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Seagrass Genome Elucidates How Flowering Plant Re-adapted to Marine Environment

NEW YORK (GenomeWeb) – A team of international researchers has sequenced the genome of the seagrass Zostera marina to gain insight into how the flowering plant re-adapted to saltwater living.

As the team led by Thorsten Reusch at the GEOMAR Helmholtz Center for Ocean Research-Kiel and Yves Van de Peer from Ghent University reported today in Nature, the Z. marina genome lost a number of genes that are integral for other angiosperms. At the same time, it regained functions that other flowering plants have lost.

Seagrasses are the only flowering plants that have returned to a marine environment and they are found throughout the temperate northern hemisphere in both the Atlantic and Pacific Oceans. There, they form underwater meadows in which a great number of species live, including sea otters, halibut, and clams, noted Susan Williams from the Bodega Marine Laboratory at the University of California, Davis, in a related Nature commentary.

But these environments are threatened, the researchers noted.

"All this makes seagrass interesting for the study of the relationship between the complex gene networks affecting temperature tolerance, like climate warming, and the mechanisms of salt tolerance through osmoregulation," said first author Jeanine Olsen, a professor of marine biology at the University of Groningen, in a statement.

She and her colleagues collected Z. marina, also known as eelgrass, from the Archipelago Sea, southwest of Finland, for sequencing. Using a combination of fosmid-ends and whole-genome shotgun approaches, they generated a 202.3-megabase Z. marina genome that encodes some 20,450 protein-coding genes. Nearly 87 percent of those protein-coding genes are supported by transcriptomic data, they noted.

Based on an analysis of synonymous substitution, the researchers reported that the Z. marina genome harbors echoes of an ancient whole-genome duplication event that they estimated took place between 72 million years and 64 million years ago —after the divergence of Zostera and the freshwater duckweed Spirodela some 135 million and 107 million years ago. This, they said, indicates a duplication event that's independent from the two reported in Spirodela.

The researchers also noted transposable element activity in the Z. marina genome and that genes gained by eelgrass tended to be closer to such elements than conserved genes.

Olsen and her colleagues then mapped those gains and losses of gene families onto a phylogenetic tree. While the researchers found that Zostera and Spirodela share a number of genes, the Zostera genome has lost a number of genes linked to its saltwater home.

For instance, it has lost all genes involved in stomatal differentiation. In land plants, stomatas on leaves are a key structure that enables them to regulate gas exchange and prevent water loss. These pores, added Bodega's Williams, aren't essential in seagrass as they don't contend as much with moisture loss and instead absorb gasses directly through their outer cell layers.

The Zostera genome has also lost genes involved in volatile synthesis and sensing pathways, including ethylene sensing, and in UV damage response. Volatile compound sensing is a defense mechanism against insects, which seagrass doesn't have to contend with as much, while UV-induced damage is also less of an issue in seagrass' dimly lit submarine environment.

At the same time, the Zostera genome has gained genes that enable it to adapt to its environment.

It has, for example, re-evolved a cell wall reminiscent of that of algae. As Olsen and her colleagues reported, the Zostera genome harbors an expansion of aryl sulfotransferases that enable it to produce sulfated polysaccharides. This further provides the seagrass cell wall with the ability to retain water and ions and cope with desiccation and osmotic stress during low tide. It further has an expanded pectin carbohydrate esterase 8 family, which, the researchers said, increases the polyanionic character of its cell wall matrix. Together these features allow for homoeostasis, nutrient uptake, and oxygen-carbon dioxide exchange through leaf epidermal cells.

"The Z. marina genome resource will markedly advance a wide range of functional ecological studies from adaptation of marine ecosystems under climate warming to unraveling the mechanisms of osmoregulation under high salinities that may further inform our understanding of the evolution of salt tolerance in crop plants," the researchers wrote.