Polymerase chain reaction, Southern blot analyses and sequencing revealed the efficient and precise transposition of piggyBac in these calli. Furthermore, we have demonstrated the excision of a selection marker from a reporter locus in T0 plants without concomitant re-integration of the transposon and at a high frequency (44.0% of excision events), even in the absence of negative selection.

The elimination of a selectable marker gene in the doubled haploid progeny of co-transformed barley plants

Following the production of transgenic plants, the selectable marker gene(s) used in the process are redundant, and their retention may be undesirable. They can be removed by exploiting segregation among the progeny of co-transformants carrying both the selectable marker gene and the effector transgene. Here we show that the doubled haploid technology widely used in conventional barley breeding programmes represents a useful means of fixing a transgene, while simultaneously removing the unwanted selectable marker gene. Primary barley co-transformants involving hpt::gfp (the selectable marker) and gus (a model transgene of interest) were produced via Agrobacterium-mediated gene transfer to immature embryos using two respective T-DNAs. These plants were then subjected to embryogenic pollen culture to separate independently integrated transgenes in doubled haploid progeny. A comparison between 14 combinations, involving two Agrobacterium strains carrying various plasmids, revealed that the highest rate of independent co-transformation was achieved when a single Agrobacterium clone carried two binary vectors. Using this principle along with Agrobacterium strain LBA4404, selectable marker-free, gus homozygous lines were eventually obtained from 1.5 per 100 immature embryos inoculated. Compared to the segregation of uncoupled T-DNAs in conventionally produced progeny, the incorporation of haploid technology improves the time and resource efficiency of producing true-breeding, selectable marker-free transgenic barley.

A hyperactive transposase of the maize transposable element activator (Ac)

Activator/Dissociation (Ac/Ds) transposable elements from maize are widely used as insertional mutagenesis and gene isolation tools in plants and more recently also in medaka and zebrafish. They are particularly valuable for plant species that are transformation-recalcitrant and have long generation cycles or large genomes with low gene densities. Ac/Ds transposition frequencies vary widely, however, and in some species they are too low for large-scale mutagenesis. We discovered a hyperactive Ac transposase derivative, AcTPase(4x), that catalyzes in the yeast Saccharomyces cerevisiae 100-fold more frequent Ds excisions than the wild-type transposase, whereas the reintegration frequency of excised Ds elements is unchanged (57%). Comparable to the wild-type transposase in plants, AcTPase(4x) catalyzes Ds insertion preferentially into coding regions and to genetically linked sites, but the mutant protein apparently has lost the weak bias of the wild-type protein for insertion sites with elevated guanine-cytosine content and nonrandom protein-DNA twist. AcTPase(4x) exhibits hyperactivity also in Arabidopsis thaliana where it effects a more than sixfold increase in Ds excision relative to wild-type AcTPase and thus may be useful to facilitate Ac/Ds-based insertion mutagenesis approaches.

Genetic engineering of cytokinin metabolism: prospective way to improve agricultural traits of crop plants

Cytokinins (CKs) are ubiquitous phytohormones that participate in development, morphogenesis and many physiological processes throughout plant kingdom. In higher plants, mutants and transgenic cells and tissues with altered activity of CK metabolic enzymes or perception machinery, have highlighted their crucial involvement in different agriculturally important traits, such as productivity, increased tolerance to various stresses and overall plant morphology. Furthermore, recent precise metabolomic analyses have elucidated the specific occurrence and distinct functions of different CK types in various plant species. Thus, smooth manipulation of active CK levels in a spatial and temporal way could be a very potent tool for plant biotechnology in the future. This review summarises recent advances in cytokinin research ranging from transgenic alteration of CK biosynthetic, degradation and glucosylation activities and CK perception to detailed elucidation of molecular processes, in which CKs work as a trigger in model plants. The first attempts to improve the quality of crop plants, focused on cereals are discussed, together with proposed mechanism of action of the responses involved.

