Tracking the sun
Sunflowers track the sun during the day, while by night they ‘reset’ themselves, ready to start the morning by facing east. A process that scientists have called heliotropism, or the following of the sun. The regulatory mechanism behind this is still unknown, even though researchers from the University of California try to shine some light on it with their latest article in Plos Biology.
Sunflowers and other heliotropic plants follow the sun. They do this by growing a bit more on the sun facing side of the stems during the day. For the ‘reset’ during the night they grow a bit more on the shade side of the stems. For a long time scientist expected that heliotropism was a combination of phototropism, a.k.a. the growing towards the light, and autostraightening, a.k.a. growing with a straight posture. So when the researchers set out to find out how heliotropism is regulated they expected to find a mechanism that overlapped with phototropism and autostraighening. They did not.
In order to find a hint of the regulatory mechanism behind heliotropism they looked to gene expression using RNA sequencing analysis. They grew two week old sunflower plants inside with a blue light coming from one side. In addition, they grew two week old sunflower plants outside. From all these plants they collected samples at multiple times during the day. From the inside plant samples were taken from the shade and light facing side of the plant. From those growing outside they took samples from the east and west side of the plant. Then they compared the outcome of the analysis of these samples.
This study shed some light on how it is not regulated
The gene expression of the samples from plants that grew inside was as expected. With genes involved in phototropism showing differences in the shade and light facing side of the plant during the day. With a different set of genes likely to be involved in autostraigthening being activated during the night. So far, so good.
But when the researchers had a look at the analysis of the outside grown plants, they found something different. They observed the gene expression following a clear day-night rhythm, no matter whether the samples came from the east of west side of the plant. Although at the same time they did see only a very subtle phototropism or autostraigthening gene expression pattern. But nothing enough to be certain about. As if the observed heliotropism was something that was simply maintained, no changes in gene expression required.
To test this hypothesis the researchers studied plants that were just placed outside. Here they noticed that during the first day activation of some of the known phototropism genes. But only during the first day outside. This effect was gone the following day.
While not discovering the regulatory mechanism of heliotropism, this study shed some light on how it is not regulated. At the very start there might be some overlap with phototropism. But as soon as heliotropism gets underway, it is maintained in a still unknown independent way.
Literature
Brooks CJ, Atamian HS, Harmer SL (2023) Multiple light signaling pathways control solar tracking in sunflowers. PLOS Biology 21(10): e3002344. https://doi.org/10.1371/journal.pbio.3002344
Plant en Zo 