By Kirsty Jackson (@kjjscience)
Plants, unlike animals, are sessile. This means that they are unable to move from where they grow. If you are a plant that has germinated into a nice nutrient rich soil with plenty of food and water this is not such a bad thing. However, if you find yourself in a spot which has a low nutrient content or is too dry, then you might find the inability to move yourself to a new patch a bit of a problem. One way to get yourself out of a sticky situation is to call on a friend who might be able to lend a hand. Plants call on their friends the arbuscular mycorrhizal fungi (AMF).
AMF are a group of fungi which form a close relationship with plants for mutual benefit. They are able to get nutrients such as phosphorus and nitrogen out of the soil and exchange them for sugars that the plant makes by photosynthesis. AMF get their name from the specialised structures they form within plant roots called arbuscules. Arbuscle translates from Ancient Greek as “little tree” which is exactly what these structures look like. They are highly branched fungal hyphae. The branches increase the surface area between the fungus and the plant that in turn increases the amount of nutrient-sugar exchange that can occur. The second half of their name, mycorrhiza, translates as “mushroom roots”. The fungal hyphae that grow out into the soil act like an extension of the plant roots.
Fossil evidence from the Rhyne Chert in Scotland shows that the plant AMF relationship dates back over 460 million years. 460 million years ago is roughly the same time as plant life was emerging from the ocean and colonising land. These early land plants did not have roots like modern plants, but short rhizoids that were just enough to allow them to grip onto surfaces. Rhizoids are not very good at nutrient uptake and it is thought that the AMF acted as the root system for the early land plants. As a result of the early evolution of the plant-AMF symbiosis approximately 80% of all modern land plants can form this association with AMF. To have survived this long in evolutionary time shows just how important this friendship is.
It is thought that the legume-rhizobia symbiosis evolved from the plant-mycorrhizal symbiosis which you can read more about here. Evidence also suggests additional benefits to the plants with AMF in their roots, such as enhanced defence against microbes and insects.
As well as AMF, there is a second group of mycorrhizal fungi collectively known as Ectomycorrhiza. Like AMF they form beneficial associations with plant roots, however the hypahe do not enter the plant roots. Many of the fungi you see in woodlands are Ectomycorrhiza including many of the edible wild mushrooms such as the cep (Boletus edulis), the chanterelle (Cantharellus cibarius) and the black truffle (Tuber melanosporum).
About the author: Kirsty Jackson is a PhD student at the John Innes Centre, Norwich. She is studying bacterial and fungal symbioses with legumes and loves all things fungi! When she isn’t in the lab she is involved with organising science outreach events. Follow her on twitter (@kjjscience)
Bapaume and Reinhardt (2012). How membranes shape plant symbioses: signaling and transport in nodulation and arbuscular mycorrhiza. Frontiers in Plant Science.