Proc. Natl Acad. Sci. USA http://doi.org/8c5 (2015)

Two billion years ago, oxygenic photosynthesis appeared in cyanobacteria. Another billion years later, endosymbiosis produced the first eukaryotic marine plants. The next step happened 450 million years ago, during the Palaeozoic era when plants began growing on land, an event that had a considerable impact on the history of life.

Jean-Michel Ané, Pierre-Marc Delaux and colleagues performed a comprehensive phylogenetic analysis of hundreds of algae and early-diverging land plants, focusing on the symbiotic signalling pathway. Most land plants today form a beneficial arbuscular mycorrhizal association with fungi, allowing them to increase their mineral nutrient and water uptake. The result was surprising: algae were prepared for the transition to land. While still in water, they had already developed parts of the molecular pathway allowing them to associate with fungi — which were already on land — to form the symbiosis necessary for their survival.

Key signalling components such as a calcium-and-calmodulin-dependent kinase were present in early chlorophyte algae, the gradual functional evolution of which was assessed by complementation of a Medicago truncatula mutant. Other downstream components of the symbiotic pathway evolved after land colonization. These discoveries fit well with the scant fossil record, and expose the successful innovations that allowed plants to thrive outside of water.