Plants and Fungi: An ancient partnership

Overlooking fungi is easy. When you go to the garden centre to buy plants, you don’t often go to buy fungi too. Yet around 90% of plants have some sort of association with fungi. The connection is through the roots.

A fairly common abbreviation in botany papers is AMF, which stands for arbuscular mycorrhizal fungus, or fungi if there’s more than one identified. The fungi live in the soil and are excellent at finding nutrients. What they cannot do though is make their own carbohydrates. Plants in contrast generally have no trouble making carbohydrates but have a limited ability to find nutrients. Building roots to search for nutrients takes energy and carbon. An arbuscular mycorrhizal fungus can enter the roots of a plant, and when they make a connection, they can trade. The fungus gives up nutrients like phosphorous and nitrogen, and in exchange, they get sugars from the plant.

It’s such a successful connection that it’s found many times over in the plant kingdom – but how far back does it go?

Kowal and colleagues looked at liverworts. They looked at a different kind of fungus, Pezoloma ericae, an ericoid mycorrhizal fungus that connects to a plant in a different way to arbuscular mycorrhizal fungi. They found that it associated with liverworts. Given liverworts don’t have actual roots, the terminology gets complicated, and the connection is described as mycorrhizal-like. The variety of liverworts that P. ericae associates with suggests that this form of partnership could date from the Triassic – 250 million years ago – but the date isn’t certain. It could be that a few liverworts have picked up the association fairly recently.

What the research shows is that mycorrhizae are a lot more gregarious than people might think. Kowal and colleagues show that non-vascular plants can partner fungi, just like the better-studied vascular plants can. That means that the possibility is definitely open for non-vascular plants to have had similar mycorrhiza-like like partners in the distant past. However, to be sure that this kind of interaction was going on, what we’d need would be a snapshot of plant roots (or root-like organs) from deep time.

That snapshot would be the Rhynie Chert.

The Rhynie Chert is a fossil bed from Rhynie – Aberdeenshire – with a large number of fossils of plants, fungi and lichens. It formed in the Early Devonian period, about 410 million years ago. This period is maybe 100 million years after plants colonised the land, so this is early material.

Research by Mills and colleagues suggests that even at this time plants and fungi were exchanging material – and it may have had significant consequences for you.

Mills and colleagues argue that developing these partnerships enabled plants to thrive and make more use of carbon dioxide – but that carbon dioxide had to come from somewhere. The authors say that it came from the atmosphere, and you can see it in the geochemical record as the levels of CO₂ in the atmosphere drop and levels of oxygen rise. By providing the phosphorous the plants needed, the fungi helped drive climate change in the Palaeozoic to a higher oxygen atmosphere – eventually allowing the evolution of us.

Strullu‐Derrien and colleagues recently wrote that the Rhynie chert was the earliest direct fossil evidence for plant-fungal interaction. However, these authors were looking at the Glomeromycotina for the more familiar arbuscules that many mycorrhizae use today.

An alternative view can be found in Field and Pressel’s recent paper, where they argue that fungi helped plants colonise the land over 500 million years ago. They have found that when they examined plants to see if they formed associations with Glomeromycotina fungi. They looked to see what plants formed associations with what fungi. Angiosperms (flowering plants) happily associate with Glomeromycotina, Ascomycota and Basidiomycota. They also found associations Glomeromycotina fungi in liverworts, so if they both inherited that trait, then the first fungi-plant partnership could be very old. They also often found Mucoromycotina fungi too.

Mucoromycotina aren’t so well studied, at least not yet. However, when you start looking for Mucoromycotina, they crop up in all sorts of places. It seems like they might be more likely to be connected with plants in deep time than the better known Glomeromycotina. Field and Pressel comment that Haplomitriopsida liverworts, plants that diverged a long while ago from their fellow liverworts, associate exclusively with Mucoromycotina fungi.

Field and Pressel draw on earlier research that suggests microbes, including fungi, colonised the land before plants. If that’s the case, then the fungi were already in position to provide the earliest plants with the nutrients they needed to thrive outside of the seas. They do point out though that we don’t know enough about what the Mucoromycotina are doing with plants and how they interact to provide nutrients. However, their research suggests that fungi may have been helping plants on land since they got on to land, up to 500 million years ago. If they have been working together for that long, it’s no surprise they’ve got everywhere in the plant kingdom.