Photosynthesis in Hornworts

Photosynthesis is the most important biological reaction on the planet. It creates the sugars and starches that we and other animals rely upon for food. Photosynthesis also helps to regulate the climate by binding up carbon from the air to keep the planet cooler. Plants need carbon dioxide, water, and light to carry out photosynthesis. 

Hornworts have a cool structure called a pyrenoid that helps to increase the rate of photosynthesis in these plants. Pyrenoids increase the concentration of carbon dioxide close to the enzyme RuBisCO, which is critical for photosynthesis. A recent study examined pyrenoid evolution in the hornworts, the bryophyte lineage most closely related to flowering plants. They asked whether the evolution of the pyrenoid in hornworts was correlated with historically low levels of carbon dioxide in the atmosphere. It is predicted that low levels of carbon dioxide in the atmosphere would put pressure on plants to evolve mechanisms that enable them to increase the concentration of carbon dioxide in their cells in order to increase rates of photosynthesis. 



Villarreal, J. & Renner, S. (2012). Hornwort pyrenoids, carbon-concentrating structures, evolved and were lost at least five times during the last 100 million years Proceedings of the National Academy of Sciences, 109 (46), 18873-18878 DOI: 10.1073/pnas.1213498109

Their results support the pyrenoid structure evolving 5 or 6 times across the hornworts (transition from blue to red in the figure below). Pyrenoid evolution does not appear to be synchronous, each time the pyrenoid evolved across the tree was at a different time in the past. If they had evolved in response to changes in the atmosphere, we would predict that they evolved at the same time. They also did not find a relationship between low atmospheric levels of carbon dioxide and pyrenoid evolution. Even when atmospheric carbon dioxide levels were low, new hornwort species evolved that did not have a pyrenoid. If the pyrenoid was really advantageous, we would predict that when the carbon dioxide levels were low only species with a pyrenoid would evolve new species. Based on this and other findings, they propose that the evolution of the pyrenoid may be related to something other than the atmospheric concentration of carbon dioxide

Figure 1 from Villarreal & Renner 2012. This shows the relationships
between different species of hornworts. The species in blue do not have pyrenoids
and the species in red do have pyrenoids. The black and white inset images show the
different types of pyrenoids found in hornwort species.