Expression ofLeafNitrate Reductase GenesfromTomatoand Tobacco inRelation toLight-Dark Regimes andNitrate Supply

Theinfluence oflight-dark cycles andnitrate supply onnitrate reductase(NR)mRNA levels wasstudied intwoplant species, tobacco (Nicotiana tabacum) andtomato (Lycopersicon esculentum) using specific NRDNA probes. Inthesameseries ofexperiments, changes inthe levels ofNR protein (NRP)byenzyme-linked immunosorbent assay and changes inthelevel ofNADH-nitrate reductase activity (NRA)werealso followed. During alight-dark cycle, itwasfound that inbothtomato and tobacco, NR mRNA accumulation increased rapidly during thedark period andreached amaximumatthebeginning oftheday, while NRP reached apeak2and4 hoursafter mRNA peaked, fortomato and tobacco, respectively. Attheendoftheday,theamount ofmRNA was decreased byafactor ofatleast 100compared tosunrise inbothspecies. Theseresults demonstrate that light isinvolved, although probably not directly, intheregulation oftheNR geneexpression atthemRNA level. ThepeakofNRA intobacco coincided withthepeakinNR mRNA accumulation (i.e. sunrise), whereas intomato thepeakofNRA was approximately 5to6hours after sunrise. There isnoobvious correlation between NRP andNRA levels during theday.Innitrogen starvation experiments, arapid decrease ofNRPandNRA wasdetected, while NR mRNA levels werenotsignificantly altered. Uponnitrate replenishment, nitrogen-starved plants accumulated NR mRNA rapidly. Theseresults suggest thattheavailability ofnitrogen affects theexpression ofNR activity atthetranscriptional aswell asatthepost-transcriptional levels.