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Plant Physiology 133:1417-1419 (2003) © 2003 American Society of Plant Biologists Light SignalingDepartment of Botany, University of Wisconsin, 430 Lincoln Drive, Madison, WI 53706
A call for papers on the topic of light signaling was broadcast and the community of photomorphogenesis workers mounted a vigorous response. Presented here is the result, a collection of three Update articles and 20 original research papers, all on the topic of how light signals influence the physiology and development of plants. Despite the generality of the solicitation mechanism, or perhaps because of it, this ensemble of papers captures the breadth and depth of the field regardless of which way one chooses to slice it. For example, the papers are distributed across the spectrum, dealing with responses to wavelength bands ranging from the UV to the far-red (see Fig. 1). Also, each of the known families of plant photoreceptors is represented: phototropins, cryptochromes, and phytochromes. The word "known" is worth a pause because some responses to light cannot yet be attributed to any of the "big three" photoreceptors or combinations of them. The papers presented here include good examples of such unfinished business, the kind that eventually lead to surprising developments. When they occur, hindsight may show their roots tracing back to these pages.
The present collection of studies also represents the breadth of the experimental stage on which photobiology is practiced: the venues range from the microfuge tube to the potato field. The subjects range in scale from a portion of a molecule, to arrays of thousands of molecules, to cells, and to whole organisms. One study did not involve carbon in any form, only silicon. Although basing studies on Arabidopsis mutants is still the rule rather than the exception in photomorphogenesis work, four other plant species of more agronomic import are put to good use in the papers presented here. The field has apparently matured to the point that information obtained from the model system can effectively inform studies of species that may better display the phenomenon of interest or have more relevance to agriculture. The Update articles serve a reader interested in an introduction to the state of the field in three main areas. Brian Parks covers the topic of how phytochromes monitor the red region of the spectrum (pp. 1437–1444). Mannie Liscum et al. cover the blue region by explaining and reviewing the phototropin and cryptochrome photoreceptors and their means of action (pp. 1429–1436). The shorter, higher energy wavelengths of the UV region can do damage and also carry morphogenic signals. These dual effects of UV are covered by Hans Frohnmeyer and Dorothee Staiger (pp. 1420–1428). A student looking for an inroad into the field of photomorphogenesis would do well to read these three Updates.
An expert looking for state of the field details will find plenty in the other 20 papers. Starting with the UV, Kucera et al. (pp. 1445–1452) show that formation of pyrimidine dimers induced in DNA by UV-B (280–320 nm) radiation play a role in the up-regulation of the
UV-A radiation (320–400 nm) and blue light induce many of the same responses because the flavin chromophores of the receptors have an absorption peak at around 360 nm in addition to peaks in the blue. The phototropins, with their two flavin-binding domains and carboxy-terminal kinase domains, are now well established UV-A/blue light receptors (Briggs and Christie, 2002
Changes in cytoplasmic Ca2+ concentrations triggered by blue light acting through phot1 (Baum et al., 1999
Chloroplasts move within mesophyll cells in response to changes in blue light irradiance sensed by phototropins (Kagawa and Wada, 2000 The cryptochromes are the other well-established blue light receptors, although here a quantitative trait locus analysis of cotyledon unfolding by Botto et al. (pp. 1547–1556) shows that cry2 influences seedling development in the absence of blue wavelengths. Perhaps cryptochromes do more than transduce blue light signals. Determining where photoreceptor proteins are located in cells and whether they move in response to light is certainly a topic du jour. Matsumoto et al. (pp. 1494–1503) show that green fluorescent protein-tagged cryptochromes of rice (Oryza sativa) are located in the cytoplasm and nucleus when expressed in Arabidopsis or rice cells. This is probably also true of the Arabidopsis cryptochromes 1 and 2, which are shown by Jiao et al. (pp. 1480–1493) to regulate the expression of a variety of transcription factors in light-grown Arabidopsis seedlings. Presumably, some of these changes in transcription factor expression are responsible for the effects of cry2 on flowering investigated by El-Assal et al. (pp. 1504–1516), who also show that cry2 expression is negatively regulated by the FLC flowering gene.
Euglena gracilis, a protist allowed in the plant club because of the chloroplast it somehow obtained, is shown by Ntefidou et al. (pp. 1517–1521) to find its way around using a completely novel photoreceptor, a photoactivated adenylate cyclase that was only recently discovered (Iseki et al., 2002 The effect of light on the ultimate length of the seedling's hypocotyl has been exploited in countless genetic screens and is responsible for more progress toward a molecular level understanding of light-sensing mechanisms than any other single approach. Now, it appears that straightforward screens for obviously long or short hypocotyls have netted all the possible genes; therefore, people are refining screens to find more subtle differences in hypocotyl length. It paid off for Hare et al., who describe LAF3, a new component in the phyA signaling chain that is located in the nuclear periphery (pp. 1592–1604). A screen based on the effects of a continuous train of light pulses led Dieterle et al. to the isolation of an allele of cop1 that displays a short hypocotyl but not the gross morphological alterations characteristic of the severe reduced function alleles that made the COP1 gene famous (pp. 1557–1564). Genes previously identified by hypocotyls length screens are now being picked apart in structure function studies. Yang et al. report (pp. 1630–1642) the surprising result that overexpressing a truncated form of HFR1, a phyA signaling element important in far-red light, causes seedlings to develop in darkness as if they had been exposed to light, a case of constitutive photomorphogenesis similar to loss-of-function alleles of cop1. At some point, the light signaling genes downstream of the photoreceptors must affect some growth control mechanism. Turk et al. present evidence (pp. 1643–1653) that metabolism of brassinolides may be one of the facets of growth control that the light signaling pathways regulate.
