Arabidopsis NPH1: a flavoprotein with the properties of a photoreceptor for phototropism

Calcium protects Trifolium repens L. seedlings against cadmium stress

The effect of calcium (Ca(2+)) on Trifolium repens L. seedlings subjected to cadmium (Cd(2+)) stress was studied by investigating plant growth and changes in activity of antioxidative enzymes. Physiological analysis was carried out on seedlings cultured for 2 weeks on half-strength Hoagland medium with Cd(2+) concentrations of 0, 400 and 600 microM, and on corresponding medium supplied with CaCl(2) (5 mM). Exposure to increasing Cd(2+) reduced the fresh weight of the upper part (stems + leaves) of the seedlings more strongly than that of the root system. In both parts of T. repens seedlings H(2)O(2) level and lipid peroxidation increased. In the upper part, Cd(2+) exposure led to a significant decrease in the activity of superoxide dismutase, catalase and glutathione peroxidase and an increase in ascorbate peroxidase activity. In contrast, the roots showed an increase in the activity of antioxidative enzymes under Cd(2+) stress. Ca(2+) addition to medium reduced the Cd(2+) accumulation, and considerably reversed the Cd(2+)-induced decrease in fresh mass as well as the changes in lipid peroxidation in the both parts of T. repens seedlings. Ca(2+) application diminished the Cd(2+) effect on the activity of antioxidative enzymes in the upper part, even though it did not significantly affect these enzymes in the roots. So the possible mechanisms for the action of Ca(2+) in Cd(2+) stress were considered to reduce Cd(2+) accumulation, alleviate lipid peroxidation and promote activity of antioxidative enzymes.

In vivo mutational analysis of YtvA from Bacillus subtilis: mechanism of light activation of the general stress response

The general stress response of Bacillus subtilis can be activated by stimuli such as the addition of salt or ethanol and with blue light. In the latter response, YtvA activates sigma(B) through a cascade of Rsb proteins, organized in stressosomes. YtvA is composed of an N-terminal LOV (light, oxygen, and voltage) domain and a C-terminal STAS (sulfate transporter and anti-sigma factor) domain and shows light-modulated GTP binding in vitro. Here, we examine the mechanism of YtvA-mediated activation of sigma(B) in vivo with site-directed mutagenesis. Constitutive off and constitutive on mutations have been identified. Disruption of GTP binding in the STAS domain eliminates light activation of sigma(B). In contrast, modification of sites relevant for phosphorylation of STAS domains does not affect the stress response significantly. The data obtained are integrated into a model for the structure of full-length YtvA, which presumably functions as a dimer.

Redox control of protein conformation in flavoproteins

Flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) are two flavin prosthetic groups utilized as the redox centers of various proteins. The conformations and chemical properties of these flavins can be affected by their redox states as well as by photoreactions. Thus, proteins containing flavin (flavoproteins) can function not only as redox enzymes, but also as signaling molecules by using the redox- and/or light-dependent changes of the flavin. Redox and light-dependent conformational changes of flavoproteins are critical to many biological signaling systems. In this review, we summarize the molecular mechanisms of the redox-dependent conformational changes of flavoproteins and discuss their relationship to signaling functions. The redox-dependent (or light-excited) changes of flavin and neighboring residues in proteins act as molecular "switches" that "turn on" various conformational changes in proteins, and can be classified into five types. On the basis of the present analysis, we recommend future directions in molecular structural research on flavoproteins and related proteins.

Light signal transduction pathway from flavin chromophore to the J alpha helix of Arabidopsis phototropin1

In the plant blue-light sensor phototropin, illumination of the chromophoric LOV domains causes activation of the serine/threonine kinase domain. Flavin mononucleotide (FMN) is a chromophore molecule in the two LOV domains (LOV1 and LOV2), but only LOV2 is responsible for kinase activation. Previous studies reported an important role of an additional helix connected to the C-terminal of LOV2 (Jalpha helix) for the function of phototropin; however, it remains unclear how the Jalpha helix affects light-induced structural changes in LOV2. In this study we compared light-induced protein structural changes of the LOV2 domain of Arabidopsis phot1 in the absence (LOV2-core) and presence (LOV2-Jalpha) of the Jalpha helix by Fourier-transform infrared spectroscopy. Prominent peaks were observed only in the amide-I region (1650 (-)/1625 (+) cm(-1)) of LOV2-Jalpha at physiological temperatures (>/=260 K), corresponding to structural perturbation of the alpha-helix. The peaks were diminished by point mutation of functionally important amino acids such as Phe-556 between FMN and the beta-sheet, Gln-575 being hydrogen-bonded with FMN, and Ile-608 on the Jalpha helix. We thus conclude that a light signal is relayed from FMN through these amino acids and eventually changes the interaction between LOV2-core and the Jalpha helix in Arabidopsis phot1.

UV-B action spectrum for UVR8-mediated HY5 transcript accumulation in Arabidopsis

Arabidopsis thaliana UV RESISTANCE LOCUS8 (UVR8) is a UV-B-specific signaling component that mediates low fluence, photomorphogenic responses to UV-B. It is required for UV-B-induced expression of the gene encoding the ELONGATED HYPOCOTYL5 (HY5) transcription factor. HY5 is a key effector of responses mediated by UVR8. In mature leaf tissue, HY5 transcript accumulation occurred rapidly in response to a brief UV-B treatment and no induction was observed in a uvr8 mutant over a broad range of UV wavelengths. In response to monochromatic light, maximal transcript accumulation occurred in wild-type plants at wavelengths 280-300 nm. HY5 transcript accumulation showed reciprocity between the fluence rate and duration of UV-B exposure, and on this basis conditions were chosen to generate an action spectrum for the UVR8 signaling pathway. Dose-response curves were produced for a range of UV wavelengths using 20 min exposure to UV and harvesting tissue 2 h after the start of illumination. Experiments using mutants defective in sinapate ester and flavonoid biosynthesis indicated that the presence of UV-absorbing compounds did not affect the construction of an action spectrum under the conditions employed. The action spectrum for the induction of HY5 by the UVR8 pathway showed a main peak at 280 nm with a smaller peak at 300 nm. The data are discussed in relation to the proposed mechanisms of UV-B photoreception.

