De-etiolated Phenotype Research Articles

Dynamic regulation of PIF5 by COP1–SPA complex to optimize photomorphogenesis in Arabidopsis

Light signal provides the spatial and temporal information for plants to adapt to the prevailing environmental conditions. Alterations in light quality and quantity can trigger robust changes in global gene expression. In Arabidopsis thaliana, two groups of key factors regulating those changes in gene expression are CONSTITUTIVE PHOTOMORPHOGENESIS/DEETIOLATED/FUSCA (COP/DET/FUS) and a subset of basic helix-loop-helix transcription factors called PHYTOCHROME-INTERACTING FACTORS (PIFs). Recently, rapid progress has been made in characterizing the E3 ubiquitin ligases for the light-induced degradation of PIF1, PIF3 and PIF4; however, the E3 ligase(s) for PIF5 remains unknown. Here, we show that the CUL4COP 1- SPA complex is necessary for the red light-induced degradation of PIF5. Furthermore, COP1 and SPA proteins stabilize PIF5 in the dark, but promote the ubiquitination and degradation of PIF5 in response to red light through the 26S proteasome pathway. Genetic analysis illustrates that overexpression of PIF5 can partially suppress both cop1-4 and spaQ seedling de-etiolation phenotypes under dark and red-light conditions. In addition, the PIF5 protein level cycles under both diurnal and constant light conditions, which is also defective in the cop1-4 and spaQ backgrounds. Both cop1-4 and spaQ show defects in diurnal growth pattern. Overexpression of PIF5 partially restores growth defects in cop1-4 and spaQ under diurnal conditions, suggesting that the COP1-SPA complex plays an essential role in photoperiodic hypocotyl growth, partly through regulating the PIF5 level. Taken together, our data illustrate how the CUL4COP 1- SPA E3 ligase dynamically controls the PIF5 level to regulate plant development.

Arabidopsis DET1 Represses Photomorphogenesis in Part by Negatively Regulating DELLA Protein Abundance in Darkness

Arabidopsis De-etiolated 1 (DET1) is one of the key repressors that maintain the etiolated state of seedlings in darkness. The plant hormone gibberellic acid (GA) also participates in this process, and plants deficient in GA synthesis or signaling show a partially de-etiolated phenotype in darkness. However, how DET1 and the GA pathway work in concert in repressing photomorphogenesis remains largely unknown. In this study, we found that the abundance of DELLA proteins in det1-1 was increased in comparison with that in the wild-type plants. Mutation in DET1 changed the sensitivity of hypocotyl elongation of mutant seedlings to GA and paclobutrazol (PAC), an inhibitor of GA synthesis. However, we did not find obvious differences between det1-1 and wild-type plants with regard to the bioactive GA content or the GA signaling upstream of DELLAs. Genetic data showed that removal of several DELLA proteins suppressed the det1-1 mutant phenotype more obviously than GA treatment, indicating that DET1 can regulate DELLA proteins via some other mechanisms. In addition, a large-scale transcriptomic analysis revealed that DET1 and DELLAs play antagonistic roles in regulating expression of photosynthetic and cell elongation-related genes in etiolated seedlings. Taken together, our results show that DET1 represses photomorphogenesis in darkness in part by reducing the abundance of DELLA proteins.

Genetic Interactions Between ArabidopsisDET1andUVH6During Development and Abiotic Stress Response

Plants must adapt to a variety of abiotic inputs, including visible light, ultraviolet (UV) light, and heat. In Arabidopsis thaliana, DE-ETIOLATED 1 (DET1) plays a role in visible light signaling, UV tolerance, and development. UV-HYPERSENSITIVE 6 (UVH6) mutants are UV and heat sensitive, as well as dwarf and pale, like det1. In this study, we examine the genetic interactions between these two genes. In dark-grown seedlings, uvh6 exhibits a weak de-etiolated phenotype but does not affect the stronger de-etiolated phenotype of det1. In the light, det1 is epistatic to uvh6 with regard to chlorophyll level, but their effect on all size parameters is additive and therefore independent. With regard to UV tolerance, det1 UV resistance is epistatic to uvh6 UV sensitivity. In heat stress experiments, det1 enhances heat-induced tissue damage in the uvh6 background but suppresses heat-induced growth inhibition. Thus, det1 acts epistatically to uvh6 with respect to de-etiolation, chlorophyll level, UV tolerance, and heat-induced growth inhibition, whereas det1 and uvh6 act additively to regulate plant size and heat-induced cell death. These data provide insight into interplay between light and heat signaling.

