Plants are often thought of as the masters of photosynthesis, the process by which sunlight, carbon dioxide and water are converted into usable energy, but when it comes to efficiency, they are beaten out by a rather surprising rival: bacteria.
Plants use resources, such as minerals and water, to promote their growth, but they also are restrained by the enzymes they need to complete photosynthesis, particularly an enzyme commonly known as RuBisCo.
Both plants and bacteria rely on RuBisCo to fix, or transform, carbon dioxide in the initial stages of photosynthesis. Unfortunately, RuBisCo can also react with oxygen, creating an unusable molecule that the plant must spend further energy to recycle. The result wastes far more nutrients than the plants need, costing both resources and money, and places a theoretical limit on crop yields.
Recently, research teams from Cornell University and Rothamsted Research in the United Kingdom began looking for ways around this barrier. They selected genes from bacteria that have evolved a way to bypass this dilemma and inserted them into plant cells, hoping that the bacterial addition would bestow the same advantages onto plants and provide food crops a way to boost yields under the pressures imposed by climate change.
"If proved effective, this technology would decrease the amount of key nutrients like nitrogen and, most notably, water needed by the plant, while increasing the yield," said Lin Myat, a postdoctoral fellow of molecular biology and genetics at Cornell and lead on the study. Both nutrients are valuable additions to any crop plant, especially under the pressure of increasing droughts.
In some key food crops, such as wheat or rice, the unwanted RuBisCo reaction happens roughly one-quarter of the time. While some crop plants like corn have devised ways to reduce the likeliness of this wasteful reaction, they require additional energy to do so. With a growing population to feed and limited resources, finding new ways to avoid the RuBisCo problem without expending extra energy in crop plants has become an increasingly studied topic.
Cyanobacteria's survival skill helps tobacco
In the last decade, advances in crop yields have reached a stalemate. Most obvious ways to increase productivity had been exhausted or have proved harmful to the climate in excess, such as nitrogen fertilizers, forcing researchers to look for alternative methods (ClimateWire, June 11).
"We'd got to the point where we'd run the course on traditional methods for increasing crop yields about 10 years ago, like selective breeding and fertilizers," Myat said.
"After that, we had to begin to look for ways to incorporate new machinery into the plants' cells themselves." This is when the genes from bacteria began to be seen as a possible solution.
While plants remain in the dark on how to make RuBisCo more efficient, cyanobacteria came up with a solution to the problem millions of years ago as early oceans gained oxygen. Cyanobacteria had to find a way to limit their own RuBisCo from reacting with the unwanted oxygen, so they developed specialized compartments, known as carboxylases, with a faster-working form of RuBisCo and a fancy new trick.
While the exact method remains unclear, carboxysomes are somehow able to raise the concentration of CO2 around their RuBisCo enzymes to the point where it outcompetes oxygen, leading to fewer unsuccessful reactions and saving the bacteria energy.
Before determining the practicality of using this photosynthetic shortcut in plants, Myat and his team first needed to prove the capsule of carboxylases could form in plant cells at all. Using a process where chosen genes are forced into the leaf's stomata, or tiny gas exchange openings, they tagged the genes with a florescent tracker and then injected them into tobacco plant leaves.
Injecting energy efficiency into crops?
By tracking the inserted genes, they found that the proteins did in fact act as they were meant to, forming the same carboxylase shell they make in bacteria.
"We were able to show that we can put carboxylase components into plant cells they will assemble as they do in cyanobacteria," Myat said. "Now we can begin adding the internal components of the carboxylase to see if it actually works at making the plant more efficient."
Application of this technology is far off, Myat said, but as climate change continues to place pressure on both farmers and industry to find more sustainable practices, carboxylases could offer a way to cut back on the amount of nutrients consumed and wasted by crops, while producing larger yields.
Yet many questions remain about the feasibility of the process, most importantly whether plants will adopt cyanobacteria RuBisCo from the introduced carboxylases or continue to produce their own inefficient form, making the technology counterproductive. The next trials will involve removing the tobacco plant's own RuBisCo to see how it handles the bacterial version.
But even if plants reject the foreign enzyme's help, there could still be practical applications for Myat's findings. The empty carboxylase shell could be used as a delivery method for other molecules or genes into plant cells.
"There is a lot we could do with this, but it is really going to be a step-by-step learning process," Myat concluded, "and we won't know how big or practical these advantages are until we've made it through each of these steps."
The research was published online this week in the Plant Journal.