Novel blue light-sensitive proteins from a metagenomic approach

A microarray-based approach was used to screen a soil metagenome for the presence of blue light (BL) photoreceptor-encoding genes. The microarray carried 149 different 54-mer oligonucleotides, derived from consensus sequences of light, oxygen and voltage (LOV) domain BL photoreceptor genes. Calibration of the microarrays allowed the detection of minimally 50 ng of genomic DNA against a background of 2-5 microg of genomic DNA. Identification of a positive cosmid clone was still possible for an amount of 0.25 ng against a background of 10 microg of labelled DNA clones. The array could readily identify targets carrying 4% sequence mismatch. Using the LOV microarray, up to 1200 library clones in concentrations of c. 20 ng each with a c. 40 kb insert size could be screened in a single batch. After calibration and reliability controls, the microarray was probed with cosmid-cloned DNA from the thermophilic fraction of a soil sample. From this approach, a novel gene was isolated that encodes a protein consisting of several Per-Arnt-Sim domains, a LOV domain associated to a histidine kinase and a response regulator domain. The novel gene showed highest similarity to a known sequence from Kineococcus radiotolerans SRS30216 (58% identity for the LOV domain only) and to a gene from Methylibium petroleiphilum PM1 (57% identity). The gene, designated as ht-met1 (Hamburg Thermophile Metagenome 1), was isolated and fully sequenced (3615 bp). ht-met1 is followed by a second open reading frame encoding a Fe-chelatase, an arrangement quite frequent for BL photoreceptors. The LOV domain region of ht-met1 was subcloned and expressed yielding a fully functional, flavin-containing LOV domain. Irradiation generated the typical LOV photochemistry, with the transient formation of a flavin-protein photoadduct. The dark recovery lifetime was found as tau(REC) = 120 s (20 degrees C) and is among the fastest ones determined so far for bacterial LOV domains.

Interaction specificity of Arabidopsis 14-3-3 proteins with phototropin receptor kinases

Phototropin receptor kinases play an important role in optimising plant growth in response to blue light. Much is known regarding their photochemical reactivity, yet little progress has been made to identify downstream signalling components. Here, we isolated several interacting proteins for Arabidopsis phototropin 1 (phot1) by yeast two-hybrid screening. These include members of the NPH3/RPT2 (NRL) protein family, proteins associated with vesicle trafficking, and the 14-3-3 lambda (lambda) isoform from Arabidopsis. 14-3-3lambda and phot1 were found to colocalise and interact in vivo. Moreover, 14-3-3 binding to phot1 was limited to non-epsilon 14-3-3 isoforms and was dependent on key sites of receptor autophosphorylation. No 14-3-3 binding was detected for Arabidopsis phot2, suggesting that 14-3-3 proteins are specific to phot1 signalling.

Understanding phototropism: from Darwin to today

Few individuals have had the lasting impact on such a breadth of science as Charles Darwin. While his writings about time aboard the HMS Beagle, his study of the Galapagos islands (geology, fauna, and flora), and his theories on evolution are well known, less appreciated are his studies on plant growth responses to a variety of environmental stimuli. In fact, Darwin, together with the help of his botanist son Francis, left us an entire book, 'The power of movements in plants', describing his many, varied, and insightful observations on this topic. Darwin's findings have provided an impetus for an entire field of study, the study of plant tropic responses, or differential growth (curvature) of plant organs in response to directional stimuli. One tropic response that has received a great deal of attention is the phototropic response, or curvature response to directional light. This review summarizes many of the most significant advancements that have been made in our understanding of this response and place these recent findings in the context of Darwin's initial observations.

Time-resolved Fourier transform infrared study on photoadduct formation and secondary structural changes within the phototropin LOV domain

Phototropins are plant blue-light photoreceptors containing two light-, oxygen-, or voltage-sensitive (LOV) domains and a C-terminal kinase domain. The two LOV domains bind noncovalently flavin mononucleotide as a chromophore. We investigated the photocycle of fast-recovery mutant LOV2-I403V from Arabidopsis phototropin 2 by step-scan Fourier transform infrared spectroscopy. The reaction of the triplet excited state of flavin with cysteine takes place with a time constant of 3 micros to yield the covalent adduct. Our data provide evidence that the flavin is unprotonated in the productive triplet state, disfavoring an ionic mechanism of bond formation. An intermediate adduct species was evident that displayed changes in secondary structure in the helix or loop region, and relaxed with a time constant of 120 micros. In milliseconds, the final adduct state is formed by further alterations of secondary structure, including beta-sheets. A comparison with wild-type adduct spectra shows that the mutation does not interfere with the functionality of the domain. All signals originate from within the LOV domain, because the construct does not comprise the adjacent Jalpha helix required for signal transduction. The contribution of early and late adduct intermediates to signal transfer to the Jalpha helix outside of the domain is discussed.

Hormonal interactions during root tropic growth: hydrotropism versus gravitropism

Terrestrial plants have evolved remarkable morphological plasticity that enables them to adapt to their surroundings. One of the most important traits that plants have acquired is the ability to sense environmental cues and use them as a basis for governing their growth orientation. The directional growth of plant organs relative to the direction of environmental stimuli is a tropism. The Cholodny-Went theory proposes that auxin plays a key role in several tropisms. Recent molecular genetic studies have strongly supported this hypothesis for gravitropism. However, the molecular mechanisms of other tropisms are far less clear. Hydrotropism is the response of roots to a moisture gradient. Since its re-discovery in 1985, root hydrotropism has been shown to be common among higher plant species. Additionally, in some species, gravitropism interferes with hydrotropism, suggesting that both shared and divergent mechanisms mediating the two tropisms exist. This hypothesis has been supported by recent studies, which provide an understanding of how roots sense multiple environmental cues and exhibit different tropic responses. In this review, we focus on the overlapping and unique mechanisms of the hormonal regulation underlying gravitropism and hydrotropism in roots.

A conserved glutamine plays a central role in LOV domain signal transmission and its duration

Light is a key stimulus for plant biological functions, several of which are controlled by light-activated kinases known as phototropins, a group of kinases that contain two light-sensing domains (LOV, light-oxygen-voltage domains) and a C-terminal serine/threonine kinase domain. The second sensory domain, LOV2, plays a key role in regulating kinase enzymatic activity via the photochemical formation of a covalent adduct between a LOV2 cysteine residue and an internally bound flavin mononucleotide (FMN) chromophore. Subsequent conformational changes in LOV2 lead to the unfolding of a peripheral Jalpha helix and, ultimately, phototropin kinase activation. To date, the mechanism coupling bond formation and helix dissociation has remained unclear. Previous studies found that a conserved glutamine residue [Q513 in the Avena sativa phototropin 1 LOV2 (AsLOV2) domain] switches its hydrogen bonding pattern with FMN upon light stimulation. Located in the immediate vicinity of the FMN binding site, this Gln residue is provided by the Ibeta strand that interacts with the Jalpha helix, suggesting a route for signal propagation from the core of the LOV domain to its peripheral Jalpha helix. To test whether Q513 plays a key role in tuning the photochemical and transduction properties of AsLOV2, we designed two point mutations, Q513L and Q513N, and monitored the effects on the chromophore and protein using a combination of UV-visible absorbance and circular dichroism spectroscopy, limited proteolysis, and solution NMR. The results show that these mutations significantly dampen the changes between the dark and lit state AsLOV2 structures, leaving the protein in a pseudodark state (Q513L) or a pseudolit state (Q513N). Further, both mutations changed the photochemical properties of this receptor, in particular the lifetime of the photoexcited signaling states. Together, these data establish that this residue plays a central role in both spectral tuning and signal propagation from the core of the LOV domain through the Ibeta strand to the peripheral Jalpha helix.

Oligomeric structure of LOV domains in Arabidopsis phototropin

Oligomeric structures of the four LOV domains in Arabidopsis phototropin1 (phot1) and 2 (phot2) were studied using crosslinking. Both LOV1 domains of phot1 and phot2 form a dimer independently on the light conditions, suggesting that the LOV1 domain can be a stable dimerization site of phot in vivo. In contrast, phot1-LOV2 is in a monomer-dimer equilibrium and phot2-LOV2 exists as a monomer in the dark. Blue light-induced a slight increase in the monomer population in phot1-LOV2, suggesting a possible blue light-inducible dissociation of dimers. Furthermore, blue light caused a band shift of the phot2-LOV2 monomer. CD spectra revealed the unfolding of helices and the formation of strand structures. Both light-induced changes were reversible in the dark.

Design and signaling mechanism of light-regulated histidine kinases

Signal transduction proteins are organized into sensor (input) domains that perceive a signal and, in response, regulate the biological activity of effector (output) domains. We reprogrammed the input signal specificity of a normally oxygen-sensitive, light-inert histidine kinase by replacing its chemosensor domain by a light-oxygen-voltage photosensor domain. Illumination of the resultant fusion kinase YF1 reduced net kinase activity by approximately 1000-fold in vitro. YF1 also controls gene expression in a light-dependent manner in vivo. Signals are transmitted from the light-oxygen-voltage sensor domain to the histidine kinase domain via a 40 degrees -60 degrees rotational movement within an alpha-helical coiled-coil linker; light is acting as a rotary switch. These signaling principles are broadly applicable to domains linked by alpha-helices and to chemo- and photosensors. Conserved sequence motifs guide the rational design of light-regulated variants of histidine kinases and other proteins.

Phytochrome A regulates the intracellular distribution of phototropin 1-green fluorescent protein in Arabidopsis thaliana

It has been known for decades that red light pretreatment has complex effects on subsequent phototropic sensitivity of etiolated seedlings. Here, we demonstrate that brief pulses of red light given 2 h prior to phototropic induction by low fluence rates of blue light prevent the blue light-induced loss of green fluorescent protein-tagged phototropin 1 (PHOT1-GFP) from the plasma membrane of cortical cells of transgenic seedlings of Arabidopsis thaliana expressing PHOT1-GFP in a phot1-5 null mutant background. This red light effect is mediated by phytochrome A and requires approximately 2 h in the dark at room temperature to go to completion. It is fully far red reversible and shows escape from photoreversibility following 30 min of subsequent darkness. Red light-induced inhibition of blue light-inducible changes in the subcellular distribution of PHOT1-GFP is only observed in rapidly elongating regions of the hypocotyl. It is absent in hook tissues and in mature cells below the elongation zone. We hypothesize that red light-induced retention of the PHOT1-GFP on the plasma membrane may account for the red light-induced increase in phototropic sensitivity to low fluence rates of blue light.

Perturbation of the ground-state electronic structure of FMN by the conserved cysteine in phototropin LOV2 domains

In LOV2, the blue-light sensitive domain of phototropin, the primary photophysical event involves intersystem crossing (ISC) from the singlet-excited state to the triplet state. The ISC rate is enhanced in LOV2 as compared to flavin mononucleotide (FMN) in solution, which likely results from a heavy-atom effect of a nearby conserved cysteine, C450. Here, we applied fluorescence line narrowing (FLN), resonance Raman (RR) and Fourier-transform infrared (FTIR) spectroscopy to investigate the electronic structure of FMN bound to Avena sativa LOV2 (AsLOV2), its C450A mutant and Adiantum LOV2 (Phy3LOV2). We demonstrate that FLN is the method of choice to obtain accurate vibrational spectra on highly fluorescent flavoproteins. The vibrational spectrum of AsLOV2-C450A showed small but significant shifts with respect to those of wild type AsLOV2 and Phy3LOV2, with a systematic down-shift of Ring I vibrations, upshifts of Ring II and III vibrations and an upshift of the C2=O mode. These trends are similar to those in FMN model systems with an electron-donating group substituted at Ring I, known to induce a quinoid character to the electronic structure of oxidized flavin. Thus, enhancement of the ISC rate in LOV2 is induced through weak electron donation by the cysteine which mixes the FMN pi-electrons with the heavy sulfur orbitals, manifesting itself in a quinoid character of the ground electronic state of oxidized FMN. The proximity of the cysteine to FMN thus not only enables formation of a covalent adduct between FMN and cysteine, but also facilitates the rapid electronic formation of the reactive FMN triplet state.

Autophagy regulated by day length determines the number of fertile florets in wheat

The wheat spikelet meristem differentiates into up to 12 floret primordia, but many of them fail to reach the fertile floret stage at anthesis. We combined microarray, biochemical and anatomical studies to investigate floret development in wheat plants grown in the field under short or long days (short days extended with low-fluence light) after all the spikelets had already differentiated. Long days accelerated spike and floret development and greening, and the expression of genes involved in photosynthesis, photoprotection and carbohydrate metabolism. These changes started while the spike was in the light-depleted environment created by the surrounding leaf sheaths. Cell division ceased in the tissues of distal florets, which interrupted their normal developmental progression and initiated autophagy, thus decreasing the number of fertile florets at anthesis. A massive decrease in the expression of genes involved in cell proliferation, a decrease in soluble carbohydrate levels, and an increase in the expression of genes involved in programmed cell death accompanied anatomical signs of cell death, and these effects were stronger under long days. We propose a model in which developmentally generated sugar starvation triggers floret autophagy, and long days intensify these processes due to the increased carbohydrate consumption caused by the accelerated plant development.

Evolution and phylogenetic utility of the PHOT gene duplicates in the Verbena complex (Verbenaceae): dramatic intron size variation and footprint of ancestral recombination

A well-resolved species level phylogeny is critically important in studying organismal evolution (e.g., hybridization, polyploidization, adaptive speciation). Lack of appropriate molecular markers that give sufficient resolution to gene trees is one of the major impediments to inferring species level phylogenies. In addition, sampling multiple independent loci is essential to overcome the lineage sorting problem. The availability of nuclear loci has often been a limiting factor in plant species-level phylogenetic studies. Here the two PHOT loci were developed as new sources of nuclear gene trees. The PHOT1 and PHOT2 gene trees of the Verbena complex (Verbenaceae) are well resolved and have good clade support. These gene trees are consistent with each other and previously generated chloroplast and nuclear waxy gene trees in most of the phylogenetic backbone as well as some terminal relationships, but are incongruent in some other relationships. Locus-specific primers were optimized for amplifying and sequencing these two loci in all Lamiales. Comparing intron size in the context of the gene trees shows dramatic variation within the Verbena complex, particularly at the PHOT1 locus. These variations are largely caused by invasions of short transposable elements and frequent long deletions and insertions of unknown causes. In addition, inspection of DNA sequences and phylogenetic analyses unmask a clear footprint of ancestral recombination in one species.

Genetic and physical interaction suggest that BARREN STALK 1 is a target of BARREN INFLORESCENCE2 in maize inflorescence development

Organogenesis in plants is controlled by polar auxin transport. In maize (Zea mays), barren inflorescence2 (bif2) encodes a co-ortholog of the serine/threonine protein kinase PINOID (PID), which regulates auxin transport in Arabidopsis. In this paper, we report that the basic helix-loop-helix transcription factor BARREN STALK1 (BA1) is a putative target of BIF2, revealing a previously unknown function of BIF2 in the nucleus. Both bif2 and ba1 are required for axillary meristem initiation during inflorescence and vegetative development in maize. Using a yeast two-hybrid approach, we identified BA1 as an interacting partner with BIF2. We confirmed the interaction by in vitro pull-down assays, and demonstrated that BIF2 phosphorylates BA1 in vitro. Previously, RNA in situ hybridization showed that bif2 and ba1 are both expressed during axillary meristem initiation. Here, we heterologously expressed BIF2 and BA1, and found that they co-localize in the nucleus. Based on these findings, we propose that in addition to regulating auxin transport at the cell periphery, BIF2 also functions in the nucleus by interacting with BA1 to promote axillary meristem initiation. Double mutant analysis is consistent with these results, showing that bif2 and ba1 have overlapping as well as unique roles in inflorescence development.

Human and Drosophila cryptochromes are light activated by flavin photoreduction in living cells

Cryptochromes are a class of flavoprotein blue-light signaling receptors found in plants, animals, and humans that control plant development and the entrainment of circadian rhythms. In plant cryptochromes, light activation is proposed to result from photoreduction of a protein-bound flavin chromophore through intramolecular electron transfer. However, although similar in structure to plant cryptochromes, the light-response mechanism of animal cryptochromes remains entirely unknown. To complicate matters further, there is currently a debate on whether mammalian cryptochromes respond to light at all or are instead activated by non–light-dependent mechanisms. To resolve these questions, we have expressed both human and Drosophila cryptochrome proteins to high levels in living Sf21 insect cells using a baculovirus-derived expression system. Intact cells are irradiated with blue light, and the resulting cryptochrome photoconversion is monitored by fluorescence and electron paramagnetic resonance spectroscopic techniques. We demonstrate that light induces a change in the redox state of flavin bound to the receptor in both human and Drosophila cryptochromes. Photoreduction from oxidized flavin and subsequent accumulation of a semiquinone intermediate signaling state occurs by a conserved mechanism that has been previously identified for plant cryptochromes. These results provide the first evidence of how animal-type cryptochromes are activated by light in living cells. Furthermore, human cryptochrome is also shown to undergo this light response. Therefore, human cryptochromes in exposed peripheral and/or visual tissues may have novel light-sensing roles that remain to be elucidated.

Vision in animals is generally associated with light-sensitive rhodopsin pigments located in the eyes. However, animals ranging from flies to humans also possess ancient visual receptors known as cryptochromes in multiple cell types. In this work, we study the mechanism of light sensing in two representative animal cryptochromes: a light-sensitive Drosophila cryptochrome (Dmcry) and a presumed light-insensitive mammalian cryptochrome from humans (Hscry1). We expressed recombinant cryptochromes to high levels in living cells, irradiated the cells with blue light, and analyzed the proteins' response to irradiation with electron paramagnetic resonance and fluorescence spectroscopic techniques. Photoreduction of protein-bound oxidized FAD cofactor to its radical form emerged as the primary cryptochrome photoreaction in living cells, and was correlated with a light-sensitive biological response in whole organisms. These results indicate that both Dmcry and Hscry1 are capable of undergoing similar light-driven reactions and suggest the possibility of an as-yet unknown photo-perception role for human cryptochromes in tissues exposed to light.

Cryptochromes are blue-light-absorbing receptors found in plants, animals, and humans. In mammals, they are not thought to respond to light, but this study demonstrates contrary evidence that indeed, human cryptochromes undergo a photochemical transformation in response to light.

Crystallization and preliminary X-ray diffraction analysis [correction of anaylsis] of the LOV1 domains of phototropin 1 and 2 from Arabidopsis thaliana

Phototropin is a blue-light receptor protein in plants that is responsible for phototropic responses, stomata opening and photo-induced relocation of chloroplasts. Higher plants such as Arabidopsis thaliana have two isoforms of phototropin: phototropin 1 and phototropin 2. Both isoforms comprise a tandem pair of blue-light-absorbing light-oxygen-voltage domains named LOV1 and LOV2 in the N-terminal half and a serine/threonine kinase domain in the C-terminal half. The LOV1 domain is thought to function as a dimerization site. In the present study, recombinant LOV1 domains of A. thaliana phototropin 1 and phototropin 2 were crystallized. The crystal of the LOV1 domain of phototropin 1 belonged to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 61.2, b = 64.9, c = 70.8 A, and diffracted X-rays to a resolution of 2.1 A. The crystal of the LOV1 domain of phototropin 2 belonged to space group P2(1), with unit-cell parameters a = 32.5, b = 66.5, c = 56.7 A, beta = 92.4 degrees , and diffracted X-rays to beyond 2.0 A resolution. In both crystals, two LOV1 domains occupied the crystallographic asymmetric unit.

Blue light-induced autophosphorylation of phototropin is a primary step for signaling

Phototropins are autophosphorylating protein kinases of plant-specific blue light receptors. They regulate various blue light responses, including phototropism, chloroplast movements, hypocotyl growth inhibition, leaf flattening, and stomatal opening. However, the physiological role of autophosphorylation remains unknown. Here, we identified phosphorylation sites of Ser or Thr in the N terminus, Hinge1 region, kinase domain, and C terminus in Arabidopsis phototropin1 (phot1) by liquid chromatography-tandem mass spectrometry in vivo. We substituted these Ser or Thr residues with Ala in phot1 and analyzed their functions by inspecting the phot1-mediated responses of stomatal opening, phototropism, chloroplast accumulation, and leaf flattening after the transformation of the phot1 phot2 double mutant. Among these sites, we found that autophosphorylation of Ser-851 in the activation loop of the kinase domain was required for the responses mentioned above, whereas the phosphorylation of the other Ser and Thr, except those in the activation loop, was not. Ser-849 in the loop may have an additional role in the responses. Immunological analysis revealed that Ser-851 was phosphorylated rapidly by blue light in a fluence-dependent manner and dephosphorylated gradually upon darkness. We conclude that autophosphorylation of Ser-851 is a primary step that mediates signaling between photochemical reaction and physiological events.

Hydrogen peroxide is involved in high blue light-induced chloroplast avoidance movements in Arabidopsis

One of the most important functions of blue light (BL) is to induce chloroplast movements in order to reduce the damage to the photosynthetic machinery under excess light. Hydrogen peroxide (H(2)O(2)), which is commonly generated under various environmental stimuli, can act as a signalling molecule that regulates a number of developmental processes and stress responses. To investigate whether H(2)O(2) is involved in high-fluence BL-induced chloroplast avoidance movements, a laser scanning confocal microscope and a luminescence spectrometer were used to observe H(2)O(2) generation in situ with the assistance of the fluorescence probe dichlorofluorescein diacetate (H(2)DCF-DA). After treatment with high-fluence BL, an enhanced accumulation of H(2)O(2), indicated by the fluorescence intensity of DCF, can be observed in leaf cells of Arabidopsis thaliana. Exogenously applied H(2)O(2) promotes the high-fluence BL-induced chloroplast movements in a concentration-dependent manner within the range of 0-10(-4) M, not only increasing the degree of movements but also accelerating the start of migrations. Moreover, the high-fluence BL-induced H(2)O(2) generation and the subsequent chloroplast movements can be largely abolished by the administration of the H(2)O(2)-specific scavenger catalase and other antioxidants. In addition, in-depth subcellular experiments indicated that high-fluence BL-induced H(2)O(2) generation can be partly abolished by the addition of diphenyleneiodonium (DPI), which is an NADPH oxidase inhibitor, and the blocker of electron transport chain dichlorophenyl dimethylurea (DCMU), respectively. The results presented here suggest that high-fluence BL can induce H(2)O(2) generation at both the plasma membrane and the chloroplast, and that the production of H(2)O(2) is involved in high-fluence BL-induced chloroplast avoidance movements.

Shade avoidance

The threat to plant survival presented by light limitation has driven the evolution of highly plastic adaptive strategies to either tolerate or avoid shading by neighbouring vegetation. When subject to vegetational shading, plants are exposed to a variety of informational signals, which include altered light quality and a reduction in light quantity. The former includes a decrease in the ratio of red to far-red wavelengths (low R : FR) and is detected by the phytochrome family of plant photoreceptors. Monitoring of R : FR ratio can provide an early and unambiguous warning of the presence of competing vegetation, thereby evoking escape responses before plants are actually shaded. The molecular mechanisms underlying physiological responses to alterations in light quality have now started to emerge, with major roles suggested for the PIF (PHYTOCHROME INTERACTING FACTOR) and DELLA families of transcriptional regulators. Such studies suggest a complex interplay between endogenous and exogenous signals, mediated by multiple photoreceptors. The phenotypic similarities between physiological responses habitually referred to as 'the shade avoidance syndrome' and other abiotic stress responses suggest plants may integrate common signalling mechanisms to respond to multiple perturbations in their natural environment.

In vivo phosphorylation site mapping and functional characterization of Arabidopsis phototropin 1

Phototropins (phot1 and phot2) are blue-light receptor kinases controlling a range of responses that optimize the photosynthetic efficiency of plants. Light sensing is mediated by two flavin-binding motifs, known as LOV1 and LOV2, located within the N-terminal region of the protein. Photoexcitation via LOV2 leads to activation of the C-terminal kinase domain and consequently receptor autophosphorylation. However, knowledge of the in-vivo phosphorylation sites for Arabidopsis phototropins is lacking and has impeded progress in elucidating the functional significance of receptor phosphorylation. We have purified phot1 from Arabidopsis and identified the in-vivo sites of receptor phosphorylation by liquid chromatography tandem mass spectrometry. Arabidopsis-derived phot1 binds flavin mononucleotide as chromophore and is phosphorylated at four major sites located upstream of LOV2 (Ser(58), Ser(85), Ser(350), and Ser(410)), three of which are induced by blue light. Nevertheless, structure-function analysis indicates that the biological activity of phot1 can be attributed to a modular unit comprising the LOV2-kinase region of the protein. Thus, peptide regions upstream of LOV2, including the sites of receptor phosphorylation identified here, do not appear to be important for receptor signaling. By contrast, these regions may be necessary for maximizing stomatal performance and possibly light-induced relocalization of phot1.

Intracellular distribution of phototropin 1 protein in the short-day plant Ipomoea nil

Phototropin 1 (phot1) is a blue-light Ser/Thr receptor kinase that contains two LOV domains. It is a plasma membrane-associated protein that mediates phototropism, blue-light induced chloroplast movement, and stomatal opening. The aim of the present work was to analyze the intracellular localization of phot1 protein in Ipomoea nil seedlings. In cotyledon and hypocotyl cells of etiolated seedlings, phot1 was specifically localized in the plasma membrane regions, whereas in light-treated seedlings, it was homogeneously distributed throughout the whole cytoplasm, excluding cell nuclei and vacuoles. Phot1 was also localized in cotyledon epidermal and guard cells. Such a localization pattern suggests a light-dependent intracellular distribution of phot1 in Ipomoea nil. On the basis of the spatial distribution, the possible role of phot1 is also discussed.

Blue light diminishes interaction of PAS/LOV proteins, putative blue light receptors in Arabidopsis thaliana, with their interacting partners

The light, oxygen, or voltage (LOV) domain that belongs to the Per-ARNT-Sim (PAS) domain superfamily is a blue light sensory module. The Arabidopsis thaliana PAS/LOV PROTEIN (PLP) gene encodes three putative blue light receptor proteins, PLPA, PLPB, and PLPC, because of its mRNA splicing variation. PLPA and PLPB each contain one PAS domain at the N-terminal region and one LOV domain at the C-terminal region, while the LOV domain is truncated in PLPC. RNA gel blot analysis showed that PLP mRNA was markedly expressed after exposure to salt or dehydration stress. Yeast two-hybrid screening led to the isolation of VITAMIN C DEFECTIVE 2 (VTC2), VTC2-LIKE (VTC2L), and BEL1-LIKE HOMEODOMAIN 10 proteins (BLH10A and BLH10B) as PLP-interacting proteins. The molecular interaction of PLPA with VTC2L, BLH10A or BLH10B, and that of PLPB with VTC2L were diminished when yeasts were grown under blue light illumination. Furthermore, the possible binding of flavin chromophore to PLPA and PLPB was demonstrated. These results imply that the LOV domain of PLPA and PLPB functions as a blue light sensor, and suggest the applicability of these interactions to blue light-dependent switching in transcriptional regulation in yeast or other organisms.

Decoding of light signals by plant phytochromes and their interacting proteins

Phytochromes are red/far-red light photoreceptors that convert the information contained in external light into biological signals. The decoding process starts with the perception of red light, which occurs through photoisomerization of a chromophore located within the phytochrome, leading to structural changes that include the disruption of intramolecular interactions between the N- and C-terminal domains of the phytochrome. This disruption exposes surfaces required for interactions with other proteins. In contrast, the perception of far-red light reverses the photoisomerization, restores the intramolecular interaction, and closes the interacting surfaces. Light information represented by the concentration of opened interacting surfaces is converted into biological signals through the modulating activity of interacting proteins. This review summarizes plant phytochromes, phytochrome-interacting proteins, and signal transmission from phytochromes to their interacting proteins.

Phototropin receptor kinase activation by blue light

Phototropins (phot1 and phot2) are blue light-activated serine/threonine protein kinases that function to mediate a variety of adaptive processes that serve to optimize the photosynthetic efficiency of plants and thereby promote their growth. Light sensing by the phototropins is mediated by a repeated motif located within the N-terminal region of the protein designated the LOV domain. Although phototropins possess two LOV photosensors (LOV1 and LOV2), recent biophysical and structure-function analyses clearly indicate that the LOV2 domain plays a predominant role in regulating phototropin kinase activity owing to specific protein changes that occur in response to LOV2 photoexcitation. In particular, the central beta-sheet scaffold plays a role in propagating the photochemical signal generated from within LOV2 to protein changes at the surface that are necessary for kinase activation.

Disruptions in AUX1-dependent auxin influx alter hypocotyl phototropism in Arabidopsis

Phototropism represents a differential growth response by which plant organs can respond adaptively to changes in the direction of incident light to optimize leaf/stem positioning for photosynthetic light capture and root growth orientation for water/nutrient acquisition. Studies over the past few years have identified a number of components in the signaling pathway(s) leading to development of phototropic curvatures in hypocotyls. These include the phototropin photoreceptors (phot1 and phot2) that perceive directional blue-light (BL) cues and then stimulate signaling, leading to relocalization of the plant hormone auxin, as well as the auxin response factor NPH4/ARF7 that responds to changes in local auxin concentrations to directly mediate expression of genes likely encoding proteins necessary for development of phototropic curvatures. While null mutations in NPH4/ARF7 condition an aphototropic response to unidirectional BL, seedlings carrying the same mutations recover BL-dependent phototropic responsiveness if co-irradiated with red light (RL) or pre-treated with either ethylene. In the present study, we identify second-site enhancer mutations in the nph4 background that abrogate these recovery responses. One of these mutations--map1 (modifier of arf7 phenotypes 1)--was found to represent a missense allele of AUX1--a gene encoding a high-affinity auxin influx carrier previously associated with a number of root responses. Pharmacological studies and analyses of additional aux1 mutants confirmed that AUX1 functions as a modulator of hypocotyl phototropism. Moreover, we have found that the strength of dependence of hypocotyl phototropism on AUX1-mediated auxin influx is directly related to the auxin responsiveness of the seedling in question.

