Regulation by light and metabolites of ferredoxin-dependent glutamate synthase in maize.

The regulation of Fd-glutamate synthase (Fd-GOGAT, EC 1.4.1.7) and NADH-glutamate synthase (NADH-GOGAT, EC 1.4.1.14) was investigated in maize (Zea mays L. cv. DEA) (1) during development starting from 7- to 11-day-old seedlings, (2) by treatment of 7-day-old etiolated leaves with intermittent light pulses to activate (red) and inactivate (far-red) phytochromes and (3) in 7-day-old green leaves grown under 16-h light/8-h dark cycles. Fd-GOGAT mRNA accumulated 4-fold, and the enzyme polypeptide (3-fold) and activity (3-fold) also increased in leaf cells, while NADH-GOGAT activity remained constantly low. Leaf-specific induction of Fd-GOGAT mRNA (3-fold) occurred in etiolated leaves by low fluence red light, and far-red light reversibly repressed the mRNA accumulation. Red/far-red reversible induction also occurred for Fd-GOGAT polypeptide (2-fold) and activity (2-fold), implicating the phytochrome-dependent induction of Fd-GOGAT. In contrast, NADH-GOGAT activity remained constant, irrespective of red/far-red light treatments. Fd-GOGAT showed diurnal changes under light/dark cycles with the maximum early in the morning and the minimum in the afternoon at the levels of mRNA, enzyme polypeptide and activity. Gln diurnally changed in parallel with Fd-GOGAT mRNA. The induction of Fd-GOGAT provides evidence that light and metabolites are the major signal for the Gln and Glu formation in maize leaf cells.