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| The ectomycorrhizosphere of forest trees: stress management at the soil-root interface |
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J. Garbaye
UMR INRA-UHP 1136 Interactions Arbres-Microorganismes, INRA Nancy, F-54280 Champenoux
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In their natural environments, plants are almost always submitted to severe physical stresses (drought, extreme temperatures, nutrient starvation, anoxia, etc.) which impair their fitness. Periods during which all growth factors are optimum simultaneously are extremely rare and short-lasting. Taking the time factor into account is therefore a key condition to describe and understand plant communities. Because plants cannot move to escape adverse conditions (as most animals do), they evolved in developing a very wide range of defence and adaptation mechanisms. We propose to discuss some of them which concern forest trees at population, community or even ecosystem levels.
Research in temperate and boreal forest ecosystems, which are dominated by ectomycorrhizal, social tree species, recently yielded a number of interesting results and new concepts on symbiosis-based plant strategies to cope with stresses. First of all, isotope studies have demonstrated that ectomycorrhizal trees in a same stand share many fungal symbionts (upper Basidio- and Ascomycetes) and are linked through the hyphal network (the so-called "Wood Wide Web"), tranfering nutrients and photosynthates among neighbouring trees. This fact fundamentaly modifies our understanding of interactions between trees, adding mutualistic relations to competitive ones.
The development of molecular typing and of micro-physiological tests on individual ectomycorrhizal tips revealed the huge diversity in space and time (taxonomical as well as functional) of the symbiotic fungi in a forest stand, and even on the root system of a single tree. This leads to the important concept of seasonal functional complementarity. The role of ectomycorrhizal fungi on water uptake, water transfer and protecting fine roots against drought has been particularly documented.
At a very different space scale, the extension of ectomycorrhiza to their associated bacterial communities led to the concepts of ectomycorrhizosphereeffect (structuration of the bacterial functional diversity in the immediate vicinity of the symbiotic organ) and mycorrhiza-helper bacteria (MHBs, which specifically enhance ectomycorrhizal colonization). Recent evidence even suggest that some ectomycorrhizal fungi contain symbiotic intra-fungal bacteria, as already demonstrated with some arbuscular endomycorrhizal fungi (Zygomycetes).
This new insight in the complex organization of ectomycorrhizal forest ecosystems will be discussed in terms of evolutionary implications as well as of practical application to forest management.
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