Fresh view of light signal transduction in plants

Molecular Lesions in BRI1 and Its Orthologs in the Plant Kingdom

Brassinosteroids (BRs) are an essential group of plant hormones regulating numerous aspects of plant growth, development, and stress responses. BRI1, along with its co-receptor BAK1, are involved in brassinosteroid sensing and early events in the BR signal transduction cascade. Mutational analysis of a particular gene is a powerful strategy for investigating its biochemical role. Molecular genetic studies, predominantly in Arabidopsis thaliana, but progressively in numerous other plants, have identified many mutants of the BRI1 gene and its orthologs to gain insight into its structure and function. So far, the plant kingdom has identified up to 40 bri1 alleles in Arabidopsis and up to 30 bri1 orthologs in different plants. These alleles exhibit phenotypes that are identical in terms of development and growth. Here, we have summarized bri1 alleles in Arabidopsis and its orthologs present in various plants including monocots and dicots. We have discussed the possible mechanism responsible for the specific allele. Finally, we have briefly debated the importance of these alleles in the research field and the agronomically valuable traits they offer to improve plant varieties.

Sensory circuitry controls cytosolic calcium-mediated phytochrome B phototransduction

Although Ca²⁺ has long been recognized as an obligatory intermediate in visual transduction, its role in plant phototransduction remains elusive. Here, we report a Ca²⁺ signaling that controls photoreceptor phyB nuclear translocation in etiolated seedlings during dark-to-light transition. Red light stimulates acute cytosolic Ca²⁺ increases via phyB, which are sensed by Ca²⁺-binding protein kinases, CPK6 and CPK12 (CPK6/12). Upon Ca²⁺ activation, CPK6/12 in turn directly interact with and phosphorylate photo-activated phyB at Ser80/Ser106 to initiate phyB nuclear import. Non-phosphorylatable mutation, phyB^S80A/S106A, abolishes nuclear translocation and fails to complement phyB mutant, which is fully restored by combining phyB^S80A/S106A with a nuclear localization signal. We further show that CPK6/12 function specifically in the early phyB-mediated cotyledon expansion, while Ser80/Ser106 phosphorylation generally governs phyB nuclear translocation. Our results uncover a biochemical regulatory loop centered in phyB phototransduction and provide a paradigm for linking ubiquitous Ca²⁺ increases to specific responses in sensory stimulus processing.

Light signaling and UV-B-mediated plant growth regulation

Light plays an important role in plants' growth and development throughout their life cycle. Plants alter their morphological features in response to light cues of varying intensity and quality. Dedicated photoreceptors help plants to perceive light signals of different wavelengths. Activated photoreceptors stimulate the downstream signaling cascades that lead to extensive gene expression changes responsible for physiological and developmental responses. Proteins such as ELONGATED HYPOCOTYL5 (HY5) and CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) act as important factors which modulate light-regulated gene expression, especially during seedling development. These factors function as central regulatory intermediates not only in red, far-red, and blue light pathways but also in the UV-B signaling pathway. UV-B radiation makes up only a minor fraction of sunlight, yet it imparts many positive and negative effects on plant growth. Studies on UV-B perception, signaling, and response in plants has considerably surged in recent times. Plants have developed different strategies to use UV-B as a developmental cue as well as to withstand high doses of UV-B radiation. Plants' responses to UV-B are an integration of its cross-talks with both environmental factors and phytohormones. This review outlines the current developments in light signaling with a major focus on UV-B-mediated plant growth regulation.

A wheat COP9 subunit 5-like gene is negatively involved in host response to leaf rust

The COP9 (constitutive photomorphogenesis 9) signalosome (CSN) is a protein complex involved in the ubiquitin proteasome system and a common host target of diverse pathogens in Arabidopsis. The known derubylation function of the COP9 complex is carried out by subunit 5 encoded by AtCSN5A or AtCSN5B in Arabidopsis. A single CSN5-like gene (designated as TaCSN5) with three homeologues was identified on the long arms of wheat (Triticum aestivum L.) group 2 chromosomes. In this study, we identified and characterized the function of TaCSN5 in response to infection by the leaf rust pathogen. Down-regulation of all three TaCSN5 homeologues or mutations in the homeologues on chromosomes 2A or 2D resulted in significantly enhanced resistance to leaf rust. Enhanced leaf rust resistance corresponded to a seven-fold increase in PR1 (pathogenesis-related gene 1) expression. Collectively, the data indicate that the wheat COP9 subunit 5-like gene acts as a negative regulator of wheat leaf rust resistance.

Ca²⁺ is involved in phytochrome A-dependent regulation of the succinate dehydrogenase gene sdh1-2 in Arabidopsis

The mechanism of transduction of the phytochrome signal regulating the expression of succinate dehydrogenase in Arabidopsis has been investigated. Using the phytochrome mutants of Arabidopsis, it is demonstrated that the inhibition of succinate dehydrogenase in the light may result from the phytochrome A-dependent modulation of Ca²⁺ amount in the nuclear fraction of leaves. This leads to the activation of expression of the gene pif3 encoding the phytochrome-interacting factor PIF3, which binds to the promoter of the gene sdh1-2 encoding the SDHA subunit of succinate dehydrogenase and suppresses its expression. It is concluded that Ca²⁺ ions are involved in the phytochrome A-mediated inhibition of succinate dehydrogenase activity in the light.

