Calcium/calmodulin-dependent and -independent phytochrome signal transduction pathways

Isolation and analysis of the soybean SGA2 gene (cDNA), encoding a new member of the plant G-protein family of signal transducers

We have isolated a cDNA clone from Glycine max, named SGA2, coding for a G alpha-subunit protein. The encoded polypeptide, SG alpha2, shows a molecular mass of 45 kDa and contains most of the conserved regions involved in guanine nucleotide binding and hydrolysis. Comparison at the nucleotide and amino acid sequence levels with the other plant G alpha's shows a high degree of conservation (>85% similarity). Phylogenetic analysis of these plant genes with the other G alpha's from different species clearly indicate that those proteins represent a new member of the heterotrimeric G-protein family, named Gp. Tissue localization of SGA2 transcripts in root, stem and leaf organs shows that this gene is widely expressed throughout the plant although it is most abundant in the vascular tissues of all these organs. Furthermore, the transcript is more abundant in young tissues and organ primordia than mature tissues. The high degree of sequence conservation among the plant G alpha's and the differences to other species of other kingdoms, suggest that plant G proteins may function in specialized signalling processes.

Evidence that the plastid signal and light operate via the same cis-acting elements in the promoters of nuclear genes for plastid proteins

Nuclear-encoded genes for proteins of the photosynthetic machinery represent a particular subset of genes. Their expression is cooperatively stimulated by discrete factors including the developmental stage of plastids and light. We have analyzed in transgenic tobacco the plastid- and light-dependent expression of a series of 5' promoter deletions of various nuclear genes from spinach, of fusions of defined promoter segments with the 90-bp 35S RNA CaMV minimal promoter, as well as with mutations in sequences with homologies to characterized cis-elements, to address the question of whether the plastid signal and light operate via the same or different cis-acting elements. In none of the 160 different transgenic lines (representing 32 promoter constructs from seven genes) analyzed, could significant differences be identified in the responses to the two regulatory pathways. The data are compatible with the idea that both signals control the expression of nuclear genes for plastid proteins via the same cis-acting elements.

Light-induced phase shifting of the circadian conidiation rhythm is inhibited by calmodulin antagonists in Neurospora crassa

The effects of calmodulin antagonists and inhibitors of protein kinases and phosphatases on light-induced phase shifting were investigated in Neurospora crassa. Calmodulin antagonists, namely, trifluoperazine, chlorpromazine, and W-7, almost completely inhibited the light-induced phase shifting without having any effect on the circadian clock itself. Chlorpromazine was less effective in inhibiting the light-induced phase shifting than trifluoperazine. W-5, a dechlorinated analogue of W-7, failed to inhibit the light-induced phase shifting at the same concentration as that at which W-7 was effective. These results suggest that calmodulin is required during signal transduction from the light-perceiving system to the circadian clock in N. crassa. Inhibitors of protein phosphorylation did not inhibit the light-induced phase shifting, although these inhibitors completely inhibited mycelial growth. Trifluoperazine partially inhibited the phosphorylation of three proteins when phosphorylation was assayed in vitro.

G proteins regulate dihydropyridine binding to moss plasma membranes

The role of calcium as an activator and regulator of many biological processes is linked to the ability of the cell to rapidly change its cytoplasmic calcium levels. Calcium acts as an intracellular messenger in hormone-induced bud formation during the development of the moss Physcomitrella patens. Calcium transport and ligand binding studies have implicated plasma membrane-localized 1, 4-dihydropyridine (DHP)-sensitive calcium channels in this increase in cellular calcium. To understand the regulation of the moss calcium channel, we investigated the involvement of GTP binding regulatory proteins (G proteins). Guanosine 5'-(gamma-thio)triphosphate (GTPgammaS), a nonhydrolyzable GTP analog that locks G proteins into their active state, stimulated DHP binding to high affinity receptors in the moss plasma membrane. DHP binding was measured as the ability of the DHP agonist Bay K8644 or the DHP antagonist nifedipine to compete with the DHP arylazide [3H]azidopine for binding to moss plasma membranes. G protein stimulation of binding was seen when competition was carried out with either nifedipine or Bay K8644. G proteins regulated the rates of association and dissociation of bound [3H]azidopine, and stimulation was dependent on GTPgammaS concentration. Guanosine 5'-(beta-thio)diphosphate, a GDP analog that locks G proteins into their inactivated state, did not affect the dose dependence of either the agonist or the antagonist. These results suggest that G proteins may act via a membrane-delimited pathway to regulate calcium channels in the moss plasma membrane.

