Investigating the symbiosis between plants and specific microorganisms that inhabit their leaves could have an important bearing on understanding plant adaptation in an era of climate change. “We are trying to understand how plants are diversifying and why some plants are doing well and some are not and how they are possibly adapting to different climatic changes,” says the EU-supported Symbiosis project’s coordinator, Nina Rønsted. Scientists studied plant-specific bacteria that reside within the leaves of certain plant species. Dr Brecht Verstraete, who received a two-year EU Marie Skłodowska-Curie Individual Fellowship to work with Professor Rønsted, found the symbiosis exists in only a few groups of plants. One of them is the Vangueria ‘tribe’ of the coffee family Rubiaceae. The genus has around 600 species. ‘It is in this group of plants that most of the occurrences of the symbiosis has been detected,’ he notes. “What we found is that every species hosting these bacteria appears to have its own unique bacterial species – it is a one-to-one relationship.” DNA analysis and dating Plants were collected during expeditions, mainly to East Africa and South Africa. The DNA was extracted from the leaves and certain parts of the DNA were sequenced. “We got hold of around 160 species, half of them containing bacteria and half of them not, and then compared the DNA sequences so that we could see how fast they were evolving.” They diversified and created new species over time, Dr Verstraete explains. Looking at the DNA sequences using the molecular clock – a technique that uses the mutation rate of DNA molecules to determine when, in prehistory, mutations occurred and then calibrating them using fossils of that particular plant where available, he was able to date when the symbiosis with the bacteria appeared. The plant fossils were dated using geological dating methods including carbon dating. “We linked the fossil data with the DNA sequence to date how old the species are,” Dr Verstraete explains. Such techniques have been used to investigate plants or animals separately, but this was the first time they have been used to establish a link between plants and the plant-specific bacteria. Mutations occurred as Africa was cooling Dr Verstraete established the origin of the bacterial symbiosis at around 11 million years ago in the Late Miocene era. Around that time there was a global cooling and an overall decrease in carbon dioxide in the atmosphere. When the African continental plate collided with the Eurasian continent, Africa became drier, and the coast-to-coast rainforest that covered the continent shrank and was replaced by open grassy plains or savannas. “This was a very tough time for plants, as the mammals, the big grazers, were everywhere wanting to eat them,” says Professor Rønsted. The bacteria in the leaves may have kept the animals away so the plants survived. Professor Rønsted, describes it as a kind of ‘chemical defence’ compared to defences such as spines or thorns. “It turns out that the plants don’t do very well when the bacteria are not there,” Professor Rønsted says. “The plant seems to be dependent on interaction with these bacteria and when you compare the lineages of the ones with these bacteria with the ones without, we find many more species. We know the plants that have these bacteria create more species faster although the extinction rate seems to be the same.” This is an advantage for the plants associated with these bacteria, Professor Rønsted says. “Where there is a lot of variation, there is a very good basis for adaptation if it should be needed.” Knowing that plants with the bacterial symbiosis may be better at adapting to climate change, for example, can be useful for more effective nature management in the future, she explains.
Symbiosis, DNA, climate change, plant adaptation, evolution, Africa, coffee, fossils, botany