Engineering drought-resistant crops with Crassulacean acid metabolism (CAM) photosynthesis

New research, published this week in Plant Cell, investigated water-saving alternatives for photosynthesis in temperate environments, which are likely to become hotter and drier in the future. 

Drought causes major crop losses in many regions of the world, and climate change threatens to exacerbate the occurrence of drought in temperate as well as arid regions. The international team analysed the potential for engineering drought-resistant plants via introduction of Crassulacean acid metabolism (CAM). They used a sophisticated mathematical modelling approach to study the effects of introducing CAM photosynthesis into C3 plants.

Most plants, including some major crops such as rice, wheat, oats, and barley, use C3 carbon fixation, in which CO2 taken up during the day through stomatal pores in the leaf is used immediately in light-driven photosynthesis reactions. Unfortunately, this leads to significant water loss through these pores in hot, dry conditions. CAM is an alternative carbon fixation pathway that temporally separates CO2 uptake from carbon fixation, allowing the plant to open stomata for CO2 uptake in the cool of the night and store the carbon internally. The CAM plant then closes its stomata during the heat of the day to minimize water loss, and releases the stored CO2 inside the leaf cells to be used for light-driven photosynthesis during the day.

Dr Nadine Töpfer, who carried out this work during her Marie-Curie Postdoctoral Fellowship with us here in Oxford, led on this research using simulations across a range of temperature and relative humidity conditions. The team found that vacuolar storage capacity in a leaf is a major factor that limits water-use efficiency during CAM and that the environmental conditions shapes the occurrence of different phases of the CAM cycle. Mathematical modeling also identified an alternative CAM cycle that involves mitochondrial isocitrate dehydrogenase as a potential contributor to initial carbon fixation at night.

Dr Töpfer – who is now based at the Leibniz Institute of Plant Genetics and Crop Plant Research - said: "Modelling is a powerful tool for exploring complex systems and it provides insights that can guide lab and field-based work. I believe that our results will provide encouragement and ideas for the researchers who aim to transfer the water-conserving trait of CAM plants into other species."

Their results revealed not only that the water-saving potential of CAM photosynthesis strongly depends on the environment, with the daytime environment more important than that at night, but also that alternative metabolic modes, distinct from those of the naturally occurring CAM cycle, may be beneficial under certain conditions such as during shorter days with less extreme temperatures. This timely work provides valuable insights that will help us prepare for the challenges of growing food crops in increasingly hot and dry temperate environments.