Abstract
Nitrate acts as both a nitrogen source and a signal regulating root system architecture (RSA), but whether extracellular Ca²⁺ participates in nitrate-mediated root developmental plasticity remains unclear. Here, a factorial hydroponic system combining nitrate availability with extracellular Ca²⁺ manipulation was used to dissect their interactive effects on wheat seedling RSA. Nitrate availability strongly reshaped root development, with optimal supply promoting primary root elongation, lateral root formation, and overall root system expansion, whereas nitrate deficiency and excess nitrate disrupted these traits. Strikingly, the effects of extracellular Ca²⁺ availability depended on nitrate status: reducing extracellular Ca²⁺ had limited effects under nitrate deficiency but strongly suppressed primary root elongation, lateral root formation, and root system expansion under nitrate supply, shifting RSA toward a shorter and less branched architecture. Similar responses to Ca deficiency and EGTA treatment further supported a specific role of extracellular Ca²⁺ availability in nitrate-dependent RSA regulation. Collectively, these findings support a nitrate–extracellular Ca²⁺–RSA regulatory framework in which extracellular Ca²⁺ functions as a nitrate-dependent component of root developmental plasticity, providing a mechanistic basis for understanding coordinated nutrient signaling and improving root-mediated nitrogen acquisition in crops.
