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As the Earth rotates and the Sun moves across the sky from east to west, sunflowers turn their bright yellow faces to follow it. They move during the day in order to track the Sun according to their biology, so they can maximize the benefits of sunlight through photosynthesis.
Knowing the mechanisms behind this movement is a mystery to scientists, which is what the study conducted by plant biologists at the University of California Davis It was published in the journal PLOS Biology in its issue issued on October 31 of last year.
Photoresponse
In the past, sunflowers’ ability to move toward light sources was thought to be driven primarily by a light-dependent response pathway known as phototropism, the statement from UC Davis says.
Photobending is the ability of plants to grow toward a light source. Sunflower flower heads swing by growing a little more on the east side of the stem, pushing the head west during the day and a little more on the west side at night, so the head swings back toward the east.
The University of California Davis statement adds that, because plants are rooted in one place, they cannot move if the light they need to make food is blocked by a neighbor or if they are in the shade, and plants depend on growth or elongation to move toward the light, and there are several Molecular systems behind it.
One of the best-known molecular systems is the photoresponse. Proteins called phototropins sense the blue light that falls unevenly on the seedlings, and growth hormones are redistributed in the plant, ultimately causing it to bend toward the light.
Botanists have long wondered how heliotropism works to allow a plant to follow the sun’s path across the sky while always keeping its face turned toward the sun. Although it was initially assumed to be a type of phototropism, scientists at the University of California Davis have discovered that heliotropism is its own distinct mechanism that is more complex and detailed, and involves the activation of a large number of genes through possible genetic rewiring.
Verifying hypotheses
To verify the hypothesis in explaining the reasons for sunflowers to bend and turn towards the sun, the scientific team that conducted the study used sunflowers grown in the laboratory and others grown outdoors under sunlight, in order to find out the genes that are activated when the two groups of plants were exposed to their light sources.
The researchers observed that indoor sunflowers grown directly toward a blue light source in the laboratory activated phototropin-related genes.
Flowers that grew outdoors and swung their heads in the sun had a different pattern of gene expression, with sunflowers having no obvious differences in phototropin molecules between one side of the stem and another.
“We were surprised when we were studying how these flowers track the sun every day,” says study co-author and plant biologist Stacey Harmer at the university. “We report in this paper that sunflower plants use different molecular pathways to initiate and maintain tracking movements, and that photoreceptors known to trigger “In plant bending, they seem to play a secondary role in this wonderful process.”
On the other hand, the team also blocked blue, ultraviolet, red, or far-red light using shadow boxes, so the experiment showed no effect on the response to heliotropism.
This shows that there are likely multiple pathways that respond to different wavelengths of light, and so researchers turned to identifying the genes involved in heliotropism.
To achieve this goal, the researchers moved the sunflowers grown in the laboratory outside, then began tracking the sun on their first day, where a huge wave of gene expression initially appeared on the shaded side of the plant, which did not happen in the following days.
“This suggests that a kind of ‘rewiring’ is occurring in the plant,” Harmer said. “In addition to eliminating some of the processes that underlie how sunflowers track the sun, this work is also important for designing future experiments with plants to understand their mechanisms,” Harmer said. “Things you determine in a controlled environment like a growth chamber may not work in the real world. Moving plants from indoors to outdoors has led to a wave of gene expression.”
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Sources:
- Statement issued by the University of California – Davis
- How Sunflowers See the Sun
- https://www.ucdavis.edu/curiosity/news/how-sunflowers-see-sun
- The study published in the journal PLOS Biology
- Multiple light signaling pathways control solar tracking in sunflowers
- https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.3002344
