How much can we improve on Nature? Fixing defective proteins and pathways is one thing, but in those cases we’re trying to get back to what the function should be (and what it is in healthy organisms). But what about “better than healthy”? That’s a tricky area to enter, because (for one thing) billions of years of evolutionary pressure have given us few opportunities for obvious improvements – and it also means that some of the ones that look obvious might not be such a good idea (see “Chesterton’s fence” for general thoughts on this in human affairs). At the extreme end of things, this is what unnerves people – and should – about using a technique like CRISPR to not only fix disease-causing mutations, but to enhance otherwise normal embryos in some way that we find valuable.
But let’s take a look at something less controversial: fixing plants. Photosynthesis has had plenty of time to evolve, although when you look at one of the key enzymes (RuBisCO), you wonder. It is, by enzyme standards, amazingly slow and inefficient. Its turnover is pitiful, and a good part of the time it grabs an oxygen instead of a carbon dioxide, because it’s too damn sloppy to be able to tell them apart. When that happens, RuBisCO does something completely inappropriate with it, producing 2-phosphoglycolate, which is not just useless but actively toxic. The photorespiration pathway exists just to clear out the 2-PG that keeps getting blindly pumped out from photosynthesis.
I have made the case for the prosecution there, but in RuBisCO’s defense, no one has ever been able to engineer anything better. And many have tried. Evolution has, in fact, had plenty of time to do just that, and it’s possible that what we’re seeing, as brutal and bumbling as it may look, is the most elegant solution available (at least in that particular part of the landscape, short of wholesale re-engineering – and if you’re thinking about that, it’s worth noting that no such alternate carbon-dioxide-fixer photosynthetic pathway seems to ever have emerged, either). It’s very likely that the ability to distinguish carbon dioxide from oxygen and the ability to actually turn over the enzymatic reaction are working against each other, and that what we see is the best compromise that a billion years could come up with.
But outside of the enzyme’s structure, there is apparently room for improvement. Evolution has already hit on the C4 pathway versus the older C3 version, improving RuBisCO function by giving it an environment that’s lower in oxygen in the first place. The problem there is that you need extra enzymatic steps to work C4, which costs you extra ATP, so while C4 plants win out at higher temperatures or under stress, C3 plants will outcompete them under other conditions. But what about fixing that photorespiration pathway? That’s what this new paper from Illinois shows. They’ve engineered in glycolate-handing enzymes in the pathway from bacteria and/or algae in an attempt to tune things up, and after many constructs, promoters, and combinations, they appear to have something.
They now have field-tested plants (tobacco as a test case, since it has easy genetics, is robust, and produces large amounts of seed) whose light-use efficiency is up 17% compared with wild-type, with 25% more biomass produced. That goes up even further to 40% biomass gain if they also engineer in RNA interference for a wild-type glycolate transporter, forcing the plant to go more through the new enzymes. What’s more, they’re optimistic that this can be translated to other more useful C3 plants, including food sources. This will also allow these plants to thrive even more if carbon dioxide levels keep increasing (and allow them to take even more of it up). It looks like the plants produce larger leaf area earlier in their life cycle, and the hope is that this will allow for larger biomass showing up as seeds (for grain crops) and tubers (as with potatoes).
This is good news. Increased crop yields would allow us to feed the Earth’s population while plowing up less of the planet than we would have to otherwise, and figuring out how to engineer plants to use carbon dioxide more effectively is no bad thing, either. So while always thinking that we can rig up something better than evolution is not the way to go, just assuming that we have to take the cards we’re dealt isn’t sound, either. Anyone who works in biomedicine has already put their bet down against that second attitude, anyway!
Now, what you think of such means might be another matter. To take up that question, let me recommend Charles Mann’s The Wizard and the Prophet, which is a history of the Green Revolution and more besides. Mann has a nice discussion of just these sorts of foodcrop-based issues, but (as he discusses) the questions raised go far beyond the borders of the simple little area of growing enough food for everyone. We’ll be hitting those soon enough in biomedicine, at the rate things are going. . .