The Phytochromes, a Family of Red/Far-red Absorbing Photoreceptors

Abstract

Because plants are photo-auxotrophic they are particularly sensitive to their light environment. To fine-tune their development according to light intensity, direction, spectral quality, and periodicity they possess a multiplicity of light sensors (1Kendrick R.E. Kronenberg G. Photomorphogenesis in Plants. Martinus Nijhoff Publishers, Dordrecht, Netherlands1994Crossref Google Scholar). InArabidopsis there are eight identified photoreceptors, but this list is still incomplete. It includes three UV-A/blue light receptors (phototropin, a photoreceptor to sense light direction, and two cryptochromes that mediate many photomorphogenic responses (2Briggs W.R. Huala E. Annu. Rev. Cell Dev. Biol. 1999; 15: 33-62Crossref PubMed Scopus (330) Google Scholar, 3Cashmore A.R. Jarillo J.A. Wu Y.J. Liu D. Science. 1999; 284: 760-765Crossref PubMed Scopus (785) Google Scholar)) and five phytochromes (phy) 1The abbreviations used are:phyphytochrome(s)BLDbilin lyase domainATEN-terminal extensionPRDPAS (PER/ARNT/SIM)-related domainHKRDhistidine kinase-related domainPYPphotoactive yellow proteinbHLHbasic helix-loop-helixnamed phyA–phyE that absorb mainly red/far-red light, with phyA also responding to broad-spectrum light (UV-A to far-red) of very low intensity (4Quail P.H. Boylan M.T. Parks B.M. Short T.W. Xu Y. Wagner D. Science. 1995; 268: 675-680Crossref PubMed Scopus (651) Google Scholar). All these photoreceptors bind to a chromophore, which for the phytochromes is a linear tetrapyrrole (phytochromobilin) (5Lagarias J.C. Rapoport H. J. Am. Chem. Soc. 1980; 102: 4821-4828Crossref Scopus (259) Google Scholar). Because many light effects are induced by the co-action of several photoreceptors and because some photoreceptors regulate multiple aspects of photomorphogenesis, a genetic approach was instrumental for dissecting the specific roles of individual photoreceptors (1Kendrick R.E. Kronenberg G. Photomorphogenesis in Plants. Martinus Nijhoff Publishers, Dordrecht, Netherlands1994Crossref Google Scholar). As a consequence, research has concentrated on a few species that are particularly well suited for molecular genetic studies, in particularArabidopsis (6Bevan M. Bancroft I. Mewes H.W. Martienssen R. McCombie R. Bioessays. 1999; 21: 110-120Crossref PubMed Scopus (28) Google Scholar). phytochrome(s) bilin lyase domain N-terminal extension PAS (PER/ARNT/SIM)-related domain histidine kinase-related domain photoactive yellow protein basic helix-loop-helix Phytochromes were originally defined as the receptors responsible for red, far-red reversible, plant responses (7Parker M.W. Hendricks S.B. Borthwick H.A. Went F.W. Am. J. Bot. 1949; 36: 194-204Crossref Google Scholar, 8Borthwick H.A. Hendricks S.B. Parker M.W. Toole E.H. Toole V.K. Proc. Natl. Acad. Sci. U. S. A. 1952; 38: 662-666Crossref PubMed Google Scholar, 9Borthwick H.A. Hendricks S.B. Parker M.W. Proc. Natl. Acad. Sci. U. S. A. 1952; 38: 929-934Crossref PubMed Google Scholar). Photobiological experiments led to the proposal that phy exists in two spectral forms: the inactive Pr form (red light absorbing) phototransforms into the active Pfr form (far-red light absorbing) upon absorption of red light. This reaction can be reversed when Pfr is converted to Pr upon absorption of far-red light. Purification of phy from plants confirmed the existence of those two spectrally interconvertible forms (10Butler W.L. Norris K.H. Siegelman H.W. Hendricks S.B. Proc. Natl. Acad. Sci. U. S. A. 1959; : 1703-1708Crossref PubMed Google Scholar). phy are classified into two groups; type I (phyA in Arabidopsis) is light-labile and type II (phyB–phyE in Arabidopsis) is light-stable (11Hirschfeld M. Tepperman J.M. Clack T. Quail P.H. Sharrock R.A. Genetics. 1998; 149: 523-535Crossref PubMed Google Scholar). Numerous recent reviews cover phy-mediated photomorphogenesis in detail (12Nagy F. Schaefer E. EMBO J. 2000; 19: 157-163Crossref PubMed Scopus (71) Google Scholar, 13Lin C. Plant Physiol. 2000; 123: 39-50Crossref PubMed Scopus (168) Google Scholar, 14Neff M.M. Fankhauser C. Chory J. Genes Dev. 2000; 14: 257-271Crossref PubMed Google Scholar, 15Deng X.W. Quail P.H. Semin. Cell Dev. Biol. 1999; 10: 121-129Crossref PubMed Scopus (169) Google Scholar, 16Osterlund M.T. Ang L.H. Deng X.W. Trends Cell Biol. 1999; 9: 113-118Abstract Full Text Full Text PDF PubMed Scopus (113) Google Scholar, 17Karniol B. Chamovitz D.A. Curr. Opin. Plant Biol. 2000; 3: 387-393Crossref PubMed Scopus (39) Google Scholar, 18Hughes J. Lamparter T. Plant Physiol. 1999; 121: 1059-1068Crossref PubMed Scopus (77) Google Scholar, 19Cashmore A.R. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 13358-13360Crossref PubMed Scopus (15) Google Scholar). Photobiological and genetic studies have revealed that this small gene family plays important roles in seed germination, seedling de-etiolation, neighbor perception and avoidance, and the transition from vegetative to reproductive growth (induction of flowering). At the molecular and