Sunflowers follow the sun, but how?

Sunflowers use a different mechanism to ‘see’ and track the sun as it crosses the sky than previously thought, researchers report.

“This was a total surprise to us,” said Stacey Harmer, professor of plant biology at the University of California, Davis and senior author of the paper in PLOS biology.

Most plants exhibit phototropism: the ability to grow toward a light source. Plant scientists had assumed that sunflowers’ heliotropism, the ability to track the sun, would be based on the same basic mechanism, which is controlled by a molecule called phototropin and responds to light on the blue end of the spectrum.

Sunflowers wave their heads by growing a little more on the east side of the stem during the day (pushing the head to the west) and a little more on the west side at night, so that the head swings back to the east. Harmer’s lab at the UC Davis College of Biological Sciences has previously shown how sunflowers use their internal circadian clock to anticipate sunrise and coordinate floret opening with the appearance of pollinating insects in the morning.

In the new study, graduate student Christopher Brooks, postdoctoral researcher Hagatop Atamian and Harmer looked at which genes were turned on (transcribed) in sunflowers grown indoors in laboratory growth chambers, and in sunflowers grown outdoors in sunlight.

Indoors, sunflowers grew straight towards the light, activating genes associated with phototropin. But the plants grown outdoors, which swung their heads back and forth with the sun, showed a very different pattern of gene expression. There was no apparent difference in phototropin between one side of the stem and the other.

The researchers have not yet identified the genes involved in heliotropism.

“It appears that we have ruled out the phototropin pathway, but we have not found a clear smoking weapon,” says Harmer.

Blocking blue, ultraviolet, red, or far-red light with shadow boxes had no effect on the response to heliotropism. This shows that there are likely multiple pathways that respond to different wavelengths of light to achieve the same goal. Upcoming work will look at protein regulation in the plants.

Sunflowers learn quickly. When plants grown in the lab were moved outdoors, they started following the sun from day one, Harmer says. That behavior was accompanied by a burst of gene expression on the shade side of the plant, which did not occur again in subsequent days. That suggests there’s some kind of “rewiring” going on, she says.

Beyond revealing previously unknown pathways for light perception and growth in plants, the discovery has broad relevance, Harmer says.

“Things you define in a controlled environment like a growth chamber may not work in the real world,” she says.

Atamian is now an assistant professor at Chapman University.

Source: UC Davis