Ectomycorrhizal Symbiosis can Enhance Plant Nutrition through Improved Access to Discrete Organic Nutrient Patches of High Resource Quality

Host favoured by fungal foraging

How do plants get the most out of the soil in which they are rooted? In many species, lateral roots proliferate in, and/or grow towards, zones of higher nutrient availability. However, growth is often too slow in the competitive soil environment for roots to benefit from transient and local increases in nutrient content. In such situations, Tibbett (Bournemouth University) and Sanders (University of Leeds) (pp. 783-789) suggest that plants in a mutualistic relationship with an ectomycorrhizal fungus are at a distinct advantage. They tested this suggestion with a hybrid Salix (willow) and an ectomycorrhizal symbiont, Hebeloma syrjense. In initial experiments under axenic conditions in Petri dishes, H. syrjense rapidly colonized and initiated breakdown of autoclaved bean cotyledons (model nutrient patches), but what would happen in soil? The authors used willow plants that had an established ectomycorrhizal symbiosis with H. syrjense and similar aged non-mycorrhizal plants. The willows were grown in nutrient-poor soil in which the autoclaved bean cotyledons had been buried. The results were very clear. The mycorrhizal willows were much greener and had higher leaf chlorophyll, nitrogen and phosphorus contents than non-mycorrhizal plants. Leaf biomass, however, showed no difference between mycorrhizal and non-mycorrhizal plants. The bean cotyledons were again extensively colonized by the fungus and the invading hyphae could be observed to connect to the mycorrhizal roots via the mycelial network. These data thus support the authors' suggestion that mycorrhizae are important in enabling plant roots to exploit nutrient patches that might otherwise be out of reach. However, we can also ask what the fungus gains from the relationship. It is generally held that while the fungal partner provides inorganic nutrients to the plant, the plant provides carbohydrate to the fungus. Certainly in these experiments, the fungal partner was able to outcompete other soil micro-organisms in colonizing and exploiting the nutrient patches. There is more to this relationship than meets the eye.

Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk