Plant-Microorganism Interactions: Effects on the Tolerance of Plants to Biotic and Abiotic Stresses - Crop Improvement: New Approaches and Modern Techniques - page 216

Functional Diversity of AMF as a Determinant in the
Ability of AMF to Increase Plant Tolerance to Stress
Conditions
Arbuscular mycorrhizal fungi form the most common mutualistic relationship in
nature with the roots of approximately 80 % of terrestrial plants, with a presumed
origin of approximately 460 million years ago (INVAM 2012 ). The intimacy of
mycorrhizal associations provides a seamless morphological and physiological in-
tegration, resulting in increased functional compatibility. Fungal hyphae act as an
extension of the plant root system, conferring increased absorption of water and nu-
trients to plants, while the plant provides the fungus with photo-assimilates, allow-
ing it to complete its life cycle, which only occurs in the presence of the host in the
case of AMF (Smith and Read 2008 ). Although this symbiosis is often considered
to be mutualistic because the AMF receive carbon from the plant, the net effect on
the plant capacity varies from mutualistic to parasitic (Kiers and van der Heijdenet
2006 ), depending on the ecological conditions and plant-fungus combinations.
Spores, fragments of colonised roots and the extraradicular mycelium of soils are
the primary potential sources of inoculum, contributing the colonisation of plants.
The relative contribution of each type of propagule to the colonisation of plant roots
is difficult to determine.
Colonising ability (Avio et al. 2006 ) is used to describe the ability of AMF to
propagate inside the plant roots. As such, it should be considered to be a measure of
the '' steady state '', differing from the level of colonisation observed in a particular
segment of the root at a given time. The dynamic colonisation process requires a
continuous signal exchange during the growth of hyphae and roots. Different AMF
can colonise a particular host species at the same level, whereas the symbiotic ef-
fectiveness, measured as the growth response, can vary substantially (Smith et al.
2004 ). Abiotic and biotic factors influence the symbiotic effectiveness between the
two partners at the organismal and cellular level. At the community level , abiotic
factors such as the availability of soil nutrients; the micro and macroclimate, includ-
ing light and moisture (Staddon et al. 2003 ); and biotic factors such as community
composition (Klironomos et al. 2000 ) indirectly influence symbiotic effectiveness.
Interactions with pathogens and parasites affect carbon gain at the community level
and the organismal levels.
Because of the lack of evidence for “taxonomic specificity”, the different sym-
biotic responses of the host plant to the various AMF isolates suggest the existence
of a “functional specificity” (Finlay 2004 ). This specificity is related to the balance
between benefits and costs of the fungus for the host, which is sometimes attrib-
uted to differences in the colonisation degree or the efficiency of nutrient transport
between fungus and plant. There may be a preferential fungus-plant association at
a certain stage of plant development, which is modulated by the physiology and
ecology of the plant through mechanisms of evolutionary convergence between
symbionts (Pawlowska 2004 ). Therefore, a functional mycorrhiza results from
the seamless morphological and physiological integration of partners, reflecting