Blue Light Sensing in Higher Plants

Abstract

Plants have evolved a range of sophisticated mechanisms to adapt and respond to their natural habitat. For example, plants rely heavily upon the surrounding light environment to direct their growth and development. Several different photoreceptor families are known to mediate the effects of light on plant development (1Ahmad M. Curr. Opin. Plant Biol. 1999; 2: 230-235Crossref PubMed Scopus (40) Google Scholar, 2Batschauer A. Cell. Mol. Life Sci. 1999; 55: 153-166Crossref PubMed Scopus (44) Google Scholar, 3Maheshwari S.C. Khurana J.P. Sopory S.K. J. Biosci. 2000; 24: 499-514Crossref Scopus (7) Google Scholar). These include the phytochrome (phy) family of photoreceptors, which monitor the red (600–700 nm) and far-red (700–750 nm) regions of the solar spectrum (4Neff M.M. Fankhauser C. Chory J. Genes Dev. 2000; 14: 257-271Crossref PubMed Google Scholar). In addition to the phytochromes, many important aspects of plant development are regulated by specific blue (390–500 nm) and/or UV-A (320–390 nm) light-absorbing receptors (5Briggs W.R. Huala E. Annu. Rev. Cell Dev. 1999; 15: 33-62Crossref PubMed Scopus (330) Google Scholar, 6Cashmore A.R. Jarillo J.A. Wu Y-J. Liu D. Science. 1999; 284: 760-765Crossref PubMed Scopus (783) Google Scholar, 7Lin C. Trends Plant Sci. 2000; 5: 337-342Abstract Full Text Full Text PDF PubMed Scopus (191) Google Scholar). Currently two classes of blue light receptors have been identified in plants: the cryptochromes and the phototropins. Here we briefly review the most recent advances in our understanding of blue light perception and signaling with an emphasis on the cryptochrome and phototropin photosensory systems.Cloning of the HY4 Gene Encoding Cryptochrome 1Our present knowledge of blue light perception would not be possible without the isolation of photoregulatory mutants. The rationale behind the genetic approach has been to screen for mutants with altered responses to light. For example, when dark-grownArabidopsis seedlings are transferred to light, hypocotyl elongation is dramatically suppressed (Fig.1A). This response is mediated by blue, red, and far-red light in Arabidopsis (8Khurana J.P. Kochhar A. Tyagi A.K. Crit. Rev. Plant Sci. 1998; 17: 465-539Crossref Google Scholar). Screening for an elongated hypocotyl phenotype in white light has resulted in the isolation of both phytochrome and blue light regulatory mutants. One of these long hypocotyl mutants, cry1(originally designated hy4), is specifically impaired in its ability to respond to blue light (Fig. 1A).The HY4 gene was isolated through the use of ahy4 mutant allele tagged with a T-DNA insertion (9Ahmad M. Cashmore A.R. Nature. 1993; 366: 162-166Crossref PubMed Scopus (1009) Google Scholar). Examination of the deduced amino acid sequence revealed that theHY4 gene encodes a 75-kDa protein with significant sequence homology to microbial DNA photolyases (Fig.2). Sequence homology is highest in the regions associated with chromophore binding. The HY4 gene was therefore proposed to encode a photoreceptor that mediates the blue light inhibition of hypocotyl elongation (9Ahmad M. Cashmore A.R. Nature. 1993; 366: 162-166Crossref PubMed Scopus (1009) Google Scholar). Based on further characterization, the hy4 protein was named cryptochrome 1 (cry1), a term previously introduced to describe the enigmatic nature of plant blue/UV-A photoreceptors and their presumed prevalance in lower plants and fungi (cryptogams) (10Gressel J. Photochem. Photobiol. 1977; 30: 749-754Crossref Scopus (63) Google Scholar).Figure 2Protein structures of E. colitype I amino amino Arabidopsis amino Arabidopsis amino amino of and are in a for photolyases are to be the for the cryptochromes 6Cashmore A.R. Jarillo J.A. Wu Y-J. Liu D. Science. 1999; 284: 760-765Crossref PubMed Scopus (783) Google Scholar). are blue in both and that the of DNA to nm) A. PubMed Scopus Google Scholar). I and photolyases mediate the of photolyases the of In the two blue/UV-A light-absorbing (Fig. The is the of the and to the of the to the The a a is the of the protein and an light to the the C. M. Cashmore A.R. Science. PubMed Scopus Google Scholar, A. A. PubMed Scopus Google Scholar). The protein a that is in the photolyases (Fig. the chromophore to a that light C. M. Cashmore A.R. Science. PubMed Scopus Google Scholar). The of is with the that light to C. M. D. Cashmore A.R. Sci. A. PubMed Scopus Google Scholar). In has been to a when A. A. PubMed Scopus Google the are impaired in a of growth J.A. PubMed Scopus Google Scholar). The photoreceptor to the blue light of by the of and J.A. Plant Sci. Scopus Google photoreceptors have been identified in plant M. Mol. 1998; PubMed Scopus Google and Plant Mol. Biol. PubMed Scopus Google and to be the plant of cryptochrome homology to (Fig. Arabidopsis a designated and and A. A. PubMed Scopus Google Scholar, A. Mol. PubMed Scopus Google Scholar). In addition to has been to blue inhibition of hypocotyl elongation and M. Jarillo J.A