Plant-Microorganism Interactions: Effects on the Tolerance of Plants to Biotic and Abiotic Stresses - Crop Improvement: New Approaches and Modern Techniques - page 228

rhizal plants at regions far from the infection sites, indicating the occurrence of sys-
temic resistance (Cordier et al. 1998 ; Pozo et al. 2002 ; Selosse et al. 2004 ).
The successful establishment of mycorrhiza is essential for the control of nema-
todes and has a negative effect on the reproduction of these organisms (Cordier
et al. 1998 ; Elsen et al. 2003 ; Vos et al. 2012 ).
The bioprotector effect of the AMF to plant pathogens might be related to the
induction of localised or systemic resistance (Cordier et al. 1998 ; Pozo et al. 2002 ;
Elsen et al. 2008 ). When colonised by AMF, plants exhibit biochemical, physiologi-
cal and molecular alterations related to the plant defence system as symbiosis is
established (Garcia-Garrido and Ocampo 2002 ; De Gara et al. 2003 ; Selosse et al.
2004 ). However, the plant defence responses are limited, transient, and restricted
to specific cells, but the plant reactions have physiological similarities with the
reactions observed during colonisation by pathogens (Garcia-Garrido and Ocampo
2002 ; Lambais et al. 2003 ). Mycorrhizal colonisation acts as the primary system of
plant defence against pathogens (Elsen et al. 2008 ). Plants with higher antioxidant
activities are more tolerant to different stresses, and mycorrhizae increase the activ-
ity of antioxidant enzymes, such as peroxidise, catalase and superoxide dismutase
(Costa 2003 ; Lambais et al. 2003 ; Meira 2004 ; Arfaoui et al. 2007 ).
Peroxidase is an enzyme that is transiently induced and subsequently suppressed
during mycorrhizal colonisation. The peroxidases catalyse the oxidative polymeri-
sation of phenylpropanoids for the production of lignin and are involved in cross-
linking the proteins of the cell wall, thus contributing to increased rigidity (Siddiqui
and Mahmood 1995 ; Hol and Cook 2005 ; del Río et al. 2006 ). Consequently, hydro-
gen peroxide plays an important role in strengthening the cell wall and the systemic
induction of defence genes (del Río et al. 2006 ).
Phenylalanine ammonia lyase is a key enzyme in the phenylpropanoid pathway,
which is responsible for the deamination of L-phenylalanine to form trans -cinnamic
acid and ammonia. Trans -cinnamic acid is incorporated in different phenolic com-
pounds to produce phytoalexins, which are antimicrobial compounds that are closely
related to the resistance of plants to pathogens (Wuyts et al. 2006 ; Arfaoui et al. 2007 ).
The study of the mechanisms involved in the AMF bioprotection to nematodes
has been limited due to obligatory biotrophism and parasitism of both. AMF can be
considered to be biological control agents; however, the diversity of responses to
the combination AMF-nematode and plants is unique. Generalisations are hindered
because these interactions are dependent on the host, nematode species, AMF spe-
cies and combinations of nematode and AMF initial inoculum density, soil fertility
and nematode inoculum.
Conclusions and Future Perspectives
The various interactions that occur between plant roots and microorganisms in the
soil are of importance to ecosystems. The understanding of these interactions can
greatly benefit agriculture through the manipulation of populations of common mi-
croorganisms that inhabit the soil associated with roots, which is a promising area