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Quillwort discovery could lead to water-efficient crops

Quillwort discovery could lead to water-efficient crops

Quillworts are an ancient group of small aquatic plants that have largely been ignored by modern botanists. But researchers at the Boyce Thompson Institute have sequenced the first quillwort genome and uncovered secrets of the plant’s method of photosynthesis. Their findings could potentially lead to the engineering of crops that use water and carbon dioxide more efficiently.

Most plants breathe in carbon dioxide and use sunlight to turn carbon dioxide into sugar during the day. Then they stop breathing when the sun sets. 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. The strategy – called Crassulacean acid metabolism or CAM photosynthesis – helps the plants save water.

Daytime water loss isn’t a problem for quillworts. Instead they 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, researchers assembled a genome for I. taiwanensis. They found some similarities between quillwort and land plant CAM photosynthesis, but also a number of differences.

“As aquatic plants, Isoetes have evolved CAM photosynthesis in a fundamentally different environment than terrestrial plants in dry habitats,” said Fay-Wei Li, project lead and an adjunct assistant professor of plant biology at the Boyce Thompson Institute. “The results tell us there are more evolutionary pathways to CAM than we previously thought.”

The researchers used the genome to identify CAM pathway genes and to examine their expression patterns, including how those patterns changed in 24 hours. One notable difference between CAM in quillworts and terrestrial plants is in the function of phosphoenolpyruvate carboxylase. All plants have two types of that enzyme – plant-type, long known for its essential role in photosynthesis; and bacterial-type, which resembles the enzyme found in bacteria.

In all other plants, bacterial-type phosphoenolpyruvate carboxylase plays a role in a range of metabolic processes but not photosynthesis, said David Wickell, a doctoral student in Li’s laboratory and first author on the study.

“In Isoetes both types appear to be involved in CAM – something that hasn’t been found in any other plant," he said. "That points to a distinct role for bacterial-type phosphoenolpyruvate carboxylase in aquatic CAM.”

All plants have the multiple components of CAM, which is why the process has evolved so many times, Li said. But aquatic and terrestrial plants recruited different versions of those components, possibly to meet the needs imposed by their differing environments.

The research team also found that expression levels of a few circadian regulators peaked at different times of day in quillworts than in terrestrial plants. That indicates a circadian clock might regulate CAM functions differently in Isoetes. The findings could potentially be used to engineer crops to withstand environmental stresses.

“It would boil down to manipulating the circadian clock genes that regulate CAM components to help plants become more efficient at conserving water or making better use of the available carbon dioxide,” Wickell said.

The research recently was published in "Nature Communications." Visit nature.com and search for “quillworts” for more information.

Michael Haas is a freelance writer. 

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