Grass holds the secret to more efficient crops?

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Why is Brachypodium distachyon, a relative of wheat, and other grasses so successful? It’s in their genes – and when they’re turned on and off, a new study shows.
Credit: Monty Rakusen / Getty Images

Why are grasses are one of the most successful plant families on Earth, found from sun-baked deserts to snow-capped peaks?

Some biologists think it’s because they quickly let gas in and out of their leaves, so photosynthesise more efficiently. But their secret weapon is also in the genetic blueprint of less widespread broad-leafed plants, new research shows – and switching it on could enable biologists to create crops with higher yields and resistance to climate change.

Stanford University’s Michael Raissig and Emily Abrash and colleagues in the US found broad-leafed plants and grasses use the same genes to produce stomata – openings in leaves that allow water and gases to pass through – but they’re turned on and off at different times to create different shaped cells.

The work “could be harnessed to improve growth performance in grasses that humans use for food or fuel”, says study co-author Dominique Bergmann, also from Stanford University.

Like us, plants must take in and release gases to survive. Tiny pores dotted on leaves – stomata, Greek for “mouths” – allows this gas exchange.

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A comparison of the dumbbell-shaped stomata found in grasses and the kidney-shaped stomata found in many other plants. The shape of grass stomata is considered one of the reasons for their evolutionary success.
Credit: Michael Raissig and Dominique Bergman

Stomata are ancient structures too, found in fossils dating back some 400 million years.

Gases such as carbon dioxide, oxygen and water vapour can diffuse in or out when stomata are open, and this is controlled by a pair of guard cells. When solutes such as potassium are pumped in, water rushes in too. And as they swell, guard cells curve outwards and bingo – stomata are open.

But not all stomata are the same.

Broad-leafed plants have kidney-shaped guard cells whereas those on leaf blades of grasses, such as wheat and bamboo, are dumbbell shaped.

Dumbbell-shaped guard cells are more efficient because they require fewer solutes and less water to open the same aperture size as kidney-shaped guard cells.

They’re also able to respond to environmental fluctuations, and thus optimise gas exchange, much more rapidly.

A 2007 Australian study in Plant Physiology of four plants found the grass species Triticum aestivum’s stomata popped open and closed an order of magnitude faster than any of the three non-grass species.

So do grasses have a different set of genes that initiate production of dumbbell guard cells or do they share the same genes as broad-leafed plants, but use them differently?

This was the question Raissig, Abrash and colleagues set out to explore.

They screened the genome of the grass Brachypodium distachyon, a relative of wheat, and found its dumbbell stomata genes were the same as the kidney-shaped Arabidopsis.

But its regulatory systems were different. In particular, B. distachyon used a different network of “transcription factors” – proteins that bind to specific DNA sequences and control which genes are turned on and off.

This determined how many stomata a plant produced, where they were put and their shape.

The findings, Bergmann says, comprise a “universal toolkit for building stomata” which may be used to improve crop yields and drought resistance.

The work was published in the Proceedings of the National Academy of Sciences

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