Induction of telomere-mediated chromosomal truncation and behavior of truncated chromosomes in Brassica napus

Engineered minichromosomes could be stably inherited and serve as a platform for simultaneously transferring and stably expressing multiple genes. Chromosomal truncation mediated by repeats of telomeric sequences is a promising approach for the generation of minichromosomes. In the present work, direct repetitive sequences of Arabidopsis telomere were used to study telomere-mediated truncation of chromosomes in Brassica napus. Transgenes containing alien Arabidopsis telomere were successfully obtained, and Southern blotting and fluorescence in situ hybridization (FISH) results show that the transgenes resulted in successful chromosomal truncation in B. napus. In addition, truncated chromosomes were inherited at rates lower than that predicted by Mendelian rules. To determine the potential manipulations and applications of the engineered chromosomes, such as the stacking of multiple transgenes and the Cre/lox and FRT/FLP recombination systems, both amenable to genetic manipulations through site-specific recombination in somatic cells, were tested for their ability to undergo recombination in B. napus. These results demonstrate that alien Arabidopsis telomere is able to mediate chromosomal truncation in B. napus. This technology would be feasible for chromosomal engineering and for studies on chromosome structure and function in B. napus.

Engineering complex metabolic pathways in plants

Metabolic engineering can be used to modulate endogenous metabolic pathways in plants or introduce new metabolic capabilities in order to increase the production of a desirable compound or reduce the accumulation of an undesirable one. In practice, there are several major challenges that need to be overcome, such as gaining enough knowledge about the endogenous pathways to understand the best intervention points, identifying and sourcing the most suitable metabolic genes, expressing those genes in such a way as to produce a functional enzyme in a heterologous background, and, finally, achieving the accumulation of target compounds without harming the host plant. This article discusses the strategies that have been developed to engineer complex metabolic pathways in plants, focusing on recent technological developments that allow the most significant bottlenecks to be overcome.

Construction of rice mini-chromosomes by telomere-mediated chromosomal truncation

Telomere truncation has been shown to be an efficient technology for the creation of mini-chromosomes that can be used as artificial chromosome platforms for genetic engineering. Artificial chromosome-based genetic engineering is considered to be superior to the existing techniques of randomized gene integration by Agrobacterium or biolistic-mediated genetic transformation. It organizes multiple transgenes as a unique genetic linkage block for subsequent manipulations in breeding. Telomere truncation technology relies on three components: the telomere sequence that mediates chromosomal truncation, a selection marker that allows the selection of transgenic events, and a site-specific recombination system that can be used to accept future genes into the mini-chromosome by gene targeting. These elements are usually pre-assembled before transformation, a process that is both time and labor consuming. We found in this research that the three elements could be mixed to transform plant cells in a biolistic transformation, and produced efficient chromosomal truncations and mini-chromosomes in rice. This system will allow rapid construction of mini-chromosomes with a flexible selection of resistant markers, site-specific recombination systems and other desirable elements. In addition, a rice telotrisomic line was used as the starting material for chromosomal truncations. Mini-chromosomes from the truncations of both the telocentric chromosome and other chromosomes were recovered. The mini-chromosomes remained stable during 2 years of subculture. The construction of mini-chromosomes in rice, an economically important crop, will provide a platform for future artificial chromosome-based genetic engineering of rice for stacking multiple genes.

Recovery of a telomere-truncated chromosome via a compensating translocation in maize

Maize-engineered minichromosomes are easily recovered from telomere-truncated B chromosomes but are rarely recovered from A chromosomes. B chromosomes lack known genes, and their truncation products are tolerated and transmitted during meiosis. In contrast, deficiency gametes resulting from truncated A chromosomes prevent their transmission. We report here a de novo compensating translocation that permitted recovery of a large truncation of chromosome 1 in maize. The truncation (trunc-1) and translocation with chromosome 6 (super-6) occurred during telomere-mediated truncation experiments and were characterized using single-gene fluorescent in situ hybridization (FISH) probes. The truncation contained a transgene signal near the end of the broken chromosome and transmitted together with the compensating translocation as a heterozygote to approximately 41%-55% of progeny. Transmission as an addition chromosome occurred in ~15% of progeny. Neither chromosome transmitted through pollen. Transgene expression (Bar) cosegregated with trunc-1 transcriptionally and phenotypically. Meiosis in T1 plants revealed eight bivalents and one tetravalent chain composed of chromosome 1, trunc-1, chromosome 6, and super-6 in diplotene and diakinesis. Our data suggest that de novo compensating translocations allow recovery of truncated A chromosomes by compensating deficiency in female gametes and by affecting chromosome pairing and segregation. The truncated chromosome can be maintained as an extra chromosome or together with the super-6 as a heterozygote.

