Plant-Microorganism Interactions: Effects on the Tolerance of Plants to Biotic and Abiotic Stresses - Crop Improvement: New Approaches and Modern Techniques - page 211

ars into the external mycelium distributed throughout the soil (Bago et al. 2003 ).
The symbiosis between plants and AMF also results in the reduction of physiologi-
cal losses through stress (Munier-Lamy et al. 2007 ) and consequently faster growth,
leading to economy input and a reduction of environmental contamination (Huang
et al. 2009 ). Moreover, these fungi can act as potential biological control agents,
reducing the effects and damage from plant pathogens through indirect means or
increased nutrition and plant resistance (Meira-Haddad 2008 ; Folli-Pereira et al.
2012 ). AMF also play an important role in the aggregation of soil particles.
PGPRs have been traditionally used as inducers of systemic resistance to dis-
eases in plants. Currently, new PGPRs have been proposed for use in agricultural
crops. Studies concerning the specific interactions of symbiotic microorganisms
and pathogens have demonstrated the complexity of rhizospheric interactions in-
volving both mycorrhizal and non-mycorrhizal fungi, beneficial and pathogenic
bacteria, the plant and the soil. It is no longer possible to study only the isolated
microorganism without considering the complexity of its habitat. The association of
PGPRs with other microorganisms such as AMF is both economical and practical.
PGPRs colonise plant roots and promote the development of AMF through the
enhanced absorption of P and N (Artursson et al. 2006 ; Richardson et al. 2009 ).
However, there are limited data concerning the PGPR hyphal inoculation of AMF
(Hartmann et al. 2009 ).
The strength of the physiological phases of bacterial binding to AMF hyphae
varies, including a weak electrostatic binding in the first stage, followed by a strong
bond in the second stage, which is related to the production of cellulose and other
extracellular bacterial products (Artursson et al. 2006 ). Indeed, mutant bacteria
are unable to produce these products in the presence of the AMF hyphae (Arturs-
son et al. 2006 ). Some bacterial strains such as Pseudomonas spp. can colonise
both plant roots and AMF hyphae, suggesting that the mechanisms of the related
processes could be relatively similar.
The association of gram-positive bacteria with AMF is high compared with
gram-negative bacteria, although this relationship has not been verified (Artursson
et al. 2005 ). The significance of these interactions is due to the synergistic inter-
action of some important PGPRs, including the gram-positive Bacillus spp ., with
AMF (Francis et al. 2010 ). The enzymes of soil bacteria and AMF can also influ-
ence the decomposition of organic matter in the soil (de Boer et al. 2005 ).
There are a large number of bacteria, including PGPR and Rhizobium , which pro-
mote the activity and development of AMF (Frey-Klett et al. 2007 ; Richardson et al.
2009 ). These mycorrhizal helper bacteria are usually fungus-specific (Rillig et al.
2005 ) and promote the growth of specific AMF during symbiosis with the host plant.
This specificity has been attributed to the size and surface rugosity of the spore (Bha-
radwaj et al. 2008 ). Thus, Frey-Klett et al. ( 2007 ) proposed the term “mycorrhizal
auxiliary bacteria” to describe a broader concept than “mycorrhizal helper bacteria”
(MHB), which includes the beginning of the arbuscular mycorrhizal symbiosis and
the effects of MHB on the biocontrol of other species in terms of specificity. MHB
may influence the germination of spores, affect the spore wall (de Boer et al. 2005 ),