Scientists at the University of Geneva have cracked open a 400-million-year-old survival code—one that shows how the earliest land plants learned to shield themselves from the sun's most dangerous rays. By studying Marchantia polymorpha, a humble liverwort that belongs to one of Earth's most ancient plant lineages, an international team led by Roman Ulm has revealed how the fundamental architecture of plant protection against ultraviolet-B radiation was assembled, then refined, over eons.
Sunlight is essential: it powers photosynthesis, the chemical reaction that lets plants feed themselves and give us oxygen. But that same sunlight carries a threat. UV-B radiation damages DNA and cell membranes, much as it does in human skin. It can also sabotage the very cellular machinery that drives photosynthesis. Over millions of years, plants evolved a sophisticated early-warning system—a photoreceptor called UVR8 that acts like a smoke detector for harmful ultraviolet light. When UVR8 senses UV-B, it triggers a cascade of molecular signals that alters gene expression and ramps up protective molecules throughout the plant.
What makes this new research so significant is that it tracks how this defense mechanism changed—and what stayed the same—across evolutionary time. The researchers found that the core mechanism of UVR8 activation and deactivation is remarkably conserved. The basic building blocks that allowed liverworts to survive harmful sunlight more than 400 million years ago are still present in modern flowering plants like Arabidopsis thaliana. This ancestral design proved so elegant that evolution essentially kept it.
But here's where the story gets more nuanced. While the foundation remained stable, the way different regulatory proteins interact with this system shifted significantly. Take the protein called SPA. In flowering plants like Arabidopsis, SPA works closely with a central regulator called COP1 to control growth in response to light stress. In Marchantia, however, SPA plays a strikingly different role—one that actually constrains the plant's UV-B protective response. Marchantia mutants lacking the SPA protein even show greater tolerance to UV-B radiation, suggesting the ancestral version of SPA functioned more as a brake than an accelerator.
"Our results suggest that while the fundamental 'building blocks' of the system were already present very early in plant evolution, their organization and regulation have been progressively reshaped," explains Roman Ulm, the Full Professor in the Department of Plant Sciences at UNIGE who led the study. As postdoctoral researchers Yuanke Liang and Roman Podolec note, the evolutionary journey of this system reveals how plants didn't invent protection mechanisms from scratch—they inherited basic components and then gradually rewired them to fine-tune responses to environmental challenge.
In a world where climate change is reshaping light exposure patterns and atmospheric conditions, understanding how plants have adapted their defenses across deep time offers crucial insight. These findings, published in Plant Physiology, could help scientists anticipate how modern plants might respond to shifting light regimes and UV exposure in decades to come. The answer, it turns out, may lie in how plants have already solved this problem over hundreds of millions of years.