Plant-Microorganism Interactions: Effects on the Tolerance of Plants to Biotic and Abiotic Stresses - Crop Improvement: New Approaches and Modern Techniques - page 210

thus indicating the occurrence of systemic resistance. AMF play an important role
in enhancing the absorption of water and nutrients, conferring an enhanced hydra-
tion and nutritional status to the plant (Pozo et al. 2010 ; Folli-Pereira et al. 2012 ).
The benefits of microorganisms might help plants in terms of tolerance to several
biotic and abiotic stress factors. Thus, it is necessary to understand each symbiosis
to ensure the best utilisation of the benefits to plants.
AMF and Growth Promoter Bacteria as Potential Factors
Involved in Plant Tolerance to Stress
Plant growth promoting rhizobacteria (PGPRs) are bacteria that increase plant
growth through interactions with plant roots. PGPRs represent a functionally ac-
tive portion of the soil biota present inside the plant root, in the rhizosphere, or
on the rhizoplane. The literature indicates their potential as a component of the
sustainable management of agricultural systems, conferring characteristics such as
greater resistance to biotic and abiotic stress conditions to the host and promoting
plant growth, leading to their widespread biotechnological use. PGPRs can enhance
growth of plants through a variety of mechanisms: (a) production of plant hor-
mones; (b) controlling pathogens; and (c) increasing tolerance to stress conditions
(Kohler et al. 2006 , 2009 ; Jalili et al. 2009 ; del Mar Alguacil et al. 2009 ).
The mechanisms of plant growth promotion include direct actions, such as bio-
logical nitrogen fixation, production of plant growth regulator hormones, and the
solubilisation of inorganic phosphate, and indirect actions, such as biological con-
trols, production of siderophores and allelochemicals and the induction of local and
systemic resistance. The use of PGPRs in biotechnology has intensified with the
production of antibiotics and other bioactive molecules and the application of these
microorganisms in bioremediation processes and transgenic techniques.
These mechanisms are potentially applicable in the field for the quantitative and
qualitative improvement of agricultural production. The use of these microorgan-
isms in agriculture depends on knowledge of their diversity, plant-bacteria interac-
tion mechanisms, and the ability to maintain, manipulate, and modify beneficial
populations under field conditions.
PGPRs, especially those belonging to the fluorescent Pseudomonas group,
have been well studied but have a disadvantage relative to Bacillus . Bacillus have
developed a resistance structure, called the endospore, which is produced under ad-
verse conditions and enables the bacterium to withstand these conditions. Once the
conditions change in favour of the microorganism, its reproduction cycle allows it
to express its beneficial characteristics to the host plant.
Arbuscular mycorrhizal associations in turn have important consequences for
nutrient cycling in soil, providing plants with essential nutrients such as phosphorus
when they are scarce or if they have low mobility in the soil solution. In exchange,
photosynthetic carbon is transported to the soil through the transfer of sugar from
the root to the AMF, which subsequently translocates this carbon as lipids and sug-