Chasing the Sun

Sunflowers, true to their name, follow the sun with their bright yellow blooms. Their stems bend as they track the solar movement from east to west; then, when the sun sets, the flowers reorient themselves toward the east to await the next sunrise.

This phenomenon, called heliotropism, is one of many ways that light can control the movement of plants. But researchers still don’t fully understand how cells control these light-triggered behaviors at the molecular level. In a recent study in PLOS Biology, plant biologists at the University of California, Davis, used gene expression profiling in sunflowers to get a closer look at the molecules at play during different light tracking behaviors.¹

“Because plants are rooted in their environment, they are just so adept at anticipating and responding to environmental cues,” said Stacey Harmer, a plant biologist at the University of California, Davis, and coauthor of the study. “It isn't the way that animals learn, but it's still super impressive.”

Harmer previously studied how sunflowers’ light-seeking movement is controlled by their internal “clocks”—molecular pathways that help plants execute cyclical, time-dependent behaviors.² Sunflowers are an especially good species to study because they are so big and move so dramatically. In an earlier study, her team found that in the morning, when the sun is still to the east, cells on the west side of the stem swell up with water and grow longer to allow the stem to bend to the east. Then, when the sun crosses to the west in the afternoon, cells on the east side of the stem grow longer.

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In their new study, her team wanted to learn more about the molecular pathways that were active in sunflowers when they tracked the sun and other light sources. To do this, they first grew the plants in a lab setting where they could control the position, amount, and type of light that the plants received. Even with artificial light, the sunflowers bent toward the light, and automatically bent away from the light after 10 hours, mimicking their carefully timed night time reorientation. The researchers then collected pieces of the lit and shady sides of the stem and measured the amount of RNA encoding various proteins and pathways. As they suspected from previous work in other plant species, the auxin pathway was more active on the shady side of the stem where it could promote growth and bending toward the light.³

However, Harmer was surprised to see that the genes expressed while the plant straightened away from the light were not the same as those expressed while the plant bent toward the light, and they did not include the auxin pathway. Her team moved sunflowers outside into a field to measure genes expressed during solar tracking, and found that they were also different from the genes expressed as the plants bent toward a stationary light indoors. In their new surroundings, it only took the plants one day to learn to track the sun and reorient at night.

“Even though they don't have brains to anticipate the timing and the direction of sunrise, they seem to learn this very quickly,” said Harmer.

Based on the diversity of genes expressed in the plants under different conditions, Harmer concluded that the plants’ ability to turn toward light may be controlled by different molecular pathways than those used to track the sun, and there may even be multiple pathways in play.

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Mannie Liscum, a plant biologist at the University of Missouri who was not involved in the study, was intrigued to see the comparisons between the bending, straightening, and solar tracking behaviors. But, he also noted that Harmer’s results in this study seemed to mirror previous studies, including his own prior work in cabbage and related plants.⁴ He pointed out that studying RNA alone might not be enough to conclusively say that the pathways are different. “Gene expression is not the end-all, be-all,” said Liscum.

To better understand the pathways, Harmer plans to look at differences in proteins next. She thinks that the pathways in sunflowers may also be relevant in other plants, although more studies are required.

“Working in plants, we have our expectations upturned all the time,” said Harmer. “It has been fascinating and frustrating at the same time.”