Genetic Evidence for an Indispensable Role of Somatic Embryogenesis Receptor Kinases in Brassinosteroid Signaling

The Arabidopsis thaliana Somatic Embryogenesis Receptor Kinases (SERKs) consist of five members, SERK1 to SERK5, of the leucine-rich repeat receptor-like kinase subfamily II (LRR-RLK II). SERK3 was named BRI1-Associated Receptor Kinase 1 (BAK1) due to its direct interaction with the brassinosteroid (BR) receptor BRI1 in vivo, while SERK4 has also been designated as BAK1-Like 1 (BKK1) for its functionally redundant role with BAK1. Here we provide genetic and biochemical evidence to demonstrate that SERKs are absolutely required for early steps in BR signaling. Overexpression of four of the five SERKs—SERK1, SERK2, SERK3/BAK1, and SERK4/BKK1—suppressed the phenotypes of an intermediate BRI1 mutant, bri1-5. Overexpression of the kinase-dead versions of these four genes in the bri1-5 background, on the other hand, resulted in typical dominant negative phenotypes, resembling those of null BRI1 mutants. We isolated and generated single, double, triple, and quadruple mutants and analyzed their phenotypes in detail. While the quadruple mutant is embryo-lethal, the serk1 bak1 bkk1 triple null mutant exhibits an extreme de-etiolated phenotype similar to a null bri1 mutant. While overexpression of BRI1 can drastically increase hypocotyl growth of wild-type plants, overexpression of BRI1 does not alter hypocotyl growth of the serk1 bak1 bkk1 triple mutant. Biochemical analysis indicated that the phosphorylation level of BRI1 in serk1 bak1 bkk1 is incapable of sensing exogenously applied BR. As a result, the unphosphorylated level of BES1 has lost its sensitivity to the BR treatment in the triple mutant, indicating that the BR signaling pathway has been completely abolished in the triple mutant. These data clearly demonstrate that SERKs are essential to the early events of BR signaling.

Overexpression of Brassica rapa NGATHA1 Gene Confers De-Etiolation Phenotype and Cytokinin Resistance on Arabidopsis thaliana

Brassica rapa NGATHA1 (BrNGA1) encodes a B3-type transcription factor. By analyzing Arabidopsis overexpressors of BrNGA1 (BrNGA1ox), we have previously demonstrated that BrNGA1 may be involved in negative regulation of cell proliferation during lateral organ and root growth. In the present study, we have found that BrNGA1ox seedlings grown in the dark display de-etiolation phenotypes, such as short hypocotyls, open and elongated cotyledons, and developing true leaves. BrNGA1ox seedlings as well as adult plants and calli are also resistant specifically to exogenous cytokinins. These data raise the possibility that the de-etiolation phenotypes of BrNGA1ox seedlings may result from an alteration in cytokinin response. We set out to test whether the de-etiolation phenotype is due to cytokinin overproduction or constitutively activated cytokinin response. First, BrNGA1ox was crossed to the CKX2ox plant, an overexpression line of CYTOKIN OXIDASE 2, which is responsible for degradation of active cytokinins. We found, however, no difference in the de-etiolation and shoot growth phenotypes between BrNGA1ox and BrNGA1ox CKX2ox plants. Next, we measured the transcripts level of ARR5 and ARR7, frequently employed as molecular markers for cytokinin signaling and yet found no difference in their transcripts levels of the wild-type and BrNGA1ox seedlings and shoots. These data indicate that biological role of BrNGA1 involved in de-etiolation seems to be associated with neither cytokinin overproduction nor its altered signaling. Possible molecular mechanisms by which BrNGA1 may interfere with cytokinin responses and etiolation are discussed.

