These processes also included the reinforcement and cross-linking of cell walls and cell defense compounds production. The production of high levels of ROS also induced synthesis of antioxidant compounds and detoxifying activities of ROS such as SOD, peroxidases and other antioxidant like phenolics compounds. The production of ROS can be achieved by the action of the RBOH and/or apoplastic peroxidases,. The phenilpropanoid metabolism is another defensive mechanism. Phenols play an important role as antioxidant and in the modification of the properties of cell walls, limiting polysaccharide degradation by exogenous enzymes and increasing cell wall rigidity. Some phenylpropanoids can polymerize and form defensive structures, such as lignin. Gayoso et al. concluded that Verticillium dahliae infection had a clear influence on phenolic metabolism in tomato, the increase in total phenolics being detected after 2 h inoculation in the resistant lines. So, a higher content of these compounds is indicative that strong defense reactions are being displayed by the plant. Nitric oxid is a highly reactive signal molecule, but the origin of NO in plants remains mainly unclear. In the cytosol the Nitrate reductase catalyzes the reduction of nitrate to nitrite using NADH. The NR-mediated NO production can be induced by LDN-193189 biotic or abiotic factors, including elicitors from fungal plant pathogens. More recently, a nitrite: NO reductase was discovered in PM from plant roots, involved in NO formation. In plants, NO is involved in morphogenetic and physiological processes including responses to biotic or abiotic stresses. Therefore, NO is involved in plant-pathogen and plant symbioses interactions, as well as plant responses induced by elicitors. High concentrations of NO can have a synergistic effect with ROS leading to defense reactions. In the reactions induced by pathogens or their avirulent strains, the O22 produced can react with NO to form peroxynitrite, an even more reactive agent to many pathogens. Nevertheless, the role of NO and ROS in symbiotic and pathogen interactions remains unclear. So, the ability of plants to sense and respond to the attack of fungal pathogens is one of the first events in the evolutionary process of land plants. Linked to this process is interesting to note the capacity developed by plants to establish fungal symbiosis, which demonstrates the ability to differentiate between pathogenic and symbiotic interactions. The involvement of oxidant and antioxidant systems in mycorrhizal symbiosis is well known. For instance, ROS production and activation of NOX/RBOH has been evidenced during mycorrhizal symbiosis, while Fester and Hause suggested that ROS play a role in the control of mycorrhizal interactions. Lambais et al. showed an induction of SOD activities in the establishment of arbuscular mycorrhiza, this activation perhaps associated with the high levels of H2O2 observed in bean roots colonized by Rhizophagus irregularis. These authors concluded that the production of H2O2 by SODs could be associated with fungal recognition and activation of the plant defense. Baptista et al. showed that two of three H2O2 peaks detected in the early.