Cadmium inhibits the induction of high-affinity nitrate uptake in maize (Zea mays 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 mu M): 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.