Peer into a plant cell and you’ll find the engine at its heart is a stowaway. In size, shape, and the genes it carries it resembles a bacterium: a cyanobacterium, to be precise.
It’s a very valuable stowaway. Known as a chloroplast, this cyanobacteria-like interloper carries the machinery for photosynthesis, the process by which plants use sunlight to power the synthesis of sugar. As a side effect, photosynthesis also produces the oxygen that we breathe and require for cellular respiration.
The evolution of this stowaway is believed to have occurred by a process called endosymbiosis, whereby a cyanobacterium was engulfed by a larger single-celled eukaryote – a more complex kind of cell that contains discrete organelles surrounded by membranes. Eventually a symbiotic relationship formed between the two which led to the development of photosynthetic plant cells. Until recently, however, the date of this key event in the history of life on Earth remained unknown.
Enter Dr Patricia Sánchez-Baracaldo of the University of Bristol and colleagues. In research outlined in a paper published in the Proceedings of the National Academy of Science, they analysed 29 different cyanobacterial genome sequences to conduct a ‘molecular clock analysis’ to determine the exact period at which the cyanobacteria were engulfed.
The study revealed that the chloroplast diverged from its closest relative, the cyanobacterium Gloeomargarita, around 2.1 billion years ago. It also showed that the divergence occurred in a freshwater environment, rather than in a marine environment as previously thought, and that it took another 200 million years for the first photosynthetic eukaryote to evolve.
Dr Geoff McFadden, a researcher at the University of Melbourne who was not involved in the study, says it’s a significant step forward in our understanding.
“This study has a lot of credence due the number of genetic sequences used,”says McFadden. “It has provided a lot of evidence for a time point which was not previously known.”
McFadden did question the large gap in time between the endosymbiosis event and the evolution of the photosynthetic eukaryotes, suggesting that it is an area that should be further explored.
Ariella Heffernan-Marks in a Melbourne-based science writer.
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