Production of selectable marker gene-free Cavendish banana (Musa spp.) using a steroid-inducible recombinase platform.

Genetic improvement of commercially accepted banana cultivars is strongly reliant on the ability to introduce genes that encode important agro-traits such as disease resistance. In most cases this can only be achieved using a transgenic approach. Public and regulatory acceptance of these events would greatly increase with "clean" single copy integration events free of the selectable marker gene and extraneous vector backbone. This would also allow for the successive addition of new genes and traits as they become available. In this study, we used the pMarker Free 1 (pMF1) vector containing the green fluorescent protein (gfp) reporter gene to assess the effectiveness of steroid-inducible recombination and positive/negative dual selection to regenerate transgenic Cavendish banana plants that were potentially free of the selectable marker gene. By examining the interaction of two different Agrobacterium strains with two different cultivars of Cavendish banana, namely Williams and Grand Naine, we describe a transformation and regeneration strategy that successfully produced marker-free, single transgene copy, gfp-expressing events. The system will provide a useful means of serially improving banana into the future.

Gene editing the phytoene desaturase alleles of Cavendish banana using CRISPR/Cas9.

Bananas are a staple food source and a major export commodity worldwide. The Cavendish dessert banana is a triploid AAA genome type and accounts for around 47% of global production. Being essentially sterile, genetic modification is perhaps the only pathway available to improve this cultivar. In this study, we used the CRISPR/Cas9 gene editing system to deliver a self-cleaving polycistronic guide RNA (gRNA) designed to target exon 1 of the Phytoene desaturase (PDS) gene in the Cavendish cultivar "Williams". Genotyping of 19 independent events showed a 100% PDS modification rate primarily in the form of insertions (1-105 nt) or deletions (1-55 nt) (indels) at the predicted cleavage site. Tri-allelic disruptive modifications were observed in 63% of plants and resulted in both albinism and dwarfing. Pale green (16%) and wildtype green (21%) phenotypes generally correlated with in-frame indels in at least one of the three PDS alleles. Editing efficiency was dependent on both target site selection and Cas9 abundance. This is the first report of a highly effective CRISPR/Cas9 modification system using a polycistronic gRNA in Cavendish banana. Such an editing platform will be of considerable utility for the development of disease resistance and novel agro-traits in this commercially important cultivar into the future.

Functional assessment of plant and microalgal lipid pathway genes in yeast to enhance microbial industrial oil production.

As promising alternatives to fossil-derived oils, microbial lipids are important as industrial feedstocks for biofuels and oleochemicals. Our broad aim is to increase lipid content in oleaginous yeast through expression of lipid accumulation genes and use Saccharomyces cerevisiae to functionally assess genes obtained from oil-producing plants and microalgae. Lipid accumulation genes DGAT (diacylglycerol acyltransferase), PDAT (phospholipid: diacylglycerol acyltransferase), and ROD1 (phosphatidylcholine: diacylglycerol choline-phosphotransferase) were separately expressed in yeast and lipid production measured by fluorescence, solvent extraction, thin layer chromatography, and gas chromatography (GC) of fatty acid methyl esters. Expression of DGAT1 from Arabidopsis thaliana effectively increased total fatty acids by 1.81-fold above control, and ROD1 led to increased unsaturated fatty acid content of yeast lipid. The functional assessment approach enabled the fast selection of candidate genes for metabolic engineering of yeast for production of lipid feedstocks.

Improved molecular tools for sugar cane biotechnology.

Sugar cane is a major source of food and fuel worldwide. Biotechnology has the potential to improve economically-important traits in sugar cane as well as diversify sugar cane beyond traditional applications such as sucrose production. High levels of transgene expression are key to the success of improving crops through biotechnology. Here we describe new molecular tools that both expand and improve gene expression capabilities in sugar cane. We have identified promoters that can be used to drive high levels of gene expression in the leaf and stem of transgenic sugar cane. One of these promoters, derived from the Cestrum yellow leaf curling virus, drives levels of constitutive transgene expression that are significantly higher than those achieved by the historical benchmark maize polyubiquitin-1 (Zm-Ubi1) promoter. A second promoter, the maize phosphonenolpyruvate carboxylate promoter, was found to be a strong, leaf-preferred promoter that enables levels of expression comparable to Zm-Ubi1 in this organ. Transgene expression was increased approximately 50-fold by gene modification, which included optimising the codon usage of the coding sequence to better suit sugar cane. We also describe a novel dual transcriptional enhancer that increased gene expression from different promoters, boosting expression from Zm-Ubi1 over eightfold. These molecular tools will be extremely valuable for the improvement of sugar cane through biotechnology.