Airy shoots
Plants grow towards the light. A phenomenon that keeps fascinating researchers. Now Swiss researchers show in Science that having airy spaces in young shoots helps plants to determine from which side the light is coming.
Phototropism is the phenomenon of growing towards the light. Plants are using receptors that are sensitive to blue light to determine from which side the light is coming from. Blue light is turning these receptors on, so they can influence auxin distribution. When there is an equal amount of blue light from all sides, then no auxin gradient is formed. But when one side of the shoot is receiving more blue light then only the receptors of that side of the shoot turn on, enabling an auxin gradient to form. This in turn influences shoot growth: the shoot bends towards the light. But how exactly the blue light receptors perceive the difference in the amount of blue light was still unknown.
At least till now. Swiss researchers stumbled on a tale cress seedling with a transparent young shoot. When they studied the effect of this transparency they noticed that this plant was showing less phototropism. It still reacted to blue light, but very slowly. And often not as expected. This, it turned out, was not the fault of its blue light receptors, these still functioned fine.
Airy shoots create the necessary scattering of light
During the unravelling of this mystery, the researchers noticed one thing: transparent seedlings did not float in water. A thing that normal seedlings do, suggesting they contain more air. To analyse this the researchers studied slices of shoots under the microscope. By normal seedlings the space where three cells join is hollow. In contrast, in the transparent seedlings this space appeared to be filled.
Subsequently the researchers studied the effect these hollow spaces on light penetration. While in normal seedlings the hollow spaces strongly scatter the light. This did not occur in the transparent seedlings. The hollow, air containing spaces enable the forming of a stronger blue light gradient.
Lastly the researchers studied the effect of the blue light gradient on the activation of the blue light receptors. They observed that when normal seedlings receive blue light from one side, only the blue light receptors of that site are turn on. In contrast in the transparent seedlings the blue light receptors across the shoot were activated. They did not observe a blue light gradient.
Airy shoots create the necessary scattering of light. Only in this way a light gradient that is strong enough for blue light receptors to detect can be formed. Allowing plants to grow towards the light.
Literature
Ganesh M. Nawkar et al., (2023) Air channels create a directional light signal to regulate hypocotyl phototropism. Science382,935-940. DOI: 10.1126/science.adh9384
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