Investing in cell wall growth for improved photosynthesis

 

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The Brandizzi and Sharkey labs have found that changes in methylated pectin content, a type of cell wall carbohydrate, affect how malleable plant cell walls are, which in turn highly influence plant growth and photosynthetic performance.

The study is published in the journal Plant Physiology.

Leaves, the powerhouses of  photosynthesis in the plant world, come in various shapes and sizes – large and thin, small and thick, or with cells densely or loosely packed – depending on how their cells expand and grow.

“Specifically, the plant  cell wall – which is rigid in order to protect the cell – controls how big each cell grows, and by extension, how the totality of cells are distributed within a leaf,” says Sarathi Weraduwage, a post-doc in the Sharkey lab and lead co-author. “Ultimately, how well a plant develops is very sensitive to the makeup of the leaf cell wall and the resulting leaf size.”

 

A critical carb for flexibility

Cell walls are built mainly with carbohydrates, and the study examined one, methylated pectin, which controls the cell wall’s flexibility and ability to expand. Working with Arabidopsis plants – the lab guinea pig of plants – the researchers altered the level of  enzymes that synthesize the methylated pectin to examine the effect on cell walls.

“Suppressing the gene responsible for producing that enzyme drastically reduced the amount of methylated pectin. The result was rigid cell walls, leading to small, dense, and tightly packed leaf cells, with a significant reduction in air spaces between the cells. Ultimately, plant leaves were smaller than their genetically unaltered counterparts.”

 

Bottom, right: supressing both genes led to smaller tightly packed cells.
Bottom, left: overexpressing one gene led to larger, loosely packed cells
© 2016 American Society of Plant Biologists

 

On the other hand, inducing the plant to produce more enzymes led to more malleable cell walls, larger and loosely packed leaf cells with more airspaces between them. The leaves ended up bigger.

 

A wise carbon investor

The next step was to determine how the changes affected photosynthetic performance.

“We saw that the smaller leaves had a difficult time absorbing carbon dioxide – the source of carbon for photosynthesis – from the atmosphere, because their cells were tightly packed,” Sarathi says. “Interestingly, we found this prominent in older leaves, as the younger ones fully rivaled their larger counterparts in photosynthetic performance rates.”

“So that ruled out the idea smaller leaves were either the cause or result of poor photosynthetic performance.”

It turned out that the changes in leaf area were a result of how much carbon – plants’ energy currency – was invested in expanding the leaf. Plants, like any system with limited resources, have to allocate the available carbon to their best advantage, whether for growth, defense, or other activities.

“Cells with more methylated pectin can expand, which creates a demand for more and more carbon for cell growth, resulting in leaf area increase. On the other hand, cells with low methylated pectin cannot grow, so the plant does not invest carbon in leaf expansion, resulting in smaller leaves.”

 

Packed crops tend to compete for sunlight. By Bs0u10e0, CC BY-SA 2.0

 

Looking to improve plant performance

Here is the twist. Although photosynthetic performance didn’t directly affect leaf size, leaf size did determine how big the plant could grow.

“We did see that larger leaf area, due to more methylated pectin in cell walls, allowed for more sunlight to be captured for photosynthesis, which in turn increased the supply of carbon available to the plant.”

The result: bigger plants.

“This is a pretty dramatic discovery,” Sarathi adds. “The relationship between one carbohydrate at the cell level and the final size and shape of an entire plant is very strong.”

And Sarathi is excited in the potential for this discovery to someday unlock additional plant horsepower. Photosynthesis is, after all, the source of energy for most life on our planet. And in the effort to help feed billions more people, or even to power our cars and planes with biofuels, scientists are looking at different ways to make the process work better.

“Look at crop plants. Farmers tend to pack them close together to save space, so the crops end up competing for sunlight. Maybe someday we can fine-tune crops to have thin and large leaves that can better capture light and atmospheric carbon dioxide, leading to greater photosynthetic efficiency and ultimately, more energy produced.”


Fellow MSU scientists Federica Brandizzi, Thomas D. Sharkey, Sang-Jin Kim, Luciana Renna, and Fransisca C. Anozie contributed to the discovery and publication.