A role for protein kinase CK2 in plant development: evidence obtained using a dominant‐negative mutant

Protein kinase CK2 is an evolutionary conserved Ser/Thr phosphotransferase composed of two distinct subunits, alpha (catalytic) and beta (regulatory), that combine to form a tetrameric complex. Plant genomes contain multiple genes for each subunit, the expression of which gives rise to different active holoenzymes. In order to study the effects of loss of function of CK2 on plant development, we have undertaken a dominant-negative mutant approach. We generated an inactive catalytic subunit by site-directed mutagenesis of an essential lysine residue. The mutated open reading frame was cloned downstream of an inducible promoter, and stably transformed Arabidopsis thaliana plants and tobacco BY2 cells were isolated. Continuous expression of the CK2 kinase-inactive subunit did not prevent seed germination, but seedlings exhibited a strong phenotype, affecting chloroplast development, cotyledon expansion, and root and shoot growth. Prolonged induction of the transgene was lethal. Moreover, dark-germinated seedlings exhibited an apparent de-etiolated phenotype that was not caused by disruption of the light-signalling pathways. Short-term induction of the CK2 kinase-inactive subunit allowed plant survival, but root growth and lateral root formation were significantly affected. The expression pattern of CYCB1;1::GFP in the root meristems of mutant plants demonstrated an important decrease of mitotic activity, and expression of the CK2 kinase-inactive subunit in stably transformed BY2 cells provoked perturbation of the G1/S and G2 phases of the cell cycle. Our results are consistent with a model in which CK2 plays a key role in cell division and cell expansion, with compelling effects on Arabidopsis development.

Phytochrome-Regulated PIL1 Derepression is Developmentally Modulated

We define the photoresponsiveness, during seedling de-etiolation, of PHYTOCHROME-INTERACTING FACTOR 3-LIKE 1 (PIL1), initially identified by microarray analysis as an early-response gene that is robustly repressed by first exposure to light. We show that PIL1 mRNA abundance declines rapidly, with a half-time of 15 min, to a new steady-state level, 10-fold below the initial dark level, within 45 min of first exposure to red light. Analysis of phy-null mutants indicates that multiple phytochromes, including phyA and phyB, impose this repression. Conversely, PIL1 expression is rapidly derepressed by subsequent far-red irradiation of previously red light-exposed seedlings. However, the magnitude of this derepression is modulated over time, in a biphasic manner, in response to increasing duration of pre-exposure to continuous red light: (i) an early phase (up to about 6 h) of relatively rapidly increasing effectiveness of far-red reversal of repression, as declining phyA levels relieve initial very low fluence suppression of this response; and (ii) a second phase (beyond 6 h) of gradually declining effectiveness of far-red reversal, to only 20% of maximal derepression, within 36 h of continuous red light exposure, with no evidence of circadian modulation of this responsiveness, an observation in striking contrast to a previous report for entrained, green seedlings exposed to vegetative shade. These data, together with analysis of phytochrome signaling mutants and overexpressors with aberrant de-etiolation phenotypes, suggest that the second-phase decline in robustness of PIL1 derepression is an indirect consequence of the global developmental transition from the etiolated to the de-etiolated state, and that circadian coupling of derepression requires entrainment.

The ABA1 gene and carotenoid biosynthesis are required for late skotomorphogenic growth in Arabidopsis thaliana