The aurea and yellow-green-2 mutants of tomato are deficient in phytochrome chromophore synthesis

The phytochrome-deficient aurea mutant of tomato has been widely used for the study of both phytochrome function and the role of other photoreceptors in the control of development in higher plants. To date the exact nature of the aurea mutation has remained unknown, though this information is clearly important for the interpretation of these studies. It has been proposed that aurea and yellow-green-2, another mutant of tomato that has a similar phenotype to aurea, could be deficient in phytochrome chromophore synthesis. We have examined this hypothesis by measuring the activity of the enzymes committed to phytochrome chromophore synthesis in these mutants. The approach takes advantage of a recently developed high pressure liquid chromatography-based assay for the synthesis of the free phytochrome chromophore, phytochromobilin from its immediate precursors biliverdin IXalpha and heme. Isolated etioplasts from aurea and yellow-green-2 seedlings were specifically unable to convert biliverdin IXalpha to 3Z-phytochromobilin and heme to biliverdin IXalpha, respectively. In addition, the level of total noncovalently bound heme in the mutants was the same as in wild type seedlings. Together, these results identify both aurea and yellow-green-2 as mutants that are deficient in phytochrome chromophore synthesis.

Fourier-transform infrared spectroscopy of phytochrome: difference spectra of the intermediates of the photoreactions

The photocycle of 124 kDa phytochrome A from Avena sativa was studied by Fourier-transform infrared spectroscopy at low temperatures. Difference spectra between the parent state Pr and the intermediates of the Pr-->Pfr pathway, i.e. lumi-R, meta-Ra, and meta-Rc, and between Pfr and the intermediates of the Pfr-->Pr pathway, lumi-F and meta-F, were obtained in 1H2O and 2H2O for the first time. Each spectrum shows characteristic spectral features which allow a clear distinction between the different intermediates. A general feature is that greater changes occur with increasing temperature, i.e. at the later steps of the photoreactions. Nevertheless, the changes in the spectral regions of the protein (amide I and amide II) were found to be surprisingly small, excluding larger conformational changes of the protein. All spectra of the intermediates are characterized by a strong negative band around 1700 cm-1. This band is tentatively assigned to the C = O stretch of ring D of the chromophore. Since it is not observed in the difference spectra between the parent states, it is concluded that ring D is located in a similar molecular environment in Pr and Pfr. In the photoproducts lumi-R and lumi-F, this band undergoes an upshift to 1720 cm-1. The high frequencies suggest that the chromophore is protonated in these intermediates as well as in Pr and Pfr.

Phosphorylation/dephosphorylation steps are key events in the phytochrome-mediated enhancement of nitrate reductase mRNA levels and enzyme activity in maize

We provide evidence to show that the increase in nitrate reductase (NR) transcript level stimulated by red light is mediated via a phosphorylation-dependent step. The light-stimulated enhancement of NR transcript level was significantly inhibited by H-7, a protein kinase inhibitor, whereas okadaic acid (OKA), a phosphatase inhibitor, had no effect. Phorbol myristate acetate (PMA), an activator of protein kinase C (PKC) enhanced the NR transcript level in dark-grown leaves. No correlation between changes in NR transcript level and NR activity (NRA) was observed. Inhibition of NRA by OKA and stimulation by H-7 indicated that NRA is increased by dephosphorylating the enzyme. We have identified a protein kinase (C type) that can phosphorylate the purified NR in vitro without the involvement of other accessory proteins. By in vivo labelling with 32P and immunoprecipitation of NR with NR antibodies it was found that in the presence of OKA most NR protein (NRP) was present in phosphorylated state, while with H-7 the reverse was seen. The red (R) and far-red (FR) light reversible experiments suggested that phytochrome (Pfr, an active form) stimulation of NRA is mediated by dephosphorylation of the enzyme, suggesting that Pfr regulates both NR transcription and NRA via phosphorylation/dephosphorylation steps controlled by separate signal transduction pathways.

