Gene Silencing in Plants: Transgenes as Targets and Effectors. In: Kempken F, Jung C, editors. Genetic Modification of Plants. Berlin: Springer Berlin Heidelberg; 2010. pp. 79-101. [DOI: 10.1007/978-3-642-02391-0_5]

Targeted insertion of large DNA sequences by homology-directed repair or non-homologous end joining in engineered tobacco BY-2 cells using designed zinc finger nucleases

Targeted integration of recombinant DNA fragments into plant genomes by DNA double-strand break (DSB) repair mechanisms has become a powerful tool for precision engineering of crops. However, many targeting platforms require the screening of many transgenic events to identify a low number of targeted events among many more random insertion events. We developed an engineered transgene integration platform (ETIP) that uses incomplete marker genes at the insertion site to enable rapid phenotypic screening and recovery of targeted events upon functional reconstitution of the marker genes. The two marker genes, encoding neomycin phosphotransferase II (nptII) and Discosoma sp. red fluorescent protein (DsRed) enable event selection on kanamycin-containing selective medium and subsequent screening for red fluorescent clones. The ETIP design allows targeted integration of donor DNA molecules either by homology-directed repair (HDR) or non-homologous end joining (NHEJ)-mediated mechanisms. Targeted donor DNA integration is facilitated by zinc finger nucleases (ZFN). The ETIP cassette was introduced into Nicotiana tabacum BY-2 suspension cells to generate target cell lines containing a single copy locus of the transgene construct. The utility of the ETIP platform has been demonstrated by targeting DNA constructs containing up to 25-kb payload. The success rate for clean targeted DNA integration was up to 21% for HDR and up to 41% for NHEJ based on the total number of calli analyzed by next-generation sequencing (NGS). The rapid generation of targeted events with large DNA constructs expands the utility of the nuclease-mediated gene addition platform both for academia and the commercial sector.

Development of glyphosate-resistant alfalfa (Medicago sativa L.) upon transformation with the GR79Ms gene encoding 5-enolpyruvylshikimate-3-phosphate synthase

MAIN CONCLUSION

The glyphosate-resistant gene, GR79Ms, was successfully introduced into the genome of alfalfa. The transgenic events may serve as novel germplasm resources in alfalfa breeding. Weed competition can reduce the alfalfa yield, generating new alfalfa germplasm with herbicide resistance is essential. To obtain transgenic alfalfa lines with glyphosate resistance, a new synthetic glyphosate-resistant gene GR79Ms encoding 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) was introduced into alfalfa germplasm by Agrobacterium tumefaciens-mediated transformation. In total, 67 transformants were obtained. PCR and Southern blot analyses confirmed that GR79Ms was successfully inserted into the genome of alfalfa. Reverse transcription-PCR and western blot analyses further demonstrated the expression of GR79Ms and its product, GR79Ms EPSPS. Moreover, two homozygous transgenic lines were developed in the T2 generation by means of molecular-assisted selection. Herbicide tolerance spray tests showed that the transgenic plants T0-GR1, T0-GR2, T0-GR3 and two homozygous lines were able to tolerate fourfold higher commercial usage of glyphosate than non-transgenic plants.

Characterization of the stress-inducible OsNCED3 promoter in different transgenic rice organs and over three homozygous generations

To be effective in crop biotechnology applications, gene promoters need to be stably active over sequential generations in a population of single-copy transgenic lines. Most of the stress-inducible promoters characterized in plants thus far have been analyzed at early (T₀, T₁ or T₂) generations and/or by testing only a small number of transgenic lines. In our current study, we report our analysis of OsNCED3, a stress-inducible rice promoter involved in ABA biosynthesis, in various organs and tissues of transgenic rice plants over the T(2-4) homozygous generations. The transgene copy numbers in the lines harboring the OsNCED3:gfp construct were determined and six single- and two double-copy transgenic lines were analyzed for promoter activity in comparison with the Wsi18, a stress-inducible promoter previously characterized. The exogenous promoter activities were found to be significantly enhanced in the roots and leaves, whereas zero or low levels of activity were evident in grains and flowers, under drought and high-salinity conditions. The highest induction levels of gfp transcripts in the OsNCED3:gfp plants upon drought treatments were 161- and 93-fold in leaves and roots, respectively, and these levels were comparable with those of gfp transcripts in the Wsi18:gfp plants. A comparison of the promoter activities between the T₂-T₄ plants revealed that comparable activity levels were maintained over these three homozygous generations with no evidence of silencing. Thus, our results provide the OsNCED3 promoter that is stress-inducible in a whole rice plant except for in the aleurones and endosperm and stably active over three generations.

Analysis of the APX, PGD1 and R1G1B constitutive gene promoters in various organs over three homozygous generations of transgenic rice plants

We have previously characterized the constitutively active promoters of the APX, PGD1 and R1G1B genes in rice (Park et al. 2010 in J Exp Bot 61:2459-2467). To have potential crop biotechnology applications, gene promoters must be stably active over many generations. In our current study, we report our further detailed analysis of the APX, PGD1 and R1G1B gene promoters in various organs and tissues of transgenic rice plants for three (T₃₋₅) homozygous generations. The copy numbers in 37 transgenic lines that harbor promoter:gfp constructs were determined and promoter activities were measured by real-time qPCR. Analysis of the 37 lines revealed that 15 contained a single copy of one of the three promoter:gfp chimeric constructs. The promoter activity levels were generally higher in multi-copy lines, whereas variations in these levels over the T₃₋₅ generations studied were observed to be smaller in single-copy than in multi-copy lines. The three promoters were further found to be highly active in the whole plant body at both the vegetative and reproductive stages of plant growth, with the exception of the APX in the ovary and R1G1B in the pistil and filaments where zero or very low levels of activity were detected. Of note, the spatial activities of the PGD1 promoter were found to be strikingly similar to those of the ZmUbi1, a widely used constitutive promoter. Our comparison of promoter activities between T₃, T₄ and T₅ plants revealed that the APX, PGD1 and R1G1B promoters maintained their activities at comparable levels in leaves and roots over three homozygous generations and are therefore potentially viable alternative promoters for crop biotechnology applications.