Ac as a tool for the functional genomics of rice

To examine whether the maize autonomous transposable element Ac can be used for the functional analysis of the rice genome, we used Southern blot analysis to analyze the behaviour of Ac in 559 rice plants of four transgenic families through three successive generations. All families showed highly active transposition of Ac, and 103 plants (18.4%) contained newly transposed Ac insertions. In nine of the 12 independent transpositions analyzed, their germinal transmission was detected. Partial sequencing of 99 Ac-flanking sequences revealed that 21 clones exhibited significant similarities with protein-coding genes in databases and four of them matched rice cDNA sequences. These results indicate preferential Ac transposition into protein-coding rice genes. To examine the feasibility of PCR-based screening of gene knockouts in rice Ac plants, we prepared bulked genomic DNA from the leaves of approximately 6000 rice Ac plants and pooled the DNA according to a three-dimensional matrix. Of 14 randomly selected genes, two gene knockouts were identified, and one encoding a rice cytochrome P450 (CYP86) gene was shown to be stably inherited to the progeny. Together, these results suggest that Ac can be efficiently used for the functional analysis of the rice genome.

Selection of marker-free transgenic plants using the isopentenyl transferase gene

We have developed a new plant vector system for repeated transformation (called MAT for multi-auto-transformation) in which a chimeric ipt gene, inserted into the transposable element Ac, is used as a selectable marker for transformation. Selectable marker genes conferring antibiotic or herbicide resistance, used to introduce economically valuable genes into crop plants, have three major problems: (i) the selective agents have negative effects on proliferation and differentiation of plant cells; (ii) there is uncertainty regarding the environmental impact of many selectable marker genes; (iii) it is difficult to perform recurrent transformations using the same selectable marker to pyramid desirable genes. The MAT vector system containing the ipt gene and the Ac element is designed to overcome these difficulties. When tobacco leaf segments were transformed and selected, subsequent excision of the modified Ac produced marker-free transgenic tobacco plants without sexual crosses or seed production. In addition, the chimeric ipt gene could be visually used as a selectable marker for transformation of hybrid aspen (Populus sieboldii x Populus grandidentata). The chimeric ipt gene, therefore, is an attractive alternative to the most widely used selectable marker genes. The MAT vector system provides a promising way to shorten breeding time for genetically engineered crops. This method could be particularly valuable for fruit and forest trees, for which long generation times are a more significant barrier to breeding and genetic analysis.

Amplification of Ac in tomato is correlated with high Ac transposition activity

We have assayed the transposition activity of the maize transposable element Ac in transgenic tomato plants that had a single copy of Ac. We found that Ac elements were in either a high or low activity state and that an Ac insertion could cycle from low to high activity within a generation. The different transposition activities were not simply due to the chromosomal position of the element, because the same Ac insertion had different levels of activity in sibling plants. Transposition activity was measured by two methods, one genetic and one physical; both assays gave similar results for each plant studied. Notably, plants with active Ac elements had progeny with amplified Ac copy number, while no amplification was detected in lines containing Ac in a low activity state. Analysis of lines with amplified elements revealed that the elements could be either clustered or dispersed. Our results were consistent with amplification being the result of transposition.

Enhanced frequency of transposition of the maize transposable element Activator following excision from T-DNA in Petunia hybrida

Many of the systems currently employed for heterologous transposon tagging in plants rely on an excision assay to monitor transposon activity. We have used the streptomycin phosphotransferase (SPT) reporter system to assay Ac activity in Petunia hybrida. In other species, such as tobacco or Arabidopsis, excision of Ac from the SPT gene in sporogenous tissue gives rise to streptomycin-resistant seedlings in the following generation. The frequency of fully streptomycin-resistant seedlings in petunia was low (0.4%) but molecular analysis of these indicated that the actual excision frequency may be as low as 0.05%. This indicates that the SPT assay is not a reliable selection criterion for germinal excision in petunia. Extensive molecular screening for reinsertion of Ac was consistent with a low primary transposition frequency (0%-0.6%). In contrast to these findings, the progeny of confirmed germinal transpositions for three independent transformants showed frequent transposition to new sites (9.5%-17.0%). This suggests a high frequency of secondary transposition compared with primary transposition from the T-DNA. Segregation analysis indicates that the high transposition activity is closely associated with transposed copies of Ac. No evidence was found for an altered methylation state for Ac following transposition. The implications of these results for heterologous transposon tagging in petunia are discussed in the context of the reliability of excision reporter systems in general.

