The way a young sunflower turns its bright yellow head to follow the movements of the Sun across the sky each day can be quite dramatic, in terms of plant activity. Now scientists have been surprised to discover that it does this through a novel genetic mechanism that defies previous research into the sunflower's distinctive solar swivel.
"This was a total surprise for us," said Stacey Harmer, professor of plant biology at the University of California (UC), Davis and senior author on the paper.
Plants express phototropism, which literally translates to ‘light turning.’ This complex molecular, biochemical and cellular process facilitates plant growth towards a blue-spectrum light stimulus, to maximize all-important photosynthesis.
The common sunflower (Helianthus annuus), however, expresses a different kind of process, heliotropism, which means ‘Sun turning,’ which has long been thought of as a specialized type of phototropism. The process sees plants bend from east to west during the day, following the Sun’s path, and can be seen in flowers such as daisies, morning glories and poppies.
However, the biologists from UC Davis have found that the sunflower, in fact, has a completely different set of genes at play in its special brand of heliotropism, and the transcriptional regulation demonstrates a totally different mechanism that allows the plant to move. It was previously thought that this behavior would be very similar to the phototropin-driven phototropism mechanism.
"We seem to have ruled out the phototropin pathway, but we did not find a clear smoking gun,” Harmer said.
In the study, Harmer, graduate student Christopher Brooks and postdoctoral researcher Hagatop Atamian looked at what genes switched on in sunflowers indoors when grown in the laboratory, and which ones transcribed naturally in outdoor solar light.
They found that, indoors, the plants grew directly towards the light, activating the genes associated with phototropins, the blue light receptor protein kinases that facilitates phototropism. But when grown outdoors, the sunflowers demonstrated a completely different pattern of gene expression, with no differences in phototropins across the sides of the stems (in plants, this difference generally drives the stem’s movement towards the light stimuli).
“The dissimilarity between gene expression in heliotropic plants in natural light and phototropic plants in monochromatic blue light leads us to suggest that multiple photoreceptors contribute to heliotropic movements in sunflower,” the researchers noted in the paper.
The team also blocked blue, ultraviolet, red and far-red light, separately, with shadeboxes, and saw it did not impede natural heliotropism, which suggests the sun-salute process is a complex pathway involving a combination of light wavelengths.
Interestingly, when the sunflowers were moved from indoors to out, they instantly began tracking the Sun on the first day. With this, came a supercharged amount of gene expression on the shaded side of the stem, which dissipated after that first day, suggesting to the researchers that the sunflower was doing some “rewiring” as it switched gears to quickly adapt to its new light source and the different molecular pathways it required.
This also sheds light on research, showing that laboratory-based studies may only provide a snapshot of the whole picture.
“Things that you define in a controlled environment like a growth chamber may not work out in the real world,” Harmer said.
While the team has not yet identified the genes involved in the sunflower’s special heliotropism, they’ll be working towards it, looking at protein regulation in the plants.
The research was published in the journal PLOS Biology. To see how the sunflower follows the Sun, check out this time-lapse video below.
Source: UC Davis