Sunlight provides the energy necessary for photosynthesis and growth, but it also exposes plants to harmful ultraviolet-B (UV-B) radiation. Plants must therefore strike a delicate balance between growth and protection. By studying Marchantia polymorpha, a plant similar to some of the earliest land plants, an international team led by scientists from the University of Geneva (UNIGE) sheds light on the evolution of fundamental UV-B perception mechanisms and plant adaptation strategies to light stress. In a context where climate change is altering light exposure conditions, these findings, published in Plant Physiology, could prove particularly valuable.
Light is essential for photosynthesis, the process by which plants produce organic molecules (sugars) and release oxygen. However, it can also be harmful. Like in humans, UV-B radiation can cause DNA damage and harm cell membranes. It can also disrupt the cellular systems responsible for photosynthesis. Over the course of evolution, plants have developed a protective system based on a key photoreceptor called UVR8, which allows them to detect UV-B radiation. When this sensor absorbs UV-B light, it triggers a cascade of molecular reactions that alter the expression of numerous genes, as well as the production of molecules involved in protection and acclimation.
In flowering plants such as Arabidopsis thaliana (thale cress), this signalling pathway involves several regulatory proteins that control genes related to growth and tolerance to light stress. But how did this defence system evolve? The laboratory of Roman Ulm, Full Professor in the Department of Plant Sciences at the Section of Biology, Faculty of Science, UNIGE, focused on the liverwort Marchantia polymorpha, a species belonging to a lineage that emerged when the first plants began colonising land more than 400 million years ago.
An ancestral defence system
The researchers show that the core mechanism activating UVR8 is remarkably conserved between Marchantia and modern flowering plants. This ancestral core includes both the activation of the UVR8 photoreceptor by UV-B radiation and its deactivation mechanism.
However, the study also reveals important changes in how these components interact. “Our work shows that in Marchantia polymorpha, certain regulatory proteins play roles that differ from those observed in more recent plants. For example, the SPA protein, which works together with the central regulator COP1 in controlling plant growth in Arabidopsis, plays a very different role in Marchantia. While it strongly contributes to developmental regulation in flowering plants, its influence appears much more limited in this ancestral liverwort. Marchantia mutants lacking SPA even show increased tolerance to UV-B, suggesting that this protein acts here as a brake on the protective response,” explain Yuanke Liang and Roman Podolec, postdoctoral researchers in Roman Ulm’s laboratory and co-first authors of the study.
“Our results suggest that while the fundamental ‘building blocks’ of the system were already present very early in plant evolution, their organisation and regulation have been progressively reshaped,” summarises Roman Ulm.
By providing new insights into how plants adapted to light over evolutionary time, this study contributes to a better understanding of plant resilience to environmental stress. In the context of climate change, such knowledge could help anticipate how plants will respond to changing light.