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How aquatic plants changed the face of the Earth
Scientists use the term “terrestrialisation” to describe the process whereby plants first began to colonise the land. This unprecedented evolutionary leap is thought to have occurred between 500 and 470 million years in the past, not so long ago on geological time scales.
"Studies of the oldest fossils of early land plants, namely spores (reproductive units), suggest that this transition took place 470 million years ago," says Christine Strullu-Derrien, a palaeobotanist specialising in the origin of plants and fungi at the Paris-based ISYEB institute of systematics, evolution and biodiversity1. "However, according to molecular phylogenetics, which ìs used to study the evolutionary history of living organisms by analysing their DNA or proteins, it dates back at least 500 million years."
The first plants to successfully colonise dry land were species without flowers or seeds that evolved from a group of freshwater algae. This colonisation is thought to have occurred exclusively from lakes, rivers and ponds – and not from seas or oceans. "In fact, the green algae that are most closely related to land plants are found only in fresh water, and not in the marine environment," points out Pierre-Marc Delaux, at the LRSV laboratory for plant science research2 in Toulouse (southwestern France). "So the most likely hypothesis is that their common ancestor also evolved in fresh water."
Symbiotic association
But, above all, this colonisation would probably never have happened had there not been microscopic fungi in the soil. According to a hypothesis drawn up in the 1980s based on the study of fossils, it was thanks to a symbiotic association with fungi that the first plants – devoid of roots, and therefore unable to extract minerals from the soil – were able to survive.
This symbiosis, called a mycorrhiza (from the ancient Greek μύκης / múkēs , “fungus”, and ῥίζα / rhíza, “root"), is a win-win partnership that lets fungi provide plants with soil resources (nutrients and water), thanks to their fine filaments that grow deep into the ground; and in return enables plants to supply fungi with sugars produced via photosynthesis, as well as with organic compounds, including lipids, that they cannot produce themselves.
Signalling genes
In 2021, new phylogenetic studies carried out by Delaux and his colleagues confirmed the hypothesis. They showed that present-day vascular plants (plants with roots and stems) possess symbiotic genes, which suggests that their common ancestor must also have contained them. The researchers then inoculated a highly divergent non-vascular plant species, a moss called Marchantia paleacea, with a fungus, and analysed the expression of the plant's genome over time. "We observed that the symbiosis triggered various processes, including the production of lipids," Delaux explains. "As a second piece of evidence, we also showed that if these symbiotic genes are removed from Marchantia paleacea, it loses its ability to associate effectively with soil fungi."
To reach this conclusion, the researchers used CRISPR molecular scissors, a powerful tool used in molecular biology to cut DNA with great precision. And finally, in a recently-published piece of research, the same team identified several “signalling” genes that vascular and non-vascular plants have shared for millions of years, "and which are there to tell them to activate the symbiotic network at the cell level", Delaux says. "This research shows that these genes have been handed down from generation to generation in every land plant, all the way back to their common terrestrial ancestor, which already had this symbiotic ability 450 million years ago."
New environment, new constraints
To survive and flourish in their new environment, the first terrestrial plants also had to adapt to various conditions specific to life on dry land, such as "limited water availability, significant temperature variations, exposure to harmful ultraviolet (UV) rays from the Sun and to pathogens, and so on", explains Hugues Renault, a researcher in plant biology at the Institute for Plant Molecular Biology (IBMP)3, in Strasbourg (northeastern France).
In land plants, hydrophobic polymers deposited on the surface of various tissues (such as the cuticle, the thin film that covers their outer walls) are known to help limit water loss and resist stress caused by extreme environmental conditions. This means that they could have been innovations caused by terrestrialisation.
In 2024, Renault and his colleagues studied a family of genes (CYP73) that generate molecules with antioxidant and anti-UV properties, and others that form protective extracellular barriers such as the cuticle. When they inactivated these genes in several species of land plant, they observed anomalous development in them, as well as greater sensitivity to drought. "Our results show that plants were able to leave the water partly due to CYP73 genes, which emerged in a common ancestor of land plants and have been strictly conserved ever since," says Renault.
Vertebrates, soils, and atmosphere: how the Earth was transformed
The migration of plants out of the water completely changed the face of our planet. "Whereas up until 500 million years ago, life on Earth was limited to aquatic habitats, the conquest of dry land by plants helped to extend the range of environments capable of supporting life," Strullu-Derrien points out. "Between 419 and 358.9 million years ago, another group of organisms, the vertebrate animals, which fed on plants, emerged from the water."
The spread of plants across the land also had a major impact on the soil of our planet. "The decomposition of plant tissue as it died off led to the formation of humus, the upper layer of the soil, which is crucial for its fertility," explains Brigitte Meyer-Berthaud, a palaeobotanist emeritus at the Botany and Modelling of Plant Architecture and Vegetation (AMAP) laboratory4, in Montpellier (southern France). "Plant roots also helped to break up rocks. In addition, these same roots secreted an acidic solution that dissolved the bedrock, producing clays that contributed to fertility."
Besides, the proliferation of plants on Earth drastically changed the composition of its atmosphere. "Thanks to photosynthesis, which enables plants to make organic matter by capturing carbon dioxide (CO2) from the air, the CO2 content of the atmosphere was reduced by a factor of ten, according to some computer models," Meyer-Berthaud explains. "Plants also helped to cool the atmosphere via the process of evapotranspiration, in which some of the water in plants is returned to the surrounding atmosphere as water vapour."
We can therefore thank the spread of plants across the world's landmasses for the emergence of the terrestrial ecosystems we know today. ♦
See also:
The Earth, precariously balanced
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Author
A freelance science journalist for ten years, Kheira Bettayeb specializes in the fields of medicine, biology, neuroscience, zoology, astronomy, physics and technology. She writes primarily for prominent national (France) magazines.