All these results clearly show that O2" and H2O2 play an important role in the first step of nodule organogenesis. Furthermore, several experimental data suggest that another activated species - nitric oxide (NO') - should also be considered. Indeed, (i) the presence of a leghemoglobin-nitric oxide complex in young soybean nodules (Mathieu et al. 1998), (ii) the detection of NO" with a fluorescent probe in alfalfa nodules and (iii) the effect of a NO" scavenger on the nodulation process, strongly suggest that NO' is also involved in the oxidative burst described above and plays a role in the establishment of the symbiosis.
This oxidative burst in response to symbiotic infection can be consistent with rhizobia being initially perceived as invaders by the plant. However the plant reaction appears to limit only the infection and does not lead to the rejection of invading symbiont (Vasse et al. 1993). H2O2 and peroxidase may be involved in the regulation of infection (Cook et al. 1995), but a question arises: why does the oxidative burst not trigger plant defense reactions? It can be suggested that rhizobia inhibit signaling pathways, leading to the deleterious defense cascade. The prolonged maintenance of the oxidative burst might keep the plant's options open, allowing it, in the absence of the appropriate bacterial signal, to switch to a defense response. On the other hand, the oxidative burst can be considered as a process necessary for the establishment of the symbiotic interaction. In this framework, it could trigger the expression of plant and/or bacterial genes, which are essential for the nodulation process. It has been reported that an oxidative burst at the very early stage of infection is necessary for the induction of Ripl peroxidase in the root of Medicago truncatula (Ramu et al. 1999). Moreover, Nod factors stimulate the oxidative burst and pharmacological effectors that inhibit the burst were shown to block induction of plant marker genes for nodulation (D.R. Cook, personal communication). In any case, it appears that ROS play a key role in the early steps of nodule development.
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