Air channel networks can influence how leaves operate

Understanding how carbon dioxide moves through pores could help develop crops to adapt to climate change

Botanists have long known that plants breathe through tiny openings in leaves called stomata that take in carbon dioxide and release oxygen.

But until now, it was never clear how these air channels form their intricate patterns in the right places to deliver a steady flow of CO2 to every plant cell.

In a new study at the University of Sheffield in the United Kingdom, scientists collaborated with colleagues at the University of Nottingham and Lancaster University to show that the movement of CO2 through pores set the shape of the air channel network. In addition, those networks could be targeted to further develop crops to adapt to climate change.

Wheat has been bred for centuries by people who unknowingly were influencing how plants breathe with fewer pores on their leaves and using less water.

“Our initial interest was in understanding the complex pattern of air channels in the leaf,” said Andrew Fleming with the University of Sheffield’s Institute for Sustainable Food. “Everyone knew they were there, but surprisingly few people had asked the question of exactly how they formed and what controlled them. At the same time, I was fortunate to have a colleague working on stomata, so linking the two together was a pretty natural thing to do. The linkage of stomata to internal airspace was one of those obvious things that everyone knew from simple observation but not many people had asked the ‘how’ question.”

The fundamental problem all organisms face is how to get air to their cells. In animals, air moves through a circulatory system that transports the oxygen around the body. Plants don’t have that system. They must deliver air to every single cell.

The how question was partially answered when researchers showed that the movement of CO2 determined the shape and scale of the air channel network. But what decides the right places in a leaf for air channels to form for CO2 to flow evenly is still a mystery.

“We would really like to find out,” said Fleming. “We believe there must be a series of local interactions where a cell gets exposed to air, then decides whether to separate from its neighbour, allowing a bit of air to get to the next cell, which then also decides what to do. It’s a bit like a relay race. It is the decision process that we still don’t understand, plus we only have a basic idea of how the cells separate.”

He said the final step of getting a functional stomate involves the partial separation of two cells to form a very small hole in the leaf surface.

“We believe this is a key event,” he said. “This leads to the exposure of the inner leaf cells to the outside air. Consecutive exposure of the cells to air leads to the process of air channel formation.”

Using CT imaging, researchers saw how the development of stomata in different leaf structures initiated the expansion of air spaces. They also found that the stomata actually need to be exchanging gases for the air spaces to expand.

The imaging was done at the University of Nottingham, where colleagues developed a new technique to visualize the cellular structure of the leaf in 3D, allowing them to see how the network of air spaces controlled the leaf’s behaviour. It confirms how historic breeding of wheat has lowered the plants’ water use. Today, growing crops to use even less water is even more essential.

“What breeders have done is select leaves which have a specific internal structure, which is at least partly responsible for the good water-use characters that modern wheat has,” Fleming said.

He said it raises many questions: is further improvement of these leaf characters possible, do any ancient plants, which may not have been used to breed modern wheat, have good leaf structures and if so, can scientists re-breed wheat to include these ancient characteristics, making our modern wheat better still at water use? As well, how will the effects of climate change influence the water use of present-day wheat?

Researchers are also exploring genetic modification options. He said one colleague has generated GM wheat with fewer stomata, which need less water to make the same amount of grain. That promising work has been duplicated in rice and barley.

The research was published in Nature Communications.

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