Coaxing bacterial genes to replace a notorious slowpoke of an enzyme in tobacco plants could be a step toward raising yields in food crops.
Biologists have grumbled for decades about the clunky pace and wasteful mistakes of the enzyme nicknamed Rubisco (for D-ribulose-1,5-bisphosphate carboxylase/oxygenase). A version of the enzyme orchestrates a key step in capturing carbon dioxide from the air for photosynthetic organisms from pond scum to redwoods. A large group of green plants, including soybeans, rice and wheat, has some of the least efficient Rubisco of all, ultimately limiting their productivity.
A way to rev up Rubisco may be a step nearer, thanks to gene replacement engineered by researchers at Cornell University and Rothamsted Research in Harpenden, England. Genes for a peppier Rubisco, borrowed from a cyanobacterium, created working enzymes in laboratory tobacco, the researchers report September 17 in Nature. Tobacco served as a botanical lab rat, but researchers hope what they are learning will someday make food plants more efficient.
More work needs to be done before researchers can make plants thrive with borrowed Rubisco genes, says coauthor Maureen Hanson, a molecular geneticist at Cornell. Still, she’s pleased with demonstrating that the transplanted genes work. “If you can’t get this to work,” she says, “you have to give up on the whole project.”
This work looks “highly significant,” says Dean Price, whose lab at Australian National University in Canberra also explores ways to boost photosynthesis. Putting another species’ Rubisco genes into a plant isn’t new, but this time, researchers persuaded genes from cyanobacteria to make useful quantities of the enzyme.
If researchers can push plants such as soybeans or wheat to photosynthesize as efficiently as cyanobacteria do, crop yields might jump 36 to 60 percent, says Stephen Long of the University of Illinois at Urbana-Champaign. Trends suggest the world will need to double supplies of rice, wheat and soy by 2050 to feed the booming population, he says.
The Rubisco enzyme is “slow and confused,” says plant biochemist Spencer Whitney, also at Australian National University. It can capture either carbon dioxide for photosynthesis, or oxygen, which short-circuits the usual energy capture and creates compounds the cell has to clean up. Rubisco’s early forms probably arose more than 3 billion years ago when CO2 dominated Earth’s atmosphere. But in today’s oxygen-rich atmosphere, it easily grabs wasteful O2 instead.
Cyanobacteria minimize such waste by creating their own CO2 world. They encase their Rubisco in very tiny compartments where CO2 concentrates. There, with minimal oxygen temptation, the risk of mistakes is low. So cyanobacteria can use Rubisco forms that aren’t particularly discriminating but churn out their products fast.
Getting Rubisco genes to work in a new species poses quirky challenges. For instance, the eight large subunits of the Rubisco enzyme are normally encoded by genes in the chloroplast, but genes for the enzyme’s eight small subunits lie in the cell nucleus.
Molecular biologist Myat Lin in Hanson’s lab coped by putting the cyanobacterial genes for both kinds of subunits plus some helpers into the tobacco chloroplasts. Chloroplasts have so many copies of some genes, around 2,500 instead of typically fewer than 10 in the cell nucleus, that cells produce chloroplast proteins in great abundance.
The transfer worked well enough for cyanobacterial Rubisco to sustain photosynthesis on its own in the tobacco plants. And the transferred version captured more carbon per unit of enzyme than the plants’ usual Rubisco. However, the modified tobacco plants don’t have genes for CO2-concentrating compartments. Even when the researchers kept the plants in chambers with 22 times the CO2 of normal air, the tobacco plants still didn’t grow fast. For tobacco to take advantage of its new enzyme, researchers hope to add compartments, Hanson says.
There are certainly other strategies for upgrading Rubisco, notes Howard Griffiths of the University of Cambridge. His lab is trying to cajole a plant version of the enzyme — with superior discrimination for CO2 over O2 — to function in loose aggregations that might provide some of the benefits of compartments.