Plant-Microorganism Interactions: Effects on the Tolerance of Plants to Biotic and Abiotic Stresses - Crop Improvement: New Approaches and Modern Techniques - page 222

Driedzic 2007 ). It is synthesised at high levels in many plant species in response to
several types of environmental stresses, acting not only as an osmoprotectant but
also as a stabiliser of proteins and membranes (Oishi and Ebina 2005 ). This com-
pound appears to be a critical determinant of stress tolerance. Its accumulation is
induced under stress conditions, and this accumulation is correlated with the toler-
ance level (Wu et al. 2008 ).
The role of arbuscular mycorrhizae in attenuating the stress caused by heavy
metals in plants growing in contaminated soils has been recognised (Göhre and
Paszkowski 2006 ). Improvement of the nutritional status and reduced or altered
metal absorption are among the greatest benefits related to mycorrhizae on host
plants under heavy metal stress (Andrade et al. 2008 ). AMF can alter the concentra-
tion of metals in plants through the immobilisation of metal in the cell wall of in-
tra or extraradicular hyphae, metal chelation through compounds secreted by AMF
such as glomalin (Vodnik et al. 2008 ), or metallic compartmentalisation in fungal
cells. Thus, these fungi act as a filter for metal, reducing local concentrations in the
soil and creating a suitable environment for plant growth in soils contaminated with
metals (Göhre and Paszkowski 2006 ).
The mycorrhizal association can alter the metal absorption in plants (Andrade
et al. 2008 ), with reports of both the increase and reduction of metal concentrations
in plant tissues. As a consequence of physiological alterations, mycorrhizal plants
perform better under metal stress conditions (Paradi et al. 2003 ). At the molecular
level, the expression of some genes related to plant tolerance to heavy metals is al-
tered through arbuscular mycorrhizal symbiosis. However, the global mechanisms
by which the fungi reduce the phytotoxicity of the metal in plants have not been
fully elucidated, and the results f some studies show conflicting results, depending
on specific plant/fungal species/metal interactions.
The literature has reported several detoxification mechanisms in plants, but the
mechanisms associated with AMF vary among plant species. Variation is also ob-
served for the metal used, applied concentration, plant organ, and duration of the
exposure (Gratão et al. 2008 ).
Abiotic Stress and its Influence on the AMF Community
in the Soil
Mycorrhizae are complex symbioses formed by several components that determine
the colonisation rate, incidence of propagules and the effects and functions of the
symbiosis for plants and ecosystems. The primary components of this system are
the fungus, the plant, and the environment (soil), which have strong interrelation-
ships and interdependences.
Arbuscular mycorrhizae (AMs) are of widespread occurrence in superior plants,
and AMF are prevalent among fungi normally found in the rhizosphere or among
root colonisers. It is estimated that most plant species (approximately 250,000
species) are capable of forming AMs. Therefore, this association has widespread