Sea plant key to salt tolerance

Seagrasses, which evolved on land and have since migrated back into the world's oceans, might hold the key to future food security, according to scientists in Saudi Arabia.

These plants are the largest and oldest living organisms on Earth. They create the underwater equivalent of a lawn, spreading by using runner-like projections of their roots, called rhizomes, to rapidly colonise large areas.Clumps more than ten kilometre across and containing plants that took root 30-100,000 years ago have been identified in the Mediterranean.

They can also reproduce sexually and release seeds into the water using a specialised underwater pollination process that they've evolved over the 300 million years since they left their land-living relatives behind and - with the exception of Antarctica - colonised every coastline around the globe.

Now scientists have successfully decoded the genome of one species of these plants to find out how they have adapted from a life on land to an underwater world dominated by salt water.

Writing in Nature, King Abdullah University of Science and Technology (KAUST) researcher Carlos Duarte and his colleagues, studied specimens of "eel grass"  - Zostera marina - from Finland.

Their analysis shows that the plants are powered by 20450 genes, which is roughly the same as a human, and, compared with land plants, they've ditched large numbers of the genes encoding functions they no longer need: UV resistance and red-light sensing genes are gone, they have cast off the genes that control structures called stomata, which are the pores on the undersides of leaves that plants use to breathe, they have also abandoned a host of defence systems and also the chemical communication system that land-dwelling plants use to talk to each other.

In their place, these seagrasses have evolved new genes that allow them to add specialised sugars to control salt balance, nutrient uptake and gas exchange to their walls of their cells, and a new form of pollen that works underwater.

The work is important on two levels. First, these plants are ubiqutious and provide important habitats and food sources for an abundance of ocean-dwellers. They're also under threat from pollution, marine exploitation and climate change.

As the research team highlight in their Nature paper this week, the DNA readout from these plants will enable biologists to develop "sensitive molecular indicators of the physiological status" of the plants, which will in turn help to inform conservation strategies.

More selfishly, the work could also keep humanity fed in future. Not only do these plants produce nutritious, tasty seeds that can be harvested, the close relationship between seagrasses, cereals and rice plants means that the genetic strategies evolved by seagrasses to enable them to survive in seawater that have been identified by this study could be bred into food crops to make them more drought and salt tolerant.

More than 45% of the world's population are fed by crops grown on irrigated soil. Over time, the steady increase in salts in these soils caused by irrigation processes, as well as the effects of climate change, affect soil productivity and the efficiency of crop growth.

Anticipating this need, and breeding plants capable of tolerating this saltier future is a priority.

"This could alleviate the pressure on fresh water resources that are already stretched and will otherwise constrain our capacity to produce enough food to feed a rising population," explains Duarte.