A novel Arabidopsis MYB-like transcription factor, MYBH, regulates hypocotyl elongation by enhancing auxin accumulation

Critical responses to developmental or environmental stimuli are mediated by different transcription factors, including members of the ERF, bZIP, MYB, MYC, and WRKY families. Of these, MYB genes play roles in many developmental processes. The overexpression of one MYB gene, MYBH, significantly increased hypocotyl elongation in Arabidopsis thaliana plants grown in the light, and the expression of this gene increased markedly in the dark. The MYBH protein contains a conserved motif, R/KLFGV, which was implicated in transcriptional repression. Interestingly, the gibberellin biosynthesis inhibitor paclobutrazol blocked the increase in hypocotyl elongation in seedlings that overexpressed MYBH. Moreover, the function of MYBH was dependent on phytochrome-interacting factor (PIF) proteins. Taken together, these results suggest that hypocotyl elongation is regulated by a delicate and efficient mechanism in which MYBH expression is triggered by challenging environmental conditions such as darkness, leading to an increase in PIF accumulation and subsequent enhanced auxin biosynthesis. These results indicate that MYBH is one of the molecular components that regulate hypocotyl elongation in response to darkness.

Maize non-photosynthetic ferredoxin precursor is mis-sorted to the intermembrane space of chloroplasts in the presence of light

Preprotein translocation across the outer and inner envelope membranes of chloroplasts is an energy-dependent process requiring ATP hydrolysis. Several precursor proteins analyzed so far have been found to be imported into isolated chloroplasts equally well in the dark in the presence of ATP as in the light where ATP is supplied by photophosphorylation in the chloroplasts themselves. We demonstrate here that precursors of two maize (Zea mays L. cv Golden Cross Bantam) ferredoxin isoproteins, pFdI and pFdIII, show distinct characteristics of import into maize chloroplasts. pFdI, a photosynthetic ferredoxin precursor, was efficiently imported into the stroma of isolated maize chloroplasts both in the light and in the dark. In contrast pFdIII, a non-photosynthetic ferredoxin precursor, was mostly mis-sorted to the intermembrane space of chloroplastic envelopes as an unprocessed precursor form in the light but was efficiently imported into the stroma and processed to its mature form in the dark. The mis-sorted pFdIII, which accumulated in the intermembrane space in the light, could not undergo subsequent import into the stroma in the dark, even in the presence of ATP. However, when the mis-sorted pFdIII was recovered and used for a separate import reaction, pFdIII was capable of import into the chloroplasts in the dark. pFNRII, a ferredoxin-NADP+ reductase isoprotein precursor, showed import characteristics similar to those of pFdIII. Moreover, pFdIII exhibited similar import characteristics with chloroplasts isolated from wheat (Pennisetum americanum) and pea (Pisum sativum cv Alaska). These findings suggest that the translocation of precursor proteins across the envelope membranes of chloroplasts may involve substrate-dependent light-regulated mechanisms.

The hypocotyl chloroplast plays a role in phototropic bending of Arabidopsis seedlings: developmental and genetic evidence

Chloroplasts of guard cells and coleoptiles have been implicated in the sensory transduction of blue light. The present study was aimed at establishing whether the chloroplast of the hypocotyl from Arabidopsis, another blue light-responding organ, has similar characteristics to that of sensory-transducing guard cell and coleoptile chloroplasts. Results showed that the phototropic curvature and arch length induced by blue light in Arabidopsis seedlings matched the distribution of mature chloroplasts in the bending hypocotyl. The bending arch consistently included the region of the hypocotyl containing mature chloroplasts, and never extended beyond that region. Manipulation of the extent of greening of dark-grown hypocotyls by varying red light pretreatments elicited blue light-stimulated curvatures and arch lengths that depended on the duration of the red light pretreatment and on the distribution of mature chloroplasts in the hypocotyl. Albino psd2 mutants of Arabidopsis, which lack mature chloroplasts, are devoid of phototropic sensitivity under conditions in which wild-type seedlings show large curvatures. The star mutant of Arabidopsis has a delayed greening and a delayed phototropic response as compared with wild type. Measurements of photosynthetic oxygen evolution and carbon fixation, dark respiration, and light-dependent zeaxanthin formation in the hypocotyl showed features similar to those of guard cells and coleoptiles, and distinctly different from those of mesophyll tissue. These results indicate that the hypocotyl chloroplast has characteristics similar to those associated with guard cell and coleoptile chloroplasts, and that phototropic bending of Arabidopsis hypocotyls appears to require mature chloroplasts.

