Excessive nitrate enhances cadmium (Cd) uptake by up-regulating the expression of OsIRT1 in rice (Oryza sativa)

• Excessive NO3− fertilizer supply increases Cd uptake and accumulation in rice without aggravating toxicity symptom.
• Excessive NO3− enhances Cd uptake by increasing net Cd2+ influx in elongation zone of rice roots.
• Excessive NO3− substantially enhances Cd uptake by up-regulating the expression of OsIRT1 in rice.

In order to pursue higher yield of rice, higher amount of NO3− is often used in rice production, but limited information is available on cadmium (Cd) uptake promoted by nitrate (NO3−) in rice. Here, hydroponic experiments with integration of three NO3− levels (0 mM, 2.86 mM and 5.72 mM simplify as 0, NO3− and 2NO3−) and two Cd concentrations (0 and 100 μM) were conducted. Phenotypes and physiological indexes related to Cd uptake were measured; content changes of nitrogen, Cd and Fe also were measured. In addition, net Cd2+ flux was monitored and quantitative RT-PCR was applied to determine the expression of Fe/Cd transporters. The results showed that although excessive NO3− had no significant promoting effect on rice growth in absence or presence of Cd, it enhanced Cd influx in elongation zone of rice roots and increased Cd accumulation in rice plants and grains. Excessive NO3− also increased OsIRT1 expression and Fe accumulation in rice seedlings in the absence or presence of Cd. In conclusion, excessive NO3− increases OsIRT1 expression and Cd influx in elongation zone of rice roots, and then it increased Cd and Fe uptake and accumulation in rice plants and grains, which raises an increased-risk for human health.