Where Does the Light Come From? Optical Properties Allow Plants to “See” Light

How can plants know where light originates from when they lack visual organs? An innovative study led by Professor Christian Fankhauser at UNIL and colleagues at EPFL has combined expertise in biology and engineering to reveal that a light-sensitive plant tissue uses the optical properties of the air-water interface to create a light gradient that is ‘visible’ to the plant. The findings were published in Science.

Most living things, including microorganisms, plants, and animals, can identify the source of light even when they lack an organ of sight similar to the eye. This is quite helpful for getting oneself oriented or positioned as best one can be in the surroundings. Plants are extremely sensitive to light intensity because they utilize this information to arrange their organs in a process called phototropism.

This makes it possible for them to absorb more of the sun's energy, which they subsequently use to fuel the process of photosynthesis, which is essential for the synthesis of almost all of the food humans eat.

The optical characteristics of photosensitive plant tissue are still unknown despite the fact that the photoreceptor that triggers phototropism has long been identified.

A surprising tissue feature that enables plants to detect directional light cues was discovered by a multidisciplinary study published in Science, combining the expertise of teams led by Dr. Sc. Christian Fankhauser (full professor and director of the Integrative Genomics Centre in the Faculty of Biology and Medicine at UNIL), Dr. Andreas Schüler (head of the Nanotechnology for Solar Energy Conversion group at EPFL’s Solar Energy and Building Physics Laboratory), and UNIL’s Electron Microscopy Center.

It all started with the observation of a mutant of the model species Arabidopsis thaliana, the thale cress, whose stem was surprisingly transparent.”

Christian Fankhauser, Study Lead, Full Professor and Director, Integrative Genomics Centre, Faculty of Biology and Medicine, University of Lausanne

These plants did not react appropriately to light. The UNIL biologist subsequently made the decision to enlist the assistance of his EPFL colleague Andreas Schüler to compare the mutant samples’ unique optical properties with those of the wild-type samples in more detail.

“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,” Fankhauser added.

However, what function do these channels loaded with air serve? They allow the light gradient to be established by the photosensitive stem so that the plant can “read” it. The plant can then identify where the light source is coming from. The difference in optical properties of water and air, which comprise most biological tissue, is the cause of this phenomenon.

More specifically, air and water have different refractive indices. This leads to light scattering as it passes through the seedling. We have all observed this phenomenon when admiring a rainbow.”

Martina Legris, Study Co-First Author and Postdoctoral Fellow, University of Lausanne

The scientists’ research has found a breakthrough mechanism that allows organisms to understand where the light is coming from, allowing them to orient their organs, such as leaves, to optimum light capture for photosynthesis. In addition to the production of light gradients, the study improved the understanding of the formation of air-filled intercellular channels, which serve a variety of functions in plants.

These channels, among other things, enhance gas exchange and allow users to withstand hypoxia (a decrease in the amount of oxygen) in the event of floods. Their growth from the embryonic stage to adulthood remains a mystery. The genetic resources used in this study will be beneficial in understanding the genesis and maintenance of these fascinating structures.