Seeing red (and far-red)
by Cherie Winner | © Washington State University
Ask crop scientist Michael Neff about plant growth, and he won't talk about rainfall or fertilizer. He'll talk about what the plants see.
"What I've been interested in forever is how plants use light as a source of information," says Neff. "Plants have photoreceptors that are completely independent of photosynthesis and chloroplasts, that read their environment and say, 'I am in full sunlight, I'm in the shade of another plant, I've got plants that are growing too close to me,'" and so on. The photoreceptors then trigger a host of hormonal reactions that influence how tall the plant will grow.
Neff thinks it's possible to boost crop yields by manipulating that system. He's especially interested in shade avoidance, the tendency of plants to grow away from shade. When a seedling in the shade grows long and leggy or a young tree leans away from its larger neighbor, that's shade avoidance. It comes into play with crops, because plants that are grown close together shade each other to some extent.
That's bad news, because shaded plants grow taller, and as a stem lengthens, it also weakens. Add the weight of a seed head or fruit, some rain, and a brisk wind, and the plant can fall over. In very bad cases, whole fields of crops can end up on the ground. They become hard to harvest; if conditions are right, they may even rot.
"And then you've lost your crop," says Neff. "Even though you had a potentially big yield, you've lost it."
For decades, largely due to the work of Washington State University wheat breeders Orville Vogel and Bob Allan, wheat farmers have relied on dwarf and semi-dwarf varieties that were genetically selected to stay short. But most crop species aren't available in dwarf varieties. Neff thinks the light-sensing system might provide a solution. If we can figure out how to reduce shade avoidance in a species, he says, the crop could be planted closer together but still remain short and sturdy.
To do that, he first has to understand how shade avoidance works. There's a lot more to it than a simple "grow toward the light" strategy. The light under a tree or among crowded crop plants isn't just dimmer than light in the open. It's also a different color. Sunshine is full-spectrum light. It includes all the visible colors, plus some that humans can't see. Plants use the blue and red wavelengths of light for photosynthesis. Light that has passed through or bounced off of leaves has lost much of its blue and red light and has relatively more green and far-red light. We can see the green light—that's why leaves look green to us—but we can't see the far-red.
Plants can see it, though. A photoreceptor protein called phytochrome B (PhyB) detects red light, far-red light, and the relative amounts of each.
"It's a great photoreceptor for reading whether you're under the shade of another or near another plant, because that light that is reflected off your neighbors is enriched in far red," says Neff. When PhyB senses a greater proportion of far-red light, it signals the plant that it's in shade. That spurs the plant to grow taller in an effort to get beyond the shade to a sunnier spot.
PhyB would be a good target for crop breeders to tinker with, except for one thing: Plants with dysfunctional PhyB do the opposite of what's wanted. They always shade-avoid, growing long and leggy even when they're in full sunlight.
So Neff and his students have taken a different approach. They start with a plant strain that has a nonfunctional form of PhyB. Then they randomly create mutations in the plant's other genes and look for one that compensates for the lack of PhyB. Such mutations are easy to identify: You collect seeds that might carry one, let them sprout, and look for seedlings that are shorter than the others. The short individuals are not shade-avoiding as much as their parents.
Whenever Neff identifies such a mutation, he and his students study it to figure out why it doesn't shade-avoid and whether it's a good possibility for plant breeders to work with.
His team has already identified dozens of genes that are involved in converting the signal from PhyB into growth instructions, and they're making good progress on developing candidate genes that could be introduced into crop plants.
Neff makes sure his students know that they carry the legacy of Vogel and Allan, who developed many of the dwarf and semi-dwarf varieties that are now the mainstays of the wheat industry.
"They should all understand that. I certainly like to impress upon them the hallowed ground that they are working on here," he says.
At the same time, they catch his enthusiasm about the revolutionary insight that plants, do, in a sense, see—and they see the world differently than we do. Neff loves to show students and visitors landscape photos shot on infrared film, which records the far-red wavelengths of light we can't see.
"All the plants look silver, like you need sunglasses to look at these things," he marvels. "What a plant sees, with its photoreceptors, is blinding far-red light being reflected off of the other plants.
"That's what the plant sees of the plant world."
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