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.

Calcium spiking in plant root hairs responding to Rhizobium nodulation signals

SUMMARY

Rhizobium lipochitooligosaccharide signal molecules stimulate multiple responses in legume host plants, including changes in host gene expression, cell growth, and mitoses leading to root nodule development. The basis for signal transduction in the plant is not known. We examined cytoplasmic free calcium in host root hairs using calcium-sensitive reporter dyes. Image analysis of injected dyes revealed localized periodic spikes in cytoplasmic calcium levels that ensued after a characteristic lag following signal application. Structural features of the signal molecules required to cause nodulation responses in alfalfa are also essential for stimulating calcium spiking. A nonnodulating alfalfa mutant is defective in calcium spiking, consistent with the possibility that this mutant is blocked in an early stage of nodulation signal perception.

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.

Molecular genetics of cellular differentiation in leaves

Leaves of green plants vary widely in morphology. However, the underlying cell types and structures observed in leaves of different species are remarkably similar. Although we can adequately describe leaf development in morphological terms we cannot yet explain interactions at the cellular level. In recent years molecular genetics has been used extensively to address a variety of developmental questions. The isolation of a wide variety of mutants disrupted in numerous aspects of leaf ontogeny has led to the cloning of genes involved in various developmental processes. In this review we consider advances that have been made in understanding shoot apical meristem organization, leaf initiation and the development of leaf form. In particular we concentrate on progress, that has been made in understanding cellular differentiation in the epidermis, and within the interior of the leaf, namely the photosynthetic cells and the vasculature. CONTENTS Summary 533 I. Introduction 533 II. Shoot growth 533 III. Leaf initiation 534 IV. Development of leaf form 536 V. Cellular differentiation 537 VI. Perspectives 548 VII. Acknowledgements 549 VIII. References 549.

Guanosine triphosphate-binding proteins on the plasmalemma of spinach leaf cells

The molecular mechanism of light perception through phytochrome is not well understood. This red-light photosensor has been implicated in various physiological processes, including the photoinduction of flowering. A few recent studies have shown that phytochrome initiates signal transduction chains via guanosine triphosphate (GTP)-binding proteins (G-proteins). We show here by different approaches that G-proteins exist in spinach (Spinacia oleracea L. cv. Nobel). Binding of GTP on the plasmalemma has been partially characterized and its possible regulation by red light examined by in-vitro assays. These experiments indicate a clear regulation of GTP binding by red light and also by Mastoparan. At least three G-proteins or protein subunits were found to be associated with the plasmalemma of leaf cells. The use of an antibody raised against an animal Gβ subunit confirmed the presence of heterotrimeric G-proteins. Separation of a crude membrane extract by free-flow electrophoresis also showed that some G-proteins could exist on the tonoplast.

Phytochrome regulates phosphorylation of a protein with characteristics of a nucleoside diphosphate kinase in the crude membrane fraction from stem sections of etiolated pea seedlings

The molecular mechanism of light signal perception via phytochrome was analysed using the third internodes of etiolated pea seedlings irradiated with red or red followed by far-red light. A crude membrane fraction prepared from the tissue was labelled by [gamma-32P]ATP at 4 x 10(-8) M for 15 s at 0 degree C, and the proteins were separated by two-dimensional gel electrophoresis. The phosphorylation of a protein with a molecular mass of about 15 kDa in the crude membrane fraction increased with an increase in the intensity of red light irradiation (10, 50 and 100 mumol m-2 s-1) for 20 s. Successive irradiation with red light (100 mumol m-2 s-1 for 20 s) and subsequent irradiation with far-red light reduced the phosphorylation of the protein, depending on the intensity of the far-red light (from 0.1 to 8 mumol m-2 s-1 for 180 s). A plasma membrane purified from the crude membrane fraction from red light irradiated tissue showed a rapid phosphorylation of the 15 kDa protein by 4 x 10(-8) M [gamma-32P]ATP at 0 degree C for 7 s, and subsequent addition of ATP, GTP, ADP or GDP at 10(-5) or 10(-6) M efficiently removed the phosphoryl group of the 15 kDa protein. The 15 kDa protein was autophosphorylated in the gel following separation by sodium dodecylsulphate (SDS) polyacrylamide gel electrophoresis. The partially purified 15 kDa protein included nucleoside diphosphate kinase (NDP kinase) activity, as well as cross-reactivity with an antibody specific to rat NDP kinase as assayed by immunostaining and crossreactivity with an antibody specific to ricet NDP kinase as assayed by immunoprecipitation.

