Few plantations attract more attention than sunflower plantations. It is a spectacle to watch as hundreds of them simultaneously follow the movements of the sun as it crosses the sky. Because of this ability, called heliotropism, the flower head is always arranged to face our star. But how these plants manage to “see” the sun’s movements is a mystery.
Because plants are rooted in one place, they cannot get up and move if a neighbor blocks their light or they sprout in a shady spot. Instead, they rely on growth or stretching to get closer to the light. There are several molecular systems that enable such reactions. The best known of these is phototropism: the ability to grow towards a light source.
Scientists believed that sunflower heliotropism relies on the same basic mechanism, controlled by a molecule called phototropin, which responds to light at the blue end of the spectrum.
However, biologists from the University of California in Davis (California, USA) explain this Tuesday in the journal “PLOS Biology” that sunflowers use a novel and different mechanism than previously thought, which is much more complex. “It was a complete surprise to us,” says Stacey Harmer, a professor of plant biology at UC Davis and lead author of the paper.
From east to west
Sunflowers move their heads by growing a little more on the east side of the stem during the day (pushing the head west) and a little more on the west side at night, causing the head to turn east. Harmer’s lab at the UC Davis School of Biological Sciences has previously shown how sunflowers use their internal body clock to anticipate sunrise and coordinate flower opening with the appearance of pollinating insects in the morning.
In the new study, researchers examined which genes were activated (transcribed) in sunflowers grown indoors in laboratory growth chambers and in sunflowers grown outdoors in sunlight.
Indoors, the sunflowers grew directly toward the light, activating genes associated with phototropin. But plants grown outdoors that moved their heads with the sun showed a completely different pattern of gene expression. There was no obvious difference in phototropin levels between one side of the stalk and the other.
Researchers have not yet identified the genes involved in heliotropism. “We seem to have ruled out the phototropin pathway, but we haven’t found any clear evidence,” says Harmer.
Blocking blue, ultraviolet, red, or far-red light with shadow boxes had no effect on the heliotropism response. This shows that there are likely multiple pathways that respond to different wavelengths of light to achieve the same goal. Sunflowers “use different molecular pathways to initiate and maintain tracking movements, and the photoreceptors best known for causing plant curvature appear to play a minor role in this remarkable process,” the researcher points out. The next work will analyze the regulation of proteins in plants.
“Rewired”
Sunflowers learn quickly. When the lab-grown plants were brought outside, they began following the sun from day one. This behavior was accompanied by a burst of gene expression on the shade side of the plant, which was not repeated in the following days. This suggests that some sort of “rewiring” is taking place.
According to Harmer, the discovery is not only highly relevant, but also reveals previously unknown pathways to light perception and plant growth. “Things defined in a controlled environment like a growth chamber may not work in the real world,” he notes.