ABOLITION OF POSTTRANSCRIPTIONAL REGULATION OF NITRATE REDUCTASE PARTIALLY PREVENTS THE DECREASE IN LEAF NO3- REDUCTION WHEN PHOTOSYNTHESISIS INHIBITED BY CO2 DEPRIVATION, BUT NOT IN DARKNESS

The activity of nitrate reductase (NR) in leaves is regulated by light and photosynthesis at transcriptional and posttranscriptional levels. To understand the physiological role of these controls, we have inves tigated the effects of light and CO2 on in vivo NO3- reduction in tran sgenic plants of Nicotiana plumbaginifolia lacking either transcriptio nal regulation alone or transcriptional and posttranscriptional regula tion of NR. The abolition of both levels of NR regulation did not modi fy the light/dark changes in exogenous (NO3-)-N-15 reduction in either intact plants or detached leaves. The same result was obtained for N- 15 incorporation into free amino acids in leaves after (NO3-)-N-15 was supplied to the roots, and for reduction of endogenous NO3- after tra nsfer of the plants to an N-deprived solution. In the light, however, deregulation of NR at the posttranscriptional level partially prevente d the inhibition of leaf (NO3-)-N-15 reduction resulting from the remo val of CO2 from the atmosphere. We concluded from these observations t hat in our conditions deregulation of NR in the transformants investig ated had little impact on the adverse effect of darkness on leaf NO3- reduction, and that posttranscriptional regulation of NR is one of the mechanisms responsible for the short-term coupling between photosynth esis and leaf NO3- reduction in the light.