A wound-induced promoter driving npt-II expression limited to dedifferentiated cells at wound sites is sufficient to allow selection of transgenic shoots

Yr36 confers partial resistance at temperatures below 18°C to U.K. isolates of Puccinia striiformis

Wheat yellow (stripe) rust, caused by the obligate biotrophic fungus Puccinia striiformis f. sp. tritici, is a continual threat to wheat fields worldwide. New isolates with increased virulence have recently emerged driving breeding efforts to incorporate disease resistance genes which confer potentially more durable, albeit partial, resistance. Yr36 is one such locus which was recently cloned (WKS1) and described as a high-temperature adult-plant gene being effective only at temperatures above 25°C. We examined the potential use of Yr36 at temperatures below 25°C. Field experiments in the United Kingdom across 2 years show that lines carrying Yr36 provide slow rusting resistance to the yellow rust pathogen. Juvenile and adult Yr36 isogenic lines showed partial resistance at temperatures below 18°C under control environment conditions in tetraploid and hexaploid genetic backgrounds, but not at seedling stage, when inoculated with U.K. P. striiformis isolates. This partial resistance phenotype was similar to that observed previously at temperatures ≥25°C. Transgenic complementation tests and ethyl methanesulfonate mutants showed that the low-temperature partial resistance was due to the WKS1 gene. This study indicates that Yr36 has the potential to be an effective source of partial resistance in temperate wheat growing regions.

Visualisation of stromules in transgenic wheat expressing a plastid-targeted yellow fluorescent protein

Stromules are stroma-filled tubules that extend from the plastids in all multicellular plants examined to date. To facilitate the visualisation of stromules on different plastid types in various tissues of bread wheat (Triticum aestivum L.), a chimeric gene construct encoding enhanced yellow fluorescent protein (EYFP) targeted to plastids with the transit peptide of wheat granule-bound starch synthase I was introduced by Agrobacterium-mediated transformation. The gene construct was under the control of the rice Actin1 promoter, and EYFP fluorescence was detected in plastids in all cell types throughout the transgenic plants. Stromules were observed on all plastid types, although the stromule length and abundance varied markedly in different tissues. The longest stromules (up to 40 μm) were observed in epidermal cells of leaves, whereas only short beak-like stromules were observed on chloroplasts in mesophyll cells. Epidermal cells in leaves and roots contained the highest proportion of plastids with stromules, and stromules were also abundant on amyloplasts in the endosperm tissue of developing seeds. The general features of stromule morphology and distribution were similar to those shown previously for tobacco (Nicotiana tabacum L.) and arabidopsis (Arabidopsis thaliana (L.) Heynh.).

Expression of Cry1Ac in transgenic tobacco plants under the control of a wound-inducible promoter (AoPR1) isolated from Asparagus officinalis to control Heliothis virescens and Manduca sexta

Expression of cry1Ac gene from Bacillus thuringiensis (Bt) was evaluated under the control of a wound-inducible AoPR1 promoter from Asparagus officinalis in transgenic tobacco plants. The leaves of transgenic plants were mechanically wounded to evaluate the activity of the AoPR1 promoter in driving the expression of Cry1Ac protein at the wound site. Our results indicate that mechanical wounding of transgenic plants was effective in inducing the expression of Cry1Ac protein. As a result of this induction, the accumulated levels of Cry1Ac protein increased during 6-72 h post-wounding period. The leaves of transgenic tobacco plants were evaluated for resistance against Heliothis virescens and Manduca sexta in insect bioassays in two different ways. The detached tobacco leaves were either fed directly to the insect larvae or they were first mechanically wounded followed by a 72 h post-wounding feeding period. Complete protection of mechanically wounded leaves of transgenic plants was observed within 24 h of the bioassay. The leaves of transgenic plants fed directly (without pre-wounding) to the larvae achieved the same level of protection between 24 and 72 h of the bioassay.

Characterisation of a new reporter system allowing high throughput in planta screening for recombination events before and after controlled DNA double strand break induction

DNA double strand breaks (DSBs) are created either by DNA damaging reagents or in a programmed manner, for example during meiosis. Homologous recombination (HR) can be used to repair DSBs, a process vital both for cell survival and for genetic rearrangement during meiosis. In order to easily quantify this mechanism, a new HR reporter gene that is suitable for the detection of rare recombination events in high-throughput screens was developed in Arabidopsis thaliana. This reporter, pPNP, is composed of two mutated Pat genes and has also one restriction site for the meganuclease I-SceI. A functional Pat gene can be reconstituted by an HR event giving plants which are resistant to the herbicide glufosinate. The basal frequency of intra-chromosomal recombination is very low (10(-5)) and can be strongly increased by the expression of I-SceI which creates a DSB. Expression of I-SceI under the control of the 35S CaMV promoter dramatically increases HR frequency (10,000 fold); however the measured recombinant events are in majority somatic. In contrast only germinal recombination events were measured when the meganuclease was expressed from a floral-specific promoter. Finally, the reporter was used to test a dexamethasone inducible I-SceI which could produce up to 200x more HR events after induction. This novel inducible I-SceI should be useful in fundamental studies of the mechanism of repair of DSBs and for biotechnological applications.

