How plants determine where light comes from

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Wild-type (left) and mutant (right) Arabidopsis thaliana seedlings with light fr
Wild-type (left) and mutant (right) Arabidopsis thaliana seedlings with light from the right. The mutant does not react to the light source. Martina Legris © CIG-UNIL

With no visual organs, how can a plant know where light is coming from? In an original study combining biological and engineering expertise, the team led by Prof. Christian Fankhauser at the University of Lausanne, in collaboration with colleagues at EPFL, has deciphered a novel mechanism using the interface between air and water to generate a gradient of light "visible" to the plant. These results have been published in the journal "Science".

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 a vision organ comparable to the eye. This information is invaluable for orienting oneself or positioning oneself optimally in the environment. This is particularly important for plants, which use the direction of light to optimize the position of their organs, a phenomenon known as phototropism. This enables them to better capture the sun’s rays, which they then transform into chemical energy through the process of photosynthesis, a device essential for their survival as well as that of the vast majority of the food chain, including humans.

A partnership between biologists and engineers

Although the photoreceptor that initiates phototropism has long been known, the optical properties of photosensitive plant tissue remained a mystery until now. A multidisciplinary study published in Science, combining the expertise of the teams of DrSc. Christian Fankhauser (full professor and director of the Centre intégratif de génomique at the University of Lausanne’s Faculty of Biology and Medicine), DrSc. Andreas Schüler (head of the Nanotechnology for Solar Energy Conversion group at EPFL’s Solar Energy and Building Physics Laboratory) and the Electron Microscopy Centre at the University of Lausanne, provides a clearer picture.

A mutant form with intriguing transparency

It all started with the observation of a mutant of the model species Arabidopsis thaliana, the Lady’s Slipper, whose stem was astonishingly transparent", explains Christian Fankhauser, who led the work. The biologist from the University of Lausanne then decided to enlist the skills of his fellow EPFL engineer Andreas Schüler, in order to further study the specific optical properties of the two types of sample: mutant versus wild-type. 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 in a part of the stem known as the hypocotyl. In mutant specimens, the air is replaced by an aqueous liquid, giving them a translucent appearance’, continues the researcher. Additional analyses carried out in partnership with the University of Lausanne’s electron microscopy center have confirmed these results.

A rainbow gradient of light

But what purpose do such air-filled channels serve? They enable the photosensitive organ to establish a light gradient that can be ’read’ by the plant. The plant 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. Specifically, air and water have very different refractive indices. This leads to a scattering of light as it passes through the hypocotyl of the seedling. We’ve all seen this phenomenon when admiring a rainbow", explains Martina Legris , postdoctoral fellow in Prof. Fankhauser’s group and co-first author of the study.

Thanks to their research, the scientists have revealed a novel mechanism that enables living organisms to perceive where light is coming from, and thus be able to reorient themselves optimally. The study also provided a better understanding of the formation of air-filled intercellular channels, whose functions in plants, in addition to the formation of light gradients, can be very diverse. Among other things, these channels promote gas exchange and help resist hypoxia (reduced oxygen levels) in the event of flooding. Their ontogeny, i.e. their development from the embryonic stage to adulthood, is still poorly understood. Perhaps the subject of a future study’, concludes Christian Fankhauser.