INVESTIGATIONS ON THE NATURE OF THE PHYTOCHROME-INDUCED TRANSMITTER FOR THE REGULATION OF NITRATE REDUCTASE IN ETIOLATED LEAVES OF MAIZE

A lag phase of 30 min for phytochrome-mediated stimulation in the indu ction of nitrate reductase (NR) in etiolated leaves of Zea mays var. ' Ganga 5', was eliminated by a 2 h pre-irradiation with red light, supp lied in the absence of nitrate. Earlier, photoreversibility of NR acti vity was found to be lost completely by 2 h and a transmitter with a l ife span of 12 h was proposed to mediate the phytochrome effect on NR (Sharma and Sopory, 1984). Low temperature (0 degrees C) treatment pre vented the loss of the photoreversibility. Almost complete photorevers ibility was observed after 2 h, suggesting that the formation of the t ransmitter requires active metabolism. Tungstate, given 2 h after red light treatment, inhibited the increase in the enzyme activity by red light, suggesting that the transmitter is not inactive NR itself, whic h becomes activated by nitrate. Analysis of steady-state levels of the NR transcript revealed that NR mRNA is not induced in response to P-f r formation in the absence of nitrate treatment, suggesting that it co uld not be the transmitter mediating the phytochrome effect on NR acti vity. Red light was found to increase the uptake of Ca-45(2+) by isola ted maize protoplasts. When supplied exogenously calcium increased NR activity by 48% of that obtained by red light irradiation. However, ex ogenous addition of phorbol myristate acetate (PMA), an activator of p rotein kinase C, was found to increase the NR transcript level to the same extent obtained after 5 min of red light irradiation. The results suggest that phytochrome may be acting through calcium along with mes sengers like diacylglycerol, generated through the phosphoinositide (P I) cycle to stimulate the induction of NR.