Polyamines: Key elements in the rhizobia-legume symbiosis?

Polyamines (PAs) are ubiquitous low molecular weight, molecules found in all living cells with participation in a variety of processes. The establishment of symbiotic interactions between soil nitrogen fixing bacteria (rhizobia) and leguminous plants, as well as the formation of the root nodules where the rhizobia are hosted, depends on the production of PAs by both partners. In the bacteria, PAs provide certain traits that facilitate the recognition between the symbionts, while in the plant, PAs participate in the production of signaling molecules like hydrogen peroxide (H2O2) and nitric oxide (NO), detected during the initial stages of the symbiosis. Once the symbiosis is established, the high amount and variety of PAs in the root nodules together to their participation in the metabolic reactions supporting nitrogen fixation, suggest their involvement in the nodule functioning. In addition, PAs are key players in the response to abiotic stresses contributing to the adaptation of the symbiosis to salinity, specially, by an increment in the production of PAs in the bacteroids and their interactions with other anti-stress plant hormones. Considering the importance of biological nitrogen fixation in soil fertility and agriculture productivity in a context of climate change, in this review we describe the participation of PAs to the rhizobia-legume interaction and the nitrogen fixation process, as well as its involvement in the response to salinity in the symbiosis.