The source of nitrogen conditions transcriptomic responses to water deficit in common bean roots

Drought stress reduces plant growth and yield of crops. Common bean (Phaseolus vulgaris L.) establishes symbiosis with rhizobia, ensuring an adequate nitrogen supply without fertilizers. However, the relationship with rhizobia is constrained by limited water availability which inhibits both nitrogen fixation and plant growth. In addition, physiological and molecular responses of common bean to drought are conditioned by the form of nitrogen assimilated. Therefore, understanding the molecular mechanism(s) triggered in common bean under water-deficit conditions is relevant to identify the best strategies to resist drought stress. With the objective of understanding the molecular responses of roots and nodules from common bean to water-deficit stress, plants cultivated under N₂-fixation or nitrate fertilization were exposed to ten days of water deprivation. Afterwards, transcriptomic analysis was performed in roots, while metabolome profiling was carried out in roots and nodules. Physiological results showed that under water-deficit, N₂-fixing plants increased their root biomass more than nitrate-fertilized plants. Furthermore, water-deficit stress induced more transcriptional changes in nitrate-fertilized plants than in N₂-fixing plants, including a larger number of transcription factors in these plants compared with the N₂-fixing plants. On the other hand, roots from N₂-fixing plants accumulated more metabolites with potential protective functions such as allantoin, proline, raffinose, abscisic acid, and flavonoids in response to water-deficit stress than plants fertilized with nitrate, indicating that symbiosis might facilitate a faster and more efficient response to water-deficit stress. Moreover, common bean nodules exposed to water-deficit stress accumulated proline and erythritol, but reduced their content of maltose, pyruvic acid and allantoin compared to their respective controls. Taken collectively, these findings suggest that, despite the inhibition of nodule activity, N₂-fixing plants respond better to water-deficit stress than nitrate-fertilized plants.