Secrets of seagrass the saltwater crop
Seagrasses are marine plants which actually evolved first on land and have since migrated back into the world's oceans. They might hold the key to freshwater shortages, according to scientists in Saudi Arabia. They've decoded the DNA of one of these plant species and by comparing the genetic sequence with land-living relatives of seagrasses, including rice, they can see which genes have been altered to endow the plants with the ability to thrive in seawater. Now they know this, the same genes can be bred into food plants so that they can tolerate much saltier soils, as Carlos Duarte explains to Chris Smith...
Carlos - We have published our analysis of the DNA sequence of a group of plants called seagrasses that can grow fully submerged in the water that allow us to understand the changes that higher plants underwent to be able to colonise this environment, and that has implications for our hopes to be able to grow crop plants that will feed the world in the future. Not with fresh water, which is becoming increasingly scarce, but with saltwater, seawater.
Chris - Now, when you say "colonised the ocean," do you mean that these plants didn't come from the ocean in the first place? The plants we see in the sea - this group of them - came out of the sea and went back again?
Carlos - So we're talking about flowering plants, and flowering plants evolved from marine ancestors that then first colonised freshwater and wetland type of environments, and then they went on from there to colonise dry land, and they also went back and recolonised the ocean, where their ancestors had originated from.
Chris - How did you know they did that?
Carlos - Because we have molecular tools and looking at the structure of their DNA, we can establish the ancestry of the different groups of plants and by that ancestry we know which ones have evolved first, and which one descended from the previous ones.
Chris - How would I recognise a seagrass if I saw one?
Carlos - They are rather inconspicuous to us and they are just like grass! Our audience can probably see them when they walk around the beaches in low tide, and they would be plants anywhere from 4-5 cm in size up to 2 m length leaves.
Chris - How do they reproduce?
Carlos - They are flowering plants, so they reproduce sexually but all of them are clonal also. They are able to produce many copies along the stem, called a rhizome, that lies horizontally on the sediment. And we also discovered that some of these plants - seagrasses - they are the largest and the longest-living organisms still present on Earth. In the Mediterranean we found a set of particular clones that extended over more than 10 km of sea floor - so they were massive. Many, many times the weight of Blue Whales and we also calculated their age to be about between 30,000 and 100,000 years old, the plants that we see living today. So, they are very long-lived organisms and very large.
Chris - Given that these plants are very similar to grasses and cereals and rice which, let's face it, feeds the majority of humans on Earth: given those similarities, how can you exploit that, and what you've learned from the genome, to potentially turn these plants into food crops? Is that possible?
Carlos - Well that may be possible. In fact, looking at the use of these plants by different cultures, at least two of the sixty species have been used, and are being used by some cultures as a source of food and staples.
Chris - Already?
Carlos - Already, yes. One of them is eelgrass. This produces seeds that are very similar to rice and Seri Indians in Baja California, they traditionally harvest these seeds and use them in the same way that we use rice today. And another plant that grows in Southeast Asia and that one produces very large seeds that have the shape and the size of a nut...
Chris - Are they any good? Do they taste nice?
Carlos - Actually, I have eaten them and they are quite nice. And they are harvested and also eaten by people in the Philippines and other regions in Southeast Asia.
Chris - So we could, potentially, sort of increase that as a food source. But what about, also, borrowing from biology and taking the knowledge these plants have evolved to tolerate those environments, and bringing that back into land dwelling plants, given the close relationship, to confer the ability to grow in really quite poor soils or salt dominated soil?
Carlos - Yes. So, indeed, that was one of the drivers of our research was to try to understand how these plants were able to cope with full strength seawater and grow in full strength seawater. So we could then transfer these mechanisms to some of the crop plants, and the candidate target plant will be rice because rice is also an aquatic plant, and if we will look at the family tree of higher plants, rice is actually very close to seagrass. So now that we have discovered seagrass are able to cope and grow in full strength seawater, we are hoping that we might be able to find ways in which we can assist rice to be able to be grown in increasingly brackish, perhaps all the way to full strength seawater, and alleviate the pressures on freshwater resources that are capping already and will constrain in the future the capacity to produce food to feed an increasing population on Earth.