As the world’s population increases, the spectre of severe food shortages is growing, with the United Nations predicting1 that food production will need to double by 2050. It has been proposed that cyanobacteria — which obtain their energy from a highly efficient form of photosynthesis — might hold the key to increasing the yield of our most important crops and vegetables. On page 547 of this issue, Lin et al.2 report a major step towards realizing this possibility, finding that cyanobacteria can be used to improve photosynthesis in the leaves of crops.
Photosynthesis harnesses sunlight to convert carbon dioxide into simple sugars. Rubisco, the key enzyme for CO2 fixation into sugar, is inefficient because it cannot easily discriminate between oxygen and CO2 and so wastes energy by fixing O2. The enzyme evolved at a time when O2 levels in the atmosphere were much lower than they are today, and there was therefore little evolutionary pressure to select for an ability to discriminate between the two molecules. Photosynthetic organisms have evolved to circumvent the problem of rising atmospheric O2 levels in two ways: first, by making more of a slower-acting version of Rubisco with an improved ability to discriminate; or second, by using various ‘add-ons’, called CO2-concentrating mechanisms (CCMs), to elevate CO2 levels in the vicinity of the enzyme.
Most crops have adopted the first strategy, making Rubisco possibly the most abundant enzyme on Earth. This approach, however, results in a 30% reduction in photosynthetic efficiency through the associated O2 fixation. That can be partly ameliorated by raising CO2 levels around the leaf3 in a manner conceptually similar to adding a CCM. There is currently increased focus on the second strategy — if a CCM could be introduced into crops, it might turbocharge photosynthetic CO2 fixation. CCMs have evolved independently in cyanobacteria, microalgae and some plants (mostly those regarded by us as weeds). Although several types of CCM are being considered for introduction into crops, Lin and colleagues’ work focuses on the cyanobacterial CCM.