Leaf development and phytochrome modulate the activation ofpsbD-psbC transcription by high-fluence blue light in barley chloroplasts

Activation ofpsbD transcription by light assists in maintaining the synthesis of the PS II reaction center protein, D2, which is photodamaged in plants exposed to high light. In this study, the photosensory pathways and mechanisms that regulate the expression of thepsbD-psbC light-responsive promoter, LRP, were investigated during barley (Hordeum vulgare L.) seedling development. Accumulation ofpsbD-psbC mRNAs in response to light was observed in apical sections of primary leaves with little or no increase in mRNAs in basal sections. In both 4.5- and 7.5-day-old etiolated seedlings, blue light was most effective for activating mRNA accumulation from thepsbD-psbC LRP. However, the response of the LRP to red light increased 7-fold in 7.5-day relative to 4.5-day-old seedlings. Blue light preferentially activatedpsbD-psbC transcription, while red light was most effective for activating total plastid transcription and the expression of genes encoding the small (RbcS) and large (rbcL) subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase and Chl-a/b-binding protein (Lhcb). The stimulatory effects of red light onpsbD-psbC expression were partially reversed, and of blue light were not reversed, by subsequent pulses of far-red light. In contrast, continuous far-red light given together with blue light enhancedpsbD-psbC transcription in a synergistic manner. These observations indicate that phytochrome modulates the effects of high-fluence blue light onpsbD-psbC transcription by affecting total plastid transcription.

Transcription of tobacco phytochrome-A genes initiates at multiple start sites and requires multiple cis-acting regulatory elements

Promoter regions of the Nicotiana tabacum PHYA1 and PHYA2 genes display 89% sequence identity. Analysis of the 5' ends of both the PHYA1 and the PHYA2 transcript revealed multiple, distinct mRNA species, each differing in length and in abundance. The levels of the major PHYA1, PHYA2 transcripts were found to be auto-regulated by phytochrome. This auto-regulation was most efficient in 2-8-day old seedlings. Furthermore, we examined the expression pattern of the PHYA1-GUS reporter gene, containing a 4700 bp PHYA1 promoter fragment or its 5'-deletion derivatives, by GUS histochemistry and by RNase protection assays in transgenic tobacco plants. Our data indicate that the PHYA1 promoter contains three regions which are necessary for the maximum level and regulated expression. We show that a 264 bp promoter fragment contains a cis-regulatory element(s) responsible for expression in the root tips of transgenic seedlings. The major cis-regulatory elements required for high-level transcription and expression in other organs are located in separate regions of the PHYA1 promoter. These data indicate the contribution of multiple cis-regulatory elements for the maximum and regulated expression of tobacco genes coding for phytochrome A protein.

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.

Signal-transduction pathways controlling light-regulated development in Arabidopsis

All metazoan cells are able to make decisions about cell division or cellular differentiation based, in part, on environmental cues. Accordingly, cells express receptor systems that allow them to detect the presence of hormones, growth factors and other signals that manipulate the regulatory processes of the cell. In plants, an unusual signal-light-is required for the induction and regulation of many developmental processes. Past physiological and molecular studies have revealed the variety and complexity of plant responses to light but until recently very little was known about the mechanisms of those responses. Two major breakthroughs have allowed the identification of some photoreceptor signalling intermediates: the identification of photoreceptor and signal transduction mutants in Arabidopsis, and the development of single-cell microinjection assays in which outcomes of photoreceptor signalling can be visualized. Here, we review recent genetic advances which support the notion that light responses are not simply endpoints of linear signal transduction pathways, but are the result of the integration of a variety of input signals through a complex network of interacting signalling components.