Highly efficient Agrobacterium-mediated transformation of wheat via in planta inoculation

This chapter details a reproducible method for the transformation of spring wheat using Agrobacterium tumefaciens via the direct inoculation of bacteria into immature seeds in planta as described in patent WO 00/63398 (1). Transformation efficiencies from 1 to 30% have been obtained and average efficiencies of at least 5% are routinely achieved. Regenerated plants are phenotypically normal with 30-50% of transformation events carrying introduced genes at single insertion sites, a higher rate than is typically reported for transgenic plants produced using biolistic transformation methods.

Heat-inducible production of beta-glucuronidase in tobacco hairy root cultures

The production of beta-glucuronidase (GUS) driven by the Arabidopsis small heat shock protein 18.2 promoter in liquid cultures of transgenic tobacco (Nicotiana tabacum) hairy roots is reported. Clone GD-3, showing high GUS heat induction and a moderate growth rate, was selected from 436 clones for study. Treatment of GD-3 with heat shock at 36-42 degrees C for 2 h then recovery at 27 degrees C resulted in an increase in GUS specific activity, while higher heat-shock temperatures led to a decline. These results were in accordance with the change in esterase activity, a measure of tissue viability. Using 2 h of 42 degrees C heat shock and a recovery phase at 27 degrees C, GUS specific activity increased rapidly and reached a maximum of 267.6 nmol 4-methylumbelliferyl beta-D-glucuronic acid (MU) min-1 mg-1 protein at 24 h of recovery. When tissues were continuously heated at 42 degrees C and tested without a recovery period, GUS mRNA was detectable at 2 h and peaked at 5 h, but GUS activity was not seen until 10 h and did not peak until 28 h; in addition, the maximum activity was lower than that seen after heat shock for only 30 min or 2 h, followed by recovery. This shows that recovery at normal temperature is crucial for the heat-inducible heterogeneous expression system of transgenic hairy roots. Multiple heat-shock treatments showed that this system was heat reinducible, although a gradual decline in GUS specific activity was seen in the second and third cycles.

Plant transformation by coinoculation with a disarmed Agrobacterium tumefaciens strain and an Escherichia coli strain carrying mobilizable transgenes

Transformation of Nicotiana tabacum leaf explants was attempted with Escherichia coli as a DNA donor either alone or in combination with Agrobacterium tumefaciens. We constructed E. coli donor strains harboring either the promiscuous IncP-type or IncN-type conjugal transfer system and second plasmids containing the respective origins of transfer and plant-selectable markers. Neither of these conjugation systems was able to stably transform plant cells at detectable levels, even when VirE2 was expressed in the donor cells. However, when an E. coli strain expressing the IncN-type conjugation system was coinoculated with a disarmed A. tumefaciens strain, plant tumors arose at high frequencies. This was caused by a two-step process in which the IncN transfer system mobilized the entire shuttle plasmid from E. coli to the disarmed A. tumefaciens strain, which in turn processed the T-DNA and transferred it to recipient plant cells. The mobilizable plasmid does not require a broad-host-range replication origin for this process to occur, thus reducing its size and genetic complexity. Tumorigenesis efficiency was further enhanced by incubation of the bacterial strains on medium optimized for bacterial conjugation prior to inoculation of leaf explants. These techniques circumvent the need to construct A. tumefaciens strains containing binary vectors and could simplify the creation of transgenic plants.

Characterization of a tobacco Bright Yellow 2 cell line expressing the tetracycline repressor at a high level for strict regulation of transgene expression

Manipulating the expression of a transgene in transient and stable transformed cells is a requirement for many functional analyses. We have investigated the use of the tetracycline-dependent gene expression system developed by Gatz et al. (1992) in tobacco (Nicotiana tabacum L. cv Bright Yellow 2 [BY2]) cells, the most widely used plant cell culture. We have selected a BY2 cell line, named BY2-tetracycline repressor (tetR) 17, which expresses the tetR at a high level, and have evaluated the capacity of this cell line to suppress the expression of a green fluorescent protein reporter gene under the control of the "Triple-Op" promoter in the absence of tetracycline in a large number of independent transformants. The ability to induce the expression of green fluorescent protein after treatment by anhydrotetracycline in the same transformants was also analyzed. BY2-tetR17 cells were demonstrated to be excellent recipient cells for recovery of clonal cell lines with a highly controlled regulation of the introduced transgene.