AUREOCHROME, a photoreceptor required for photomorphogenesis in stramenopiles

A blue light (BL) receptor was discovered in stramenopile algae Vaucheria frigida (Xanthophyceae) and Fucus distichus (Phaeophyceae). Two homologs were identified in Vaucheria; each has one basic region/leucine zipper (bZIP) domain and one light-oxygen-voltage (LOV)-sensing domain. We named these chromoproteins AUREOCHROMEs (AUREO1 and AUREO2). AUREO1 binds flavin mononucleotide via its LOV domain and forms a 390-nm-absorbing form, indicative of formation of a cysteinyl adduct to the C(4a) carbon of the flavin mononucleotide upon BL irradiation. The adduct decays to the ground state in approximately 5 min. Its bZIP domain binds the target sequence TGACGT. The AUREO1 target binding was strongly enhanced by BL treatment, implying that AUREO1 functions as a BL-regulated transcription factor. The function of AUREO1 as photoreceptor for BL-induced branching is elucidated through RNAi experiments. RNAi of AUREO2 unexpectedly induces sex organ primordia instead of branches, implicating AUREO2 as a subswitch to initiate development of a branch, but not a sex organ. AUREO sequences are also found in the genome of the marine diatom Thalassiosira pseudonana (Bacillariophyceae), but are not present in green plants. AUREOCHROME therefore represents a BL receptor in photosynthetic stramenopiles.

NPY1, a BTB-NPH3-like protein, plays a critical role in auxin-regulated organogenesis in Arabidopsis

Auxin is an essential regulator for plant development. To elucidate the mechanisms by which auxin regulates plant development, we isolated an Arabidopsis mutant naked pins in yuc mutants 1 (npy1) that develops pin-like inflorescences and fails to initiate any flowers in yuc1 yuc4, a background that is defective in auxin biosynthesis. The phenotypes of npy1 yuc1 yuc4 triple mutants closely resemble those of Arabidopsis mutants pin-formed1 (pin1), pinoid (pid), and monopteros (mp), which are defective in either auxin transport or auxin signaling. NPY1 belongs to a large family of proteins and is homologous to NON-PHOTOTROPIC HYPOCOTYL 3 (NPH3), a BTB/POZ protein that regulates phototropic responses along with the protein kinase PHOT1 (Phototropin 1). We demonstrate that NPY1 works with the protein kinase PID, which is homologous to PHOT1, to regulate auxin-mediated plant development. The npy1 pid double mutants fail to form any cotyledons, a phenotype that is also observed in yuc1 yuc4 pid triple mutants. Interestingly, both auxin-regulated organogenesis and phototropic responses require an auxin response factor (ARF). Disruption of ARF7/NPH4 leads to nonphototropic hypocotyls and arf5/mp forms pin-like inflorescences. Whereas the PHOT1/NPH3 pathway is regulated by light, our data suggest that the PID/NPY1 pathway may be regulated by auxin synthesized by the YUC flavin monooxygenases. Our findings put YUCs, PID, and NPY1 into a genetic framework for further dissecting the mechanisms of auxin action in plant development.

Molecular structure and regulation of phototropin kinase by blue light

Phototropin (phot) is a blue light photoreceptor in plants that mediates phototropism, chloroplast movement, stomata opening and leaf expansion. The phot molecule has two photoreceptive domains, LOV 1 and 2, in the N-terminal half and the C-terminal half forms Ser/Thr kinase. Phot acts as a blue light-regulated protein kinase. Each LOV domain binds a FMN and undergoes a unique cyclic reaction upon blue light absorption that induces conformational changes in the protein moiety and leads to regulation of the kinase activity, in which LOV2 plays a predominant role in the switching and LOV1 acts to attenuate the light sensitivity. Phot kinase is classified into the AGC kinase group since the consensus amino acid residues and the motifs are well conserved except for the lack of the hydrophobic motif and the presence of additional amino acid sequence in the activation loop. Secondary structure prediction and 3D structure simulation show a alpha/beta fold of the phot kinase similar to that of the catalytic subunit of PKA. The additional sequence forms an extra helix and loops. Docking simulation of the LOV2 domain with phot kinase provided useful information regarding the molecular mechanism underlying the photoregulation of phot kinase.

The C-terminal kinase fragment of Arabidopsis phototropin 2 triggers constitutive phototropin responses

Phototropins mediate various blue-light responses such as phototropism, chloroplast relocation, stomatal opening and leaf flattening in plants. Phototropins are hydrophilic chromoproteins that are mainly bound to the plasma membrane. One of two phototropins in Arabidopsis thaliana, phot2, associates with the Golgi apparatus in a light-dependent manner. In this study, we analyzed the biological activities of the N-terminal photosensory and C-terminal kinase domains of phot2. For this purpose, these domains were fused to green fluorescent protein (GFP) and ectopically expressed in the wild-type and a phot1 phot2 double mutant of Arabidopsis. The kinase domain fused to GFP (P2CG) was localized to the plasma membrane and the Golgi apparatus, whereas the photosensory domain fused to GFP (P2NG) was uniformly localized in the cytosol. Hence, the kinase domain rather than the photosensory domain is responsible for the membrane association. Interestingly, the P2CG plants exhibited constitutive blue-light responses even in dark conditions, i.e. stomata were open and chloroplasts were in the avoidance position. By contrast, P2CG with a mutation that abolishes the kinase activity (P2C[D720/N]G) failed to exhibit these responses. phot2 kinase is therefore suggested to be correctly localized to functional sites in the cell and to trigger light signal transduction through its kinase activity. In contrast to P2CG, P2NG did not affect the phot2 responses, except for partial inhibition of the phototropic response caused by the endogenous phototropins.

ZEITLUPE is a circadian photoreceptor stabilized by GIGANTEA in blue light

The circadian clock is essential for coordinating the proper phasing of many important cellular processes. Robust cycling of key clock elements is required to maintain strong circadian oscillations of these clock-controlled outputs. Rhythmic expression of the Arabidopsis thaliana F-box protein ZEITLUPE (ZTL) is necessary to sustain a normal circadian period by controlling the proteasome-dependent degradation of a central clock protein, TIMING OF CAB EXPRESSION 1 (TOC1). ZTL messenger RNA is constitutively expressed, but ZTL protein levels oscillate with a threefold change in amplitude through an unknown mechanism. Here we show that GIGANTEA (GI) is essential to establish and sustain oscillations of ZTL by a direct protein-protein interaction. GI, a large plant-specific protein with a previously undefined molecular role, stabilizes ZTL in vivo. Furthermore, the ZTL-GI interaction is strongly and specifically enhanced by blue light, through the amino-terminal flavin-binding LIGHT, OXYGEN OR VOLTAGE (LOV) domain of ZTL. Mutations within this domain greatly diminish ZTL-GI interactions, leading to strongly reduced ZTL levels. Notably, a C82A mutation in the LOV domain, implicated in the flavin-dependent photochemistry, eliminates blue-light-enhanced binding of GI to ZTL. These data establish ZTL as a blue-light photoreceptor, which facilitates its own stability through a blue-light-enhanced GI interaction. Because the regulation of GI transcription is clock-controlled, consequent GI protein cycling confers a post-translational rhythm on ZTL protein. This mechanism of establishing and sustaining robust oscillations of ZTL results in the high-amplitude TOC1 rhythms necessary for proper clock function.

