The world’s biggest and oldest living plant found in Western Australia

Move over Big Banana, make way Big Pineapple. The largest and oldest-known living plant on Earth has been discovered in Australia.

Posidonia australis is an ancient and incredibly resilient seagrass that has been discovered in Shark Bay, Western Australia. It’s at least 4,500 years old and spans 180km of shallow ocean.

While looking at the genetic diversity of Shark Bay’s seagrass meadows, Australian researchers discovered that the meadows were in fact a single plant or ‘clone’ of the same individual. The exciting finding was published in Proceedings of the Royal Society B.

“We often get asked how many different plants are growing in seagrass meadows and this time we used genetic tools to answer it,” says senior author Dr Elizabeth Sinclair from the University of Western Australia (UWA).

The team sampled seagrass shoots across Shark Bay, a UNESCO World Heritage Area, and generated a ‘fingerprint’ using 18,000 genetic markers.

“The answer blew us away – there was just one,” says lead author Jane Edgeloe from UWA. “That’s it – just one plant has expanded over 180km in Shark Bay, making it the largest-known plant on Earth.”

P. australis has polyploidy, meaning it has a complete sets of chromosomes. This is common in plants, as well as some fish, amphibians and leeches. Posidonia also has the ability to reproduce through cloning that is achieved through vegetative growth via extension of horizontal rhizomes (underground plant stems).

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

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

To calculate the age of the polyploidy individual, researchers used the total estimated area of the Shark Bay P. australis meadows (around 200km2), and divided it by a conservative range of annual rhizome extension (15cm to 35cm a year), to get a minimum age of 4500 years.

“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,” says Sinclair.

While the combination of polyploidy and clonality have allowed the seagrass to expand into a large geographic range across ecologically diverse habitats, researchers are surprised it has been so successful.

“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,” says Sinclair.

“This single plant may in fact be sterile; it doesn’t have sex,” says co-author Dr Martin Breed from Flinders University, South Australia. “How it’s survived and thrived for so long is really puzzling. Plants that don’t have sex tend to also have reduced genetic diversity, which they normally need when dealing with environmental change.”  

A series of experiments has now been set up in the Shark Bay seagrass meadows to better understand how this plant has been able to survive, and outcompete its diploidy neighbours, for such a long time under such variable conditions.

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Map of Shark Bay, Gathaagudu, Western Australia. Distribution of persistent seagrass cover (dense and sparse) from 2016. All sites but one (Site 7) appear to be the same polyploidy individual.

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