Cadmium inhibits the induction of high-affinity nitrate uptake in maize (Zeamays L.) roots

Cadmium (Cd) detoxification involves glutathione and phytochelatins biosynthesis: the higher need of nitrogen should require increased nitrate (NO3 -) uptake and metabolism. We investigated inducible high-affinity NO3 - uptake across the plasma membrane (PM) in maize seedlings roots upon short exposure (10 min to 24 h) to low Cd concentrations (0, 1 or 10 μM): the activity and gene transcript abundance of high-affinity NO3 - transporters, NO3 - reductases and PM H+-ATPases were analyzed. Exposure to 1 mM NO3 - led to a peak in high-affinity (0. 2 mM) NO3 - uptake rate (induction), which was markedly lowered in Cd-treated roots. Plasma membrane H+-ATPase activity was also strongly limited, while internal NO3 - accumulation and NO3 - reductase activity in extracts of Cd treated roots were only slightly lowered. Kinetics of high- and low-affinity NO3 - uptake showed that Cd rapidly (10 min) blocked the inducible high-affinity transport system; the constitutive high-affinity transport system appeared not vulnerable to Cd and the low-affinity transport system appeared to be less affected and only after a prolonged exposure (12 h). Cd-treatment also modified transcript levels of genes encoding high-affinity NO3 - transporters (ZmNTR2. 1, ZmNRT2. 2), PM H+-ATPases (ZmMHA3, ZmMHA4) and NO3 - reductases (ZmNR1, ZmNADH:NR). Despite an expectable increase in NO3 - demand, a negative effect of Cd on NO3 - nutrition is reported. Cd effect results in alterations at the physiological and transcriptional levels of NO3 - uptake from the external solution and it is particularly severe on the inducible high-affinity anion transport system. Furthermore, Cd would limit the capacity of the plant to respond to changes in NO3 - availability.