Sugar control of the plant cell cycle: differential regulation of Arabidopsis D-type cyclin gene expression

In most plants, sucrose is the major transported carbon source. Carbon source availability in the form of sucrose is likely to be a major determinant of cell division, and mechanisms must exist for sensing sugar levels and mediating appropriate control of the cell cycle. We show that sugar availability plays a major role during the G(1) phase by controlling the expression of CycD cyclins in Arabidopsis. CycD2 mRNA levels increase within 30 min of the addition of sucrose; CycD3 is induced after 4 h. This corresponds to induction of CycD2 expression early in G(1) and CycD3 expression in late G(1) near the S-phase boundary. CycD2 and CycD3 induction is independent both of progression to a specific point in the cell cycle and of protein synthesis. Protein kinase activity of CycD2- and CycD3-containing cyclin-dependent kinases is consistent with the observed regulation of their mRNA levels. CycD2 and CycD3 therefore act as direct mediators of the presence of sugar in cell cycle commitment. CycD3, but not CycD2, expression responds to hormones, for which we show that the presence of sugars is required. Finally, protein phosphatases are shown to be involved in regulating CycD2 and CycD3 induction. We propose that control of CycD2 and CycD3 by sucrose forms part of cell cycle control in response to cellular carbohydrate status.

The role of the Arabidopsis ELD1 gene in cell development and photomorphogenesis in darkness

Because cell growth and differentiation are regulated by complex interactions among different signaling pathways, a growth defect affects subsequent differentiation. We report on a growth-defective mutant of Arabidopsis, called eld1 (elongation defective 1). Cell elongation was impaired in every organ examined. Later characteristics of the eld1 phenotype include defective vascular tissue differentiation, the inability to grow in soil, ectopic deposition of suberin around twisted vascular bundles, the de-etiolation phenotype, and continuation of shoot development and flowering in the dark. The dwarf phenotype of eld1 could not be rescued by treatment with exogenous growth regulators. Because defective cell elongation is the earliest and most universal feature detected in eld1 mutants, control of or activity in cell elongation may be the primary function of the ELD1 gene. The impaired cell growth results in pleiotropic effects on cell proliferation and differentiation, and the retardation in hypocotyl elongation enables growth and development in darkness.

Modulation by phytochrome of the blue light-induced extracellular acidification by leaf epidermal cells of pea (Pisum sativum l.): a kinetic analysis

Blue light induces extracellular acidification, a prerequisite of cell expansion, in epidermis cells of young pea leaves, by stimulation of the proton pumping-ATPase activity in the plasma membrane. A transient acidification, reaching a maximum 2.5-5 min after the start of the pulse, could be induced by pulses as short as 30 msec. A pulse of more than 3000 micromol m-2 saturated this response. Responsiveness to a second light pulse was recovered with a time constant of about 7 min. The fluence rate-dependent lag time and sigmoidal increase of the acidification suggested the involvement of several reactions between light perception and activation of the ATPase. In wild-type pea plants, the fluence response relation for short light pulses was biphasic, with a component that saturates at low fluence and one that saturates at high fluence. The phytochrome-deficient mutant pcd2 showed a selective loss of the high-fluence component, suggesting that the high-fluence component is phytochrome-dependent and the low-fluence component is phytochrome-independent. Treatment with the calmodulin inhibitor W7 also led to the elimination of the phytochrome-dependent high-fluence component. Simple models adapted from the one used to simulate blue light-induced guard cell opening failed to explain one or more elements of the experimental data. The hypothesis that phytochrome and a blue light receptor interact in a short-term photoresponse is endorsed by model calculations based upon a three-step signal transduction cascade, of which one component can be modulated by phytochrome.

Localization and light-dependent phosphorylation of white collar 1 and 2, the two central components of blue light signaling in Neurospora crassa

In Neurospora crassa only two white collar (wc) mutants, wc-1 and wc-2, have been described that seem to be insensitive to light. The pleiotropic phenotypes of these mutants suggest that they represent two central components of blue light signal transduction. The WC proteins have several characteristics of transcription factors consistent with an involvement in transcriptional control of light-regulated genes. Here, we present a biochemical analysis of WC1 and WC2 polypeptides in N. crassa. Using specific antisera against WC1 and WC2, respectively, the subcellular localization of the WC polypeptides was investigated. The WC1 protein was localized exclusively in the nucleus, whereas WC2 was detected in both the nuclear and cytoplasmic fractions. The nuclear localization of WC1 and WC2 was shown to be independent of light and dimerization between the two proteins. In addition, WC1 and WC2 are phosphorylated in response to light. The phosphorylation of WC1 and WC2 was dependent on functional WC1 and WC2 proteins, respectively, which clearly indicated a correlation between the light-dependent phosphorylation and the function of WC1 and WC2 in blue light signaling. However, the light-specific phosphorylation of the WC proteins revealed different kinetics. The phosphorylation of WC1 was transient whereas the WC2 phosphorylation was shown to be stable under constant light conditions. The analysis of the light-dependent phosphorylation of WC1 and WC2 in wc-2 and wc-1 mutants revealed an epistatic relationship for WC1 and WC2 with WC2 acting downstream of WC1 in the signal transduction pathway of blue light.

Interactions and intersections of plant signaling pathways

Plant signal transduction is a rapidly expanding field of research, and during the last decade a wealth of insight into how plants perceive and transmit signals as part of normal development and in response to environmental cues has been and is continuing to be unraveled. Although ?signaling cascades are often viewed as linear chains of events it is now becoming increasingly apparent, through the use of cell biological, molecular and genetic approaches, that plant signal transduction involves extensive cross-talk between different pathways. The numerous interactions and intersections which take place are potentially important to modulate and balance the various inputs from different signaling cascades so that plants can integrate all this information to execute the proper developmental responses.

