Inducible excision of selectable marker gene from transgenic plants by the cre/lox site-specific recombination system

A Cre/loxP-mediated self-activating gene excision system to produce marker gene free transgenic soybean plants

Marker-gene-free transgenic soybean plants were produced by isolating a developmentally regulated embryo-specific gene promoter, app1, from Arabidopsis and developing a self-activating gene excision system using the P1 bacteriophage Cre/loxP recombination system. To accomplish this, the Cre recombinase gene was placed under control of the app1 promoter and, together with a selectable marker gene (hygromycin phosphotransferase), were cloned between two loxP recombination sites. This entire sequence was then placed between a constitutive promoter and a coding region for either beta-glucuronidase (Gus) or glyphosate acetyltransferase (Gat). Gene excision would remove the entire sequence between the two loxP sites and bring the coding region to the constitutive promoter for expression. Using this system marker gene excision occurred in over 30% of the stable transgenic events as indicated by the activation of the gus reporter gene or the gat gene in separate experiments. Transgenic plants with 1 or 2 copies of a functional excision-activated gat transgene and without any marker gene were obtained in T0 or T1 generation. This demonstrates the feasibility of using developmentally controlled promoters to mediate marker excision in soybean.

Evaluation of CRE-mediated excision approaches in Arabidopsis thaliana

The ability of the CRE recombinase to catalyze excision of a DNA fragment flanked by directly repeated lox sites has been exploited to modify gene expression and proved to function well in particular case studies. However, very often variability in CRE expression and differences in efficiency of CRE-mediated recombination are observed. Here, various approaches were investigated to reproducibly obtain optimal CRE activity. CRE recombination was analyzed either by transforming the CRE T-DNA into plants containing a lox-flanked fragment or by transforming a T-DNA harboring a lox-flanked fragment into plants producing the CRE recombinase. Although somatic CRE-mediated excision of a lox-flanked fragment was obtained in all transformants, a variable amount of germline-transmitted deletions was found among different independent transformants, irrespective of the orientation of transformation. Also, the efficiency of CRE-mediated excision correlated well with the CRE mRNA level. In addition, CRE-mediated fragment excision was compared after floral dip and after root tissue transformation when transforming in a CRE-expressing background. Importantly, less CRE activity was needed to excise the lox-flanked fragment from the transferred T-DNA after root tissue transformation than after floral dip transformation. We hypothesize that this is correlated with the lower T-DNA copy number inserted during root transformation as compared to floral dip transformation.

GM maize from site-specific recombination technology, what next?

The term plant genetic engineering has long conveyed a highly efficient and precise process for the manipulation of plant genomes. For nearly two decades, research on recombinase-based applications has steadily advanced the surgical capabilities of plant genome rearrangements. Once considered interesting laboratory exercises, a first crop plant derived from this type of DNA acrobatics is heading to market. Originally configured for a specific application, to remove a selectable marker, it could be the first of more to come - and not just market-free plants.

A passage through in vitro culture leads to efficient production of marker-free transgenic plants in Brassica juncea using the Cre-loxP system

The Cre-loxP site-specific recombination system was deployed for removal of marker genes from Brassica juncea (Indian mustard). Excision frequencies, monitored by removal of nptII or gfp genes in F(1) plants of crosses between LOX and CRE lines, were high in quiescent, differentiated somatic tissues but extremely poor in the meristematic regions (and consequently the germinal cells) thus preventing identification and selection of marker-free transgenic events which are devoid of both the marker gene as well as the cre gene, in F(2) progeny. We show that a passage through in vitro culture of F(1 )leaf explants allows efficient development of marker-free transgenics in the F(2) generation addressing current limitations associated with efficient use of the Cre/loxP technology for marker gene removal.

Kanamycin resistance in plants: an unexpected trait controlled by a potentially multifaceted gene

Ayalew Mentewab and C. Neal Stewart Jr recently showed that an Arabidopsis kanamycin resistance gene encodes an ATP binding cassette (ABC) transporter. This Atwbc19 protein is hypothesized to prevent ribosome inactivation by translocating kanamycin into the vacuole. Because ABC transporters often recognize multiple exogenous substrates, overexpression of Atwbc19 can result in the accumulation of unexpected compounds in transgenic plants. Another potential safety issue associated with this gene is horizontal gene transfer. Thus, commercial applications are likely to be limited to methods that allow removal of the selectable marker from the transgenic plant genome.

Chemical-induced autoexcision of selectable markers in elite tomato plants transformed with a gene conferring resistance to lepidopteran insects

Marker-free transgenic tomato plants harboring a synthetic Bacillus thuringiensis endotoxin gene, cryIAc, were obtained by using a chemically regulated, Cre/loxP-mediated site-specific DNA recombination system, in which the selectable marker neomycin phosphotransferase gene flanked by two directly oriented loxP sites was located between the cauliflower mosaic virus 35S promoter and a promoterless cryIAc. Upon induction by 2 microM beta-estradiol, sequences encoding the selectable marker and cre sandwiched by two loxP sites were excised from the tomato genome, leading to activation of the downstream endotoxin gene cryIAc with high expression levels as shown by Northern blot and ELISA assay (250-790 ng g(-1) fresh wt) in T(1) generation. For transgenic line with single transgenic loci, 15% of T(1) progenies were revealed marker-free. This autoexcision strategy provides an effective approach to eliminate a selectable marker gene from transgenic tomato, thus expediting the public acceptance of genetically modified crop.

Genetic engineering of wheat--current challenges and opportunities

Wheat is one of the major staple food crops grown worldwide; however, productivity in cereal crops has not kept pace with the world population growth. A significant increase in wheat production (>40% by 2020) is needed simply to keep up with the growing demand. This increase is unlikely to be achieved by conventional plant breeding methods because of the limited gene pool available. The application of recombinant techniques to improve wheat quality and yield is not only desirable but also has potential to open up new opportunities. Although there has been significant progress in developing gene-transformation technologies for improving these traits, this remains an important challenge for plant biotechnology. Obstacles to translate the full potential of the genomic era to wheat breeding include the need to develop elite wheat varieties without selectable markers, introducing minimal or nil intergenic DNA and social and market issues concerning genetically engineered food products.