plant surface microbiology.pdf

22 Nitrogen Transport and Metabolism in
Mycorrhizal Fungi and Mycorrhizas
Arnaud Javelle, Michel Chalot, Annick Brun
and Bernard Botton
1 Introduction
1.1 Ecological Significance of Ectomycorrhizas
Unlike most other organisms, plants and fungi are restricted to their habitats,
creating potential problems when nutritional conditions become limited. To
cope with nutrient deficiencies, they have developed a variety of adaptations
that enable them to respond to their internal nutritional status as well as to
the external availability of nutrients.A strategy for plants is mycorrhizal association,
in which expanding mycorrhizal mycelia that grow outward from the
mantle into the surrounding soil is a very efficient nitrogen scavenger owing
to (1) its capacity to explore a larger soil volume than roots alone (Smith and
Read 1997), (2) its ability to provide access to nitrogenous reserves contained
in organic horizons (Chalot and Brun 1998) and (3) its greater capacity for
uptake of nitrogenous compounds (Javelle et al. 1999; Wallanda and Read
1999).
This interconnected network of hyphae (or specialized aggregates, i.e., rhizomorphs)
forms a supracellular compartment for the transport of nutrients
from sites of nutrient capture to sites of nutrient utilization and transfer. It
has been estimated that the external mycelium makes, by far, the greatest contribution
to the overall potential absorbing surface area of pine seedlings
inoculated with Pisolithus tinctorius or Cenococcum geophilum (Rousseau et
al. 1994). Fungal hyphae have a number of advantages compared with roots;
(1) hyphae have a low ratio of biomass to absorptive surface area and can easily
be regenerated (Harley 1989; Rousseau et al. 1994), (2) they have been
shown to rapidly colonize nutrient-rich sites (Carleton and Read 1991; Bending
and Read 1995) and (3) because of their small diameter, they can exploit
small pores inaccessible to roots. The symbiotic association of higher plants
with mycorrhizal fungi is considered to have been responsible for the colonization
of land by plants (Taylor and Osborn 1995).
Plant Surface Microbiology
A.Varma, L. Abbott, D. Werner, R. Hampp (Eds.)
© Springer-Verlag Berlin Heidelberg 2004