During invasion RKN cause little damage to the root as they migrate between the cells. Still, a rapid and transient production of reactive oxygen species (ROS), such as superoxide anion (O2~) and hydrogen peroxide (H2O2), is detected at the time of nematode invasion. No significant oxidative burst was detectable at the time of giant cell induction and at later time points (Melillo et al. 2006; Das et al. 2008). ROS are also produced in host plants during interaction with rhizobia notably in the infection process (Santos et al. 2001). So the bacteria have to protect themselves from these defence molecules and bacterial strains with impaired capacity to detoxify H2O2 have deficiencies in their symbiotic capacities. Despite negative effects, ROS has been reported as signalling molecules in the control of nodulation process, and a threshold level of ROS would be required for a harmonious nodule development (Pauly et al. 2006). Additional major molecules involved in the regulation of the cellular redox state, nitrogen monoxide (NO) and glutathione (GSH) are detected in response to rhizobia and required in proper nodule development (Frendo et al. 2005; del Giudice et al. 2011). Unfortunately, the role of GSH and NO in giant cell formation has not yet been investigated.
Microarray analysis showed that the successful establishment of RKN is associated with the suppression of plant defence responses in A. thaliana (Jammes et al. 2005). Most of the defence-related genes such as WRKY transcripts and genes associated to the JA/ethylene-dependent pathways (EIN3, ERF1, PR4) were repressed particularly at later stage of gall formation. Expression of maize lipoxygenase LOX3 and Arabidopsis pathogenesis-related genes PR-1, PR-2, PR3 and PR5 peaked at 7 or 9 dpi, respectively, in RKN infected roots (Gao et al. 2008; Hamamouch et al. 2010). The observed downregulation of defence genes from host plants suggests an active modulation of the plant response by RKN as reported in different plant-pathogen interactions (Abramovitch and Martin 2004). Defence suppression also appears to play an important role in symbiotic plant-microbe interactions. The NopL effector of Rhizobium sp. NGR234 suppresses PR gene expression when expressed in tobacco or L. japonicus (Bartsev et al. 2004).
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