Earth’s oldest plant is 4,500 years old. 180 kilometres of seagrass is one clonal plant

Ribbon weed, Posidonia australis, meadow in Shark Bay, Western Australia. Credit: Rachel Austin, University of Western Australia

Australian researchers have found what they consider to be the world’s largest plant.

Researchers from UWA and Flinders found 180km of ancient, hardy seagrass.

A new study published in Proceedings of the Royal Society B describes the discovery of a single plant or “clone” of the seagrass Posidonia australis in Shark Bay, WA.

Dr. Elizabeth Sinclair of UWA’s School of Biological Sciences and UWA Oceans Institute said the project began when researchers sought to know how genetically varied Shark Bay’s seagrass meadows were and which species should be collected for seagrass restoration.

Dr. Sinclair notes, “We commonly get asked how many different plants thrive in seagrass meadows.”

The team took seagrass shoots from Shark Bay’s various ecosystems and used 18,000 genetic markers to create a “fingerprint.”

“One answer blew us away” said Edgeloe. One plant in Shark Bay has grown 180km, making it the world’s largest.

200 km2 of ribbon weed meadows appear to have grown from a single seedling.

Dr. Martin Breed of Flinders University was a co-author. It’s an ecological conundrum, he argues.

“This plant may be sterile; it’s not sexual. It’s puzzling how it’s lasted so long. Dr. Breed from Flinders University says plants without sex have less genetic diversity, which they need to deal with environmental change.

“Seagrass has also experienced environmental change. Today, typical temperatures vary from 17 to 30 °C. Seawater salinity doubles. From dark to bright. These conditions stress plants. Still, it continues.

“How? We think its genes are well-suited to its local, fluctuating environment, and it has small genetic changes across its range that help it deal with local conditions, adds Dr. Breed.

This seagrass plant has twice as many chromosomes than its marine counterparts, making it a polyploid, according to Dr. Sinclair.

“When diploid ‘parent’ plants hybridise, polyploidy (doubling of chromosomes) develops. Dr. Sinclair claims the new seedling has 100% of each parent’s genome, rather than 50%.

“Polyploid plants live in severe environments, are frequently sterile, but can develop if left alone, as this huge seagrass has.

Even without flowering and seed production, it seems to be highly tough, enduring a wide range of temperatures, salinities, and strong light circumstances that would harm other plants.

Researchers have set up trials in Shark Bay to see how this plant lives in such changing settings.

Further information: Jane M. Edgeloe et al, Extensive polyploid clonality was a successful strategy for seagrass to expand into a newly submerged environment, Proceedings of the Royal Society B: Biological Sciences (2022). DOI: 10.1098/rspb.2022.0538

Journal information: Proceedings of the Royal Society B

Source: Flinders University