Expression of complete metabolic pathways in transgenic plants

Balanced gene losses, duplications and intensive rearrangements led to an unusual regularly sized genome in Arbutus unedo chloroplasts

Completely sequenced plastomes provide a valuable source of information about the duplication, loss, and transfer events of chloroplast genes and phylogenetic data for resolving relationships among major groups of plants. Moreover, they can also be useful for exploiting chloroplast genetic engineering technology. Ericales account for approximately six per cent of eudicot diversity with 11,545 species from which only three complete plastome sequences are currently available. With the aim of increasing the number of ericalean complete plastome sequences, and to open new perspectives in understanding Mediterranean plant adaptations, a genomic study on the basis of the complete chloroplast genome sequencing of Arbutus unedo and an updated phylogenomic analysis of Asteridae was implemented. The chloroplast genome of A. unedo shows extensive rearrangements but a medium size (150,897 nt) in comparison to most of angiosperms. A number of remarkable distinct features characterize the plastome of A. unedo: five-fold dismissing of the SSC region in relation to most angiosperms; complete loss or pseudogenization of a number of essential genes; duplication of the ndhH-D operon and its location within the two IRs; presence of large tandem repeats located near highly re-arranged regions and pseudogenes. All these features outline the primary evolutionary split between Ericaceae and other ericalean families. The newly sequenced plastome of A. unedo with the available asterid sequences allowed the resolution of some uncertainties in previous phylogenies of Asteridae.

Creation and validation of a widely applicable multiple gene transfer vector system for stable transformation in plant

Multiple gene transfer (MGT) technology has become a powerful tool for basic and applied plant biology research in recent years. Despite some notable successes in obtaining plant lines harbouring multiple transgenes, these methods are still generally unwieldy and costly. We report here a straightforward and cost effective strategy, utilizing commonly available restriction enzymes for the transfer of multiple genes into plants, hence greatly widening the accessibility of MGT. This methodology exploits the specific 'nested' arrangement of a pair of isocaudomer restriction enzymes (for example XbaI-AvrII-XbaI) so that through the alternate use of these two enzymes in a reiterative fashion multiple genes/constructs (up to five in this study) could be 'stacked' together with ease. In a proof-of-concept experiment, we constructed a plant transformation vector containing three reporter gene expression cassettes flanked by two matrix attachment region sequences. The expression of all three genes was confirmed in transgenic Arabidopsis thaliana. The usefulness of this technology was further validated by the construction of a plant transformation vector containing five transgenes for the production of eicosapentaenoic acid (EPA, C20∆⁵,⁸,¹¹,¹⁴,¹⁷), a polyunsaturated essential fatty acid found in fish oils that is beneficial for health. In addition, we constructed four more vectors, incorporating one seed specific and three promoters conferring constitutive expression. These expression cassettes are flanked by a different isocaudomer pair (AvrII-SpeI-AvrII) and four other unique restriction sites, allowing the exchange of promoters and terminators of choice.

Metabolic engineering of plants for artemisinin synthesis

Artemisinin, a natural compound from Artemisia annua, is highly effective in treating drug-resistant malaria. Because chemical synthesis of this natural terpenoid is not economically feasible, its only source remains as the native plant which produces only small quantities of it, resulting in a supply that is far short of demand. Extensive efforts have been invested in metabolic engineering for the biosynthesis of artemisinin precursors in microbes. However, the production of artemisinin itself has only been achieved in plants. Since, A. annua possesses only poorly developed genetic resources for traditional breeders, molecular breeding is the best alternative. In this review, we describe the efforts taken to enhance artemisinin production in A. annua via transgenesis and advocate metabolic engineering of the complete functional artemisinin metabolic pathway in heterologous plants. In both cases, we emphasize the need to apply state-of-the-art synthetic biology approaches to ensure successful biosynthesis of the drug.