Several plant hormones, including auxin, brassinosteroids and gibberellins, are required for skotomorphogenesis, which is the etiolated growth that seedlings undergo in the absence of light. To examine the growth of abscisic acid (ABA)-deficient mutants in the dark, we analysed several aba1 loss-of-function alleles, which are deficient in zeaxanthin epoxidase. The aba1 mutants displayed a partially de-etiolated phenotype, including reduced hypocotyl growth, cotyledon expansion and the development of true leaves, during late skotomorphogenic growth. In contrast, only small differences in hypocotyl growth were found between wild-type seedlings and ABA-deficient mutants impaired in subsequent steps of the pathway, namely nced3, aba2, aba3 and aao3. Interestingly, phenocopies of the partially de-etiolated phenotype of the aba1 mutants were obtained when wild-type seedlings were dark-grown on medium supplemented with fluridone, an inhibitor of phytoene desaturase, and hence, of carotenoid biosynthesis. ABA supplementation did not restore the normal skotomorphogenic growth of aba1 mutants or fluridone-treated wild-type plants, suggesting a direct inhibitory effect of fluridone on carotenoid biosynthesis. In addition, aba1 mutants showed impaired production of the beta-carotene-derived xanthophylls, neoxanthin, violaxanthin and antheraxanthin. Because fluridone treatment of wild-type plants phenocopied the phenotype of dark-grown aba1 mutants, impaired carotenoid biosynthesis in aba1 mutants is probably responsible for the observed skotomorphogenic phenotype. Thus, ABA1 is required for skotomorphogenic growth, and beta-carotene-derived xanthophylls are putative regulators of skotomorphogenesis.

Gibberellins modulate light signaling pathways to prevent Arabidopsis seedling de‐etiolation in darkness

In many plants, photomorphogenesis is the default developmental program after seed germination, and provides the key features that allow adaptation to light. This program is actively repressed if germination occurs in the absence of light, through a mechanism dependent on the E3 ubiquitin ligase activity that is encoded in Arabidopsis by COP1 (CONSTITUTIVE PHOTOMORPHOGENIC 1), which induces proteolytic degradation of transcription factors necessary for light-regulated development, such as HY5 (LONG HYPOCOTYL 5) and HYH (LONG HYPOCOTYL 5 HOMOLOG), and stabilization of transcription factors that promote skotomorphogenesis, such as PIF3 (PHYTOCHROME INTERACTING FACTOR 3). Seedlings deficient in gibberellin (GA) synthesis or signaling display a de-etiolated phenotype when grown in darkness, equivalent to the phenotype of cop1 mutants, which indicates that the switch between photo- and skotomorphogenesis is also under hormonal control. Here we provide evidence for the existence of crosstalk between GA and the COP1-mediated pathway, and identify HY5 and the PIF family as nodes of a regulatory network. This interaction occurs through distinct molecular mechanisms, based on the observation that GA signaling regulates protein stability of HY5, and the activity of PIF3.

HYPERSENSITIVE TO RED AND BLUE 1 and its C‐terminal regulatory function control FLOWERING LOCUS T expression

The red and far-red light-absorbing phytochromes and UV-A/blue light-absorbing cryptochromes regulate seedling de-etiolation and flowering responses. The signaling steps that mediate the photoreceptor regulation on key flowering genes remain largely unknown. We report that a previously identified photomorphogenic mutant, hypersensitive to red and blue 1 (hrb1), flowered late and showed attenuated expression of FLOWERING LOCUS T (FT) over both long days and short days. Transgenic plants that overexpress the full-length HRB1, or its C-terminal half, flowered early and accumulated more FT messages under short-day conditions. The transgenic plants also displayed hyposensitive de-etiolation phenotypes, and the expression of these phenotypes requires the action of PIF4. The double mutant of hrb1/cry2 showed a flowering phenotype and an FT expression pattern similar to hrb1 under long-day conditions, suggesting that HRB1 may function downstream of cry2 under long-day conditions. In contrast, hrb1/phyB-9 showed a flowering phenotype and an FT expression pattern similar to phyB-9 over both long days and short days, indicating a modulatory role of HRB1 in the flowering pathway mediated by phyB. Overexpression of HRB1 did not affect the expression of the central clock oscillators, TOC1 and CCA1. HRB1 therefore represents a signaling step that regulates FT expression downstream of red and blue light perception.

