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

Plant photomorphogenesis is regulated by at least three different type s of photoreceptors, among them the best characterized is phytochrome. Throughout their development plants express several types of phytochr ome apoproteins that are encoded by members of a small multigene famil y. 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 transf erred into tobacco. The expression characteristics of the endogenous P HY-A1, PHY-A2 genes, as well as those of the PHY-A1-GUS/CAT, PHY-A2-GU S/CAT transgenes were determined by RNase protection assays and by GUS histochemical staining. It is reported here that (i) the expression l evels of the tobacco PHY-A genes and those of the transgenes are downr egulated by light, (ii) this light-sensitive expression is mediated, a t least partially, by phytochrome and further modified by (iii) a deve lopmental programme and (iv) a well-defined organ and tissue-specific pattern. Furthermore, it is shown that the expression of the PHY-A gen es is not affected by the circadian clock. It is concluded that the ex pression of these tobacco genes is controlled by, at least, three diff erent regulatory mechanisms at the level of transcription.