THE DEVELOPMENTAL AND TISSUE-SPECIFIC EXPRESSION OF TOBACCO PHYTOCHROME-A GENES

Plant photomorphogenesis is regulated by at least three different types of photoreceptors, among them the best characterized is phytochrome. Throughout their development plants express several types of phytochrome apoproteins that are encoded by members of a small multigene family. Two genes have been isolated from Nicotiana tabacum, designated Nt-PHY-A1 and A2, encoding type-A phytochrome apoproteins. Chimeric genes, containing the promoter regions of the PHY-A1 and A2 genes fused to the GUS or to the CAT reporter genes, were then constructed and transferred into tobacco. The expression characteristics of the endogenous PHY-A1, PHY-A2 genes, as well as those of the PHY-A1-GUS/CAT, PHY-A2-GUS/CAT transgenes were determined by RNase protection assays and by GUS histochemical staining. It is reported here that (i) the expression levels of the tobacco PHY-A genes and those of the transgenes are downregulated by light, (ii) this light-sensitive expression is mediated, at least partially, by phytochrome and further modified by (iii) a developmental programme and (iv) a well-defined organ and tissue-specific pattern. Furthermore, it is shown that the expression of the PHY-A genes is not affected by the circadian clock. It is concluded that the expression of these tobacco genes is controlled by, at least, three different regulatory mechanisms at the level of transcription.