Mycorrhizal Symbiosis in the Formation of Antioxidant Compounds

Plant–microbe interactions usually occur in
the phyllosphere, endosphere and rhizosphere. The phyllosphere is the aerial (above-ground)
part of the plant and the endosphere is the in
ternal parts of the plant, including the trans
port system (Bhattacharyya and Jha, 2012).
The rhizosphere is the narrow zone of soil
that is under the direct influence of living
plant roots and their secretions and the asso
ciated soil microorganisms. Enormous amounts
of exudates can be released into the rhizo
sphere, and these significantly stimulate mi
crobial diversity and function (Hiltner, 1904).
Since 1904, when Hiltner coined the term
‘rhizosphere’, the rhizosphere processes of plants
have been widely investigated, although very
little information is available yet on the com
position of the microbial community in this
very significant volume of soil. Another def
inition of the rhizosphere has been offered by
Bringhurst et al. (2001), who described the
rhizosphere as including the region of soil
bound by plant roots and often extending a
few millimetres from the root surface. Plant root-associated free-living as well
as symbiotic rhizobacteria, and mycorrhizal
fungi in particular, are integral parts of the
rhizosphere microbiota. The filamentous act
inobacteria are also considered to be an import
ant microbial community in the rhizosphere
biota (Benizri et al., 2001), as they are able to
influence plant growth and development as
well as protect the plant roots against phyto
pathogens. Studies based on molecular tech
niques have estimated that there are more than
4000 microbial species/g soil (Montesinos,
2003). A higher microbial density, as well as
extensive metabolic activity, in the rhizo
sphere is the overall result of vigorous plant
rhizosphere-microbe interactions (Khan, 2005).
Synergistic positive interactions recorded be
tween mycorrhizal fungi and plant growth-
promoting rhizobacteria (PGPR) such as nitrogen
f
ixers, phosphate solubilizers (especially fluor
escent pseudomonads and sporulating bacilli)
and cellulose decomposers in the rhizosphere
usually lead to successful plant–rhizosphere
microbe interactions (Artursson et al., 2006). Plant–root interactions in the rhizosphere
may include root–root, root–insect and root–
microbe interactions, resulting in the produc
tion of more root exudates, which ultimately
favours maximum microbial populations in
this ecologically significant region. Rhizosphere
microbial cells are known to produce and
transmit signal molecules that allow the
whole microbial population to spread as a bio
f
ilm over the root surface and, thereby, could
initiate a concerted action when a particular
population density of soil microbes is
achieved. This phenomenon is commonly
known as quorum sensing (QS), which, in
combination with other regulatory systems,
may expand the range of environmental sig
nals that could elicit specific gene expression.