Tracing a path through photosynthesis to food security

The energy that plants capture from sunlight through photosynthesis provides the source of nearly all of humanity’s food. Yet the process of photosynthesis has inefficiencies that limit crop productivity, especially in a rapidly changing world. A new review by University of Illinois scientists and collaborators reflects on how improving photosynthesis can bring us closer to food security.

The review, which was published in Cell, was coauthored by plant biology professors Stephen Long, Amy Marshall-Colon, and Lisa Ainsworth. With chemical and biomolecular engineering professor Diwakar Shukla and colleagues at eight partner institutions, they evaluated biological strategies to improve the efficiency of photosynthesis, the process by which plants convert sunlight to sugar in crop plants.

A plant that is able to capture more sunlight can produce a greater amount of food or fuel. In a review written ten years ago, Long, Marshall-Colon, and Chinese Academy of Sciences principle investigator Xin-Guang Zhu described the promise of focusing on photosynthesis to develop higher-yield crop plants and identified potential pathways to improvement. The present review examines the progress made on each and re-emphasizes the value of this research approach. 

"The pace at which we have gained knowledge over the last decade has been incredible," Ainsworth said. "As we look at all of the challenges surrounding agriculture . . . photosynthesis has the potential to address them. The pace of research progress has accelerated over the last decade and we are poised to have exponential growth in understanding of photosynthesis in the coming decade."

Ainsworth directs Realizing Increased Photosynthetic Efficiency, an international project based in the Carl R. Woese Institute for Genomic Biology and focusing on this area of research; Long, Marshall-Colon, Shukla, and several other coauthors are RIPE investigators. RIPE is supported by Gates Agricultural Innovations (Gates Ag One).

Why focus on photosynthesis? This process, in which sunlight’s energy is captured by chlorophyll in plants and used to convert carbon dioxide and water into sugar and oxygen, is central to plant life on earth. Photosynthesis is the ultimate source of the calories we consume. But despite its central role in plant life, photosynthesis is like so many other complex systems that evolution built ad hoc and is not optimized to today’s environment. 

"What's exciting about this paper is that many of the proposed changes or proposed interventions to photosynthesis have been put into crops and tested in the field. Many have shown promise in improving daily photosynthetic carbon gain as well as yield," Ainsworth said. "The review also points to different strategies that have not yet been tested or not yet realized, the next frontier. We discuss the tools that we need to reach the next frontier."

The review emphasizes several key targets for improving photosynthesis. One of the most prominent is engineering an enzyme called Rubisco, which is responsible for taking atmospheric carbon dioxide and fixing it into sugars. Rubisco sometimes grabs oxygen instead, leading to a costly recycling cycle termed photorespiration.

The review examines the status of multiple solutions to this challenge. Solutions include using a lab-directed evolutionary process to develop a less error-prone Rubisco; inducing plants to produce more of the enzyme to maximize its throughput; prompting plants to form cell or tissue structures that help to concentrate carbon dioxide around Rubisco to block out competing oxygen molecules; and introducing synthetic pathways to circumvent the costly photorespiration strategies to keep Rubisco active under a wider range of environmental conditions.

Other targets for improved efficiency of photosynthesis relate to plants’ need to capture sunlight. Light must filter through many layers of leaves in closely planted rows of crops. Plants in the wild evolved to compete with each other, shading out those around them from light, but crop plants can be more efficient when they are cooperative, ensuring that leaves at all levels receive light. Crops can be developed to grow more efficiently by altering the angle and chlorophyll content of their leaves, allowing more sunlight to filter past the upper canopy. Other research is focusing on helping plants more quickly shift from protective mechanisms used in high-intensity light to optimal light-gathering in cloudy conditions.

Advances in all these areas have moved faster than the coauthors originally predicted, thanks in part to new technologies.

"Dynamic protein models now help us better understand protein interactions and advanced imaging means we can visualize the intercellular airspace of leaves providing novel insights,” Ainsworth said.

The publication serves as both a reflection of what has already been accomplished and a call to action. In both capacities, it has also come to represent a legacy. Both Long and Marshall-Colon passed away earlier this year. It was important to Long in particular to refocus the research community on the achievability and potential impact of improving photosynthetic efficiency in the cause of feeding the world.

"In a decade we have gone from theory or proof of concept in model crops, to field testing of food crops," Ainsworth said. "This review shows that we're not just close to enhancing photosynthesis, we are doing it. Enhancing photosynthetic efficiency is achievable and something that we can integrate now into crop improvement strategies."