Erratum

Plant, Cell & EnvironmentVolume 29, Issue 6 p. 1191-1191 Free Access Erratum This article corrects the following: Brassinosteroids and plant function: some clues, more puzzles L. L. HAUBRICK, S. M. ASSMANN, Volume 29Issue 3Plant, Cell & Environment pages: 446-457 First Published online: March 14, 2006 First published: 12 May 2006 https://doi.org/10.1111/j.1365-3040.2006.01535.xAboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat The paper ‘Brassinosteroids and plant function: some clues, more puzzles’ by Haubrick & Assmann (2006), contains a number of errors. The following is the corrected text. Table 1: For the BES1/BZR2 and BZR1 genes, the cell locus should be listed as C→Nu, to indicate the movement of the gene product from the cytoplasm to the nucleus in response to application of brassinolide. Table 1 footnote should read: Cellular locations were identified by green fluorescent protein (GFP) gene fusions and confocal microscopy or by subcellular fractionation and immunoblot. Proteins that have not been specifically localized in these ways were labelled as NA (data not available). Abbreviations for cell loci: C, cytosol; ER, endoplasmic reticulum; Nu, nucleus; PM, plasma membrane; V, vacuole. Other abbreviations: ABA, abscisic acid; BL, brassinolide; BR, brassinosteroid; DEB, 1,2:3,4-diepoxybutane; EMS, ethyl methane sulphonate; T-DNA, transfer DNA. *In very young cells DET3: GFP was detected in the PM and ER. In mature cells fluorescence coincided with the vacuolar membrane, and possibly the cytosol. Figure 1 legend should read: Figure 1. Effect of brassinosteroids on Arabidopsis. (a) Exogenous application of brassinosteroid can partially rescue germination of gibberellin-deficient Arabidopsis seeds. (b) Many of the BR-related mutants exhibit a de-etiolated phenotype when grown in the dark. For wild-type seedlings, low concentrations (nM range) of brassinosteroids can stimulate hypocotyl elongation and primary root growth, while higher concentrations (µM range) of either brassinosteroids or ABA inhibit hypocotyl elongation and primary root growth. Roots of several BR response mutants of Arabidopsis (bin2, bri1, bin5) are hypersensitive to ABA and exhibit increased inhibition of primary root growth in response to exogenous application of ABA. Abbreviations: ABA, abscisic acid; BR, brassinosteroid; bin2 and bin5, brassinosteroid-insensitive 2 and 5; bri1, brassinosteroid-insensitive 1; cpd, constitutive photomorphogenesis and dwarfism; det2, de-etiolated 2. The following reference should be included: Khripach V., Zhabinskii V. & de Groot A. (2000) Twenty years of brassinosteroids: steroidal plant hormones warrant better crops for the XXI century. Annals of Botany86, 441–447. The publisher would like to apologize for these errors and for any confusion caused. REFERENCE Haubrick L.L. & Assmann S.M. (2006) Brassinosteroids and plant function: some clues, more puzzles. Plant, Cell & Environment 29, 446– 457. Wiley Online LibraryCASPubMedWeb of Science®Google Scholar Volume29, Issue6June 2006Pages 1191-1191 ReferencesRelatedInformation

Arabidopsis SHORT HYPOCOTYL UNDER BLUE1 contains SPX and EXS domains and acts in cryptochrome signaling.

Photomorphogenesis is regulated by red/far-red light-absorbing phytochromes and blue/UV-A light-absorbing cryptochromes. We isolated an Arabidopsis thaliana blue light mutant, short hypocotyl under blue1 (shb1), a knockout allele. However, shb1-D, a dominant allele, exhibited a long-hypocotyl phenotype under red, far-red, and blue light. The phenotype conferred by shb1-D was caused by overaccumulation of SHB1 transcript and recapitulated by overexpression of SHB1 in Arabidopsis. Therefore, SHB1 acts in cryptochrome signaling but overexpression may expand its signaling activity to red and far-red light. Consistent with this, overexpression of SHB1 enhanced the expression of PHYTOCHROME-INTERACTING FACTOR4 (PIF4) under red light. PIF4 appears to specifically mediate SHB1 regulation of hypocotyl elongation and CHLOROPHYLL a/b BINDING PROTEIN3 or CHALCONE SYNTHASE expression under red light. Overexpression of SHB1 also promoted proteasome-mediated degradation of phytochrome A and hypocotyl elongation under far-red light. Under blue light, shb1 suppressed LONG HYPOCOTYL IN FAR-RED LIGHT1 (HFR1) expression and showed several deetiolation phenotypes similar to hfr1-201. However, the hypocotyl and cotyledon-opening phenotypes of shb1 were opposite to those of hfr1-201, and HFR1 acts downstream of SHB1. SHB1 encodes a nuclear and cytosolic protein that has motifs homologous with SYG1 protein family members. Therefore, our studies reveal a signaling step in regulating cryptochrome- and possibly phytochrome-mediated light responses.