C4 GENE EXPRESSION

C4 plants, including maize, Flaveria, amaranth, sorghum, and an amphibious sedge Eleocharis vivipara, have been employed to elucidate the molecular mechanisms and signaling pathways that control C4 photosynthesis gene expression. Current evidence suggests that pre-existing genes were recruited for the C4 pathway after acquiring potent and surprisingly diverse regulatory elements. This review emphasizes recent advances in our understanding of the creation of C4 genes, the activities of the C4 gene promoters consisting of synergistic and combinatorial enhancers and silencers, the use of 5' and 3' untranslated regions for transcriptional and posttranscriptional regulations, and the function of novel transcription factors. The research has also revealed new insights into unique or universal mechanisms underlying cell-type specificity, coordinate nuclear-chloroplast actions, hormonal, metabolic, stress and light responses, and the control of enzymatic activities by phosphorylation and reductive processes.

Blue light and abscisic acid independently induce heterophyllous switch in marsilea quadrifolia

In natural habitats Marsilea quadrifolia L. produces different types of leaves above and below the water level. In aseptic cultures growth conditions can be manipulated so that leaves of the submerged type are produced continuously. Under such conditions the application of either blue light or an optimal concentration of abscisic acid (ABA) induced the development of aerial-type leaves. When fluridone, an inhibitor of ABA biosynthesis, was added to the culture medium it did not prevent blue light induction of aerial leaf development. During blue light treatment the endogenous ABA level in M. quadrifolia leaves remained unchanged. However, after the plants were transferred to an enriched medium, the ABA level gradually increased, corresponding to a transition in development from the submerged type of leaves to aerial leaves. These results indicate that the blue light signal is not mediated by ABA. Therefore, in the regulation of heterophyllous determination, discrete pathways exist in response to environmental signals.

The phytochrome family: dissection of functional roles and signalling pathways among family members

There is considerable evidence that individual members of the five-membered phytochrome family of photoreceptors in Arabidopsis have differential functional roles in controlling plant photomorphogenesis. Emerging genetic evidence suggests that this differential activity may involve initially separate signalling pathway branches specific to individual family members.

A fusca gene homologue in mammalian tissues: Developmental regulation in the rat testes

By differential screening of a rat pineal cDNA library we identified earlier a novel transcript having a 57% nucleotide homology and a 45% amino acid identity with a plant fusca-gene (fus6) to which a corresponding human sequence (gps1) has recently been reported. Expression of this mammalian fusca homologue (mfh) was seen in a variety of mammalian tissues, including kidney, pineal and retina, but it was particularly strong in the testes. Northern blot analysis demonstrated that the rat testicular mfh message increases markedly from day 28 onwards. Additionally, by in situ hybridization, mfh was localized primarily to the seminiferous tubules with a stage-dependent distribution pattern, a result which was confirmed by immunohistochemistry with antibodies raised against a synthetic MFH oligopeptide. Western blotting also revealed strong signals of the expected molecular weight in testicular extracts from several species. In view of its homology to fus6, a plant gene known to be involved in repressing photomorphogenesis in darkness, the conservation of mfh in mammals suggests a potential function for MFH in signaling pathways involved in the regulation of mammalian differentiation and development.

Phytochrome-mediated light signals are transduced to nucleoside diphosphate kinase in Pisum sativum L. cv. Alaska

To clarify the molecular mechanism for the transduction of light signals in plants, we have established an in vitro system that uses crude membrane and soluble fractions of stem sections of etiolated Pisum sativum L. cv. Alaska after irradiation by red light, or sequential application of red and far-red light to the stem section. In a previous report (T. Hamada et al., J. Photochem. Photobiol. B: Biol. 33 (1996) 143-151) the labelling of proteins in membrane fraction by [gamma-32P] ATP at 0 degree C for 15 s and subsequent separation of proteins by two-dimensional electrophoresis allowed unambiguous identification of a heavily phosphorylated protein spot at 18 kDa (p18). In the present study we have confirmed the former results in the membrane fraction, and obtained the result that an increase in the phosphorylation of p18 by red-light irradiation is observed in the soluble fraction. Further, we have provided evidence that the p18 in the soluble fraction is purified and identified as nucleoside diphosphate (NDP) kinase by Western blotting, immuno-precipitation, amino acid sequencing and cDNA analysis. Purified p18 shows autophosphorylation activity and strong phosphorylating activity against myelin basic protein (MBP), a substrate of MAP (mitogen activated protein) kinase. The results show that phytochrome-mediated light signals are transduced to NDP kinase, which may elicit signals by providing high concentrations of, for example, GTP from GDT and ATP, by the autophosphorylation and by the protein kinase activity similar to MAP kinase.

