Figure 1. Regulatory mechanism of Arabidopsis NRT2.1 and the principle of luciferase (LUC) assay.

(A) The known regulatory mechanism of NRT2.1 in Arabidopsis. Nitrate directly binds to and activates NLP. Activated NLP (NLP*) directly binds to the promoters of NRT2.1 and NIGT1, enhancing their expression. Subsequently, NIGT1 suppresses the expression of not only NRT2.1 but also NIGT1 genes. (B) Mode of action of LUC. The level of LUC protein increases upon the activation of NRT2.1 promoter (NRT2.1pro). LUC protein catalyzes the conversion of luciferin to an excited state (oxyluciferin) with the aid of Mg²⁺. Subsequently, light is emitted during the transition of oxyluciferin to the ground state.