Every leaf on Earth runs on one enzyme, and it is not very good at its job. Rubisco grabs carbon dioxide from the air and stitches it into sugar, the first step of almost all photosynthesis. It is also famously sluggish and easily distracted, sometimes grabbing oxygen instead of CO2 and wasting the plant's energy. Because it works so slowly, plants have to make enormous amounts of it. Rubisco is thought to be the most abundant protein on the planet. Speeding it up, even a little, has been a decades-long dream for anyone who wants crops to grow faster.
A team led by Spencer Whitney at the Australian National University just took a concrete step toward that goal. Writing in Nature Plants, they describe a way to hunt through Rubisco's genetic sequence for rare mutations that make the enzyme faster or easier to build, then move the best ones into a living plant. The winning versions made tobacco grow dramatically quicker.
Letting bacteria do the searching
The clever part is the screen. Testing Rubisco variants one at a time inside plants would take years per candidate. So the researchers built a system in Escherichia coli, the workhorse gut bacterium, that only lets cells survive and grow when the plant Rubisco they carry actually fixes carbon well. Feed the bacteria millions of mutant versions, and the ones with better enzymes outcompete the rest. The useful mutations rise to the top on their own.
Two changes stood out. One, a swap of a single amino acid called Met-116-Leu, sped up the core chemical reaction of several different plant Rubiscos by 25 to 40 percent. The other, Ala-242-Val, did something less glamorous but arguably just as valuable. It made the enzyme far easier to assemble, boosting how much functional plant Rubisco the bacteria could produce by two to ten times. Rubisco is notoriously hard to fold correctly, so a mutation that improves manufacturing is a real prize.
The tobacco surprise
Then came the test that matters. The team used plastome transformation, a technique that edits the DNA inside chloroplasts, to write these mutations directly into tobacco's own rbcL gene. And here the story takes a turn. Editing the native tobacco gene did nothing. No change in Rubisco levels, no change in photosynthesis, no change in growth.
That could have been the end of it. Instead the researchers tried a different setup: tobacco plants carrying a low-abundance hybrid Rubisco borrowed from Arabidopsis, the lab world's favorite little weed. In that background, the mutations came alive. Plants making the M116L version grew about 75 percent faster during their exponential growth phase compared with plants carrying the unmutated hybrid enzyme. The A242V change lifted both Rubisco production and plant growth by roughly 50 percent.
What this does and does not prove
It is worth being careful about what happened here. The 75 percent jump was measured against a deliberately weak hybrid enzyme, not against a normal, fully productive tobacco plant. So this is a proof that the screen can find growth-boosting mutations, not a finished crop trait ready for a field. The plants were grown in controlled conditions, and tobacco is a lab model, not a staple food. Getting a similar lift in rice or wheat, under sun and drought and pests, is a much harder problem that this paper does not claim to solve.
What the work does deliver is a faster way to explore Rubisco's vast space of possible sequences. Most past efforts could only test a handful of variants at a time. A bacterial screen that surfaces both catalytic speed and easier assembly, and a plant system to confirm the hits, together form a pipeline that can be run again and again. The authors are frank that these particular mutations are modest. Their point is that the method should turn up bigger ones. After decades of Rubisco resisting improvement, having a repeatable way to search is the part worth watching.
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