1,4-Dihydropyridine binding sites in moss plasma membranes. Properties of receptors for a calcium channel antagonist

An increase in cytoplasmic calcium is an early event in hormone (cytokinin)-induced vegetative bud formation in the moss Physcomitrella patens. Whole cell and calcium transport studies have implicated 1,4-dihydropyridine-sensitive calcium channels in this increase in cellular calcium. To understand the molecular nature of the dihydropyridine-sensitive calcium channel, we have established conditions for the binding of the arylazide 1,4-dihydropyridine, [3H]azidopine, to its receptor in moss plasma membranes. [3H]Azidopine bound specifically in a saturable and reversible manner. The KD for [3H]azidopine binding was 5.2 nM and the Bmax was 35.6 pmol/mg of protein. Association and dissociation of the receptor and [3H]azidopine were temperature-dependent, and association varied as a function of pH. Binding was inhibited by dihydropyridine, phenylalkylamine, and benzothiazepine calcium channel blockers, bepridil, lanthanum, and N-ethylmaleimide. [3H]Azidopine binding was stimulated by cations including calcium, strontium, manganese, and barium. [3H]Azidopine binding was also stimulated by cytokinin with a Km value for kinetin of 0.13 nM. These studies utilize a simple plant system to provide a biochemical framework for understanding calcium regulation during development and have implications for understanding mechanisms of signal transduction in plants.

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.

Circadian oscillations of cytosolic and chloroplastic free calcium in plants

Tobacco and Arabidopsis plants, expressing a transgene for the calcium-sensitive luminescent protein apoaequorin, revealed circadian oscillations in free cytosolic calcium that can be phase-shifted by light-dark signals. When apoaequorin was targeted to the chloroplast, circadian chloroplast calcium rhythms were likewise observed after transfer of the seedlings to constant darkness. Circadian oscillations in free calcium concentrations can be expected to control many calcium-dependent enzymes and processes accounting for circadian outputs. Regulation of calcium flux is therefore fundamental to the organization of circadian systems.

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.

In vivo characterization of phytochrome-phycocyanobilin adducts in yeast

The in vivo reconstitution of phycocyanobilin with apophytochrome leads to photoreversible adducts in living yeast cells. Investigations with the rice phytochrome A phycocyanobilin adduct (PHYA*) and the tobacco phytochrome B phycocyanobilin adduct (PHYB*) show that the protein stability in yeast is independent of the form of the photoreceptor. After in vivo assembly and irradiation with red light, 25.6% of the far-red light-absorbing form of PHYB* exhibited dark reversion with a half-life time of approximately 20 min. Control experiments with PHYA* revealed no dark reversion. The data indicate that the molecular basis for this reaction is the formation of heterodimers between the red and the far-red light absorbing form of phytochrome. Electron microscopic in situ localizations and in vitro sequestering experiments showed that phytochrome A was able to sequester in yeast. On the electron microscopic level, the sequestered areas of phytochrome from etiolated plants and yeast are indistinguishable. The sequestering reaction in yeast is independent of the formation of the far-red light absorbing form of phytochrome. Therefore, we discuss a new model for this reaction in plants. Since it is unlikely that yeast cells contain elements that distinguish between phytochrome A and B, we conclude that sequestering and dark reversion reflect intrinsic properties of phytochrome.

Phytochromes: photosensory perception and signal transduction

The phytochrome family of photoreceptors monitors the light environment and dictates patterns of gene expression that enable the plant to optimize growth and development in accordance with prevailing conditions. The enduring challenge is to define the biochemical mechanism of phytochrome action and to dissect the signaling circuitry by which the photoreceptor molecules relay sensory information to the genes they regulate. Evidence indicates that individual phytochromes have specialized photosensory functions. The amino-terminal domain of the molecule determines this photosensory specificity, whereas a short segment in the carboxyl-terminal domain is critical for signal transfer to downstream components. Heterotrimeric GTP-binding proteins, calcium-calmodulin, cyclic guanosine 5'-phosphate, and the COP-DET-FUS class of master regulators are implicated as signaling intermediates in phototransduction.

