Tobacco phytochromes: Genes, structure and expression

Photomorphogenesis in tobacco, Like in other higher plant species, is regulated by at least three different types of photoreceptors, among which the best characterized is phytochrome, Three genes, designated Nt-PHYA1, Nt-PHYA2 and Nt-PHYB1, have been isolated from the allotetraploid Nicotiana tabacum encoding phytochrome A (phyA) -like and phytochrome B (phvB) -like apoproteins, respectively. The deduced amino acid sequences of the encoded tobacco phyA and B proteins exhibit significant homology- (> 75%) to phyA and phyB proteins described in Arabidopsis and in other plant species including potato, tomato and pea, Expression patterns and levels of the tobacco PHYA- and PHYB-like genes ha re been characterized in detail, Expression of the PHYB1 gene is regulated mainly, while that of the PHYA genes is regulated partially, at the level of transcription, which is initiated at multiple start sites and produces at least three different types of transcripts differing in length and abundance, analysis of transgenic tobacco plants showed that expression patterns of the Nt-PHYA/GUS and Nt-PHYB/GUS transgenes exhibit well-defined organ/tissue-specific patterns. Transcription of the endogenous Nt-PHYA genes as well as of the Nt-PHYA/GUS transgenes is down-regulated by light, and this regulation is mediated by phytochrome. Expression of the Nt-PHYB gene is insensitive to light, Expression of several light-induced tobacco genes is mediated by phytochrome and further modulated by a circadian clock, Although phytochrome plays a significant role in synchronizing/setting this endogenous oscillator, transcription of the Nt-PHYA and Nt-PHYB genes is not regulated by the circadian rhythm in developing tobacco seedlings, Ln the absence of well-characterized mutants, the precise biological function of tobacco phyA and phyB molecules, in contrast to the phyA and phyB proteins in Arabidopsis, is not yet understood.