Plant-Microorganism Interactions: Effects on the Tolerance of Plants to Biotic and Abiotic Stresses - Crop Improvement: New Approaches and Modern Techniques - page 215

tion could be important for biostabilisation in soils contaminated with MTs. Bedini
et al. ( 2009 ) showed that the amounts of Cu, Ni, Pb and Co bound to GRSP were,
respectively 2.3, 0.83, 0.24, 0.24 % of the total amount of MTs present in contami-
nated soil, thereby reducing the bioavailability of toxic elements and, consequently,
plant stress. Vodnik et al. ( 2008 ) showed that GRSP represented 21.2 % of the or-
ganic matter in soil contaminated with MTs, which was positively correlated with
the concentrations of Pb and Zn in the soil; notably, the amount of lead bound to
GRSP ranged from 0.69 to 23.4 mg g -1 DW GRSP, which represented 0.8-15.5 %
of the total Pb in the soil.
Wright et al. ( 1996 ) hypothesised that AMF secrete glomalin into the soil, which
helps in soil aggregation. This model was directly based on the observed correla-
tion between the GRSP concentrations with the stability of soil aggregates in water.
The increase of soil aggregation would benefit both the host and associated AMF,
justifying the energy “cost” of glomalin production. Experimental evidence, though
obtained in an artificial manner, suggested that relations between the production
of glomalin, soil aggregation and the enhancement of extraradicular AMF hyphae
growth might indeed exist (Bedini et al. 2010 ). However, AMF also appear to pro-
duce GRSP in soils where organic matter is not the primary binding agent in the
soil, and GRSP and soil aggregation are not correlated (Rillig et al. 2003 ). This find-
ing suggests that the promotion of soil aggregation might not be the primary func-
tion of glomalin. In addition, the AMF communities and many other groups of soil
biota profit from an improved soil structure (Niklaus et al. 2003 ), which makes it
unlikely that the promotion of soil aggregation is the primary function of glomalin.
Using an in vitro sterile culture system, Driver et al. ( 2005 ) showed that most
(80 %) of the glomalin was contained in the fungal mycelium, rather than in the liq-
uid growth medium. It is unclear if this result translates from the artificial aqueous
culture system to the soil environment, or if it applies to fungi across the spectrum
of AMF species. However, if it does, it suggests that a primary function of glomalin
may be in the living fungus. Indeed, the putative function of glomalin is homolo-
gous to that of heat shock proteins. Based on these observations, Purin and Rillig
( 2007 ) proposed a new model for glomalin function. This model has the following
key components: (a) glomalin primarily functions as chaperone in the cell. It is
known that certain chaperones have the ability to act as a signal, resulting in greater
thermotolerance and control of spore viability; (b) in the context of soil aggregation,
the environmental function of glomalin is secondary to its primary physiological
function.
There are few reports of heat shock proteins (Hsp) that act as chaperones in
Glomeromycota, other than glomalin. Using the AMF species G. intraradices ,
Porcel et al. ( 2006 ) showed the expression of the small Hsp 30 improved plant
tolerance to water stress.