Plant-Microorganism Interactions: Effects on the Tolerance of Plants to Biotic and Abiotic Stresses - Crop Improvement: New Approaches and Modern Techniques - page 214

Glomalin in Soil: The Importance of the Soil-Plant-
Microorganism System
AMF are critical for the establishment and adaptation of plants in severely disturbed
locations, including those contaminated with heavy metals (Vallino et al. 2006 ).
They also affect the physicochemical characteristics of the substrate and contribute
to the formation and maintenance of soil structure through the aggregation of soil
particles, hyphal exudates, and residues. Moreover, mycorrhizal fungi produce glo-
malin, a protein extracted from soil as glomalin-related soil protein (GRSP) (Rillig
2004 ), which plays a key role in the stability of the soil (Bedini et al. 2009 ).
GRSP is an alkali-soluble protein material related to AMF (Rillig 2004 ; Nichols
and Wright 2006 ), whose biochemical nature has not been elucidated. As fungal
hyphae are shed (Driver et al. 2005 ), GRSP is transferred to the soil as a complex of
repeat monomer structures connected by hydrophobic interactions (Nichols 2003 ),
which bind to soil particles and stabilise aggregates (Rillig and Mummey 2006 ). In
addition, GRSP contains bound iron (0.04-8.8 %) (Nichols 2003 ), but it does not
contain phenolic compounds such as tannins (Rillig et al. 2001 ).
Glomalin contains approximately 60 % carbohydrates, comprising N linked to
oligosaccharides. It also contains Fe, which is insoluble in water. Glomalin exhibits
high hydrophobicity, which might contribute to the initiation of soil aggregation.
The amount of immunoreactive glomalin extracted from the soil is directly pro-
portional to soil aggregate stability in various regions of the world. Glomalin was
detected in large amounts in many soils (Nichols 2003 ), which has been attributed
to the fact that AMF colonise the roots of approximately 80 % of vascular plant
species and have a global distribution. Large “pools” of glomalin might result from
their high persistence in soil (Rillig et al. 2001 ).
Soil aggregates have also become an important protective environment for AMF
hyphae. In degraded soils in recovery, improved aggregation is accompanied by an
increased amount of colonised fine roots and hyphae that influence the geometric
diameter of the aggregates. Because well-aggregated soils are less affected by ero-
sion and more favourable for plant development, the effects of AMF on aggregation
contribute to agricultural productivity and sustainability and to the conservation and
functionality of natural ecosystems.
C losses in the soil result from leaching and erosion (Rillig et al. 2006 ). Stable
soil structural units (aggregates) provide resistance to erosion. The importance of
AMF in reducing erosion losses is related to their role in soil aggregation (Rillig
2004 ) and consequently in nutrient cycling through the reduction of carbon leaching
in soils (Rillig et al. 2006 ).
Cations are bound to GRSP in quantities that vary in different soils (Nichols
2003 ; Chern et al. 2007 ). Recently, González-Chávez et al. ( 2004 ) clearly showed
an increased binding capacity of GRSP to heavy metals (MTs) (Cu, Pb and Cd).
Based on the results of his investigation, it has been suggested that this sequestra-