Expression of tobacco genes for light-harvesting chlorophyll a/b binding proteins of photosystem II is controlled by two circadian oscillators in a developmentally regulated fashion

Light-induced expression of genes encoding the light-harvesting chlorophyll a/b binding proteins of photosystem II (Cab) was shown to be controlled by a circadian oscillator coupled to the red-light-absorbing plant photoreceptor phytochrome. Here we show that a red-light-insensitive oscillator is also involved in regulating the expression of the Cab genes. We provide evidence that germination leads, in a light-independent manner, to the setting and/or synchronization of endogenous oscillators and that it induces the expression of Cab genes in a circadian fashion. This circadian oscillator is not coupled to phytochrome, as it cannot be reset by red light for at least 44 h after sowing. Short red light pulses given between 12 and 44 h after sowing, however, induce new rhythms without perturbing the already free-running red-light-independent circadian oscillation. At this stage of development, the phytochrome-coupled and uncoupled circadian rhythms coexist. Both circadian rhythms are expressed and exhibit period lengths close to 24 h but are phased differently. At later stages of development (60 h or later after sowing), red light treatments synchronized these free-running rhythms and led to the appearance of a single new circadian oscillation. These data indicate that during early development the expression of single tobacco Cab genes, particularly expression of the Cab21 and Cab40 genes, is controlled in a developmentally dependent manner by two circadian oscillators.

Molecular cloning and characterization of RGA1 encoding a G protein alpha subunit from rice (Oryza sativa L. IR-36)

A cDNA clone, RGA1, was isolated by using a GPA1 cDNA clone of Arabidopsis thaliana G protein alpha subunit as a probe from a rice (Oryza sativa L. IR-36) seedling cDNA library from roots and leaves. Sequence analysis of genomic clone reveals that the RGA1 gene has 14 exons and 13 introns, and encodes a polypeptide of 380 amino acid residues with a calculated molecular weight of 44.5 kDa. The encoded protein exhibits a considerable degree of amino acid sequence similarity to all the other known G protein alpha subunits. A putative TATA sequence (ATATGA), a potential CAAT box sequence (AGCAATAC), and a cis-acting element, CCACGTGG (ABRE), known to be involved in ABA induction are found in the promoter region. The RGA1 protein contains all the consensus regions of G protein alpha subunits except the cysteine residue near the C-terminus for ADP-ribosylation by pertussis toxin. The RGA1 polypeptide expressed in Escherichia coli was, however, ADP-ribosylated by 10 microM [adenylate-32P] NAD and activated cholera toxin. Southern analysis indicates that there are no other genes similar to the RGA1 gene in the rice genome. Northern analysis reveals that the RGA1 mRNA is 1.85 kb long and expressed in vegetative tissues, including leaves and roots, and that its expression is regulated by light.

Sequence similarities of phytochrome to protein kinases: implication for the structure, function and evolution of the phytochrome gene family

Phytochrome, the best characterised plant photoreceptor, is encoded by a small multigene family within the plant kingdom. The different phytochrome types are composed of a conserved light-sensing chromophore domain of about 80 kDa and a less-conserved C-terminal domain of about 50 kDa. The C-terminus of phytochrome of the moss Ceratodon purpureus is homologous to the catalytic domain of eukaryotic serine/threonine or tyrosine protein kinases; in contrast, for all other phytochromes (conventional phytochromes) sequence similarities within the C-terminal domain to the catalytic domain of bacterial histidine kinases have been reported. We performed careful sequence comparisons of the putative catalytic domains of phytochrome with each other, with authentic serine/threonine, tyrosine and with histidine kinases. We report that conventional phytochromes exhibit structural elements of the catalytic domains of both histidine and, to a lesser extent, of serine/threonine and tyrosine kinases. The significance of these observations is discussed in the framework of the structure, function and evolution of phytochrome.

