Could we bioengineer trees to grow more quickly and store more carbon? Sophie Young explores the challenges of introducing the more efficient C4 photosynthesis into non-C4 plants Of the estimated 435,000 plant species on planet Earth, C4 photosynthesis is present in fewer than 2%. However, it accounts for approximately 25% of global primary productivity1,2. It can be considered a ‘turbocharged’ form of photosynthesis, with the most productive C4 plants having yields and maximum growth rates some 40–50% higher than the most productive C3 species. Just three of the world’s top 10 crops – maize, sorghum and sugarcane – use C4 photosynthesis. As such, it has long been recognised that if more of our most-consumed crop species used C4 photosynthesis, it could have a radical effect on global food security. However, a less well-discussed topic is how the lack of C4 photosynthesis in forest tree species, which are important both economically and in terms of carbon storage, has impacted and continues to impact habitats across Earth and global carbon sequestration. Ongoing research aims to provide insight into the possibility of engineering the C4 pathway into C3 plants and investigate the reasons behind the rarity of C4 photosynthesis in trees. EVOLVING EFFICIENCY Since the discovery of C4 photosynthesis over 50 years ago, plant lineages from 19 different families (including the Poaceae, Chenopodiaceae and Asteraceae) have been observed to have evolved this photosynthetic pathway, representing nearly 70 independent evolutions of the C4 state2. The evolutionary transition to the complex C4 type of photosynthesis from the ancestral C3 state involves the acquisition of a number of modifications that set up a ‘carbon shuttle’ to concentrate CO2 around Rubisco, the central enzyme of photosynthesis. This CO2-concentrating mechanism is beneficial because Rubisco is inefficient at catalysing photosynthesis: when exposed to both carbon dioxide and oxygen it cannot differentiate between the two molecules. This means that it sometimes catalyses an oxygenation reaction, where oxygen rather than CO2 is combined with the RuBP molecule, creating toxic by-products that must be rescued in order to return RuBP to the Calvin cycle (see Figure 1, p32). This process, known as photorespiration, reduces photosynthetic efficiency for three reasons. First, it consumes RuBP that could otherwise be used in photosynthesis. Second, oxygen competes with CO2 for Rubisco-active sites. Third, it releases previously fixed carbon and nitrogen as CO2 and ammonia respectively. Photorespiration by C3 crops is socioeconomically costly: photorespiration by soybean and wheat crops in the US accounts for a total loss of 148 trillion calories, which is equivalent to the yearly calorie requirements of 203 million people3. This photorespiratory inhibition of photosynthesis increases with reduced atmospheric CO2 concentrations and increasing temperature, and results in reduced water- and nitrogen-use efficiency. Thus, the C4 CO2 concentrating mechanism, which largely avoids photorespiration, is most advantageous under hot and dry conditions. This is reflected by the dominance of C4 vegetation in environments such If the C4 Rice Project is successful, engineered rice could be up to 50% more productive than C3 rice Left: Sorghum, one of the few major crops that uses C4 photosynthesis Vol 67 No 3 / The Biologist / 31