Polyamines oxidation is required in the symbiotic interaction Medicago truncatula–Sinorhizobium meliloti but does not participate in the regulation of polyamines level under salinity

Polyamines (PAs) are small nitrogen polycationic compounds involved in the response to salinity as well as in biotic interactions. Legumes are salt sensitive species characterised by their ability to establish symbiotic interactions with nitrogen fixing bacteria known as rhizobia that colonize the roots inducing the formation of nodules where the nitrogen fixation occurs. The content of PAs can be regulated among others by the catabolic enzymes diamine oxidase (DAO) and polyamine oxidase that additionally, are a source of H2O2, required in the regulation of biotic interactions including the rhizobia-legume symbiosis. Therefore, the role of PAs catabolism in the symbiosis between Medicago truncatula and Sinorhizobium meliloti under salt stress conditions was assessed in this work by the inhibition of DAO with the specific inhibitor aminoguanidine (AG). Treatment with AG limited the symbiosis, reducing root nodule formation and nitrogenase activity. Under salt stress conditions, the reduction of the PAs levels in nodules, together with glutamic acid (Glu), support that synthesis of PAs would be involved in the regulation of PAs levels rather than degradation. In addition, the content of H2O2 in mature nodules and leaves is regulated by catalase activity (CAT), instead of PAs oxidation, while H2O2 produced by PAs catabolism seems to be involved in the inhibition of the symbiosis establishment.