Ancient fossil reveals how plants and fungi first developed on land

A microscopic structure frozen in time for more than 400 million years offers hints about the origin of one of the greatest partnerships in the history of life on Earth.

For billions of years, life on Earth lived solely in water. It was only around half a billion years ago that plants first started to grow on land – but exactly how these organisms first made that transition has remained a mystery.

New research, published in the journal New Phytologist, adds to a growing body of evidence that fungi had an important role to play.

Evidence of the collaboration between plants and fungi, known as a mycorrhiza, was found in a 407-million-year-old fossil from Scotland’s Windyfield Chert. Inside the preserved tissues of an ancient plant was a tiny structure that allows plants and fungi to share nutrients, called an arbuscule.

The scientists have named this fungus as a new species, Rugososporomyces lavoisierae, in honour of Marie-Anne Paulze de Lavoisier, a French scientist who was a pioneer of physiology and modern chemistry in the late 1700s. Our scientist, Dr Christine Strullu-Derrien, led the research.

“Mycorrhizas are very rare in the fossil record and have never been found in the Windyfield Chert before,” Christine says. “The presence of the arbuscule shows that the fungus wasn’t parasitising on the plant or feeding on it after death – instead, there was a symbiotic association.”

“The fungus would have provided minerals like phosphorus in return for sugars from the plant in a way that benefits them both.”

Dr Paul Kenrick, one of our fossil plant experts who co-authored the research, adds that it’s “extraordinary” to find such ancient evidence of a symbiotic relationship.

“Dry land would have been a very challenging environment for plants to live in,” Paul explains. “Most species didn’t even have roots around 400-million-years-ago, making it difficult for them to obtain the nutrients they need.”

“It appears that symbioses were a necessary part of allowing plants to adapt to life on land. With over 85% of living plants having mycorrhizas, finding out more about how these relationships developed can contribute to our knowledge of the past, present and future.”

What is the Windyfield Chert?

The Windyfield Chert is a layer of rocks from the Scottish Highlands that date back to the Early Devonian Period. It’s thought to be a similar age to the more famous Rhynie Chert, which is found nearby.

Both the Windyfield and Rhynie Chert are world-renowned for preserving an ancient wetland ecosystem in extraordinary detail. During the Devonian these areas would probably have been similar to parts of Yellowstone National Park in the USA today, with sandy soils, hot springs and pools of water.

This ecosystem came to an abrupt end around 407 million years ago, when silica-rich water from the hot springs overflowed into the wetland and entombed the species living there. This has provided scientists with astonishing wealth of well-preserved fossils to study from a key moment in the evolution of life.

Historically, samples of the cherts have been cut into slices. These slices are thin enough that light can pass through them, allowing them to be placed under a light microscope so scientists can reconstruct what the ancient environment was like.

While this works for larger plants in the chert, traditional white light microscopes struggle to image the tiny structures of the fungi. Instead, the researchers took a new sample from the Windyfield Chert and exposed it to laser light instead.

“Light microscopy allows you to see specimens in great detail, but confocal laser scanning microscopy takes things to another level,” Paul says. “We hit the sample with lasers to make it fluoresce, and eliminate out of focus light so we can see a much crisper image of the specimens.”

“With the help of colleagues from Cambridge’s Sainsbury Laboratory, we were able to take this another step further by combining this with a process known as fluorescence lifetime imaging microscopy, or FLIM. By measuring how long it takes the fluorescence of the samples to decay, we get an idea about the chemical structure of different parts of the fossil.”

Delving into an ancient relationship

This study is the first time that a combination of confocal microscopy and FLIM has been used on fossils, and it’s revealed their structure in unprecedented detail.

Using the images, the team were able to identify signs of Rugososporomyces lavoisierae growing within a plant known as Aglaophyton majus. In modern plants the mycorrhiza would normally be found in the roots, but Aglaophyton didn’t have any.

“Very few plants had roots at this point in time,” Christine says. “Instead, they had small hair-like growths called rhizoids. So, even though mycorrhiza translates as ‘fungus root’, we found it in parts of the plant growing above the soil.”

“Today, this type of association is only seen in some liverworts and hornworts. We need to understand why this changed.”

Understanding more about this process will mean digging deeper into how the symbiotic relationship between plants and fungi first evolved. For example, it’s not currently clear whether this relationship began in the water, or once both plants and fungi moved onto land.

Studies of green algae from freshwater, the closest living relatives of plants, show that they have some of the genes that would allow a symbiotic relationship, but not all of them. This suggests that the ancestors of plants might have developed the other genes after separating from algae. But it’s still an open question.

Another major puzzle is how plants and fungi learned to cope with each other. A fungus invading a plant would normally cause an immune response, with the plant laying down chemical barriers, but symbiotic fungi dampen these defences.

The team hope that the remains of these barriers might have been preserved in the fossils from the Rhynie and Windyfield Cherts, offering new insights into the changes that led to mycorrhiza becoming so common.

“So far, no one has ever looked at the immune response in fossil plants this ancient,” Paul says. “But it’s very possible that the signs are still there, even 400 million years later.”

“It would be a world first if we could detect it, and it would give us an unprecedented insight into the co-evolution of plants and fungi.”