Cloning of a cDNA encoding an importin-alpha and down-regulation of the gene by light in rice leaves

The import of nuclear proteins into nuclei begins with recognition of nuclear localization signal-harboring proteins and binding to a nuclear pore targeting complex. A cDNA for an importin-alpha protein, a subunit of the complex, was isolated from rice plants. The amino acid sequence deduced from the nucleotide sequence of the cDNA exhibited a high homology to those of importin-alpha proteins from many organisms such as Arabidopsis thaliana, Saccharomyces cerevisiae, human, mouse, Xenopus laevis and Drosophila melanogaster. Down-regulation of the transcription by light was shown in the leaves of light- and dark-grown seedlings by RNA blot analysis. The down-regulation was specific to leaves, whereas no light effect was observed in root tissues or calli, in which higher levels of the transcript were detected.

The effect of external Ca2+ and Ca(2+)-channel modulators on red-light-induced swelling of protoplasts of Phaseolus radiatus L

Red-light-induced swelling of the protoplasts isolated from hypocotyl of etiolated mung bean (Phaseolus radiatus L.) was observed only when Ca2+ ions were present in the medium. The optimal CaCl2 concentration was 250 microM. Swelling response declined when Ca2+ was supplied into the medium after red light irradiation. The Ca(2+)-chelator EGTA eliminated the red-light-induced swelling and 45Ca2+ accumulation in the protoplasts. In contrast, A23187, a Ca(2+)-ionophore, could mimic the effect of red light in darkness. These results indicate that Ca2+ may play a role in light signal transduction. In addition, swelling response was prevented by TFP and CPZ (both are CaM antagonists), implying the involvement of CaM in red-light-induced and Ca(2+)-dependent protoplast swelling.

Molecular cloning and sequencing of the cDNA of cop1 gene from Pisum sativum

Cop1 protein plays an important role in seedling development of higher plants. The cDNA of cop1 gene from pea (Pisum sativum) was cloned and sequenced. Cop1 protein of pea is predicted to have 672 amino acids and a molecular mass of 76 kDa. Sequence comparison between Cop1 proteins of pea and Arabidopsis thaliana revealed that the two Cop1 proteins were highly homologous in the regions with functional domains and at the C-terminus. Immunoblotting performed with polyclonal antibodies against recombinant Cop1 of pea showed that Cop1 protein was present in seedlings germinated both in light and darkness.

PNZIP is a novel mesophyll-specific cDNA that is regulated by phytochrome and the circadian rhythm and encodes a protein with a leucine zipper motif

We isolated and characterized a novel light-regulated cDNA from the short-day plant Pharbitis nil that encodes a protein with a leucine (Leu) zipper motif, designated PNZIP (Pharbitis nil Leu zipper). The PNZIP cDNA is not similar to any other gene with a known function in the database, but it shares high sequence homology with an Arabidopsis expressed sequence tag and to two other sequences of unknown function from the cyanobacterium Synechocystis spp. and the red alga Porphyra purpurea, which together define a new family of evolutionarily conserved Leu zipper proteins. PNZIP is a single-copy gene that is expressed specifically in leaf photosynthetically active mesophyll cells but not in other nonphotosynthetic tissues such as the epidermis, trichomes, and vascular tissues. When plants were exposed to continuous darkness, PNZIP exhibited a rhythmic pattern of mRNA accumulation with a circadian periodicity of approximately 24 h, suggesting that its expression is under the control of an endogenous clock. However, the expression of PNZIP was unusual in that darkness rather than light promoted its mRNA accumulation. Accumulation of PNZIP mRNA during the dark is also regulated by phytochrome, since a brief exposure to red light in the middle of the night reduced its mRNA levels. Moreover, a far-red-light treatment at the end of day also reduced PNZIP mRNA accumulation during the dark, and that effect could be inhibited by a subsequent exposure to red light, showing the photoreversible response attributable to control through the phytochrome system.

The Arabidopsis HY5 gene encodes a bZIP protein that regulates stimulus-induced development of root and hypocotyl

Plant developmental processes are controlled by both endogenous programs and environmental stimuli. As a photomorphogenetic mutant, hy5 of Arabidopsis has been isolated and characterized. Our detailed characterization has revealed that the mutant is deficient in a variety of stimulus responses, including gravitropic response and waving growth of roots, as well as light-dependent hypocotyl elongation. In the roots and hypocotyl, the hy5 mutation also affects greening and specific cell proliferation such as lateral root formation and secondary thickening. Those phenotypes indicate that the HY5 gene is responsible for the regulation of fundamental developmental processes of the plant cell: cell elongation, cell proliferation, and chloroplast development. Molecular cloning of the HY5 gene using a T-DNA-tagged mutant has revealed that the gene encodes a protein with a bZIP motif, one of the motifs found in transcriptional regulators. Nuclear localization of the HY5 protein strongly suggests that the HY5 gene modulates the signal transduction pathways under the HY5-related development by controlling expression of genes downstream of these pathways.