Reorganization of microfilaments in protonemal tip cells of the moss Ceratodon purpureus during the phototropic response

The F-actin distribution in caulonemal tip cells of the moss Ceratodon purpureus was examined by rhodamine-phalloidin staining. Gravitropically-growing caulonemal tip cells of the moss possess a distinct alignment of microfilaments (MFs) in their apices. Axially oriented actin bundles run from subapical regions to the apex where they converge towards a central area of the tip, although bundles are absent from the central area itself thus forming a collar-like structure. During a unilateral red light irradiation the actin strands of the apical dome become reoriented towards the irradiated apical flank and still surround an area free of MFs, the point of prospective outgrowth. This process is closely correlated with the morphological effect of bulging and precedes the light-directed outgrowth. The collar structure is essential for the tubular growth form. In darkness, under the influence of antimicrotubule agents the structure is decomposed, the actin strands drift along the cell flanks and finally accumulate in randomly distributed areas where further growth takes place. The microtubules (MTs) are not involved in the phytochrome-mediated reorientation of the microfilaments. Unilateral red light suppresses the distorting effect of antimicrotubule drugs and restores the collar structure with a pronounced light-directed orientation. Instead, the MTs seem to be responsible for restricting the reorientation to the cell tip. This notion is based on the observation that the small area in the apical dome, which is normally the exclusive location of the light-regulated MF rearrangement, extends towards the cell base when MT inhibitors are applied before the unilateral red light irradiation. This in turn leads to a non-tubular expansion of the light-directed cell flank.

Light inactivation of Arabidopsis photomorphogenic repressor COP1 involves a cell-specific regulation of its nucleocytoplasmic partitioning

Arabidopsis COP1 acts as a repressor of photomorphogenesis in darkness, and light stimuli abrogate this suppressive action. COP1, when fused to beta-glucuronidase (GUS), is enriched in the nucleus in darkness, but not in the light, in hypocotyl cells of Arabidopsis seedlings and epidermal cells of onion bulbs. In Arabidopsis hypocotyl cells, the nuclear GUS-COP1 level changes in response to dark-light transitions and quantitatively correlates with the extent of repression of photomorphogenic development. In root cells, GUS-COP1 is constitutively nuclear, consistent with an established role of COP1 in suppressing root chloroplast development in both light and darkness. We conclude that COP1 acts inside the nucleus to suppress photomorphogenesis and that light inactivation of COP1 involves a cell type-specific control of its nucleocytoplasmic partitioning.

5-Hydroxytryptamine affects turnover of polyphosphoinositides in maize and stimulates nitrate reductase in the absence of light

Incubation of etiolated maize leaves for 5 min in 5-hydroxytryptamine increased phosphatidylinositol-4,5-bisphosphate levels but on longer incubation its level decreased and a corresponding increase in inositol-trisphosphate was observed. The increase in phosphatidylinositol-4,5-bisphosphate by 5-hydroxytryptamine was similar to that obtained after short irradiation of leaves with red light. Nitrate-inducible and phytochrome-stimulated enzyme nitrate reductase could be stimulated in darkness if the leaves were incubated in the presence of nitrate and 5-hydroxytryptamine. These results indicate that one of the initial events in phytochrome-mediated enzyme stimulation could be through the generation of 'signals' from the turnover of the phosphoinositide cycle.

Translation of the psbA mRNA of Chlamydomonas reinhardtii requires a structured RNA element contained within the 5' untranslated region

Translational regulation is a key modulator of gene expression in chloroplasts of higher plants and algae. Genetic analysis has shown that translation of chloroplast mRNAs requires nuclear-encoded factors that interact with chloroplastic mRNAs in a message-specific manner. Using site-specific mutations of the chloroplastic psbA mRNA, we show that RNA elements contained within the 5' untranslated region of the mRNA are required for translation. One of these elements is a Shine-Dalgarno consensus sequence, which is necessary for ribosome association and psbA translation. A second element required for high levels of psbA translation is located adjacent to and upstream of the Shine-Dalgarno sequence, and maps to the location on the RNA previously identified as the site of message-specific protein binding. This second element appears to act as a translational attenuator that must be overcome to activate translation. Mutations that affect the secondary structure of these RNA elements greatly reduce the level of psbA translation, suggesting that secondary structure of these RNA elements plays a role in psbA translation. These data suggest a mechanism for translational activation of the chloroplast psbA mRNA in which an RNA element containing the ribosome-binding site is bound by message-specific RNA binding proteins allowing for increased ribosome association and translation initiation. These elements may be involved in the light-regulated translation of the psbA mRNA.

