Transcriptional and posttranscriptional regulation of the glycolate oxidase gene in tobacco seedlings

The roles of light and of the putative plastid signal in glycolate oxidase (GLO) gene expression were investigated in tobacco (Nicotiana tabacum cv. S amsun NN) seedlings during their shift from skotomorphogenic to photomorpho genic development. GLO transcript and enzyme activities were detected in et iolated seedlings. Their respective levels increased three- and six-fold du ring 96 h of exposure to light. The GLO transcript was almost undetectable in seedlings in which chloroplast development was impaired by photooxidatio n with the herbicide norflurazon. In transgenic tobacco seedlings, photooxi dation inhibited the light-dependent increase in GUS activity when it was p laced under the regulation of the GLO promoter P-GLO. However, even under t hese photooxidative conditions, a continuous increase in GUS activity was o bserved as compared to etiolated seedlings. When GUS expression was driven by the CaMV 35S promoter (P-35S), no apparent difference was observed betwe en etiolated, deetiolated and photooxidized seedlings. These observations i ndicate that the effects of the putative plastid development signal and lig ht on GUS expression can be separated. Translational yield analysis indicat ed that the translation of the GUS transcript in P-GLO::GUS seedlings was e nhanced 30-fold over that of the GUS transcript in P-35S::GUS seedlings. Th e overall picture emerging from these results is that in etiolated seedling s GLO transcript, though present at a substantial level, is translated at a low rate. Increased GLO transcription is enhanced, however, in response to signals originating from the developing plastids. GLO gene expression is f urther enhanced at the translational level by a yet undefined light-depende nt mechanism.