All plants are hosts to fungi that are specialised at establishing close contact with their cells and feed on the products of photosynthesis. This mode of fungal nutrition is termed biotrophy. Some biotrophs confer advantages to their hosts and are considered to be mutualistic. Mycorrhizal fungi sustain terrestrial ecosystems through mutualisms with roots. These include ectomycorrhizas, mainly on trees, arbuscular mycorrhizas on most plants, and ericaceous mycorrhizas on ericaceous plants of nutrient-poor soils. Lichens consist of fungi that form tissues that accommodate unicellular green algae and photosynthetic prokaryotes. These photobionts provide sugars for the fungus. Lichens can therefore grow on bare illuminated surfaces and can be the primary producers in environments too cold and arid for vascular plants. Fungi that grow harmlessly inside the tissues of plants, termed endophytes, are widespread. Grass endophytes include vertically-transmitted fungi that defend their hosts against insects and grazing herbivores with toxic secondary metabolites.
