Molecular properties and biogenesis of phytochrome I and II

Previously, phytochrome was thought to consist of a single molecular species. However, physiological and spectrophotometric evidence has accumulated to indicate that there are two phytochrome pools in tissues, one of which is predominant in dark-grown tissues and rather unstable in the light, and the other present in very low concentrations but stable, even in the Pfr form, irrespective of light condition. Recently, two immunochemically distinct phytochromes I and II, PI and PII, were found in both dark- and light-grown tissues, and their comparative amino acid sequences shown to be 64% homologous. This is crucial evidence for the presence of chemically different phytochrome apoproteins in a single plant species. However, it is still an open question as to which phytochrome, PI or PII, is a component of the photolabile and photostable pools of phytochrome. Our understanding of the molecular structure of phytochrome has been greatly improved by recent, rapid progress in the cloning and characterization of phytochrome genes. The expression of PI genes is photoreversibly inhibited by the photostable Pfr pool, while that of several other genes, like Cab, appears to be induced by PI in the Pfr form. It is suggested that autoregulation of phytochrome gene expression is not so simply governed in plants as thought earlier. If there are two different phytochromes in a plant cell, the most important physiological problem to be solved is which phytochrome triggers the numerous red/far-red reversible reactions reported in the literature. Photomorphogenetic mutants and transgenic plants with engineered phytochrome genes will probably help to solve this problem in the future, and preliminary work along this line has already introduced in this article. A model of the molecular structure of pea PI dimer was proposed on the basis of small angle X-ray scattering analysis, and the model then confirmed by rotary shadowing electron microscopy. Important questions are still open, such as: what is the nature of phytochrome's partner compounds in cells (phytochrome receptor)? How is/are the phytochrome-induced signal(s) transmitted in the signal transduction chain?