There is currently much interest in planting trees to increase absorption of CO2 and fix carbon into plant biomass, thereby reducing CO2 levels in the atmosphere. It is less widely appreciated that trees have symbiotic partnerships with fungi (mycorrhizas), and that these may significantly improve growth, nutrition and water relations, depending on the soil type and environmental conditions.

 

For many decades we have been interested in the beneficial partnerships between fungi and eucalypts (Fig. 1), and in particular how they function. Many of the fungi completely envelop the fine feeder roots of eucalypts (Fig. 2) and many other tree species and grow out from them into the soil to form complex dynamic networks of fungal threads (hyphae) that may extend for considerable distances just under the soil surface. These networks are nutrient-gathering systems which 'mine' the soil for nutrients. They proliferate in nutrient patches, hydrolyse complex organic forms of nutrients converting them into simpler forms, and take these up and transport them back to the fine feeder roots of the tree, where they exchange them for carbon compounds fixed by the tree from atmospheric CO2. These fungi can siphon off 10-20% of the carbon fixed in photosynthesis to sustain their soil mycelium and fruit bodies. They therefore act as significant carbon sinks. In some species soil hyphae aggregate to form larger strand-like structures specialised for rapid transport and long-term survival in the soil. Working with blackbutt (Eucalyptus pilularis) and a very common eucalypt associate, a Pisolithus sp. (Fig. 3), we have been investigating how nutrients are transported from their sites of uptake in the growing hyphal tips back along more mature hyphae to the interfaces with the eucalypt roots. 

 

We have found that hyphae near the growing edge of an active mycelium contain a previously unrecognised cellular organelle system that is an ideal conduit for a component of this transport. It is a system of interconnected motile tubular vacuoles. Vacuoles have long been recognised as subcellular compartments that contain high levels of nutrients, especially phosphorus and nitrogen, but it was thought that they were non-motile storage structures and not in any way directly involved in the long distance transport. Working with cultures (Fig. 4) isolated from fruit bodies such as that shown in Fig. 3 we have characterised the motile vacuole system and found that it is present throughout the mycelial network extending from growing hyphal tips to mycorrhizal roots. It constantly changes form and shows complex and rapid movements. Vacuole tubules move along hyphae in either direction; they connect and disconnect with larger, more spherical and less motile vacuoles. Fig. 5, a still taken from a video sequence, shows a series of spherical vacuoles situated at intervals along a hypha connected to one another via fine tubules. The analogy is with a mass transport system of a mobile 'train' on a fixed track connecting stations. The motile vacuole system is made visible by loading it with fluorescent probes and viewing it by a fluorescence microscopy.

 

It can also be demonstrated in electron micrographs of very thin sections immobilised and preserved by freeze substitution, a very good way of preparing cells for high resolution structural studies with minimal damage (Fig. 6). Confirmation that the vacuole tubules contain relatively high levels of nutrient elements relevant to tree nutrition is obtained from X-ray analysis of sections of similar hyphae (Fig. 7). Tubular vacuoles can be recognised and are shown to contain relatively high levels of phosphorus and potassium, but not sulphur, compared to the rest of the cell. For recent reviews see publications 20 and 32.

 

Recent work in collaboration with a team at Oxford University with a much faster growing wood-rotting fungus which can also form networks across the forest floor has demonstrated how nutrients are likely to be transported in tubular vacuole systems and has confirmed their potential role in transporting nutrients for realistic distances across a soil mycelium (see publications 30 & 31).