OsCBL1 Modulates the Nitrate-Induced Phosphate Response by Altering OsNLP4 Cytoplasmic-Nucleus Shuttling

Nitrate can directly activate phosphate (Pi) starvation signaling, ultimately promoting plant growth by enhancing phosphorus absorption and utilization and optimizing the balance of nitrogen and phosphorus nutrients. However, the complex mechanisms by which plants integrate complex nutrient signals from nitrogen to phosphorus are not well understood. This study highlights the importance of Calcineurin B-like protein-1 (OsCBL1), a calcium sensor, in coordinating nitrogen and phosphorus signaling in rice. Knockdown of OsCBL1 in rice reduced the expression of genes involved in nitrate-induced Pi starvation responses. In high nitrate conditions, OsCBL1-KD plants displayed diminished biomass gain, unlike the wild-type rice, which thrived under elevated phosphate levels. In OsCBL1-KD plants, OsSPX4, a key repressor in nitrogen and phosphorus signaling, remains undegraded in the presence of nitrate due to the significantly reduced expression of OsNRT1.1B. Moreover, the OsCBL1 knockdown hampers the movement of the nitrogen-related transcription factor, OsNLP4, from the cytoplasm to the nucleus when nitrate is present. This impedes the expression of OsNRT1.1B, as OsNLP4 can directly bind to the promoter of OsNRT1.1B nitrate responsive cis-element (NRE) and activate its expression. In summary, these findings suggest that OsCBL1 plays a pivotal role in regulating OsNRT1.1B expression by managing the transport of OsNLP4 between cytoplasm and nucleus in response to nitrate availability. This regulation subsequently influences the phosphate response triggered by nitrate and optimizes the coordinated utilization of nitrogen and phosphorus.