One of the great ironies of evolution is that almost all known life depends on one of the slowest and most inefficient enzymes on Earth. Cyanobacteria have evolved a better mechanism to deal with the slow rate at which the Rubisco enzyme processes carbon dioxide than plants. Now scientists have taken the largest step towards transferring this work-around into a plant. If the process can be completed, crop yields may increase by as much as 60 percent while fertilizer and water demands decline, making it the single largest advance in crop efficiency ever contemplated.
Both plants and cyanobacteria use Rubisco to photosynthesize carbon dioxide into sugars, which can then be converted into all the other components necessary for life. Cyanobacteria have a carbon dioxide concentration mechanism to speed up the process, and Dr Ben Long of the Australian National University is working to give plants the same capacity.
Long told IFLScience each Rubisco molecule can only process around three carbon dioxide molecules a second, while many other enzymes induce hundreds of chemical reactions in the same time. Besides being painfully slow, Rubisco often fails to differentiate CO2 from oxygen, wasting precious time and energy on the wrong molecule.
Plants have responded to this inefficiency by producing enormous amounts of Rubisco. Long told IFLScience, “In a typical leaf of a plant half the protein can be Rubisco.” Plants also respond by leaving their gas-exchanging stoma wide open, drawing in as much carbon dioxide as possible, at the cost of losing a lot of water.
Thirty-five million years ago the development of the C4 pathway gave one group of plants a slightly improved mechanism for dealing with this inefficiency, but long before that, single-celled organisms had created something much better. “Cyanobacteria use what's called a 'CO2 concentrating mechanism' to deliver large amounts of the gas into their carboxysomes, where their Rubisco is encapsulated," Long said in a statement.
After thousands of years of careful breeding of crops for greater yield, accelerated in recent decades through our knowledge of plant genetics, we are starting to run out of ways to increase crop efficiency. For many plants, yield improvements are diminishing, sometimes almost to zero.
The Realizing Increased Photosynthetic Efficiency (RIPE) project, which Long is a part of, has recognized that photosynthesis inefficiency is the primary remaining limiting factor for crops. Long told IFLScience RIPE considered a long list of changes that could be made to food plants, such as transferring the C4 mechanism into rice. Introducing cyanobacteria’s concentrating mechanism into crops was considered the hardest thing to accomplish, because so many steps were involved, but also the biggest payoff if achieved.
“It’s our moonshot,” Long told IFLScience. Philanthropic organizations and governments provided funding to reflect this. In Nature Communications, Long has announced the first major step, inserting the genes for a simplified carboxysome into tobacco plants. These plants achieved the same photosynthesis as wild relatives, with far less Rubisco. Long likens this to building the Saturn 4 rocket to take humans into space.
Tobacco plants were chosen because they are well-understood and easy-to-modify model organisms, into which more limited cyanobacteria photosynthesis gene transfers have been achieved. Making the shift to food crops is among the smaller remaining obstacles. More challenging will be to adapt cyanobacteria’s mechanism for concentrating the carbon dioxide inside cells. Even then, Long says, some “tinkering” will be needed to make all the parts work together.
Modeling suggests full transfer could increase crop yields by 60 percent, if Rubisco concentration is maintained. If plants responded by producing less Rubisco, they would need less nitrogen fertilization. Fewer Rubisco molecules would also dramatically cut plants’ water consumption.