Coordination of nuclear and chloroplast gene expression in plant cells

Plastid proteins are encoded in two genomes, one in the nucleus and the other in the organelle. The expression of genes in these two compartments in coordinated during development and in response to environmental parameters such as light. Two converging approaches reveal features of this coordination: the biochemical analysis of proteins involved in gene expression, and the genetic analysis of mutants affected in plastid function or development. Because the majority of proteins implicated in plastid gene expression are encoded in the nucleus, regulatory processes in the nucleus and in the cytoplasm control plastid gene expression, in particular during development. Many nucleus-encoded factors involved in transcriptional and posttranscriptional steps of plastid gene expression have been characterized. We are also beginning to understand whether and how certain developmental or environmental signals perceived in one compartment may be transduced to the other.

Computer analysis of phytochrome sequences and reevaluation of the phytochrome secondary structure by Fourier transform infrared spectroscopy

A repertoire of various methods of computer sequence analysis was applied to phytochromes in order to gain new insights into their structure and function. A statistical analysis of 23 complete phytochrome sequences revealed regions of non-random amino acid composition, which are supposed to be of particular structural or functional importance. All phytochromes other than phyD and phyE from Arabidopsis have at least one such region at the N-terminus between residues 2 and 35. A sequence similarity search of current databases indicated striking homologies between all phytochromes and a hypothetical 84.2-kDa protein from the cyanobacterium Synechocystis. Furthermore, scanning the phytochrome sequences for the occurrence of patterns defined in the PROSITE database detected the signature of the WD repeats of the beta-transducin family within the functionally important 623-779 region (sequence numbering of phyA from Avena) in a number of phytochromes. A multiple sequence alignment performed with 23 complete phytochrome sequences is made available via the IMB Jena World-Wide Web server (http://www.imb-jena.de/PHYTO.html). It can be used as a working tool for future theoretical and experimental studies. Based on the multiple alignment striking sequence differences between phytochromes A and B were detected directly at the N-terminal end, where all phytochromes B have an additional stretch of 15-42 amino acids. There is also a variety of positions with totally conserved but different amino acids in phytochromes A and B. Most of these changes are found in the sequence segment 150-200. It is, therefore, suggested that this region might be of importance in determining the photosensory specificity of the two phytochromes. The secondary structure prediction based on the multiple alignment resulted in a small but significant beta-sheet content. This finding is confirmed by a reevaluation of the secondary structure using FTIR spectroscopy.

Plastoquinol at the quinol oxidation site of reduced cytochrome bf mediates signal transduction between light and protein phosphorylation: thylakoid protein kinase deactivation by a single-turnover flash

Redox-controlled phosphorylation of thylakoid membrane proteins represents a unique system for the regulation of light energy utilization in photosynthesis. The molecular mechanisms for this process remain unknown, but current views suggest that the plastoquinone pool directly controls the activation of the kinase. On the basis of enzyme activation by a pH shift in the darkness combined with flash photolysis, EPR, and optical spectroscopy we propose that activation occurs when plastoquinol occupies the quinol-oxidation (Qo) site of the cytochrome bf complex, having its high-potential path components in a reduced state. A linear correlation between kinase activation and accessibility of the Qo site to plastoquinol was established by quantification of the shift in the g(y) EPR signal of the Rieske Fe-S center resulting from displacement of the Qo-site plastoquinol by a quinone analog. Activity persists as long as one plastoquinol per cytochrome bf is still available. Withdrawal of one electron from this plastoquinol after a single-turnover flash exciting photosystem I leads to deactivation of the kinase parallel with a decrease in the g(z) EPR signal of the reduced Rieske Fe-S center. Cytochrome f, plastocyanin, and P(700) are rereduced after the flash, indicating that the plastoquinol at the Qo site is limiting in maintaining the kinase activity. These results give direct evidence for a functional cytochrome bf-kinase interaction, analogous to a signal transduction system where the cytochrome bf is the receptor and the ligand is the plastoquinol at the Qo site.

White collar 2, a partner in blue-light signal transduction, controlling expression of light-regulated genes in Neurospora crassa

A saturating genetic dissection of 'blind' mutants in Neurospora crassa has identified a total of two non-redundant loci (wc-1 and wc-2) each of which is required for blue-light perception/signal transduction. Previously, we demonstrated that WC1 is a putative zinc finger transcription factor able to bind specifically to a light-regulated promoter. Here, we present the cloning and characterization of the wc-2 gene. We demonstrate using mutation analysis and in vitro DNA-binding assays that WC2, the second partner of this light signal transduction system, encodes a functional zinc finger DNA-binding protein with putative PAS dimerization and transcription activation domains. This molecular genetic dissection of the second of two components of this light signal transduction system has enabled us to devise a model whereby WC1 and WC2 are proposed to interact via homologous PAS domains, bind to promoters of light-regulated genes and activate transcription. As such, this study provides the first insight into two co-operating partners in blue-light signal transduction in any organism and describes the molecular tools with which to dissect this enigmatic process.

