Q:

What is the symbiotic relationship between truffle fungi and their host trees?

I understand that truffles share a symbiotic relationship as they reside among the root systems of host trees, but I cannot find any material that specifies the details of such a relationship. In other words, what exactly do truffles get from their host trees, and what do they provide in turn?

A:

A truffle is the fruiting body of a subterranean Ascomycete fungus, predominantly one of the many species of the genus Tuber. Truffles are ectomycorrhizal fungi and form symbiotic relationships with the roots of several tree species including beech, birch, hazel, hornbeam, oak, pine, and poplar.
Mycorrhizal symbiosis is a mutualistic association between a fungus and the roots of a plant (usually a tree) in which (typically) the fungus obtains energy in the form of carbohydrates from the plant, and the plant gains the benefits of the mycelium's higher absorptive capacity for water and mineral nutrients, partly because of the large surface area of fungal hyphae, which are much longer and finer than plant root hairs, and partly because some such fungi can mobilize soil minerals unavailable to the plants' roots. The effect is thus to improve the plant's mineral absorption capabilities. 
Based on the quote below from the website of a truffle nursery in Australia (http://trufficulture.com.au/what_are_truffles.html), it sounds like truffle symbiosis is a completely standard mycorrhizal symbiotic relationship.

The truffle coats the tips of the tree roots to form mycorrhiza which
  act as an extension of the tree's root system. The tree provides the
  truffle with a source of photosynthesised carbohydrates, and in return
  the fine, thread-like filaments (mycelia) of the truffle, extract and
  trade soil minerals and nutrients which would normally be unavailable
  to the tree. Thus the mycorrhiza is able to increase the effectiveness
  of the trees roots, enabling the tree to grow in soils which would
  normally be too nutrient deficient to support them.

