Proline interacts with Ca-dependent signaling to enhance chromium tolerance in rice by manipulating nitrate reductase and sucrose phosphate synthase.

The entrance of chromium (Cr) into the agricultural system would exert a negative influence on the carbon/nitrogen metabolism (CNM) of plants. In this study, we investigated the role of exogenous proline-mediated Ca-dependent signaling in the regulation of CNM in rice subjected to Cr(VI) stress, with emphasis on the involvement of nitrate reductase (NR) and sucrose phosphate synthase (SPS). Results demonstrated that proline effectively mitigated the growth inhibition of rice imposed by Cr(VI) stress, which is achieved by a reduction in cytoplasmic Ca and Cr content and the activation of the downstream Ca-dependent signaling pathway. Additionally, proline displayed a positive effect in modulating the expression and activities of NR and SPS under Cr(VI) stress, which are attributed to the cross-regulation between calcium-dependent protein kinases (CDPKs) and 14-3-3 proteins (14-3-3s). Consequently, nitrogen use efficiency and sucrose content in rice under Cr(VI) + proline treatments were higher than Cr(VI) treatments. Gene expression variation factors underscored that the regulation of proline on NR is crucial to the Ca-dependent signaling pathway, initiated by the interaction between CDPKs and 14-3-3s in rice plants during Cr(VI) stress. These results reveal that proline interacts with Ca-dependent signaling pathways to enhance Cr tolerance in rice by regulating NR and SPS.