The world's largest plant is a 112-mile-long seagrass in Australia

A team of researchers from the University of Western Australia and Flinders University has discovered what they believe to be the largest plant on Earth: a gigantic network of seagrass meadows, Posidonia australis, off the coast of Western Australia, covering more than 77 square miles (200 square km), according to a recent study published in the journal Proceedings of the Royal Society B. 

The meadow network is essentially one single plant that has grown from just one "colonizing" seedling. And, remarkably, the ancient plant with long, bright green ribbons has been cloning itself for at least 4,500 years!

The world’s largest known plant

The researchers essentially wanted to investigate the genetic diversity of seagrass meadows in the Shark Bay area, a protected body of shallow water in Western Australia. As they were asked how many different plants are growing in seagrass meadows on numerous occasions, they decided to employ genetic tools to address the question.

The team collected samples of seagrass shoots from various habitats throughout the region. They then examined 18,000 genetic markers to create profiles of the plants and find out the answer to the question. 

“The answer blew us away – there was just one!” said lead author Jane Edgeloe, a University of Western Australia student researcher, in a press release. “That’s it, just one plant has expanded over 180 km (112 miles) 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, colonizing seedling.”

The researchers determined that it was at least 4,500 years old. And, aside from its immense size, what distinguishes this seagrass plant from other giant seagrass clones is that it contains twice as many chromosomes as its oceanic relatives, indicating that it is a polyploid.

“Whole genome duplication through polyploidy – doubling the number of chromosomes – occurs when diploid ‘parent’ plants hybridise. The new seedling contains 100 per cent of the genome from each parent, rather than sharing the usual 50 per cent,” senior author Elizabeth Sinclair, an evolutionary biologist from UWA’s School of Biological Sciences and the UWA Oceans Institute, explained. "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."

 An ecological conundrum

The previous record holder for the largest plant was Pando, an aspen tree in Utah that cloned itself into a similar colony connected by a single root system. Pando occupies only 0.4 square km (0.2 square miles), whereas seagrass is 400 times larger. However, Pando would be larger in terms of biomass, according to New Atlas.

Co-author Flinders University ecologist Martin Breed stated that the study presents "a real ecological conundrum" as they don't know how the plant survived and thrived for thousands of years.

 “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. Plants that don’t have sex tend to also have reduced genetic diversity, which they normally need when dealing with environmental change,” explained Breed. "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."

While researchers aren't sure how the plant does that, Breed believes "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". Next, the researchers want to investigate the plant further to uncover more of its secrets.