Plant-Microorganism Interactions: Effects on the Tolerance of Plants to Biotic and Abiotic Stresses - Crop Improvement: New Approaches and Modern Techniques - page 223

occurrence, except in plants that are members of the following families: Brassica-
ceae, Amarantaceae, Comelinaceae, Juncaceae, Proteaceae, Poligonaceae, Cypera-
ceae and Chenopodiaceae. Approximately 87 % of the Cruciferae (Brassicaceae),
67 % of the Chenopodiaceae, 37 % of the Poligonaceae and 4 % of the legumes do
not form AMs. Surveys conducted in various regions of the world confirm that AMs
are much more abundant than ectomycorrhizae and occur in most Phanerogams
(97 %), including almost all species of agronomic and pastoral interest and forest
species native to the tropics.
The richness of AMF varies greatly, and two to 33 species per ecosystem have
been identified. Although several studies have been conducted, the wealth, diversity
and symbiotic potential of AMF populations in Brazilian ecosystems have not yet
been sufficiently studied. The occurrence of AMF in the country includes surveys
conducted in various crops and non-cultivated ecosystems. Many of them reveal
the richness of the species, with many of them that have not yet been identified
(approximately 20 % of the species observed).
The cultivation of soil and the imposition of environmental stress cause major
changes in the structure of fungal communities through changes in the distribution
and dominance of the species. These effects are due to biotic and abiotic changes
in the edaphic environment, such as changes in the vegetation (roots) and chemical
properties of the soil, especially in the components of acidity, availability of nutri-
ents, water, salinity and heavy metal contamination. Propagules of these fungi are
present in almost all soils, and the type of vegetation and environment determine
the occurrence and degree of root colonisation. AMF have reduced occurrence or
are absent in soils that are fumigated, severely disturbed by erosion, subject to min-
ing, in areas of civil construction, under long fallow period or flooding, and those
cultivated for long periods with non-host species and high concentrations of envi-
ronmental pollutants.
The presence of heavy metals at toxic concentrations in the soil greatly influ-
ences the AMF. The excess metals reduce spore germination, mycelia growth, de-
gree of colonisation and sporulation of these fungi, causing a significant impact
on their ecology and diversity (Klauberg-Filho et al. 2005 ). Despite these effects,
more than 30 species of AMF have been identified in contaminated soils world-
wide and some at high frequencies, such as Paraglomus occultum , G. clarum ,
G. intraradices and Scutellospora pellucida , in addition to abundant colonisation
and sporulation. Even at high concentrations of toxic metals, increased colonisation
rates and spore densities have been reported (Gaur and Adhoeya 2004 ); however,
in soils contaminated with Cd, Zn, Cu and Pb, the species richness decreases with
the increasing concentration of these metals in the soil (Klauberg-Filho et al. 2002 ).
The presence of heavy metals inhibits spore germination and mycelial growth, re-
ducing the mycorrhizal colonisation of plants. Several studies provide evidence of
the different AMF behaviours in relation to excess metals in the soil, and several
isolates are known to be tolerant to multiple metal contaminants in the soil. Con-
sidering the importance of these fungi in the ecology of plants, isolates tolerant
to heavy metals are of great interest in the revegetation of areas degraded by the
accumulation of these elements.