BRASSINOSTEROID BIOSYNTHESIS INHIBITION AS A SIDE EFFECT OF THE COMMONLY USED IMIDAZOLE FUNGICIDE IMAZALIL

Dark germinating Arabidopsis thaliana seedlings showed a clearly de-etiolated phenotype on a medium with 2.5 muM imazalil (IMA). Comparable with brassinosteroid deficient mutants, hypocotyl length was reduced and cotyledons had opened. A major part of the hypocotyl length reduction could be restored by 0.1 muM epibrasinolide (EBR). Combining EBR with 2.5 muM GA(3) was necessary for a complete restoration of the hypocotyl length. The dose dependent root length reduction by IMA could not be overcome by adding EBR. These experiments suggest that one of the side effects of IMA is the inhibition of the brassinosteroid biosynthesis.

EARLY FLOWERING 4 functions in phytochrome B-regulated seedling de-etiolation.

To define the functions of genes previously identified by expression profiling as being rapidly light induced under phytochrome (phy) control, we are investigating the seedling de-etiolation phenotypes of mutants carrying T-DNA insertional disruptions at these loci. Mutants at one such locus displayed reduced responsiveness to continuous red, but not continuous far-red light, suggesting a role in phyB signaling but not phyA signaling. Consistent with such a role, expression of this gene is induced by continuous red light in wild-type seedlings, but the level of induction is strongly reduced in phyB-null mutants. The locus encodes a novel protein that we show localizes to the nucleus, thus suggesting a function in light-regulated gene expression. Recently, this locus was identified as EARLY FLOWERING 4, a gene implicated in floral induction and regulating the expression of the gene CIRCADIAN CLOCK-ASSOCIATED 1. Together with these previous data, our findings suggest that EARLY FLOWERING 4 functions as a signaling intermediate in phy-regulated gene expression involved in promotion of seedling de-etiolation, circadian clock function, and photoperiod perception.

Uncoupling brassinosteroid levels and de-etiolation in pea.

The suggestion that brassinosteroids (BRs) have a negative regulatory role in de-etiolation is based largely on correlative evidence, which includes the de-etiolated phenotypes of, and increased expression of light-regulated genes in, dark-grown mutants defective in BR biosynthesis or response. However, we have obtained the first direct evidence which shows that endogenous BR levels in light-grown pea seedlings are increased, not decreased, in comparison with those grown in the dark. Similarly, we found no evidence of a decrease in castasterone (CS) levels in seedlings that were transferred from the dark to the light for 24 h. Furthermore, CS levels in the constitutively de-etiolated lip1 mutant are similar to those in wild-type plants, and are not reduced as is the case in the BR-deficient lkb plants. Unlike lip1, the pea BR-deficient mutants lk and lkb are not de-etiolated at the morphological or molecular level, as they exhibit neither a de-etiolated phenotype or altered expression of light-regulated genes when grown in the dark. Similarly, dark-grown WT plants treated with the BR biosynthesis inhibitor, Brz, do not exhibit a de-etiolated phenotype. In addition, analysis of the lip1lkb double mutant revealed an additive phenotype indicative of the two genes acting in independent pathways. Together these results strongly suggest that BR levels do not play a negative-regulatory role in de-etiolation in pea.

The phytochrome A specific signaling component PAT3 is a positive regulator of Arabidopsis photomorphogenesis.

