Scientists have identified a region of DNA present in both C3 plants (such as wheat and soybeans) and C4 plants (including corn and sorghum) that enhances C4 plant photosynthesis, allowing them to better endure heat and drought conditions. This significant discovery sheds light on how some plants have adapted to be more efficient, and it may serve as a foundation for activating C4 photosynthesis in C3 plants. This is a crucial advancement for developing sustainable crop varieties that can cope with climate change and support a growing global population.
Over 3 billion years ago, photosynthesis first emerged in ancient bacteria in an ocean-covered Earth. Over millions of years, these bacteria evolved into plants, optimizing their functions to adapt to environmental shifts. Approximately 30 million years ago, a new, more effective method of photosynthesis appeared. While some plants, like rice, retained the older C3 photosynthesis method, species like corn and sorghum evolved a newer, more efficient method known as C4.
Currently, there are over 8,000 species of C4 plants, which thrive in hot and dry conditions and rank among the world’s most productive crops. However, the majority of plants still primarily utilize C3 photosynthesis. This leads to questions about how C4 plants evolved, and whether C3 plants could also undergo a similar transformation.
For the first time, researchers at the Salk Institute, in collaboration with the University of Cambridge, have pinpointed a significant evolutionary step that allowed C4 plants like sorghum to optimize their photosynthetic capabilities. This insight could help boost the productivity and resilience of crops such as rice, wheat, and soybeans against a changing climate.
The research findings were published in Nature on November 20, 2024.
“Investigating the differences between C3 and C4 plants is crucial not only for understanding evolutionary biology but also for maximizing crop yields amidst climate challenges,” says Professor Joseph Ecker, the study’s senior author and a prominent figure in genetics research at Salk Institute.
Approximately 95% of plants utilize C3 photosynthesis, which involves mesophyll cells—green spongy cells in leaves—transforming light, water, and carbon dioxide into sugars that fuel plant growth. Despite being prevalent, C3 photosynthesis has two notable drawbacks: 1) Oxygen is mistakenly used instead of carbon dioxide about 20% of the time, hindering efficiency and wasting energy; 2) The leaf pores remain excessively open, leading to water loss, and making plants more susceptible to drought and heat stresses.
C4 photosynthesis resolves these challenges. C4 plants utilize bundle sheath cells in addition to mesophyll cells for photosynthesis, thereby avoiding the oxygen usage errors and keeping leaf pores closed more often to reduce water loss. This method enhances photosynthesis efficiency by 50% compared to C3 plants.
So, what molecular changes enabled C3 plants to evolve into C4 plants? And can we facilitate the transition of C3 crops into C4 varieties?
To investigate these questions, Salk scientists employed advanced single-cell genomics technology to compare C3 rice with C4 sorghum. Earlier techniques were inadequate for distinguishing between closely situated cells like mesophyll and bundle sheath cells, but single-cell genomics allowed the team to assess the genetic and structural differences in each cell type from both plant species.
“We were intrigued to discover that the distinction between C3 and C4 plants doesn’t stem from the removal or addition of specific genes,” explains Ecker. “Instead, the differences occur at a regulatory level, which may facilitate efforts to activate more efficient C4 photosynthesis in C3 crops.”
All cells in an organism share the same genetic material, but the expression of different genes defines each cell’s role and function. Gene expression can be influenced by transcription factors—proteins that attach to specific DNA sequences called regulatory elements. These factors help turn nearby genes “on” or “off.”
When analyzing gene expression in rice and sorghum, the scientists found a family of transcription factors referred to as DOFs, which regulate the genes responsible for the formation of bundle sheath cells in both plants. They also observed that DOFs bind to the same regulatory elements in both species. However, in C4 sorghum, these elements not only correspond to bundle sheath identity genes but also activate photosynthesis-related genes. This suggests that C4 plants may have incorporated ancestral regulatory elements from bundle sheath genes into the photosynthesis genes, enabling DOFs to stimulate both gene sets simultaneously—allowing bundle sheath cells in C4 plants to perform photosynthesis.
These findings indicate that both C3 and C4 plants possess the essential genes and transcription factors needed for efficient C4 photosynthesis, presenting a hopeful path for scientists looking to encourage C3 plants to adopt this advanced photosynthesis method.
“We now have a framework for understanding how various plants harness solar energy to thrive in different environments,” comments Joseph Swift, a co-first author of the study and postdoctoral researcher in Ecker’s lab. “Our main goal is to turn on C4 photosynthesis, leading to more productive and resilient crops for future generations.”
The next step for the team is to assess the feasibility of engineering rice to use C4 photosynthesis instead of C3. This long-term objective comes with substantial technical hurdles addressed through a global collaboration called the “C4 Rice Project.” Meanwhile, the findings will contribute to the Salk Harnessing Plants Initiative, focusing on developing optimized crops that can withstand climate change’s impacts.
The single-cell genomics data collected during this research has also been made available to scientists globally, sparking enthusiasm for its potential answers to a longstanding evolutionary puzzle.
Additional authors include Travis Lee and Joseph Nery from Salk, along with Leonie Luginbuehl, Lei Hua, Tina Schreier, Ruth Donald, Susan Stanley, Na Wang, and Julian Hibberd from the University of Cambridge in the UK.
This research was supported by institutions including the Howard Hughes Medical Institute, the Biotechnology and Biological Sciences Research Council, the C4 Rice Project, the Bill and Melinda Gates Foundation, the Life Sciences Research Foundation, the Herchel Smith Fellowship, and the European Molecular Biology Organization.