Best practice procedures for the establishment of a C(4) cycle in transgenic C(3) plants

C(4) plants established a mechanism for the concentration of CO(2) in the vicinity of ribulose-1,5-bisphosphate carboxylase/oxygenase in order to saturate the enzyme with substrate and substantially to reduce the alternative fixation of O(2) that results in energy losses. Transfer of the C(4) mechanism to C(3) plants has been repeatedly tested, but none of the approaches so far resulted in transgenic plants with enhanced photosynthesis or growth. Instead, often deleterious effects were observed. A true C(4) cycle requires the co-ordinated activity of multiple enzymes in different cell types and in response to diverse environmental and metabolic stimuli. This review summarizes our current knowledge about the most appropriate regulatory elements and coding sequences for the establishment of C(4) protein activities in C(3) plants. In addition, technological breakthroughs for the efficient transfer of the numerous genes probably required to transform a C(3) plant into a C(4) plant will be discussed.

Towards the development of better crops by genetic transformation using engineered plant chromosomes

Plant Biotechnology involves manipulation of genetic material to develop better crops. Keeping in view the challenges being faced by humanity in terms of shortage of food and other resources, we need to continuously upgrade the genomic technologies and fine tune the existing methods. For efficient genetic transformation, Agrobacterium-mediated as well as direct delivery methods have been used successfully. However, these methods suffer from many disadvantages especially in terms of transfer of large genes, gene complexes and gene silencing. To overcome these problems, recently, some efforts have been made to develop genetic transformation systems based on engineered plant chromosomes called minichromosomes or plant artificial chromosomes. Two approaches namely, "top-down" or "bottom-up" have been used for minichromosomes. The former involves engineering of the existing chromosomes within a cell and the latter de novo assembling of chromosomes from the basic constituents. While some success has been achieved using these chromosomes as vectors for genetic transformation in maize, however, more studies are needed to extend this technology to crop plants. The present review attempts to trace the genesis of minichromosomes and discusses their potential of development into plant artificial chromosome vectors. The use of these vectors in genetic transformation will greatly ameliorate the food problem and help to achieve the UN Millennium development goals.

Engineered plant minichromosomes

The advent of transgenic technologies has met many challenges, both technical and political; however, these technologies are now widely applied, particularly for crop improvement. Bioengineering has resulted in plants carrying resistance to herbicides, insects, and viruses, as well as entire biosynthetic pathways. Some of the technical challenges in generating transgenic plant or animal materials include: an inability to control the location and nature of the integration of transgenic DNA into the host genome, and linkage of transformed genes to selectable antibiotic resistance genes used in the production of the transgene cassette. Furthermore, successive transformation of multiple genes may require the use of several selection genes. The coordinated expression of multiple stacked genes would be required for complex biosynthetic pathways or combined traits. Engineered nonintegrating minichromosomes can overcome many of these problems and hold much promise as key players in the next generation of transgenic technologies for improved crop plants. In this review, we discuss the history of artificial chromosome technology with an emphasis on engineered plant minichromosomes.

Plant B chromosomes

B chromosomes are dispensable elements of the genome that do not recombine with the A chromosomes of the regular complement and that follow their own evolutionary pathway. Here, we survey current knowledge on the DNA/chromatin composition, origin, and drive mechanisms of B chromosomes and discuss the potential research applications of supernumerary chromosomes.

Testing for genetically modified organisms (GMOs): Past, present and future perspectives

This paper presents an overview of GMO testing methodologies and how these have evolved and may evolve in the next decade. Challenges and limitations for the application of the test methods as well as to the interpretation of results produced with the methods are highlighted and discussed, bearing in mind the various interests and competences of the involved stakeholders. To better understand the suitability and limitations of detection methodologies the evolution of transformation processes for creation of GMOs is briefly reviewed.