GTP-binding proteins in plants: new members of an old family

Regulatory guanine nucleotide-binding proteins (G proteins) have been studied extensively in animal and microbial organisms, and they are divided into the heterotrimeric and the small (monomeric) classes. Heterotrimeric G proteins are known to mediate signal responses in a variety of pathways in animals and simple eukaryotes, while small G proteins perform diverse functions including signal transduction, secretion, and regulation of cytoskeleton. In recent years, biochemical analyses have produced a large amount of information on the presence and possible functions of G proteins in plants. Further, molecular cloning has clearly demonstrated that plants have both heterotrimeric and small G proteins. Although the functions of the plant heterotrimeric G proteins are yet to be determined, expression analysis of an Arabidopsis G alpha protein suggests that it may be involved in the regulation of cell division and differentiation. In contrast to the very few genes cloned thus far that encode heterotrimeric G proteins in plants, a large number of small G proteins have been identified by molecular cloning from various plants. In addition, several plant small G proteins have been shown to be functional homologues of their counterparts in animals and yeasts. Future studies using a number of approaches are likely to yield insights into the role plant G proteins play.

Identification of potential regulatory elements in the far-upstream region of the Arabidopsis thaliana plastocyanin promoter

The far-upstream region of the Arabidopsis thaliana plastocyanin (Pc) promoter acts positively on transcription. This -1580 to -710 region (relative to the translation start site) has enhancer-like properties since it is also functional when situated downstream of the gene. Using tobacco nuclear extracts, this region was tested for protein-binding sites. Two short binding sequences were identified. The AT-rich sequence separating these binding sites shows extensive homology to the sequences separating the paired GT-1-binding sites of the pea rbcS-3A promoter. The requirements for complex formation strongly suggest that a GT-1-like protein binds to the two identified boxes in the Pc promoter. Sequence comparisons revealed that both boxes fit within the moderate consensus sequence needed for GT-1-binding. This GT-1-like DNA-binding activity is present in light-grown as well as in dark-adapted plants. Therefore, the possible role for GT-1 in light regulation of transcription does not depend upon its de novo synthesis. In some of the gel mobility shift assays, an additional DNA-protein complex was formed. The formation of this complex was only observed if the heteropolymer poly(dAdT).poly(dAdT) was used as a non-specific competitor and was dependent on the CpG density of the probe used.

Two distinct cis-acting elements are involved in light-dependent activation of the pea elip promoter

Light activation of the pea (Pisum sativum) elip gene promoter was analysed in transgenic plants and in transiently transfected plant protoplasts. A series of promoter deletions fused to the gusA reporter was tested, and the results obtained by the two experimental approaches were in good agreement. We identified two nucleotide sequence elements involved in light-regulated expression of the elip gene. One element is similar to the GT1 binding site of the rbcS-3A gene, and the other resembles a G-box-like ACGT element. The region containing both elements was able to confer light responsiveness on a heterologous basic promoter. Electrophoretic mobility shift assays demonstrated that each element is specifically recognized by DNA-binding proteins present in nuclear extracts from pea seedlings. The G-box-like ACGT element is necessary but not sufficient for light inducibility, indicating that the two elements act together in confering light responsiveness.

Transcription factors in plant growth and development

The target DNA sequences of several classes of plant transcription factors, including basic leucine zipper (bZIP) proteins and Myb-related factors, have been characterized in vivo as well as in vitro. The bZIP proteins, for example, act at ACGT elements, the flanking nucleotides determining their binding specificities. Overexpression, co-suppression, and antisense technology studies of factor genes in transgenic plants have uncovered the roles of bZIP, homeodomain, and MADS box factors in plant growth and development; for example, ectopic expression of pMADS1 alone in early Petunia development is sufficient for homeotic conversion of sepals into petaloid organs.

Photosensory perception and signal transduction in plants

Genetic and molecular studies are beginning to unravel the complexities of the signaling circuitry that plants use to sense and transduce information concerning the prevailing light environment. The past year has witnessed definition of discrete photosensory roles for phytochromes A and B, the cloning of a gene encoding the first apparent blue-light photoreceptor from any organism, the cloning of genes encoding additional members of the COP/DET/FUS class of light-responsive master regulators, and evidence that G proteins, Ca2+/calmodulin, and cGMP may be signaling intermediates in phototransduction.