Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

246j 13 Microbial Dynamics in the Mycorrhizosphere with Special Reference to Arbuscular Mycorrhizae
originated in the mid-Devonian Era, and AM associations are reported in these
plants [5]. Both living and Triassic fossil Cycades contained AMF in their roots. AM
associations are ubiquitous in living angiosperms, which probably arose in the early
Cretaceous era [6]. The phylogenetic relationship between origin and diversification
of AMF and coincidence with vascular land plants was investigated by Simon et al. [7]
by sequencing ribosomal DNA genes (SS sequences) as a molecular clock to infer
dates, from 12 Glomalean fungal species. The authors estimated that AM-like fungi
originated some 354–462 million years ago, which is consistent with the hypothesis
that AMF were instrumental in the colonization of land by ancient plants. This
hypothesis is also supported by the observation that AM can now be found worldwide
in the angiosperms and gymnosperms as well as ferns, suggesting an ancestral
nature of the association.
Universal and ubiquitous fungi belonging to Glomales form symbiotic relationships
with roots of 80–90% of land plants in natural and agricultural ecosystems [5]
including halophytes, hydrophytes and xerophytes [8–10] – and, owing to greater
exploration of soil for nutrients, are known to benefit plant nutrition, growth and
survival [11]. These associations represent a key factor in the below-ground networks
that influence diversity and plant community structure [12–14], but we know very
little about the enormous AM fungal diversity in soils and their properties and
behavior in the soil [2]. However, not all plants have mycorrhizal associations and
not all AMF benefit host plants under all growth conditions [15]. The degree of
benefit to each partner in any AMF–plant host interaction depends not only on the
particular plant and AMF species involved but also on the rhizobacteria and soil
abiotic factors.
Availability of modern techniques such as root organ based methodologies and
rDNA techniques has made it possible to study AMF–host associations in some
detail to understand the comprehensive interaction scenario of plant, rhizosphere
and soil components. Much recent research showed that these symbiotic associations
have extreme genetic complexity and functioning diversity [16]. Based on SSU
rRNA molecular phylogenetic studies, AMF have been elevated to the phylum
Glomeromycota [17], a conclusion supported by the recent study [18].
Arbuscular mycorrhizal associations are important in natural and managed ecosystems
because of their nutritional and nonnutritional benefits to their symbiotic
partners. They can alter plant productivity, because AMF can act as biofertilizers,
bioprotectants or biodegraders [19]. AMF are known to improve plant growth and
health by improving their mineral intake or increasing resistance or tolerance to
biotic and abiotic stresses [20,21]. Their potential role in phytoremediation of heavy
metal contaminated soils and water is also becoming evident [22–26].
AMF modify the quality and abundance of rhizosphere microflora and alter
overall rhizosphere microbial activity. Following host root colonization, the AMF
induce changes in the host root exudation pattern, which alters the microbial equilibrium
in the mycorrhizosphere [27]. These interactions can be beneficial or harmful
to the partner microbes involved and to the plant, and sometimes may enhance
plant growth, health and productivity [28,29]. Recently, Giovannetti and Avio [30]
have reviewed and analyzed important data on the main parameters affecting AM