An international team of researchers has sequenced the genome of Isoetes taiwanensis, a species of aquatic plant endemic to Taiwan, and uncovered some secrets of the plant’s unique method of photosynthesis. Their results appear in the journal Nature Communications.
Distribution of genes and repetitive elements in Isoetes taiwanensis (center): the relatively even distributions differ from angiosperm genomes, but are similar to what have been reported in other seed-free plants; only scaffolds longer than 10 Mb are plotted. Image credit: Wickell et al., doi: 10.1038/s41467-021-26644-7.
Most plants breathe in carbon dioxide and use sunlight to turn the gas into sugar during the day, and then stop breathing when the Sun goes down.
But plants in arid regions have evolved to breathe in carbon dioxide at night, and then stop breathing during the day while they conduct photosynthesis. This strategy — called the Crassulacean Acid Metabolism (CAM) photosynthesis — helps the plants save water.
Forty years ago, quillworts — plants of the genus Isoetes — became the first group of aquatic plants discovered to use CAM photosynthesis.
Daytime water loss is clearly not a problem for the aquatic plants. Instead, quillworts use CAM to collect carbon dioxide dissolved in water and store it overnight, to avoid competing with other aquatic plants and organisms, such as algae, that deplete water levels of the gas during the daytime.
To investigate the genetic mechanisms regulating quillworts’ CAM photosynthesis process, Dr. Fay-Wei Li of Boyce Thompson Institute and colleagues sequenced and analyzed the genome of the quillwort species Isoetes taiwanensis.
They found some similarities between quillwort and land plant CAM photosynthesis, but also a number of differences.
They then used the genome to identify CAM pathway genes and to examine their expression patterns, including how those patterns changed across the day/night cycle.
One notable difference between CAM in quillworts and terrestrial plants is in the function of phosphoenolpyruvate carboxylase (PEPC).
All plants have two types of PEPC: plant-type, long known for its essential role in photosynthesis; and bacterial-type, which resembles the PEPC found in bacteria.
“In all other plants, bacterial-type PEPC plays a role in a range of metabolic processes but not photosynthesis,” said David Wickell, a Ph.D. student at Cornell University and Boyce Thompson Institute.
“In Isoetes, both types appear to be involved in CAM — something that hasn’t been found in any other plant and points to a distinct role for bacterial-type PEPC in aquatic CAM.”
“All plants have the multiple components of CAM, which is why the process has evolved so many times,” Dr. Li said.
“But aquatic and terrestrial plants recruited different versions of those components possibly to meet the needs imposed by their differing environments.”
The scientists also found that expression levels of a few circadian regulators peaked at different times of day in quillworts than in terrestrial plants, indicating the circadian clock might regulate CAM functions differently in Isoetes.
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D. Wickell et al. 2021. Underwater CAM photosynthesis elucidated by Isoetes genome. Nat Commun 12, 6348; doi: 10.1038/s41467-021-26644-7
