Regulation of transcript level and synthesis of nitrate reductase by phytochrome and nitrate in turions of Spirodela polyrhiza (L.) Schleiden.

Control by light and nitrate of the appearance of nitrate reductase (NR; EC 1.6.6.1) in the turions of Spirodela polyrhiza (L.) Schieiden was investigated during the pre-germination phase, i.e. up to 120 h after the transfer from after-ripening to germination conditions. Turions are particularily suited for this type of coaction study since control of nitrate uptake by light and long-distance transport of nitrate do not play a role. Control of NR gene expression was mainly studied between 72 and 120 h after transfer to germination conditions (phase II of the pre-germination process). It was found that the effect of light on NR appearance is exerted via phytochrome. The light effect on enzyme synthesis was only observed in the presence of nitrate. On the other hand, in darkness, the NR level decreased to almost zero even in the presence of an optimum supply of nitrate. It is concluded that in phase II a coaction of light (phytochrome) and nitrate is required to bring about NR synthesis. However, when nitrate was applied to turions - following a dark incubation without nitrate - the turions responded to nitrate even in darkness with strong NR synthesis. This response was augmented by light. The highest transcript levels were observed in the presence of both factors, light and nitrate. As a single factor, light was more effective in stimulating the transcript level than nitrate. However, no correlation between the transcript level and the rate of enzyme synthesis was observed. This is consistent with the previous conclusion (Schuster et al. 1988, Planta 174, 426-432) that in higher plants control at the transcript level is only coarse and does not determine quantitatively the output at the level of enzyme protein.