title: Cadmium inhibits the induction of high-affinity nitrate uptake in maize (Zea               mays L.) roots.
abstract: Cadmium (Cd) detoxification involves glutathione and phytochelatins biosynthesis: the higher need of nitrogen should require increased nitrate (NO(3)(-)) uptake and metabolism. We investigated inducible high-affinity NO(3)(-) 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 NO(3)(-) transporters, NO(3)(-) reductases and PM H(+)-ATPases were analyzed. Exposure to 1 mM NO(3)(-) led to a peak in high-affinity (0.2 mM) NO(3)(-) uptake rate (induction), which was markedly lowered in Cd-treated roots. Plasma membrane H(+)-ATPase activity was also strongly limited, while internal NO(3)(-) accumulation and NO(3)(-) reductase activity in extracts of Cd treated roots were only slightly lowered. Kinetics of high- and low-affinity NO(3)(-) 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 NO(3)(-) transporters (ZmNTR2.1, ZmNRT2.2), PM H(+)-ATPases (ZmMHA3, ZmMHA4) and NO(3)(-) reductases (ZmNR1, ZmNADH:NR). Despite an expectable increase in NO(3)(-) demand, a negative effect of Cd on NO(3)(-) nutrition is reported. Cd effect results in alterations at the physiological and transcriptional levels of NO(3)(-) 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 NO(3) (-) availability.
authors.affiliation: Dipartimento di Scienze Agrarie e Ambientali, Università degli Studi di Milano, Via Celoria 2, 20133, Milan, Italy. cecilia.rizzardo@unimi.it
authors.forename: Cecilia
authors.initials: C
authors.lastname: Rizzardo
authors.forename: Nicola
authors.initials: N
authors.lastname: Tomasi
authors.forename: Rossella
authors.initials: R
authors.lastname: Monte
authors.forename: Zeno
authors.initials: Z
authors.lastname: Varanini
authors.forename: Fabio F
authors.initials: FF
authors.lastname: Nocito
authors.forename: Stefano
authors.initials: S
authors.lastname: Cesco
authors.forename: Roberto
authors.initials: R
authors.lastname: Pinton
mesh_terms: D001692:Biological Transport; D002104:Cadmium / Q000032:analysis / Q000494:pharmacology*; D002462:Cell Membrane / Q000201:enzymology; D018506:Gene Expression Regulation, Plant / Q000187:drug effects*; D007700:Kinetics; D050901:Nitrate Reductase / Q000235:genetics / Q000378:metabolism; D009566:Nitrates / Q000032:analysis / Q000378:metabolism*; D009584:Nitrogen / Q000378:metabolism; D010940:Plant Proteins / Q000235:genetics* / Q000378:metabolism; D018517:Plant Roots / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D018520:Plant Shoots / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D006180:Proton-Translocating ATPases / Q000235:genetics / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics; D018749:RNA, Plant / Q000235:genetics; D036226:Seedlings / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D003313:Zea mays / Q000187:drug effects* / Q000235:genetics / Q000378:metabolism
chemical_list: D009566:Nitrates; D010940:Plant Proteins; D012333:RNA, Messenger; D018749:RNA, Plant; D002104:Cadmium; D050901:Nitrate Reductase; D006180:Proton-Translocating ATPases; D009584:Nitrogen