A “friendship” apparently formed the basis of the green revolution on our planet: Researchers present new evidence that plant colonization of the country was made possible around 450 million years ago through a symbiotic relationship with fungi. They were able to show that even the simple liverworts – which are considered living fossils – offer their fungal partners lipids as “commercial goods” – similar to the highly developed plant species. In the case of algae, however, there is no corresponding lipid biosynthesis. From this, the scientists conclude that this system goes back to the time when the first plants turned the country green with the support of their mushroom partners.
Most of today’s plant species have loyal partners: They enter into an intimate exchange relationship with soil-dwelling fungus species, through which they can obtain additional nutrients and water from the soil. In this so-called mycorrhizal symbiosis, the partners exchange nutrients via a connection between the widespread fungal network in the soil and the plant roots. In the most common form, the “trade” takes place via a complex structure inside the roots: the fungus penetrates them with its hyphae and forms a tree-like structure (arbuscle) in some cells together with the plant. The nutrient transfer is then carried out over the large surface of this structure. The fungus gives the plant water and, above all, the nutrient phosphate, which it has absorbed from the soil. In return, the plant supplies him with energy-rich substances. Studies have shown that in addition to sugar molecules, lipids are also involved.
The pioneers’ recipe for success?
As early as the 1980s, studies of plant fossils showed that the mycorrhizal symbiosis is ancient. The suspicion arose that the partnership might have made it possible for the plants to go ashore around 450 million years ago: in their new, harsh habitat, the fungi were probably able to provide the pioneering plants with water and nutrients in a crucial way. The current study by the researchers led by Mélanie Rich from the University of Toulouse supports this assumption with genetic and molecular biological evidence.
You are building on long-term research results on the genetic makeup that enable plants to enter into the complex interrelationship with fungi. So far, the focus has been on modern vascular plants (tracheophytes) such as beans, potatoes and the like. But Rich and her colleagues have now devoted themselves to plants that still resemble the pioneers of plant-based shore leave: mosses (bryophytes). It is believed that they evolved from green algae in the intertidal zone 450 million years ago.
It is known that these primeval plants can also form partnerships with fungi – but the genetic and molecular basis of these fungus-moss symbioses has not yet been explored. As part of their study, the researchers have now looked at the liverwort Marchantia paleacea as a model that forms an arbuscular mycorrhizal symbiosis with a fungus. Using genetic and molecular biological methods, the team investigated which hereditary factors and processes are active in the plants when the tree-like exchange structures arise in their cells and come into action.
Ancient principle
They were able to prove that lipids are also transferred from the plant to the fungus in liverworts. The researchers were able to make it clear that this is the same basic principle of lipid transfer as in modern vascular plants. As with these, the process is also a prerequisite for the ability to form symbiosis with mosses. The researchers were able to show this on genetically modified moss breeding lines, in which they had switched off a genetic make-up necessary for lipid transfer. The studies showed that these little plants could no longer establish a partnership with the fungus.
The researchers were also able to make it clear that the lipid biosynthesis system of vascular plants and mosses necessary for the symbiosis does not exist in the aquatic algae from which the green land pioneers are believed to have originated. In these results you see a clear indication that the development of the mycorrizal symbiosis was at the very beginning of the development history of land plants and possibly represented a prerequisite for the successful conquest of the new habitat. “Our results suggest that the common ancestor of all land plants already entered into a partnership with arbuscular mycorrhizal fungi 450 million years ago,” the scientists write.
Source: CNRS, technical article: Science, doi: 10.1126 / science.abg0929