A Nitrogen Molecular Sensing System, Comprised of the and Genes, Can Be Used to Monitor N Status in Rice.

Nitrogen (N) is an essential nutrient for plant growth and development, but its concentration in the soil is often insufficient for optimal crop production. Consequently, improving N utilization in crops is considered as a major target in agricultural biotechnology. However, much remains to be learnt about crop N metabolism for application. In this study, we have developed a molecular sensor system to monitor the N status in rice (). We first examined the role of the ureide, allantoin, which is catabolized into allantoin-derived metabolites and used as an N source under low N conditions. The expression levels of two genes involved in ureide metabolism, () and (), were highly responsive to the N status. was rapidly up-regulated under low N conditions, whereas was up-regulated under high N conditions. Taking advantage of the responses of these two genes to N status, we generated transgenic rice plants harboring the molecular N sensors, and , comprising the gene promoters driving expression of the luciferase reporter. We observed that expression of the transgenes mimicked transcriptional regulation of the endogenous and genes in response to exogenous N status. Importantly, the molecular N sensors showed similar levels of specificity to nitrate and ammonium, from which we infer their sensing abilities. Transgenic rice plants expressing the sensor showed strong luminescence under high exogenous N conditions (>1 mM), whereas transgenic plants expressing the sensor showed strong luminescence under low exogenous N conditions (<0.1 mM). High exogenous N (>1 mM) substantially increased internal ammonium and nitrate levels, whereas low exogenous N (<0.1 mM) had no effect on internal ammonium and nitrate levels, indicating the luminescence signals of molecular sensors reflect internal N status in rice. Thus, and represent N molecular sensors that operate over a physiological and developmental range in rice.