Sunflowers are famous for always turning to seek the sun. But how do sunflowers “see” the sun to follow it? As reported by AGI, new research conducted by plant biologists at the University of California, Davis, and published in PLOS Biology, shows that they use a different and innovative mechanism than previously thought.
“This was a complete surprise to us,” said Stacey Harmer, a professor of plant biology at UC Davis and the lead author of the study. Most plants exhibit phototropism, which is the ability to grow towards a source of light. Plant scientists had assumed that sunflowers’ heliotropism, the ability to follow the sun, was based on the same basic mechanism, regulated by a molecule called phototropin, which responds to light in the blue part of the spectrum.
Sunflowers turn their heads by growing a bit more on the east side of the stem, pushing the head westward during the day and a bit more on the west side during the night so that the head orients towards the east. Harmer’s laboratory at the College of Biological Sciences at UC Davis had previously shown how sunflowers use their internal circadian clock to anticipate dawn and coordinate the opening of flowers with the appearance of pollinating insects in the morning. In the new study, undergraduate student Christopher Brooks, postdoctoral researcher Hagatop Atamian, and Harmer examined which genes were activated (transcribed) in sunflowers grown in the laboratory in growth chambers and sunflowers grown outdoors in natural sunlight. In the laboratory, sunflowers grew straight toward the light, activating genes associated with phototropin.
However, the plants grown outdoors, which turned their heads to follow the sun, showed a completely 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 seems we have ruled out the phototropin pathway, but we have not found definitive evidence,” Harmer said. Blocking blue, ultraviolet, red, or infrared light with shaded boxes had no effect on the heliotropism response. This demonstrates that there are likely multiple pathways, responding to different wavelengths of light, to achieve the same goal.