Phytochrome A plays a major role in early seedling development by triggering the transition from etiolated growth to greening. Seedlings germinated under constant far-red (FR) light show a partially de-etiolated phenotype that is not seen in phyA mutants. This phytochrome A specific response was used to screen a population of T-DNA mutagenized Arabidopsis seedlings. One mutant line, pat3 (phytochrome A signal transduction3), which showed no inhibition of hypocotyl elongation under FR light conditions and no FR-induced killing response, contained a T-DNA insertion in a 609-bp ORF. The recessive mutation co-segregated with the T-DNA resistance marker and could be allelic to fhy1. A 2,248-bp genomic fragment of the PAT3 locus can complement the pat3 mutant phenotype. PAT3 transcript peaked 3 d after germination and was downregulated by light. PAT3 has no significant homology to any known protein and shows no preferential cellular localization. The protein can activate transcription in yeast when fused to the GAL4 DNA-binding domain. Our results show that PAT3 is a positive regulator of phytochrome A signal transduction.

COP1b, an isoform of COP1 generated by alternative splicing, has a negative effect on COP1 function in regulating light-dependent seedling development in Arabidopsis.

COP1 is a negative regulator of Arabidopsis light-dependent development. Mutation of the COP1 locus causes constitutive photomorphogenesis in the dark. Here, we report the identification of an isoform of the COP1 protein, named COP1b, which is generated by alternative splicing. COP1b has a 60-amino acid deletion in the WD-40 repeat domain relative to the full-length COP1. This splicing step is light-independent and takes place mostly in mature seeds and in germinating seedlings. Transgenic Arabidopsis plants that overexpress COP1b show a de-etiolated phenotype in the dark, with a short hypocotyl, open and developed cotyledons. The transgenic seedlings are adult-lethal. These phenotypes closely resemble that of severe cop-1 mutants, indicating that COP1b has a dominant negative effect on COP1 function.

The dark-adaptation response of the de-etiolated pea mutant lip1 is modulated by external signals and endogenous programs.

The lip1 mutant of pea (Pisum sativum L.) exhibits a de-etiolated phenotype. When grown in darkness, lip1 plants have several characteristics normally associated only with light-grown plants. Young wild-type (WT) seedlings accumulate high levels of transcripts from plastid-related genes (such as those encoding chlorophyll a/b-binding proteins, ferredoxin, and the small subunit of Rubisco) only in the light. In contrast, regardless of the light conditions under which the plants are grown, young mutant seedlings accumulate transcript levels equal to or greater than those seen in light-grown WT seedlings of the same age. Under some conditions, light-grown lip1 seedlings failed to respond to dark treatment. The largest response to darkness observed in the mutant occurred when older seedlings were first grown under low-light conditions before transfer to darkness. The mutant's inability to respond to darkness is not due to a gross disturbance in the circadian clock. We conclude that environmental signals (light) and endogenous programs (developmental and circadian) regulate gene expression in both WT and mutant plants. However, mutant seedlings exhibit a developmentally regulated and exaggerated response to light. In addition, the effect of the mutation may be greatest during a brief period early in development.

The Fern Ceratopteris richardii as a Lower Plant Model System for Studying the Genetic Regulation of Plant Photomorphogenesis

Photomorphogenetic research on fern gametophytes has provided important insights about pigment localization, ionic currents, and signal transduction. The first part of this article characterizes how light affects various aspects of gametophyte growth, such as spore germination, filamentous growth, and prothallial growth, in the three principal fern species used for photomorphogenetic research, namely, Adiantum capillus-veneris, Ceratopteris richardii, and Onoclea sensibilis. Although each species offers particular advantages for investigating certain photoresponses, we conclude that the process of spore germination in Ceratopteris is especially conducive to the selection of photomorphogenetic mutants. The second part describes the available Ceratopteris mutants isolated from three different selection schemes. Dark-germinating 1 (dkg1) exhibits the unique phenotype of reversed photoregulation of spore germination. Four other mutants, which are provisionally assigned the names of germ 1-4, are characterized in terms of germination percentages and prothallial growth in darkness, red light, and blue light. Germ 1 and 2 are impaired in their ability to respond to blue light, with the mutated genes apparently encoding signal transduction proteins that act close to the blue light photoreceptor. Germ 3 and 4 exhibit a de-etiolated phenotype in which the gametophytes grown in complete darkness resemble broader prothalli exposed to continuous light. Thus, the Ceratopteris mutants isolated to date complement, and perhaps extend, the range of comparable mutants in Arabidopsis seedling growth.