How do plants know which direction light is coming from?

Flowering plants tend to grow towards sunlight to position their organs to better capture light, which they then convert into chemical energy through photosynthesis.

This phenomenon is known as phototropism. In flowering plants, light direction is sensed by blue light–absorbing phototropin receptors. More activation of the photoreceptors happens towards the lit side, and less in shaded parts. And it’s this gradient of phototropin activation that allows plants to determine the origin of a light source.

But what are the physical properties of the plant tissues that allows this to exist?

Researchers from the University of Lausanne in Switzerland have zeroed in on tiny air channels inside the plant’s cells that enhance this gradient by scattering light. They’ve written up their research in a new study in the journal Science.

“It all started with the observation of a mutant of the model species Arabidopsis thaliana, the thale cress, whose stem was surprisingly transparent,” says Professor Christian Fankhauser, Director of the Integrative Genomics Centre at the University of Lausanne (UNIL), Switzerland, who led the research.

“We found that the natural milky appearance of the stems of young wild plants was in fact due to the presence of air in intercellular channels precisely located in various tissues. In the mutant specimens, the air is replaced by an aqueous liquid, giving them a translucent appearance.”

These plants failed to respond to light correctly. This is because the different optical properties of air and water in normal plant tissues would usually lead to light scattering as it passes through the seedling.

“More specifically, air and water have different refractive indices. We have all observed this phenomenon when admiring a rainbow,” explains Martina Legris, a postdoctoral fellow at UNIL and co-first author of the study.

However, the mutant lacked this property and couldn’t establish a light gradient as a result. This means that wasn’t as big of a difference in phototropic receptor activation in the lit regions, versus shaded regions of the plant, because the light could travel through it more easily.

These air-filled intracellular channels also have a range of other functions in plants, such as promoting the exchange of carbon dioxide and oxygen in the leaf and resisting hypoxia (reduction in the quantity of oxygen) in the event of flooding. 

The genetic resources used in this study will be useful better understanding their development of plants from the embryonic stage to adulthood.

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