Is this ancient seagrass the world's largest plant?

Australian researchers have located what they believe is the largest plant on Earth — and they estimate it’s at least 4500 years old.

The ancient and incredibly resilient seagrass, stretching across 180 km, was located by researchers from The University of Western Australia (UWA) and Flinders University. The discovery of the single plant or ‘clone’ of the seagrass Posidonia australis in the shallow waters of the World Heritage Area of Shark Bay in WA has been detailed in the journal Proceedings of the Royal Society B.

Senior author Dr Elizabeth Sinclair, from UWA, said the project began when researchers wanted to understand how genetically diverse the seagrass meadows in Shark Bay were, and which plants should be collected for seagrass restoration. UWA student researcher Jane Edgeloe, lead author of the study, said the team sampled seagrass shoots from across Shark Bay’s variable environments and generated a ‘fingerprint’ using 18,000 genetic markers.

“The answer blew us away — there was just one!” Edgeloe said. “Just one plant has expanded over 180 km in Shark Bay, making it the largest known plant on Earth.

“The existing 200 km² of ribbon weed meadows appear to have expanded from a single, colonising seedling.”

Co-author Dr Martin Breed, from Flinders University, said the study presented a real ecological conundrum, as this single plant “may in fact be sterile; it doesn’t have sex”.

“How it’s survived and thrived for so long is really puzzling,” Breed said. “Plants that don’t have sex tend to also have reduced genetic diversity, which they normally need when dealing with environmental change.

“Our seagrass has seen its fair share of environmental change too. Even today, it experiences a huge range of average temperatures, from 17 to 30°C; salinities from normal seawater to double that; and from darkness to extreme high light conditions. These conditions would typically be highly stressful for plants. Yet it appears to keep on going.

“How does it do it? Well, we reckon its genes are very well suited to its local, but variable, environment and it also has subtle genetic differences across its range that help it deal with the local conditions.”

Sinclair said what makes this seagrass plant unique from other large seagrass clones, other than its enormous size, is that it has twice as many chromosomes as its oceanic relatives, meaning it is a polyploid.

“Whole genome duplication through polyploidy — doubling the number of chromosomes — occurs when diploid ‘parent’ plants hybridise,” she said. “The new seedling contains 100% of the genome from each parent, rather than sharing the usual 50%.

“Polyploid plants often reside in places with extreme environmental conditions, are often sterile, but can continue to grow if left undisturbed, and this giant seagrass has done just that.

“Even without successful flowering and seed production, it appears to be really resilient, experiencing a wide range of temperatures and salinities plus extreme high light conditions, which together would typically be highly stressful for most plants.”

The researchers have now set up a series of experiments in Shark Bay to understand how this plant survives and thrives under such variable conditions.

Image caption: The Posidonia australis meadow in Shark Bay, Western Australia. Image credit: Rachel Austin, University of Western Australia.

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