Role of nitrate and nitrite in the induction of nitrite reductase in leaves of barley seedlings.
The role of NO3- and NO2- in the induction of nitrite reductase (NiR) activity in detached leaves of 8-day-old barley (Hordeum vulgare L.) seedlings was investigated.  Barley leaves contained 6 to 8 micromoles NO2-/gram fresh weight x hour of endogenous NiR activity when grown in N-free solutions.  Supply of both NO2- and NO3- induced the enzyme activity above the endogenous levels (5 and 10 times, respectively at 10 millimolar NO2- and NO3- over a 24 hour period).  In NO3(-)-supplied leaves, NiR induction occurred at an ambient NO3- concentration of as low as 0.05 millimolar; however, no NiR induction was found in leaves supplied with NO2- until the ambient NO2- concentration was 0.5 millimolar.  Nitrate accumulated in NO2(-)-fed leaves.  The amount of NO3- accumulating in NO2(-)-fed leaves induced similar levels of NiR as did equivalent amounts of NO3- accumulating in NO3(-)-fed leaves.  Induction of NiR in NO2(-)-fed leaves was not seen until NO3- was detectable (30 nanomoles/gram fresh weight) in the leaves.  The internal concentrations of NO3-, irrespective of N source, were highly correlated with the levels of NiR induced.  When the reduction of NO3- to NO2- was inhibited by WO4(2-), the induction of NiR was inhibited only partially.  The results indicate that in barley leaves in NiR is induced by NO3- directly, i.e. without being reduced to NO2-, and that absorbed NO2- induces the enzyme activity indirectly after being oxidized to NO3- within the leaf.
Aslam
M
M
Department of Agronomy and Range Science, University of California, Davis 95616, USA.
Huffaker
R C
RC
D004790:Enzyme Induction; D001467:Hordeum / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D050901:Nitrate Reductase; D009565:Nitrate Reductases / Q000378:metabolism; D009566:Nitrates / Q000378:metabolism* / Q000493:pharmacokinetics; D009572:Nitrite Reductases / Q000096:biosynthesis* / Q000378:metabolism; D009573:Nitrites / Q000378:metabolism* / Q000493:pharmacokinetics; D018515:Plant Leaves / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D013997:Time Factors; D017973:Tungsten Compounds / Q000494:pharmacology*
D009566:Nitrates; D009573:Nitrites; D017973:Tungsten Compounds; D009565:Nitrate Reductases; D009572:Nitrite Reductases; D050901:Nitrate Reductase; C045951:tungstate