The restoration of fertility in male sterile tobacco demonstrates that transgene silencing can be mediated by T-DNA that has no DNA homology to the silenced transgene

Male sterile tobacco plants expressing a pathogenesis-related (PR) beta-1,3-glucanase gene driven by the Arabidopsis thaliana A3 or A9 tapetum-specific promoter, were partially restored to fertility by retransformation with a range of pA9-driven sense and antisense PR glucanase fragments. The restored plants exhibited improved seed set. PR glucanase protein was undetectable in the anthers of these plants and there was an associated increase in microsporocyte callose, the structural target of the A3 and A9-driven PR glucanase. This phenotype was not solely dependent on interactions between sense and antisense PR glucanase transcripts since a pA9-driven restorer was also capable of down regulating a pA3-GUS construct in the absence of extensive promoter, coding region, or terminator sequence homology. Since the A3 and A9 promoters have similar temporal and spatial expression patterns, it is possible that trans-acting factors common to both promoters become limiting in the PR glucanase double transformants resulting in improved levels of fertility. An alternative hypothesis is that additional sequences present in both the silencing and target T-DNAs can mediate the silencing of adjacent non-homologous transgenes.

Efficiency of physical (light) or chemical (ABA, tetracycline, CuSO4 or 2-CBSU)-stimulus-dependent gus gene expression in tobacco cell suspensions

In this study, the efficiency of inducible promoters to switch on gene expression in the presence of inducer or to switch it off in its absence was evaluated in tobacco cell suspensions transformed with the gus gene coding sequence. Either plant (pats1A, pSalT, pIn2-2) or microbial (pMre, pTet) inducible promoters were used to drive gus expression. The inducers were light, abscisic acid, 2-CBSU, CuSO4, tetracycline, respectively. For each construct (inducible promoter-gus coding sequence), the optimal induction conditions were determined (inducer concentration, induction time, and age of cells in culture cycle before induction). The efficiency of the inducible promoter was then evaluated under optimal induction conditions. GUS-expression levels obtained under non-inducing and inducing conditions were systematically compared. Thirty or forty percent of the clones transformed with the pSalT-gus or pTet-gus construct, respectively, showed high induction rates (>1000) and GUS activities of the same order as those obtained with a constitutive system. However, basal GUS levels were always high for the pTet-gus cell lines. Seventy or eighty-five percent of the cell lines transformed with the pMre-gus or pln2-2-gus construct, respectively, had induction rates of 1.5 to 1000. The pats1A-gus construct gave very low induction rates-55% of cell lines had induction rates less than 1.5. Only the pSalt-gus construct gave both the highest induction rates and basal GUS-levels equivalent to the endogenous GUS background.

A 13-bp cis-regulatory element in the LTR promoter of the tobacco retrotransposon Tto1 is involved in responsiveness to tissue culture, wounding, methyl jasmonate and fungal elicitors

The tobacco Tto1 is one of the few active LTR-retrotransposons of plants, and its transposition is activated by tissue culture and is primarily regulated at the transcriptional level. The expression of Tto1 RNA can also be activated by various stresses, including viral infection, wounding, and treatment with jasmonate, a signal molecule of plant defence responses. It is shown here that the Tto1 LTR promoter is responsible for a high level of expression in cultured tissues of transgenic tobacco plants. We demonstrate that a 13-bp repeated motif (TGGTAGGTGAGAT) in the LTR functions as a cis-regulatory element, which confers the responsiveness to tissue culture, wounding and methyl jasmonate. Fungal elicitors also activate the promoter containing multiple copies of the 13-bp motif. Expression mediated by the 13-bp motif is activated markedly by okadaic acid and moderately by K252a, so that both phosphorylation and dephosphorylation of proteins are possibly involved in the signalling pathways. Interestingly, the 13-bp motif contains a conserved motif, Box L (also called AC-I or H-box like sequence) which has been shown to be involved in the expression of phenylpropanoid synthetic genes. Moreover, extended homologies are found between promoters of Tto1 and an asparagus defence gene, AoPR1, suggesting a possibility that the ancient insertion of an ancestral Tto1-related retrotransposon has provided some of the promoter/regulatory sequences, including the 13-bp motif-related sequence, of the AoPR1 gene. Based on the structural and functional similarity between the two promoters, a possible evolutionary role of the regulatory sequences of LTR-retrotransposons is discussed.