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How Plants Determine Where Light Comes From

Leaves in the sun
Credit: joffi/ Pixabay
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The majority of living organisms (micro-organisms, plants and animals) have the ability to determine the origin of a light source, even in the absence of an organ of vision comparable to the eye. This information is valuable for orienting yourself or positioning yourself optimally in the environment. This is particularly important for plants that use the direction of light to optimize the position of their organs, a phenomenon known as phototropism . This allows them to improve the capture of the sun's rays which they then transform into chemical energy through the process of photosynthesis, an essential device for their survival as well as that of the vast majority of the food chain, including humans.

An association between biologists and engineers

Although the photoreceptor that initiates phototropism has been known for a long time, the optical properties of photosensitive plant tissue have until now remained a mystery. A multidisciplinary study published in Science , combining the skills of DrSc teams. Christian Fankhauser (full professor and director of the Integrative Genomics Center of the Faculty of Biology and Medicine of UNIL), DrSc. Andreas Schüler (director of the Nanotechnology for Solar Energy Conversion group at the Solar Energy and Building Physics Laboratory at EPFL) and the Electron Microscopy Center at UNIL, helps us see things more clearly.

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A mutant form with intriguing transparency

“It all started from the observation of a mutant of the model species Arabidopsis thaliana , the Lady's Cress, whose stem was surprisingly transparent,” recalls Christian Fankhauser, who led the work. The UNIL biologist then decided to join the skills of his fellow EPFL engineer Andreas Schüler, in order to further study the optical properties specific to the two types of samples: mutant versus wild. “We found that the milky effect perceived in wild plants was in fact due to the presence of air in intercellular channels located in various plant tissues, and in particular at a part of the stem called the hypocotyl. In mutant specimens, the air is replaced by an aqueous liquid, which gives them a translucent appearance,” continues the researcher. Additional analyzes carried out in partnership with the UNIL Electron Microscopy Center confirmed these results.

A gradient of light like a rainbow

But what purpose can such air-filled channels serve? “They allow the photosensitive organ to establish a gradient of light “readable” by the plant. This can then determine the origin of the light source. This phenomenon is due to the different optical properties of air and water which make up the majority of living tissues. More precisely, air and water have very distinct refractive indices. This leads to diffusion of light when it passes through the hypocotyl of the seedling. We have all observed this phenomenon while admiring a rainbow,” explains Martina Legris , postdoctoral researcher in Professor Fankhauser’s group and co-first author of the study.

Thanks to their research, scientists have revealed a new mechanism, which allows living organisms to perceive where light comes from and thus be able to reorient themselves optimally. This study also served to better understand the formation of intercellular air-filled channels, whose functions in plants, in addition to the formation of light gradients, can be very diverse. “These channels promote, among other things, gas exchanges and also make it possible to resist hypoxia (reduction in the quantity of oxygen) in the event of flooding. Their ontogeny, i.e. their development from the embryonic stage to adulthood, nevertheless remains very poorly understood. The subject, perhaps, of a future study,” concludes Christian Fankhauser.

Reference: Nawkar GM, Legris M, Goyal A, et al. Air channels create a directional light signal to regulate hypocotyl phototropism. Science. 2023;382(6673):935-940. doi: 10.1126/science.adh9384

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