Plant-Microorganism Interactions: Effects on the Tolerance of Plants to Biotic and Abiotic Stresses - Crop Improvement: New Approaches and Modern Techniques - page 212

produce stimulants such as CO 2 (Carpenter-Boggs et al. 1995 ), or affect the absorp-
tion of P by the AMF (Ruiz-Lozano and Bonfante 2000 ).
Moreover, the AMF might compete for nutrients from the soil, resulting in af-
fecting changes within the bacterial community of the rhizosphere. The associa-
tion of some bacteria with AMF is specific (Artursson et al. 2005 ), suggesting that
fungal exudates stimulate communication between bacteria and AMF (Artursson
et al. 2006 ). Indeed, some genera of bacteria, including Arthrobacter and Bacillus ,
were most commonly observed in the hyphosphere or within soil around AMF hy-
phae, while Pseudomonas spp. were more distributed in the rhizosphere of Sorghum
bicolour (Artursson et al. 2005 ).
The adhesion of PGPRs to AMF is determined by the formation of biofilms
(Seneviratne et al. 2009 ). MHB, which primarily includes Bacillus and Pseudomo-
nas , can affect the functions of the AMF, influencing root permeability, root exu-
dation, AMF colonisation of the host root, and phytohormone production, thereby
mitigating the adverse effects of the environment on hyphal growth and stimulating
the growth of root hairs in plants. Some strains of rhizobia are also able to affect
the pre-symbiotic stage of fungi, influencing spore germination and hyphal growth
(Frey-Klett et al. 2007 ).
The symbiosis between AMF and plants contributes to the stability of soil aggre-
gates, including soils with high salinity (Caravaca et al. 2005 ). Stability is initiated
through macroaggregates (> 250 mm), which tangle hyphae and deposit organic
substances that assist in the subsequent stability of soil aggregates. A key factor in
the contribution of these fungi to the stabilisation of saline soils is the production
of glomalin, a glycoprotein that acts as an insoluble glue to stabilise aggregates
(Gadkar and Rillig 2006 ).
The influence of AMF on plant growth has been attributed to bacteria associated
with the mycorrhizosphere (Larsen et al. 2009 ). The production of exopolysaccha-
rides (EPSs) in response to adverse environmental conditions such as drought can
contribute to soil aggregation, leading to increased water retention in the rhizo-
sphere (Kaci et al. 2005 ), which can eventually affect the growth of AMF in these
soils. The effectiveness of the PGPR inoculation of plants leads to soil stabilisation
and promotes soil fertility (Kohler et al. 2006 ). The study of the antagonistic and
synergistic effects of different microbial inoculants when co-inoculated is a crucial
step in the development of efficient host-microorganism combinations. The inocu-
lation with rhizobacteria, alone or in combination with AMF, improves the physical
properties of the soil, even under saline stress.
Inoculation with AMF is an effective method of increasing the capacity of host
plants to establish and address stress situations, such as nutrient deficiency, drought
and soil disturbance (Caravaca et al. 2003 ). In fact, several authors have indicated
that AMF inoculation stimulates the absorption of water and nutrients, especially
N and P (Jeffries et al. 2003 ; Folli-Pereira et al. 2012 ) in the plant or enhances the
aggregation of eroded soils (Caravaca et al. 2002 ) to improve seedling performance.
In return, the mycorrhizal plants provide fungus with photosynthetic C, which is
delivered into the soil through fungal hyphae. Thus, the formation of mycorrhizae