Aquatic plant’s genome reveals evolutionary surprises

Researchers unravelled the secrets behind Quillwort’s (Isoëtes taiwanensis
Researchers unravelled the secrets behind Quillwort’s (Isoëtes taiwanensis) special kind of photosynthesis Photo: Pi-Fong Lu
International research team including Göttingen University sequence whole genome of quillwort

The quillwort is an aquatic plant belonging to an ancient lineage of vascular plants, meaning plants that have tissues to transport water, sap and nutrients. This lineage diverged from the other vascular plants more than 400 million years ago. A large international research team, including the University of Göttingen, have now sequenced the whole genome of this curious plant, uncovering the secrets of its unique method of photosynthesis, as well as shedding light on the different regulation and evolutionary history of photosynthesis in aquatic and terrestrial plants. Their research was published.

The humble quillworts are a group of about 250 small, aquatic plants that, so far, have received little attention. While most plants in this family are smallish organisms, the fossil record shows that quillworts are the living relatives of now-extinct tree-sized lycopods, which formed large forests hundreds of millions of years ago. Hence, in evolutionary terms, these are of significant scientific interest. The combined expertise and efforts of a large international consortium led by Cornell University worked together to sequence the full quillwort genome for the first time. This means the entire DNA sequence - the molecular blueprint of the organism - was assembled and analyzed, allowing researchers to understand its special photosynthesis and its evolutionary origin. 

The Göttingen University research team performed analyses that revealed clues about the different evolutionary paths that contribute to the "wood molecule", lignin. Professor Jan de Vries from the Institute of Microbiology and Genetics at the University of Göttingen explains, "We uncovered an evolutionary path to lignin that ancient lycopod trees probably followed. These trees from millions of years ago contribute to a significant portion of the fossilised biomass that we nowadays burn as fossil fuels." He goes on to say, "Understanding the paths that led to the formation of lignin, could potentially lead to finding new biosynthetic paths towards wood-like polymers."

However, there were further surprises for the researchers. The most peculiar was the way that quillworts perform photosynthesis, using a mechanism known as Crassulacean Acid Metabolism (CAM). CAM is a mechanism that allows desert plants to close their stomata (tiny holes) and ’stop breathing’ CO2 during the day whilst continuing to photosynthesise: a skill that allows them to save water. These desert plants then open their stomata at night to absorb CO2. But why would an aquatic plant need CAM photosynthesis? In fact, it looks like this is a useful adaptation to low CO2 levels underwater, allowing quillworts to collect CO2 and store it overnight. The data enabled scientists to understand how these aquatic plants regulate CAM photosynthesis to compete for carbon dioxide underwater, and how that regulation differs from terrestrial plants, which use CAM to conserve water.

The researchers in Göttingen were supported by funding from the European Research Council (ERC), the Priority Programme "MAdLand" (SPP 2237) funded by the German Research Foundation (DFG), and through integration into the International Max Planck Research School (IMPRS) for Genome Science.

Original publication: Wickell D et al Underwater CAM photosynthesis elucidated by Isoetes genome. Nature Communications 2021; DOI: 10.1038s41467-021-26644-7  and text also available here:  https://rdcu.be/cAM1C