Why do sunflowers follow the sun? Scientists discover new mechanism →

Compiled by: Gong Zixin

sunflower

Known for its ability to track the sun throughout the day

But how does it "see" the sun?

And what follows it?

New research by UC Davis plant biologists, published Oct. 31 in the journal PLOS Biology, shows they use a novel mechanism that is different from what was previously thought.

“This was a complete surprise,” said Stacey Harmer, a UC Davis professor of plant biology and senior author of the paper.

Heliotropism and phototropism

As the sun moves across the sky, the sunflower turns its face to follow the sun, then reorients itself at night to face east the next morning. This happens through a different growth pattern, with the east side of the stem growing more during the day and the west side of the stem growing more at night. This process is called heliotropism and is often considered a specialized form of phototropism.

Phototropism - the ability to grow towards a light source - is one of the most characteristic growth responses of plants. Plant scientists had hypothesized that heliotropism in sunflowers is based on the same basic mechanism, controlled by a molecule called phototropin and responding to light at the blue end of the spectrum. However, the underlying mechanism of heliotropism was unclear.

To better understand heliotropism, the researchers compared gene expression patterns in sunflowers grown in a controlled environment in a laboratory with those grown outdoors in field sunlight.

Figure 1 Phototropism and auto-straightening response of sunflower stems

Indoors, sunflowers growing directly into the light activated genes associated with phototropin. But sunflowers grown outdoors, swaying their heads in the sun, showed a completely different pattern of gene expression. There was no discernible difference in the phototropin proteins between one side of the stem and the other.

Figure 2 Transcriptome analysis of sun-tracking sunflower stems

However, some genes rapidly induced during phototropism, as well as genes involved in leaf shade growth response, were rapidly induced on the west side of the stem at the onset of phototropism, suggesting that red photoreceptors may play a role in sun tracking.

Fig. 3 Different patterns of gene expression during phototropism and heliotropism

To test the role of different photoreceptor signaling pathways in heliotropism, the researchers adjusted the light environment in which the plants initiate sun tracking. They found that blocking blue, ultraviolet, red, or far-red light with a blackout box had no effect on the heliotropism response and did not hinder the onset or maintenance of heliotropism in sunflowers in the field.

Figure 4 The onset of heliotropism shows distinct transcriptional signatures and occurs under different light conditions

Taken together, our results suggest that the transcriptional regulation of phototropism is distinct from that of phototropin-mediated phototropism. Both the onset and maintenance of heliotropism are resistant to changes in light quality and indicate that multiple light signaling pathways are involved in the regulation of sun tracking.

"We seem to have ruled out the phototropin pathway as a possibility, but we haven't found a clear smoking gun yet," Harmer said. Next steps will look at protein regulation in plants.

Data chart and reference source:
https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.3002344