Transactivation of Ds elements in plants of lettuce (Lactuca sativa)

The maize transposable element, Activator (Ac), is being used to develop a transposon mutagenesis system in lettuce, Lactuca sativa. In this paper, we describe somatic and germinal transactivation of Ds by chimeric transposase genes in whole plants. Constructs containing either the Ds element or the Ac transposase open reading frame (ORF) were introduced into lettue. The Ds element was located between either the 35S or the Nos promoter and a chimeric spectinomycin resistance gene (which included a transit peptide), preventing expression of spectinomycin resistance. The genomic coding region of the Ac transposase was expressed from the 35S promoter. Crosses were made between 104 independent R1 plants containing Ds and three independent R1 plants expressing transposase. The excision of Ds in F1 progenies was monitored using a phenotypic assay on spectinomycin-containing medium. Green sectors in one-third of the F1 families indicated transactivation of Ds by the transposase at different developmental stages and at different frequencies in lettuce plants. Excision was confirmed using PCR and by Southern analysis. The lack of green sectors in the majority of F1 families suggest that the majority of T-DNA insertion sites are not conducive to excision. In subsequent experiments, the F1 plants containing both Ds and the transposase were grown to maturity and the F2 seeds screened on medium containing spectinomycin. Somatic excision was again observed in several F2 progeny; however, evidence for germinal excision was observed in only one F2 family.

Selection for enhanced germinal excision of Ac in transgenic Arabidopsis thaliana

Gene tagging in Arabidopsis thaliana using the autonomous Ac (Activator) transposable element has so far been hampered by low frequencies of germinal transposition events. Here we describe a procedure by which the frequency of independent germinal reinsertions has been much improved by a process of long-term selection on kanamycin for the continued growth of tissues in which somatic excisions have occurred. Growth on artificial media increased the somatic excision frequency, and the long-term selection procedure channelled somatic transposition events into the germline. This resulted in an overall germinal excision frequency in the progeny of longterm selected plants of 15%, as confirmed by Southern blotting, with 63% of the plants bearing excision events having detectable reinsertions of the Ac element. This compares with a germinal excision frequency of approximately 1% when no long-term selection is employed. However, offspring from individual plants tended to have identical germinal Ac reinsertion patterns, thus the critical parameter for evaluating the system for tagging purposes is the frequency of individual plants yielding offspring with reinsertions, which was 64%. This high frequency, when coupled to the enhanced germinal transposition rate overall, easily allows the generation of a large population of plants with independent reinsertions.

Infrequent transposition of Ac in lettuce, Lactuca sativa

The maize transposable element Activator (Ac) is being used to develop a transposon mutagenesis system in lettuce, Lactuca sativa. Two constructs containing the complete Ac from the waxy-m7 locus of maize were introduced into lettuce and monitored for activity using Southern analysis and PCR amplification of the excision site. No transposition of Ac was detected in over 32 transgenic R1 plants, although these constructs were known to provide frequent transposition in other species. Also, no transposition was observed in later generations. In subsequent experiments, transposition was detected in lettuce calli using constructs that allowed selection for excision events. In these constructs, the neomycin phosphotransferase II gene was interrupted by either Ac or Ds. Excision was detected as the ability of callus to grow on kanamycin. Synthesis of the transposase from the cDNA of Ac expressed from the T-DNA 2' promoter resulted in more frequent excision of Ds than was observed with the wild-type Ac. No excision was observed with Ds in the absence of the transposase. The excision events were confirmed by amplification of the excision site by PCR followed by DNA sequencing. Excision and reintegration were also confirmed by Southern analysis. Ac/Ds is therefore capable of transposition in at least calli of lettuce.

