New insights into reactive oxygen species and nitric oxide signalling under low oxygen in plants

Plants produce reactive oxygen species (ROS) when exposed to low oxygen (O-2). Much experimental evidence has demonstrated the existence of an oxidative burst when there is an O-2 shortage. This originates at various subcellular sites. The activation of NADPH oxidase(s), in complex with other proteins, is responsible for ROS production at the plasma membrane. Another source of low O-2-dependent ROS is the mitochondrial electron transport chain, which misfunctions when low O-2 limits its activity. Arabidopsis mutants impaired in proteins playing a role in ROS production display an intolerant phenotype to anoxia and submergence, suggesting a role in acclimation to stress. In rice, the presence of the submergence 1A (SUB1A) gene for submergence tolerance is associated with a higher capacity to scavenge ROS. Additionally, the destabilization of group VII ethylene responsive factors, which are involved in the direct O-2 sensing mechanism, requires nitric oxide (NO). All this evidence suggests the existence of a ROS and NO - low O-2 mechanism interplay which likely includes sensing, anaerobic metabolism and acclimation to stress. In this review, we summarize the most recent findings on this topic, formulating hypotheses on the basis of the latest advances.