SYNTHESIS OF NITRITE REDUCTASE IS REGULATED DIFFERENTLY IN LEAF AND ROOT OF BARLEY (HORDEUM-VULGARE L)

Activity staining after non-denaturing polyacrylamide gel electrophoresis (PAGE) of extracts from nitrate-treated plants indicates the presence of a single isoform of nitrite reductase in the leaf and root of barley. Crossed and tandem-crossed immunoelectrophoresis, and immunoblotting, show that polyclonal antibodies raised against the purified leaf nitrite reductase protein recognise specifically the nitrite reductase protein in both leaf and root, suggesting that the leaf and root enzymes are immunologically similar, if not identical. Green, light-grown plants maintained in the absence of added nitrate possess low levels of nitrite reductase activity (measured with dithionite-reduced methyl viologen as electron donor) and nitrite reductase cross-reacting material in both leaf and root. Addition of nitrate in the tight to green, light-grown plants leads to a concomitant increase in the level of nitrite reductase activity and cross-reacting material in both leaf and root. When green, light-grown plants are treated with nitrate and transferred immediately to the dark, there is a concomitant increase in nitrite reductase activity and cross-reacting-material in the root, but neither increase in the leaf. We conclude that the observed increases in nitrite reductase activity are due to the net synthesis of new nitrite reductase molecules and that their synthesis is regulated differently in leaf and root: both light and nitrate together are required in the leaf but nitrate alone is able to bring about synthesis of nitrite reductase molecules in the root. Inability to synthesise nitrite reductase molecules in leaves in the dark after treatment of plants with nitrate is not due to exclusion of nitrate from the leaves. Possible mechanisms which underlie these observations are discussed.