INDUCTION OF NITRATE REDUCTASE, NITRITE REDUCTASE, AND GLUTAMINE-SYNTHETASE ISOFORMS IN SUNFLOWER COTYLEDONS AS AFFECTED BY NITRATE, LIGHT,AND PLASTID INTEGRITY

We investigated the inducibility of nitrate reductase (NR; EC 1.6.6.1) , nitrite reductase (NiR; EC 1.7.7.1), and glutamine syn thetase (GS; EC 6.3.1.2) isoforms in cotyledons of 7-day-old seedlings of sunflower (Helianthus annuus L.) in relation to light, nitrogen source (NO3-, N O2- or NH4+), and the involvement of plastids. Nitrate was absolutely (and specifically) required for NR induction, and stimulated more effe ctively than NO2- or NH4+ the synthesis of NiR and chloroplastic GS (G S(2)) over the constitutive levels present in N-free-grown seedlings. In vivo inhibition of NR activity by tungsten application to seedlings and measurements of tissue NO3- concentration indicate that NO3--depe ndent enzyme induction is elicited by NO3- per se and not by a product of its assimilatory reduction, e.g., NO2- or NH4+. In the presence of NO3-, light remarkably enhanced the appearance of NR, NiR, and GS(2), while the activity of the cytosolic GS isoform (GSI) was adversely af fected. Cycloheximide suppressed much more efficiently than chloramphe nicol the light-and NO3--dependent increase of GS(2) activity, indicat ing that sunflower chloroplastic GS is synthesized on cytoplasmic 80S ribosomes. When the plastids were damaged by photooxidation in cotyled ons made carotenoid-free by application of norflurazon, the positive a ction of light and NO3- on the appearance of NR, NiR, and GS(2) isofor m was greatly abolished. Therefore, it is suggested that intact chloro plasts are required for the inductive effect of light and NO3- and/or for the accumulation of newly formed enzymes in the organelle.