Ac-induced instability at the Xanthophyllic locus of tomato

To detect genomic instability caused by Ac elements in transgenic tomatoes, we used the incompletely dominant mutation Xanthophyllic-1 (Xa-1) as a whole plant marker gene. Xa-1 is located on chromosome 10 and in the heterozygote state causes leaves to be yellow. Transgenic Ac-containing tomato plants which differed in the location and number of their Ac elements were crossed to Xa-1 tester lines and F1 progeny were scored for aberrant somatic sectoring. Of 800 test and control F1 progeny screened, only four plants had aberrantly high levels of somatic sectors. Three of the plants had twin sectors consisting of green tissue adjacent to white tissue, and the other had twin sectors comprised of green tissue adjacent to tissue more yellow than the heterozygote background. Sectoring was inherited and the two sectoring phenotypes mapped to opposite homologs of chromosome 10; the green/yellow sectoring phenotype mapped in coupling to Xa-1 while the green/white sectoring phenotype mapped in repulsion. The two sectoring phenotypes cosegregated with different single, non-rearranged Acs, and loss of these Acs from the genome corresponded to the loss of sectoring. Sectoring was still observed after transposition of the Ac to a new site which indicated that sectoring was not limited to a single locus. In both sectored lines, meiotic recombination of the sectoring Ac to the opposite homolog caused the phenotype to switch between the green/yellow and the green/white phenotypes. Thus the two different sectoring phenotypes arose from the same Ac-induced mechanism; the phenotype depended on which chromosome 10 homolog the Ac was on. We believe that the twin sectors resulted from chromosome breakage mediated by a single intact, transposition-competent Ac element.

Transposition of the maize autonomous element Activator in transgenic Nicotiana plumbaginifolia plants

The maize autonomous transposable element Ac was introduced into haploid Nicotiana plumbaginifolia via Agrobacterium tumefaciens transformation of leaf disks. All the regenerated transformants (R0) were diploid and either homozygous or heterozygous for the hygromycin resistance gene used to select primary transformants. The Ac excision frequency was determined using the phenotypic assay of restoration of neomycin phosphotransferase activity and expression of kanamycin resistance among progeny seedlings. Some of the R0 plants segregated kanamycin-resistant seedlings in selfed progeny at a high frequency (34 to 100%) and contained one or more transposed Ac elements. In the primary transformants Ac transposition probably occurred during plant regeneration or early development. Other R0 transformants segregated kanamycin-resistant plants at a low frequency (< or = 4%). Two transformants of this latter class, containing a unique unexcised Ac element, were chosen for further study in the expectation that their kanamycin resistant progeny would result from independent germinal transposition events. Southern blot analysis of 32 kanamycin-resistant plants (R1 or R2), selected after respectively one or two selfings of these primary transformants, showed that 27 had a transposed Ac at a new location and 5 did not have any Ac element. Transposed Ac copy number varied from one to six and almost all transposition events were independent. Southern analysis of the R2 and R3 progeny of these kanamycin-resistant plants showed that Ac continued to transpose during four generations, and its activity increased with its copy number. The frequency of Ac transposition, from different loci, remained low (< or = 7%) from R0 to R3 generations when only one Ac copy was present.(ABSTRACT TRUNCATED AT 250 WORDS)

Development and characterization of a generalized gene tagging system for higher plants using an engineered maize transposon Ac

This report describes a series of transposon tagging vectors for dicotyledonous plants based on the maize transposable element Ac. This binary system includes the transposase (Ts) and the tagging element (Ds) on separate T-DNA vectors. Ts elements include versions in which transcription is driven either by the endogenous Ac promoter or by the cauliflower mosaic virus (CaMV) 35S promoter. Ds tagging element includes a gene conferring methotrexate (Mtx) resistance for selection and a supF gene to facilitate cloning of tagged sequences. The Ds element is flanked by a CaMV 35S promoter and the beta-glucuronidase (GUS) coding sequence so that GUS expression occurs upon excision of the element. We have transformed these Ts and Ds elements into tobacco and demonstrated that the Ts is functional with either promoter, and that the artificial Ds elements are capable of transposition. The amount of excision was found to depend upon both the individual Ts and Ds primary transformants used. Somatic excision of Ds was seen in up to 100% of progeny seedlings containing Ts and Ds. Germinal excision was detected in up to 48% of the progeny of plants containing both elements. Hence, this system can generate a sufficient number of events to be useful in gene tagging.

Molecular analysis of the Ubiquitous (Uq) transposable element system of Zea mays

The Uq transposable element of maize is the most widely dispersed among different maize populations and genetic testerstrains. Despite intensive genetic characterization, little is known about its molecular structure. In order to obtain information relevant to this topic, we have cloned and sequenced three ruq receptors. Surprisingly, they are all Ds1-like receptor types of the Ac-Ds transposon family. Based on our molecular data, we present a model to explain the functional differences associated with the differential expression of the Uq and Ac transposon systems.

