A study into how plants evolved to cope with climate changes millions of years ago could help scientists develop modern-day crops that are more productive and resilient to global warming.
Researchers from the University of Cambridge and the SALK Institute in California have uncovered genetic changes in the evolution of a more efficient form of photosynthesis that evolved around 30 million years ago. Their findings have key implications for ongoing efforts to improve crops for resilience to climate change. The research was published in Nature on 20 November 2024.
Two forms of photosynthesis
Photosynthesis allows life on Earth. More than 3 billion years ago bacteria first evolved the ability to photosynthesise. This original form, known as C3 photosynthesis, is still used by some of the world’s major crops including wheat and rice.
Around 30 million years ago, in response to changes in climate, some plants developed a new form of photosynthesis better suited to warm and dry conditions. This more efficient form, known as C4 photosynthesis, is used by the world’s most productive plants including crops such as maize and sorghum.
Plants that use the C4 pathway dominate the tropics and semi-tropics now because in these warm conditions it allows increased water and nitrogen use efficiencies as well as about 50% higher productivity. Understanding how the C4 pathway evolved and operates is therefore important for improving our crops for resilience to climate change.
Novel technologies provide insights into genetic changes
For decades it has been clear that although considered highly complex, the C4 pathway has evolved multiple times in land plants. One key step in the evolution of the C4 pathway was to change where in the leaf genes for photosynthesis are active.
While in C3 plants these genes are active in the mesophyll cells, in C4 plants they are active in a neighbouring cell type, the bundle sheath cells, which surround the veins. However, how genes changed during C4 evolution to allow this new complexity has not been clear.
The researchers used novel technologies allowing single cell genomics to provide insights into the genetic changes underpinning the C4 pathway and its evolution.
Dr Leonie Luginbuehl, head of the Plant Physiology and Symbiosis Group at the Department of Plant Sciences and co-lead author of the study, said “By using a technique called single cell sequencing, we were able to investigate how genes are activated in each cell of plant leaves in the two crops C3 rice and C4 sorghum. This resolution was previously not possible but crucial to understand how C4 photosynthesis evolved.”
A framework to help engineer the C4 pathway into C3 crops
Surprisingly, the team found that the mechanisms that activate genes in either the mesophyll or in the bundle sheath are in fact the same in both plant species. In particular, the study revealed that a set of regulators called the DOFs are important for gene activation in the bundle sheath of both rice and sorghum.
The researchers discovered that genes required for photosynthesis in sorghum had changed their DNA sequence such that DOF regulators can bind to these genes and activate their expression in the bundle sheath rather than the mesophyll.
A long-standing goal for the research community has been to introduce the C4 pathway into C3 crops such as rice to enhance photosynthesis and improve yield as temperatures are predicted to increase and water is becoming scarcer.
Professor Julian Hibberd, head of the Molecular Physiology group and last author of the study, says: “This discovery identifies at base pair resolution how genes changed in order for the C4 pathway to operate. Because of this precision it also provides a predictive framework to help engineer the C4 pathway into less efficient C3 crops.”
Reference: Swift, J., Luginbuehl, L.H., Hua, L. et al. Exaptation of ancestral cell-identity networks enables C4 photosynthesis. Nature (2024). https://doi.org/10.1038/s41586-024-08204-3
Image: Electron microscopy images of cross sections of leaves of sorghum, a C4 plant. Credit: Dr Tina Schreier, University of Cambridge (now Oxford).