From seed germination to flowering, light controls plant development via the pigment phytochrome

Plant growth and development are regulated by interactions between the environment and endogenous developmental programs. Of the various environmental factors controlling plant development, light plays an especially important role, in photosynthesis, in seasonal and diurnal time sensing, and as a cue for altering developmental pattern. Recently, several laboratories have devised a variety of genetic screens using Arabidopsis thaliana to dissect the signal transduction pathways of the various photoreceptor systems. Genetic analysis demonstrates that light responses are not simply endpoints of linear signal transduction pathways but are the result of the integration of information from a variety of photoreceptors through a complex network of interacting signaling components. These signaling components include the red/far-red light receptors, phytochromes, at least one blue light receptor, and negative regulatory genes (DET, COP, and FUS) that act downstream from the photoreceptors in the nucleus. In addition, a steroid hormone, brassinolide, also plays a role in light-regulated development and gene expression in Arabidopsis. These molecular and genetic data are allowing us to construct models of the mechanisms by which light controls development and gene expression in Arabidopsis. In the future, this knowledge can be used as a framework for understanding how all land plants respond to changes in their environment.

Transport of proteins in eukaryotic cells: more questions ahead

Some newly synthesized proteins contain signals that direct their transport to their final location within or outside of the cell. Targeting signals are recognized by specific protein receptors located either in the cytoplasm or in the membrane of the target organelle. Specific membrane protein complexes are involved in insertion and translocation of polypeptides across the membranes. Often, additional targeting signals are required for a polypeptide to be further transported to its site of function. In this review, we will describe the trafficking of proteins to various cellular organelles (nucleus, chloroplasts, mitochondria, peroxisomes) with emphasis on transport to and through the secretory pathway.

LIGHT CONTROL OF SEEDLING DEVELOPMENT

Light control of plant development is most dramatically illustrated by seedling development. Seedling development patterns under light (photomorphogenesis) are distinct from those in darkness (skotomorphogenesis or etiolation) with respect to gene expression, cellular and subcellular differentiation, and organ morphology. A complex network of molecular interactions couples the regulatory photoreceptors to developmental decisions. Rapid progress in defining the roles of individual photoreceptors and the downstream regulators mediating light control of seedling development has been achieved in recent years, predominantly because of molecular genetic studies in Arabidopsis thaliana and other species. This review summarizes those important recent advances and highlights the working models underlying the light control of cellular development. We focus mainly on seedling morphogenesis in Arabidopsis but include complementary findings from other species.

Brassinosteroids rescue the deficiency of CYP90, a cytochrome P450, controlling cell elongation and de-etiolation in Arabidopsis

The cpd mutation localized by T-DNA tagging on Arabidopsis chromosome 5-14.3 inhibits cell elongation controlled by the ecdysone-like brassinosteroid hormone brassinolide. The cpd mutant displays de-etiolation and derepression of light-induced genes in the dark, as well as dwarfism, male sterility, and activation of stress-regulated genes in the light. The CPD gene encodes a cytochrome P450 (CYP90) sharing homologous domains with steroid hydroxylases. The phenotype of the cpd mutant is restored to wild type both by feeding with C23-hydroxylated brassinolide precursors and by ectopic overexpression of the CPD cDNA. Brassinosteroids also compensate for different cell elongation defects of Arabidopsis det, cop, fus, and axr2 mutants, indicating that these steroids play an essential role in the regulation of plant development.

The regulation of enzymes involved in chlorophyll biosynthesis

All living organisms contain tetrapyrroles. In plants, chlorophyll (chlorophyll a plus chlorophyll b) is the most abundant and probably most important tetrapyrrole. It is involved in light absorption and energy transduction during photosynthesis. Chlorophyll is synthesized from the intact carbon skeleton of glutamate via the C5 pathway. This pathway takes place in the chloroplast. It is the aim of this review to summarize the current knowledge on the biochemistry and molecular biology of the C5-pathway enzymes, their regulated expression in response to light, and the impact of chlorophyll biosynthesis on chloroplast development. Particular emphasis will be placed on the key regulatory steps of chlorophyll biosynthesis in higher plants, such as 5-aminolevulinic acid formation, the production of Mg(2+)-protoporphyrin IX, and light-dependent protochlorophyllide reduction.

Systemin--a polypeptide defense signal in plants

Insect and pathogen attacks activate plant defense genes within minutes in nearby cells, and within hours in leaves far distant from the sites of the predator attacks. A search for signal molecules involved in both the localized and distal signalling has resulted in the identification of an 18-amino-acid polypeptide, called systemin, that activates defense genes in leaves of tomato plants when supplied at levels as low as fmols/plant. Several lines of evidence support a role for systemin as a wound hormone. As with animal polypeptide hormones, systemin is derived from a larger precursor protein, called prosystemin, by limited proteolysis. Systemin has been shown by autoradiography to be phloemmobile and, by antisense technology, to be an essential component of the wound-inducible, systemic signal transduction system leading to the transcriptional activation of the defensive genes. A search for the receptor of systemin has led to the identification in plant plasma membranes of a systemin-binding protein. However, this protein has properties not of a receptor, but of a furin-like proteinase that cleaves systemin into smaller polypeptides. Systemin and its precursor prosystemin provide prototypes for the emerging possibilities that polypeptide hormones may have broad roles in signalling environmental stress responses, and in regulating plant growth and development as well.

