The joys of being single

With the latest genetic techniques, scientists can make ever more extensive changes to an organism’s genome, including introducing whole new metabolic pathways to confer entirely new abilities. For biofuels, these techniques have been used to develop plants that produce higher yields of sugars and oil, and microbes that can convert plant material into a whole range of fuels and useful compounds.

But more extensive changes require more complex processes, which often involve building up new metabolic pathways using genes from several different organisms. They also increase the risk that the change will have various unforeseen and potentially detrimental effects on the organism. And such extensive changes are not always required, as recent research has shown that impressive results can sometimes be achieved by simply switching off a single gene.

Last year, in a paper in the Plant Biotechnology Journal, a team of Chinese scientists led by Xiao-Li Tan at Jiangsu University showed that switching off a gene called DA1 in rapeseed (Brassica napus) caused it to produce greater numbers of larger seeds, and thus more oil. DA1 codes for a receptor for a specific regulator protein. Tan and his team silenced it by introducing a modified version of DA1 that produced a non-working version of the receptor into rapeseed and then overexpressing this modified version so that it outcompeted the normal version.

This simple change caused the resultant rapeseed plants to produce 13% more seeds on average, with each seed weighing over 20% more on average. It also caused the plants to produce larger leaves, flowers and fruits, without seeming to have any detrimental effects.

Now, in a paper in the New Phytologist, a team of scientists from the UK, the US and Brazil, co-led by Rowan Mitchell at Rothamsted Research in the UK, show that switching off a gene called BAHD01 in green foxtail (Setaria viridis) makes its sugars much easier to release. They chose green foxtail for this work because it’s a model species of grass, meaning their findings should apply to other grass species. These include important biofuel feedstocks such as the cereals maize and wheat and the energy crops switchgrass and miscanthus.

BAHD01 codes for a protein involved in constructing plant cell walls in grasses by binding sugar molecules together with a component of lignin known as ferulate. Several BAHD genes are known to be involved in this process, and so offer intriguing targets for making plant cell walls easier to break down for biofuel production. But when scientists tried doing this, by switching off some of these genes in rice plants, the results were not as impressive as they’d hoped for. The reductions in ferulate concentrations varied widely between plants and tissues, with a maximum reduction of just 27% in the stems of a single line of plants.

Mitchell and his team wondered whether the problem was switching off so many genes at once, which likely had various untoward effects. So, instead, they decided to switch off just a single gene, with previous research indicating that BAHD01 might prove most effective. To do this, they utilized a technique known as RNA interference, which blocks the RNA strand the carries the coding information from a gene to the cell’s protein construction machinery.

When they did this, it reduced the concentration of ferulate in the stems of S. viridis by 60%, without any obvious detrimental effects on the grass, causing 40–60% more sugars to be released when the grass was digested in a weak acid solution. However, when they switched BAHD01 off in a different model grass species (Brachypodium distachyon), the results were far less impressive, indicating that the most effective mix of BAHD genes to switch off may may vary between grasses.

Nevertheless, these two studies amply demonstrate that much can be achieved by switching off single genes, and with the rise of the CRISPR-Cas9 genetic editing technique, scientists now have an accurate and effective way to do that (see Biofuels could benefit from CRISPR editing). Expect lots more single genes to be switched off in the future.

The views represented here are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd. or of the SCI.