Why do plants need fungi to thrive?

This question came up on our forum - and Nadia Radzman had an answer...

Nadia - This particular fungus, we call it Arbuscular Mycorrhiza fungus. Actually a lot of plants can associate with this fungus. More than 80% of land plants could have mutualistic symbiosis with this fungus. So it's a win win situation. So what happens is that if there is low phosphate in the soil, the plants would then engage in this symbiosis with the fungus. And because the fungus has this extensive network of mycelium in the soil, it can actually forage that soil for more phosphate and then transfer it to the plant. So mycelium is this thread-like extension of the fungus, it finds where the phosphate patches are that the plants couldn't find, and then transfers it to the plant. And then in return the plant, who would actually give the carbon nutrient, informs of lipids back to the fungus.

Phil - Why can't plants do that themselves though? Find that particular phosphate?

Nadia - Ah, so sometimes there is this depletion zone around the root. This is because it will take up a lot of the phosphate around it. But then there's also this particular thing where phosphate usually actually just stays in the topsoil. So obviously because the root is going down, it can access something that's really high up. So with the fungus it can actually help that.

Phil - They're almost acting like root extensions, then trying to seek out phosphates?

Nadia - Yes, exactly! Yeah. So mycorrhiza is a fungus. This association was actually back 400 million years ago. It's a very ancient symbiosis and this is even before the land plants actually acquired roots. So it actually helped defenses to gain the nutrients that it needs.

Phil - Chris Smith, did you have a question there?

Chris S - No, I was just gonna say, I like your root extension, I began to think about hair extensions and it's like this is the plant equivalent of hair extensions. This enormously increases the collecting area of the plant. And the amazing thing about all this, because I first encountered this about 20 years ago when I began to read about it and this, this whole barter economy, because the fungus and the plant are trading: I'll give you some nutrient in return for some other. But actually if the fungus tries to rip off the tree, then the tree can actually decide it's not going to trade with that fungus anymore and they can be stingy, can't they? So it's a really clever sort of relationship.

Nadia - Yes, a lot of symbiosis. I like that. They're quite stingy.

Phil - What about the second part of this question though, are these fungal relationships something that's important for farming?

Nadia - So it would clearly help the farming system in a way they'll make it more sustainable because if we think of this fungus helping the plant to acquire more phosphate when there is low phosphate around, you can have reduced usage of phosphate fertilizers. And also what's interesting is that a couple of studies also show that having this fungus around would increase the drought tolerance of this plant, and it also increases the resistance to diseases.

Phil - Is this something that farmers are currently taking into account?

Nadia - Not exactly. Because if we think that there are also pathogenic fungus around...

Phil - What does that mean - pathogenic?

Nadia - Induces diseases to plants.

Phil - So bad fungus, as opposed to the good fungus.

Nadia - I suppose that's bad fungus, yes, while arbuscular mycorrhiza is the good guy. So obviously to deter this bad fungus, the farmers would spray a lot of the crops with fungicide. And if it's a broad spectrum fungicide, what happens is that it will also kill the good guys in the soil. So to prevent that from happening, what farmers could do is actually to use very specific fungicide for a particular pathogenic fungus, the bad guy that is affecting the plant, but only in the leafs. And so the good guys, the good fungus, could thrive in the soil.

Chris S - Nadia, given how important these fungi are, is there any way that a plant has got to make sure that its offspring get the same beneficial fungus?

Nadia - Right? Not exactly. So as I've mentioned before, this particular fungus is called Arbuscular Mycorrhiza fungus. There's another type of fungus which is called Endomycorrhizal fungi. It actually associates with some orchids. So with that, this is very interesting because some orchids would actually need a particular fungus for it to germinate. So what happens is that the seed of the orchid needs to be associated with a very specific fungus and then it would eat up the fungus for it to then grow and germinate.

Phil - A fungus specifically for that one plant. I guess this must be a whole world of different possible relationships.

Nadia - Yeah. So that's why some orchids cannot be grown outside of their habitat because they need that fungus to grow.

Chris S - Chris Rogers?

Chris R - I've heard before that different plants can talk through fungi, they can pass chemicals between different plants. Is that true?

Nadia - It is, yeah. So it's pretty much a hidden network of massive threats under the soil. Right. And the way they knew about this is that a particular tree was fed with a radioactive isotope of carbon. It doesn't occur in nature. So we can trace this, right. And they could actually trace it very, very far away from that one particular tree. So it means that there must be some sort of communication, the network under the soil, the hidden network, through the fungus.

Chris S - So the interpretation is that the radioactive carbon goes into tree number one, let's call it, then trades with this fungus that's around its roots, which then carts off that particular chemical and then delivers it or deposits it or trades it with another tree miles around, and that's how the radioactivity gets spread.

Nadia - Yeah, exactly.