Characterization of the level, target sites and inheritance of cytosine methylation in tomato nuclear DNA

The tomato nuclear genome was determined to have a G + C content of 37% which is among the lowest reported for any plant species. Non-coding regions have a G + C content even lower (32% average) whereas coding regions are considerably richer in G + C (46%). 5-methyl cytosine was the only modified base detected and on average 23% of the cytosine residues are methylated. Immature tissues and protoplasts have significantly lower levels of cytosine methylation (average 20%) than mature tissues (average 25%). Mature pollen has an intermediate level of methylation (22%). Seeds gave the highest value (27%), suggesting de novo methylation after pollination and during seed development. Based on isoschizomer studies we estimate 55% of the CpG target sites (detected by Msp I/Hpa II) and 85% of the CpNpG target sites (detected by Bst NI/Eco RI) are methylated. Unmethylated target sites (both CpG and CpNpG) are not randomly distributed throughout the genome, but frequently occur in clusters. These clusters resemble CpG islands recently reported in maize and tobacco. The low G + C content and high levels of cytosine methylation in tomato may be due to previous transitions of 5mC----T. This is supported by the fact that G + C levels are lowest in non-coding portions of the genome in which selection is relaxed and thus transitions are more likely to be tolerated. This hypothesis is also supported by the general deficiency of methylation target sites in the tomato genome, especially in non-coding regions. Using methylation isoschizomers and RFLP analysis we have also determined that polymorphism between plants, for cytosine methylation at allelic sites, is common in tomato. Comparing DNA from two tomato species, 20% of the polymorphisms detected by Bst NI/Eco RII could be attributed to differential methylation at the CpNpG target sites. With Msp I/Hpa II, 50% of the polymorphisms were attributable to methylation (CpG and CpNpG sites). Moreover, these polymorphisms were demonstrated to be inherited in a mendelian fashion and to co-segregate with the methylation target site and thus do not represent variation for transacting factors that might be involved in methylation of DNA. The potential role of heritable methylation polymorphism in evolution of gene regulation and in RFLP studies is discussed.

Properties of the maize transposable element Activator in transgenic tobacco plants: a versatile inter-species genetic tool

The maize controlling element Activator (Ac) transposes autonomously from an integrated T-DNA vector to new sites in the genomes of tobacco and other heterologous plant species. Here we demonstrate that critical functions required for transposition of Ac in maize are conserved in tobacco and that Ac transposes at high frequency for at least five generations. Ac structure and terminal sequences are conserved upon transposition and a characteristic 8-bp duplication of target sequences is generated upon integration. Ac remains unmethylated, transcriptionally active, and capable to trans-activate transposition of the nonautonomous Dissociation (Ds) element throughout several generations. In tobacco, as in maize, Ac transposes adjacent to low copy or unique DNA, and transcriptional analysis of unique target DNA provides evidence that an Ac element transposed into a gene. In maize, increasing copies of Ac delay the timing and reduce the frequency of early transposition of Ac and Ds. In tobacco, increasing copies of Ac correlate with an increased frequency of Ds trans-activation. These data firmly establish that the Ac and Ds transposable elements are versatile genetic tools well suited for use as insertional mutagens and demonstrate that thorough investigation of mechanism and regulation of transposition is facilitated in heterologous settings.

A genetic analysis of mutations recovered from tomato following Agrobacterium-mediated transformation with the maize transposahle elements Activator and Dissociation

The maize autonomous transposable element, Activator (Ac), and the nonautonomous element Dissociation (Ds), were introduced into the tomato cultivars VF36 and VFNT Cherry by Agrobacterium-mediated transformation. Progeny families from 145 primary transformants were scored at the seedling stage for phenotypically variant individuals. When VF36 was transformed, 20% of families had progeny with aberrant phenotypes. The mutation frequency in VFNT Cherry transformants was lower; in this cultivar 7% of the transformants had progeny segregating for seedling mutations. The majority of the mutations showed monogenic inheritance in the R1 population, suggesting that the mutations occurred early in the transformation/regeneration process. One mutation, however, was recovered at low frequency in the R1, suggesting a late mutagenesis event. When tomato was transformed with either the Ac or Ds elements, no differences in mutation frequencies were observed. Since Ac is transpositionally active in tomato transformants while Ds is not, these numbers indicate that the mutation frequency inherent to the transformation process is higher than the mutational activity of Ac. These results demonstrate that efficient gene tagging using heterologous transposable elements will require screening for transposon-induced mutations in later generations.