Scientists have discovered molecular evidence indicating that chloroplasts began as energy-producing organelles, and later adapted to assist in carbon assimilation within plant cells.
One of the pivotal moments in the evolution of life on Earth was the process known as endosymbiosis. This is where one organism consumes another, but instead of digesting it, the first organism integrates its DNA and functions. It is widely accepted by scientists that this event occurred twice in evolutionary history, leading to the formation of mitochondria and, subsequently, the photosynthetic plastids.
A study recently published in Nature Communications investigates the origin of chloroplasts, which are plastids enabling plants to absorb carbon from the air to create their cellular structures and tissues. By examining a molecule involved in energy transport within plastids, the researchers uncovered evidence that primitive chloroplasts may have initially served the primary function of generating chemical energy for the cell, before their main role transitioned to carbon assimilation.
Chloroplasts are thought to have evolved from photosynthetic cyanobacteria; however, it remains unclear what function the original cyanobacteria served for the cells that engulfed them, according to Angad Mehta, a chemistry professor at the University of Illinois Urbana-Champaign and lead author of the new research.
“We questioned the chemical role played by the primitive symbiont that eventually became chloroplasts for the host cell,” Mehta explained. “Did it contribute to carbon assimilation, ATP synthesis, or both?”
Research indicates that the plastids found in red algae and another group of photosynthetic organisms called glaucophytes may represent earlier evolutionary forms compared to the chloroplasts found in terrestrial plants. However, current bioinformatics techniques have their limits, Mehta noted.
According to him, understanding the evolution of mitochondria and plastids hinges on their ability to generate energy. Both types of organelles produce ATP, a crucial energy-rich molecule that powers most chemical reactions in living cells. They also utilize ADP/ATP carrier translocases, located within the organellar membranes, which exchange ADP for ATP once its energy has been spent.
Mehta and his team examined variations in the activity of translocases in the plastids of land plants, red algae, and glaucophytes to gather insights into the evolution of chloroplasts.
Through a series of experiments, the researchers modified cyanobacteria to express one of three translocase types. They then created artificial endosymbiosis between these engineered cyanobacteria and budding yeast cells. By controlling the lab environment, they made the yeast reliant solely on the cyanobacterial endosymbionts for energy. Mehta’s lab initially developed this method to induce internalization of cyanobacterial endosymbionts in a 2022 study.
The findings revealed notable differences in translocase activity.
“We observed a significant difference: endosymbionts with translocases from red algae and glaucophytes could export ATP to sustain endosymbiosis, while those with chloroplast translocases imported ATP and could not meet the energy demands of the endosymbiotic cells,” Mehta noted. The translocases from land plant chloroplasts were taking in ATP and releasing ADP.
Since the plastids of red algae and glaucophytes appear to mirror a more ancient form of photosynthetic organelles, the new research suggests that chloroplasts initially had a similar function of supplying energy to the host cell. However, at some point in evolution, land plant chloroplasts seem to have shifted to using ATP derived from photosynthesis to power their own carbon-assimilation processes. It also seems that chloroplasts may even use some ATP produced by mitochondria, Mehta added.
While the new data doesn’t conclusively prove this evolutionary pathway for chloroplasts, it does bolster this perspective, according to Mehta.
“The hypothesis proposes that the original relationship between the endosymbiont and the host cell was based on ATP production and supply,” he said. “One could envision a scenario where, as these organisms evolved into land plants in oxygen-rich environments, mitochondria became specialized in producing ATP while chloroplasts optimized for carbon assimilation.”
This research was supported by the Gordon and Betty Moore Foundation and the National Institute of General Medical Sciences at the National Institutes of Health. Mehta is also associated with the Carl R. Woese Institute for Genomic Biology at the University of Illinois.
