Growth & Development

Botanists have discovered the world’s largest plant in what should be one of the world’s least plant-friendly environments

Botanists have discovered the world's largest plant in Shark Bay, but Shark Bay is unusually salty - so how could a plant thrive there?
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Jane Edgeloe and colleagues from The University of Western Australia and Flinders University have made the biggest botanical discovery of the year. They’ve found the largest organism on the planet, a seagrass living in Shark Bay. The plant is ribbon weed, Posidonia australis. But it’s not just the size that’s important. An estimated four and a half thousand years old, its survival could help scientists plan conservation efforts for seagrasses and help protect more coastlines. The key may be a combination of cloning and polyploidy.

Light green tendrils of seagrass, capped with purple tips waft in a very blue looking sea.
Ribbon weed, Posidonia australis, meadow in Shark Bay, Western Australia. Photo: Rachel Austin, University of Western Australia

Polyploidy describes the number of copies of chromosomes an organism has. You and I are diploid, and we have two copies of our chromosomes, one from our mother and one from our father. When we pass on our genes, we pass along one of these copies, so the offspring still has two copies when they’re put together. The seagrass in Shark Bay is tetraploid, meaning it has four chromosomes. It inherited the full pair of chromosomes from both parents.

Whole Genome Duplication, as it’s known, is of interest to botanists as it can radically change how a plant works. For plants at the edge of a species’s range, it can give them a whole new set of genetic tools to cope with challenging conditions. The downside is that it can often cause them new problems. Edgeloe and colleagues refer back to work by Tomáš Herben and colleagues on how polyploids reproduce.

Herben and colleagues examined around nine hundred plants to see how they reproduced. They found that polyploids were not that good at sexual reproduction. Instead, they vegetatively reproduced. That is, they grew clones of themselves. The team studying the Shark Bay seagrass knew it was a polyploid and, from earlier work published in AoB PLANTS, they knew that it wasn’t very effective at sexual reproduction. So they tested the DNA of various samples to see how much of the grass in Shark Bay was clonally reproduced.

The team sampled seagrass shoots from across Shark Bay to create a ‘fingerprint’ using 18,000 genetic markers. Looking at how this fingerprint varied across the samples would tell them what diversity there was among the plants.

“The answer blew us away – there was just one!” Jane Edgeloe said in a press release. “That’s it, just one plant has expanded over 180km in Shark Bay, making it the largest known plant on earth. The existing 200km2 of ribbon weed meadows appear to have expanded from a single, colonising seedling.”

That means the plant covers an area of around 20,000 football pitches (or 47,000 American football fields). Knowing the area it covers, and that it came from one plant, allowed the botanists to estimate its age. The team knew that ribbon grass rhizomes can grow between fifteen and thirty-five centimetres a year, but they won’t grow continuously in a straight line. Using examples from elsewhere, they estimate this plant is around four and a half thousand years old, making it about as old as the Great Pyramid of Giza. It’s this combination of age as well as area that points to something extraordinary going on in Shark Bay.

Shark Bay is right on the limit of Posidonia australis‘s range. On top of that, Shark Bay has its own local problems. There’s more evaporation than rainfall, and flow from the ocean into the bay is limited. That means the ribbon weed has to cope with unusually saline conditions. The age of the plant also means it has had to cope with changing climatic conditions.

In their paper, the team attribute the plant’s success to high heterozygosity. Heterozygosity is a measure of the different alleles, or versions of genes, that an organism has. You might have the genes for both blue and brown eyes in your genetic make-up, or maybe you have just the genes for brown eyes. Each version of your eye-colour gene is an allele. This plant has four copies of its genes so, in some cases, it could have four different alleles to draw upon to cope with different environments.

There are some potential issues with dating clonal plants, but this discovery hasn’t simply appeared from nowhere. It’s part of a long study of a series of puzzles posed by the seagrass of Shark Bay. Concluding that the seagrass is all one plant is a solution that sits well with many of these problems.

The authors conclude their paper by saying: “Exactly how the polyploid clone varies its response to local environmental conditions is unknown and the subject of further research, but its relative abundance suggests that it has evolved a resilience to variable and often extreme conditions that enable it to persist now and into the future.”

READ THE ARTICLE

Edgeloe, J.M., Severn-Ellis, A.A., Bayer, P.E., Mehravi, S., Breed, M.F., Krauss, S.L., Batley, J., Kendrick, G.A. and Sinclair, E.A. (2022) “Extensive polyploid clonality was a successful strategy for seagrass to expand into a newly submerged environment,” Proceedings of the Royal Society B: Biological Sciences, 289(1976), https://doi.org/10.1098/rspb.2022.0538

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