Phytochrome signal-transduction: characterization of pathways and isolation of mutants

The study of phytochrome signalling has yielded a wealth of data describing both the perception of light by the receptor, and the terminal steps in phytochrome-regulated gene expression by a number of transcription factors. We are now focusing on establishing the intervening steps linking phytochrome photoactivation to gene expression, and the regulation and interactions of these signalling pathways. Recent work has utilized both a pharmacological approach in phototrophic soybean suspension cultures and microinjection techniques in tomato to establish three distinct phytochrome signal-transduction pathways: (i) a calcium-dependent pathway that regulates the expression of genes encoding the chlorophyll a/b binding protein (CAB) and other components of photosystem II; (ii) a cGMP-dependent pathway that regulates the expression of the gene encoding chalcone synthase (CHS) and the production of anthocyanin pigments; and (iii) a pathway dependent upon both calcium and cGMP that regulates the expression of genes encoding components of photosystem I and is necessary for the production of mature chloroplasts. To study the components and the regulation of phytochrome signal-transduction pathways, mutants with altered photomorphogenic responses have been isolated by a number of laboratories. However, with several possible exceptions, little real progress has been made towards the isolation of mutants in positive regulatory elements of the phytochrome signal-transduction pathway. We have characterized a novel phytochrome A (PhyA)-mediated far-red light (FR) response in Arabidopsis seedlings which we are currently using to screen for specific phyA signal-transduction mutants.

Evidence for some common signal transduction events for opposite regulation of nitrate reductase and phytochrome-I gene expression by light

We have explored the possible involvement of the phosphoinositide (PI) cycle and protein kinase C (PKC) in the phytochrome (Pfr)-mediated light signal transduction pathway using nitrate reductase (NR) and phytochrome-I (PhyI) genes as model systems. We have shown earlier that phorbol myristate acetate (PMA) completely replaces the red light effect in stimulating nitrate reductase activity and transcript levels in maize. In this paper, we present detailed evidence to show that PMA mimics the red light effect and follows similar kinetics to enhance NR steady-state transcript accumulation in a nitrate-dependent manner. We also show that PMA inhibits phyI steady-state transcript accumulation in a manner similar to red light, indicating that a PKC-type enzyme(s) may be involved in mediating the light effect in both cases. Serotonin or 5-hydroxytryptamine (5-HT), a stimulator of PI turnover, was also found to mimic the red light effect in enhancing NR transcript levels and inhibiting phyI transcript accumulation, indicating the role of the PI cycle in generating second messengers for regulating the two genes. These results indicate that phytochrome-mediated light regulation of NR and phyI gene expression may involve certain common steps in the signal transduction pathway such as the PI cycle and protein phosphorylation by a PKC-type enzyme.

Developmental and tissue-specific regulation of the gene for the wheat basic/leucine zipper protein HBP-1a(17) in transgenic Arabidopsis plants

Wheat basic/leucine zipper protein HBP-1a(17) binds in vitro specifically to ACGT motif-containing cis-acting elements, such as the type I element of plant histone promoters and the G-box of hormone- and light-inducible promoters. To address the in vivo function of HBP-1a(17), we isolated and structurally analyzed the HBP-1a(17) gene and examined its expression in transgenic Arabidopsis plants. The HBP-1a(17) gene is composed of 14 exons; the basic region and leucine zipper are encoded by separate small exons, as is the case for other bZIP protein genes. The G-box of the HBP-1a(17) promoter bound specifically to HBP-1a(17) and its related HBP-1a isoforms, suggesting that the HBP-1a(17) gene may be autoregulated, although the binding affinity of these proteins in vitro is very low. In Arabidopsis plants, activation of the HBP-1a(17) promoter was highly restricted to photosynthetically active mesophyll, and guard cells and vascular bundles of vegetative leaves. Etiolation of transgenic plants resulted in inhibition of expression of the HBP-1a(17) promoter. Indeed, the HBP-1a(17) promoter contains several sequence elements homologous to cis-acting elements conserved in light-inducible promoters. It is, therefore, assumed that the HBP-1a(17) gene is light regulated and that HBP-1a(17) is involved in light-responsive gene transcription via the G-box.

Arabidopsis COP1 protein specifically interacts in vitro with a cytoskeleton-associated protein, CIP1

Arabidopsis COP1 acts inside the nucleus to suppress photomorphogenic cellular development, and light inactivation of COP1 may involve a specific control of its nuclear activity in hypocotyls and cotyledons, but not in roots, of developing seedlings. To understand the molecular mechanisms of COP1 action during light-mediated development, we initiated a screen for Arabidopsis cDNAs encoding proteins which interact directly with COP1 in vitro as a step to identify the cellular components involved. We report here the isolation and characterization of a cDNA clone encoding a protein designated CIP1 (COP1-interactive protein 1). CIP1 is predominantly alpha-helical and most likely involved in coiled-coil formation. It interacts specifically with the putative coiled-coil region of COP1 in vitro. Further, CIP1 is encoded by a single gene in Arabidopsis, and its mRNA and protein levels are not regulated by light. Immunofluorescent labeling of CIP1 in Arabidopsis seedling protoplasts demonstrated that CIP1 is part of, or associated with, a cytoskeletal structure in hypocotyl and cotyledon cells, but not in roots. Our results are consistent with a possible role of CIP1 in mediating light control of COP1 nuclear activity by regulating its nucleocytoplasmic partitioning.