Over 3 billion years ago, photosynthesis first emerged in ancient bacteria on a water-covered Earth. Over time, these bacteria evolved into plants, adapting to changes in their environment. Around 30 million years ago, a more efficient form of photosynthesis, known as C4, developed. While some plants, like rice, continue to use the older C3 method, others, such as corn and sorghum, adopted the more energy-efficient C4 process.
Today, more than 8,000 species of C4 plants thrive in hot, dry environments, making them some of the most productive crops globally. Despite this, the majority of plants still rely on the older C3 photosynthesis method.
In a significant breakthrough, researchers from the Salk Institute, in collaboration with the University of Cambridge, have uncovered a crucial step in the evolution of C4 photosynthesis. Their findings could offer a way to enhance the productivity and climate resilience of C3 crops like rice, wheat, and soybeans.
Professor Joseph Ecker, senior author of the study and Salk Institute’s Chair in Genetics, said, “Understanding the differences between C3 and C4 plants is not just a biological puzzle—it’s key to improving crop resilience as we face climate change and a growing global population.”
Currently, 95% of plants use the C3 method, in which mesophyll cells convert light, water, and carbon dioxide into sugars. However, C3 photosynthesis has two major drawbacks: oxygen is sometimes used instead of carbon dioxide, wasting energy, and pores on the leaves open too often, leading to water loss and increased vulnerability to heat and drought.
C4 photosynthesis addresses these issues by using bundle sheath cells to improve the efficiency of photosynthesis. It reduces oxygen-related mistakes and minimizes water loss by keeping leaf pores closed more often, making C4 plants 50% more efficient than their C3 counterparts.
The challenge has been understanding how C3 plants evolved into C4 plants on a molecular level—and whether C3 crops could be engineered to adopt the C4 process.
Using advanced single-cell genomics, Salk scientists compared the genetic structures of C3 rice and C4 sorghum. Unlike previous techniques, which struggled to distinguish between different cell types, this approach allowed researchers to examine genetic and structural changes in individual plant cells.
Prof. Ecker remarked, “We were surprised to find that the difference between C3 and C4 plants isn’t the addition or removal of specific genes. Instead, the change occurs at a regulatory level, which could make it easier to activate the more efficient C4 process in C3 crops.”
In both C3 rice and C4 sorghum, the scientists found that a family of proteins called DOFs regulates genes responsible for creating bundle sheath cells. In sorghum, DOFs are linked to a regulatory element that activates both bundle sheath and photosynthesis genes. This suggests that C4 plants evolved by attaching ancient regulatory elements for bundle sheath cells to photosynthesis genes, allowing both gene sets to be activated at once.
This discovery is promising for scientists working to engineer C3 plants to adopt C4 photosynthesis, making them more efficient and resilient.
Joseph Swift, co-first author and postdoctoral researcher in Ecker’s lab, said, “We now have a clearer blueprint for how plants use solar energy in different environments. The ultimate goal is to activate C4 photosynthesis in crops, improving their productivity and resilience for the future.”
The next step for the research team is to explore whether rice can be engineered to use C4 photosynthesis instead of C3, a goal that presents significant technical challenges. This work is part of the global “C4 Rice Project,” which aims to create crops better suited to climate change.
The team’s findings contribute to the Salk Harnessing Plants Initiative, which seeks to develop crops that can withstand extreme environmental conditions. Their single-cell genomics data has been shared globally, offering a new resource for scientists working to solve this long-standing evolutionary mystery.
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