Frankenplant

40% more efficient plants? Done!

What is the most common protein in the biosphere? It occurs in plants, right? Right- it is RuBisCO, the enzyme that fixes carbon dioxide from the atmosphere, is the workhorse of agriculture, and hero of the fight against global warming, should we choose to grow more plants instead of burning them down. Its full name is ribulose-1,5-bisphosphate carboxylase-oxygenase, meaning that its substrate is a five carbon sugar (ribulose) that has two phosphates attached, and the enzyme attaches a carboxyl group from CO2, but can also attach an oxygen instead (the oxygenase part). And therein lies the problem. RuBisCO is phenomenally inefficient (maybe ten reactions per second) and error-prone (using oxygen [O2] instead of CO2 roughly a quarter of the time), which is why it is made in such prodigious quantities, amounting to half the protein complement of leaves.

Plant researchers have been casting about for a long time for ways to make this core reaction more efficient. But have had no success. Indeed, an interesting paper came out a few years ago arguing that as far as this enzyme is concerned, the shape and chemical similarity between CO2 and O2 are so close that RuBisCO is perfectly optimized, exchanging speed for what specificity is possible given its substrates. It varies quite a bit in this tradeoff, depending on the specific environment, arguing that the optimization is quite dynamic over evolutionary time. One of the few innovative solutions that plants have developed is not a tweek to the enzyme, but a physiological compartment present in C4 plants (like corn), which concentrates CO2 and excludes O2, thus resolving the competitive constraint for some of their chloroplasts. Their RuBisCO enzymes are adapted to have a slightly more relaxed attitude- slightly less specific for CO2, while also almost 2-fold faster, gaining an significant advantage.

The error pathway, fixing oxygen instead of CO2, is called photorespiration, since it uses up oxygen like regular respiration, but now in a completely wasteful way. The product is phospho-glycolate instead of 3-phospho-glycerate, and the glycolate is both inhibitory to photosynthesis and difficult to dispose of. It is typically exported from the chloroplast, and bounced around between the peroxisome and mitochondrion in its way to being turned into the amino acid glycine, all at the cost of roughly twelve ATP. It is hard to believe that this waste goes on day in and day out across the biosphere, but it seems to be the case. One might note that this yet another case of the steep price of success, since RuBisCO evolved in a high CO2 environment. It was the success of the photosynthetic process that covered the earth with green and filled the atmosphere with what was to all existing life forms a poison- oxygen.

Now, a team of researchers have engineered a way around this conundrum, at least reducing the cost of glycolate recycling, if not resolving the fundamental problems of RuBisCO. They describe the import of a set of genes from other species- one from pumpkin, one from the alga Chlamydomonas, and five from the bacterium E. coli, plus a genetic suppressor of glycolate export from the chloroplast, all resulting in a far less costly recycling system for the waste product glycolate.

New pathways (red, blue) inserted into tobacco plants, plus inhibition of the glycolate transporter PLGG1. Some of the wild-type pathway for diposing of glycolate is sketched out on the right.

Firstly, glycolate export was suppressed by expressing a tiny RNA that uses the miRNA system to target and repress the gene (PLGG1) encoding the main glycolate transporter. Secondly, the researchers imported a whole metabolic system from E. coli (red part at top of diagram) that efficiently processes glycolate to glycerate, which, with a phosphorylation (one ATP) can be taken right up by the RuBisCO cycle. Lastly, they backstopped the bacterial enzymes with another pair that oxidize glycolate to glyoxylate (glycolate oxidase), and then (malate synthetase) combine two of them into malate, a normal intermediate in cellular metabolism. This was all done in tobacco plants, which, sadly, are one of the leading systems for molecular biology in plants.

Wild-type plant is on the far left, and a sample plant with all the engineered bells and whistles (AP3) is on the far right, showing noticeably more robust growth.

Combining all these technologies, they come up with plants that show biomass productivitiy 40% higher than the parent plants, as well as reducing plant stress under high light conditions. After 3 billion years of plant evolution, this is a shocking and impressive accomplishment, and can be extended to all sorts of C3 plants, like wheat and other grains (that is, non-C4 plants). Due to the number of genes involved, unintentional spread to other plants, such as weeds, is unlikely. But given the urgency of our CO2 waste problem, one wonders whether we might welcome such escapes into the wild.

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