Another way to investigate development is to find candidate genes first and then study the phenotypic consequences of knocking out the gene. Khanna et al. (pp. 1530–1538) use this reverse genetic approach to show that a gene identified by a previous microarray expression study (Tepperman et al., 2001 Devlin et al. (pp. 1617–1629) used microarray-based transcription profiling to identify phytochrome-dependent genes that may function in the shade avoidance syndrome, which can be induced with supplemental far red light and is agriculturally relevant, particularly when crops are planted at high densities. Boccalandro et al. (pp. 1539–1546) show that suppression of the shade avoidance syndrome by overexpression of phyB in potato (Solanum tuberosum) increases tuber yield at high plant densities. The comprehensive characterization of the variability in photoresponsiveness among maize lines presented by Markelz et al. (pp. 1578–1591) is a necessary step toward assessing the use of photoresponsiveness as a breeding trait. Since the early days of plant molecular biology, the regulation of gene expression by light has been an active area of research. It has reached a new level of sophistication now that multiple microarray datasets are available for study. Hudson and Quail examined such datasets and genomic sequence to identify novel promoter elements that may confer regulation by phyA on genes (pp. 1605–1616). Contrast this genome-wide analysis of light-regulated promoters with the base-by-base analysis of the CAB2 promoter performed by Maxwell et al. (pp. 1565–1577). They determined specific regions important for the influence of the DET1 and HY5 proteins on the photocontrol of this gene. The CAB2 gene must now be one of the best understood examples of light-regulated transcription. It is remarkable how a self-assembled collection of 20 papers on light signaling captured the breadth and depth of photomorphogenesis research. The project would not have been possible without the conscientious and selfless efforts of many reviewers and the good faith efforts of the authors to comply with the constructive criticism that was returned. The only people who may have worked harder to make this Focus Issue a reality are Leslie A. Csikos and Lisa M. Pergolizzi in the Plant Physiology office. Many thanks to you all. FOOTNOTES http://www.plantphysiol.org/cgi/doi/10.1104/pp.900097. * E-mail spalding{at}wisc.edu; fax 608–262–7509. LITERATURE CITED
Ballaré CL (2003) Stress under the sun: spotlight on ultraviolet-B responses. Plant Physiol 132: 1725-1727
Babourina O, Newman I, Shabala S (2002) Blue light-induced kinetics of H+ and Ca2+ fluxes in etiolated wild-type and phototropin-mutant Arabidopsis seedlings. Proc Natl Acad Sci USA 99: 2433-2438 Baum G, Long JC, Jenkins GI, Trewavas AJ (1999) Stimulation of the blue light phototropic receptor NPH1 causes a transient increase in cytoplasmic Ca2+. Proc Natl Acad Sci USA 96: 554-559 Briggs WR, Christie JM (2002) Phototropins 1 and 2: versatile plant blue-light receptors. Trends Plant Sci 7: 204-210[CrossRef][ISI][Medline] Doyle MR, Davis SJ, Bastow RM, McWatters HG, Kozma-Bognár L, Nagy F, Miller AJ, Amasino RM (2002) The ELF4 gene controls circadian rhythms and flowering time in Arabidopsis thaliana. Nature 419: 74-77[CrossRef][Medline]
Harada A, Sakai T, Okada K (2003) phot1 and phot2 mediate blue light-induced transient increase in cytosolic Ca2+ differently in Arabidopsis leaves. Proc Natl Acad Sci USA 100: 8583-8588 Iseki M, Matsunaga S, Murakami A, Ohno K, Shiga K, Yoshida C, Sugai M, Takahashi T, Hori T, Watanabe M (2002) A blue-light-activated adenylyl cyclase mediates photoavoidance in Euglena gracilis. Nature 415: 1047-1051[CrossRef][Medline] Kagawa T, Wada M (2000) Blue light induced chloroplast relocation in Arabidopsis thaliana as analyzed by microbeam irradiation. Plant Cell Physiol 41: 84-93 Kinoshita T, Shimazaki K-I (1999) Blue light activates the plasma membrane H+-ATPase by phosphorylation of the C-terminus in stomatal guard cells. EMBO J 18: 5548-5558[CrossRef][ISI][Medline]
Kircher S, Kozma-Bognár L, Kim L, Ádám É, Harter K, Schäfer E, Nagy F (1999) Light quality-dependent nuclear import of the plant photoreceptors phytochrome A and B. Plant Cell 11: 1445-1456 Matsushita T, Mochizuki N, Nagatani A (2003) Dimers of the N-terminal domain of phytochrome B are functional in the nucleus. Nature 424: 571-574[CrossRef][Medline]
Sakai T, Kagawa T, Kasahara M, Swartz TE, Christie JM, Briggs WR, Wada M, Okada K (2001) Arabidopsis nph1 and npl1: blue light receptors that mediate both phototropism and chloroplast relocation. Proc Natl Acad Sci USA 98: 6969-6974
Stoelzle S, Kagawa T, Wada M, Hedrich R, Dietrich P (2003) Blue light activates calcium-permeable channels in Arabidopsis mesophyll cells via the phototropin signaling pathway. Proc Natl Acad Sci USA 100: 1456-1461
Tepperman JM, Zhu T, Chang HS, Wang X, Quail PH (2001) Multiple transcription-factor genes are early targets of phytochrome A signaling. Proc Natl Acad Sci USA 98: 9437-9442 Related articles in Plant Physiol.:
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-1,3-glucanase gene. In this case, what is typically considered damage (dimer formation) is part of the light signaling mechanism leading to altered gene expression. A recent Focus Issue of Plant Physiology (August 2003) and accompanying editorial by Carlos Ballaré (2003
N105, Activates a Branch Pathway of Light Signaling in Arabidopsis