Photoreceptors of higher plants.

Abstract

Key words: Arabidopsis mutant – Blue-light receptor –Phytochrome – Signal transductionIntroductionPlants sense the quality, quantity, direction and durationof the incident light and use it as a signal to optimizegrowth and development for the ambient light condi-tions during their whole life cycle. All aspects ofdevelopment are influenced by light, in particular,seedling development and the transition from growthunder soil (dark) to growth above the ground (light).Light inhibits the rate of hypocotyl growth, stimulatesthe opening of the hook, the expansion of the cotyle-dons, induces the di•erentiation of leaves and chloro-plasts, and the formation of pigments and trichomes. Allof these processes are accomplished by and depend onthe di•erential expression of a large number of genes.Several other aspects of plant growth and developmentare a•ected by light, such as seed germination, sensingand response to neighboring plants, phototropism (thebending response in relation to the direction of the light)and induction of flowering (Kendrick and Kronenberg1994; Millar et al. 1994; Quail 1994a; Smith 1995; Choryet al. 1996; McNellis and Deng 1995; Chory 1997).The light is perceived by a complex system ofdi•erent photoreceptors, which detect di•erent lightqualities over a wide spectral range, but with overlap-ping action spectra. These photoreceptors include thephytochromes which are the best-characterized photo-receptors and are responsible for the detection of far-redlight (FR) and red light (R), but sense also blue and UVlight. The blue/UV-A photoreceptors, as the nameindicates, sense the blue and UV-A part of the spectrumand have attracted much more interest in recent yearsmostly due to successful molecular cloning followed byfurther functional characterization (Ahmad and Cash-more 1993; Batschauer 1993; Ho•man et al. 1996; Linet al. 1996b; Huala et al. 1997). The UV-B photorecep-tors, which, for example, regulate the formation of UV-shielding pigments (Beggs and Wellmann 1994; Quail1994a; Christie and Jenkins 1996), have so far not beencharacterized at the molecular level.Physiological and biochemical approaches in the pastdecades have given a broad foundation for the under-standing of how light influences plant growth anddevelopment. Genetics, molecular genetics and cellbiological studies have led in recent years to a majorbreakthrough in the characterization of the photorecep-torsandtheirresponsemodes,andintheidentificationofcomponents of the light signal transduction chains. Sincemostofthesestudieswereforobviousreasonsperformedwith Arabidopsis thaliana, this review concentrates onrecent advances in Arabidopsis research, knowing thatthe biology of higher plants has aspects which can not allbe studied with a single species. Several excellent reviewson the same topic have been published recently, and Irefer the reader to these for additional information orother points of view (Furuya 1993; Quail 1994a,b, 1997;Short and Briggs 1994; Whitelam and Harberd 1994;Jenkins et al. 1995; McNellis and Deng 1995; Quail et al.1995; Smith 1995; Ahmad and Cashmore 1996b; Choryet al. 1996; Furuya and Scha¨fer 1996; Briggs and Liscum1997a,b; Chory 1997; Mustilli and Bowler 1997).The phytochromesThe phytochrome photoreceptor family is very likelysolely responsible for the detection of R and FR.Phytochromes are pigments consisting of a polypeptideof about 125 kDa carrying a chromophore moiety, thephytochromobilin, which is a linear tetrapyrrole co-valently attached to a conserved cysteine residue in the