Monday, September 22, 2014

Go Speed Rubisco, Go!

In addition to generating oxygen for us to breath, plants are also the main source of biologically useable carbon in our diets. The enzyme responsible for taking carbon dioxide and adding it to carbohydrate production is Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase). Rubisco is probably the most abundant protein on Earth, accounting for up to 50% of the protein in leaves. In this case, it is compensating for something... it has to be so prolific because Rubisco is very, very slow.

As Rubisco is the gate keeper for fixing carbon dioxide into edible carbon, researchers have long been interested in finding a way to speed it up which would ultimately increase crop yields. Scientists have been trying to find a way to engineer Speed Rubisco. Last week, a paper came out that described two lines of transgenic tobacco that had been outfitted with a faster Rubisco from a cyanobacteria (Lin et al., 2014).

First, I need to explain that the fact that they were able to switch the tobacco Rubisco genes with that of the cyanobacteria is fantastic. It might sound incredibly simple, after all we've been adding genes from one species into another for a long time, but, the gene for the active part of Rubisco is found in the chloroplast and plunking a gene into the genome of chloroplast is not easy.

Most of commonly held beliefs about genes are based upon nuclear genes. But that is not the only place where DNA is found in a plant cell. Plants have 3 locations where genetic material is located: the nucleus (purple), the mitochondria (orange) and the chloroplast (green). Targeting genes in the nucleus is easy, targeting to the chloroplast not so much.
Plant cell showing only genome containing organelles
For this study, they swapped out the native tobacco large subunit gene with a construct containing both the large and small units of Rubisco from the cyanobacteria, as well as, either the protein that helps Rubisco fold or a protein that supports Rubisco in the cyanobacteria. This resulted in functional cyanobacteria Rubisco being the only type of Rubisco. The original tobacco Rubisco was completely knocked out. And their transgenic lines had a higher rate of carbon fixation then normal tobacco!

You might be thinking BINGO! But this is far from the slam dunk to creating super crops. First, the cyanobacteria Rubisco, while faster, is much more sensitive to oxygen inhibition. To overcome this, the researchers grew their transformed tobacco under higher CO2 concentrations than are currently present in our atmosphere, in fact they grew their plants at more than 20x current atmospheric conditions. The cyanobacteria get around this problem by having special compartments, carboxysomes, where Rubisco and CO2 are concentrated together within the cell. One future solution to this problem would be to create carboxysomes within the tobacco.

Then even under the higher CO2, the transformed plants still grew slower than normal tobacco. Since the goal here is to create crops with higher yields, slowing the growth is not helpful. The cyanobacteria Rubisco was expressed at low levels compared to wild type (12-18% of leaf protein vs. ~50%) which might account for the slower growth. Increasing the amount of Rubisco might help bring the growth rate back up.

This is a really cool first step. But like everything in science, there are still a lot of things left to do!

Source: Lin, et al., 2014. A faster Rubisco with potential to increase photosynthesis in crops. Nature: Published online 17 Sept.

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