Plant-Microorganism Interactions: Effects on the Tolerance of Plants to Biotic and Abiotic Stresses - Crop Improvement: New Approaches and Modern Techniques - page 213

can affect the microbial population in the rhizosphere directly or indirectly through
changes in exudation patterns or fungal exudates. Moreover, soil microorganisms
might affect the formation and function of arbuscular mycorrhiza (AM). Growth-
promoting mycorrhizal helper bacteria are known to stimulate the mycelial growth
of AMF or improve the establishment of the mycorrhizal association (Toro et al.
1997 ).
The combined inoculation of beneficial microorganisms in the soil rhizosphere
reduces the need for agricultural chemicals that are harmful to the environment;
consequently, these microorganisms are gaining more attention for establishing
sustainable agroecosystems. Indeed, microorganisms are active at the soil-plant in-
terface, where microcosm systems such as the rhizosphere are developed (Cordier
et al. 2000 ). Carbon flows are essential to the functioning of the rhizosphere. Many
microbial interactions are responsible for key environmental processes, such as bio-
geochemical nutrient cycles and the maintenance of plant health and soil quality.
The effectiveness of microorganisms as modifiers of soil fertility and facilitators
of plant development has been verified through the analysis of alterations in nutri-
tional status and plant development. The combined inoculation of selected micro-
organisms in the rhizosphere has been recommended to maximise the growth and
nutrition of plants. The study of the antagonistic and synergistic effects of different
microbial inoculants when co-inoculated is a crucial step for the development of
effective microorganism-host combinations. It has been reported that the double in-
oculation of Glomus intraradices and Bacillus subtilis promotes the establishment
of the AMF and increases the plant biomass and P accumulation (Toro et al. 1997 ).
Inoculation with both growth-promoting bacteria and AMF produced decreased
Na and increased K absorption in lettuce leaves, increasing the salinity tolerance
of plants (Kohler et al. 2009 ). The PGPR strain Pseudomonas mendocina produces
exopolysaccharides (Kohler et al. 2006 ) that bind to cations, including Na, thereby
decreasing the content of Na available for absorption by plants.
AMF might influence bacterial communities in the soil, including PGPRs that
are involved in soil aggregation through the exudation of carbon derived from pho-
tosynthesis in the mycorrhizosphere. However, the mechanisms underlying changes
in the soil matrix and their significance for soil aggregation are poorly understood.
Unlike AMF, which exert a strong influence on the scale of macroaggregates, rhi-
zobacteria directly influence the formation and stabilisation of microaggregates.
Thus, the AMF-mediated alteration of prokaryotic communities could indirectly
influence aggregation processes at smaller scales than macroaggregates. In drought
conditions, the formation of aggregates in the soil and the consequent soil stabilisa-
tion are essential for the increased accumulation of water in the soil, which conse-
quently increases plant productivity during water stress. In addition, PGPRs and
AMF produce phytohormones that contribute to increased development and root
growth, and plant roots contribute to the stability of soil aggregates directly through
the root “material” and indirectly through the stimulation of microbial activity in
the rhizosphere.