Visible light regulates neurite outgrowth of nerve cells

The neurite outgrowth of PC12 cells on collagen-coated glass plates under light emitting diode (LED) irradiation at several wavelengths (i.e., 455, 470, 525, 600, 630, 880 and 945 nm) was investigated. No neurite outgrowth was observed during cultivation under irradiation from the lamp of an inverted light microscope through filters (yielding mixed light at ca. 525 nm and more than 800 nm), whereas neurite outgrowth was observed during cultivation in the dark. When these cells were irradiated with monochromatic LED light, neurite outgrowth was slightly, but not completely, suppressed at 455, 525, 600, 630, 880 and 945 nm, as was observed in the case of mixed light. Long connected neuronal outgrowths (e.g., 3 mm length) were observed with LED light at 470 nm and 1.8 mW/cm(2) intensity. No such outgrowths were observed at other LED light wavelengths (i.e., 455, 525, 600, 630, 880 and 945 nm). Irradiation at 470 nm may have caused specific responses to transductional signals in these cells that led to the connection of neuronal outgrowths between cells. Not only suppressed neurite outgrowth but also long connected neurite outgrowths were observed when PC12 cells were cultured under several different wavelengths of light.

Structural basis for light-dependent signaling in the dimeric LOV domain of the photosensor YtvA

The photosensor YtvA binds flavin mononucleotide and regulates the general stress reaction in Bacillus subtilis in response to blue light illumination. It belongs to the family of light-oxygen-voltage (LOV) proteins that were first described in plant phototropins and form a subgroup of the Per-Arnt-Sim (PAS) superfamily. Here, we report the three-dimensional structure of the LOV domain of YtvA in its dark and light states. The protein assumes the global fold common to all PAS domains and dimerizes via a hydrophobic interface. Directly C-terminal to the core of the LOV domain, an alpha-helix extends into the solvent. Light absorption causes formation of a covalent bond between a conserved cysteine residue and atom C(4a) of the FMN ring, which triggers rearrangements throughout the LOV domain. Concomitantly, in the dark and light structures, the two subunits of the dimeric protein rotate relative to each other by 5 degrees . This small quaternary structural change is presumably a component of the mechanism by which the activity of YtvA is regulated in response to light. In terms of both structure and signaling mechanism, YtvA differs from plant phototropins and more closely resembles prokaryotic heme-binding PAS domains.

Photochemical Intermediates of Arabidopsis Phototropin 2 LOV Domains Associated with Conformational Changes

The photochemical reactions of Arabidopsis phototropin 2 light- oxygen-voltage domain 2 (LOV2) with the linker region (LOV2-linker), without the linker (LOV2), and LOV1 were studied using the time-resolved transient grating (TG) and transient lens (TrL) methods. Although the absorption spectra did not change after the formation of the adduct species, a small volume expansion process with a time constant of 9 ms was observed for LOV2. For the LOV2-linker, at 293 K, a volume contraction process with a time constant of 140 mus was observed in addition to a volume expansion process with 9 ms and the diffusion coefficient change with 2 ms. The reaction intermediate species were characterized on the basis of their thermodynamic properties, such as changes in enthalpy, thermal expansion, and heat capacity. For the first intermediate (S(390)), the values of these properties were similar to those of the ground state for both LOV2 and LOV2-linker. A relatively large thermal expansion volume (0.09 cm(3)mol(-1)K(-1)) and a positive heat capacity change (4.7 kJ mol(-1)K(-1)) were detected for the intermediates of LOV2-linker. These characteristic features were interpreted in terms of structural fluctuation and exposure of hydrophobic residues in the linker domain, respectively. The enthalpy change of S(390) of the LOV1 domain was significantly greater than changes for the LOV2 or LOV2-linker samples. Data from this study support a major conformational change of the linker region in the photochemical reaction of phototropin.

Primary processes during the light-signal transduction of phototropin

Phototropin is a blue-light photoreceptor in plants that mediates phototropism, chloroplast relocation, stomata opening and leaf expansion. Phototropin molecule has two photoreceptive domains named LOV1 (light-oxygen-voltage) and LOV2 in the N-terminus and a serine/threonine kinase domain in the C-terminus, and acts as a blue light-regulated kinase. Each LOV domain binds a flavin mononucleotide as a chromophore and undergoes unique cyclic reactions upon blue-light absorption that comprises a cysteinyl-flavin adduct formation through a triplet-excited state and a successive adduct break to revert to the initial ground state. The molecular reactions underlying the photocycle are reviewed and one of the probable molecular schemes is presented. Adduct formation alters the secondary protein structure of the LOV domains. This structural change could be transferred to the linker between the kinase domain and involved in the photoregulation of the kinase activity. The structural changes as well as the oligomeric structures seem to differ between LOV1 and LOV2, which may explain the proposed roles of each domain in the photoregulation of the kinase activity. The photoregulation mechanism of phototropin kinase is reviewed and discussed in reference to the regulation mechanism of protein kinase A, which it resembles.

Regulation of Phototropic Signaling in Arabidopsis via Phosphorylation State Changes in the Phototropin 1-interacting Protein NPH3

Phototropism, or the directional growth (curvature) of various organs toward or away from incident light, represents a ubiquitous adaptive response within the plant kingdom. This response is initiated through the sensing of directional blue light (BL) by a small family of photoreceptors known as the phototropins. Of the two phototropins present in the model plant Arabidopsis thaliana, phot1 (phototropin 1) is the dominant receptor controlling phototropism. Absorption of BL by the sensory portion of phot1 leads, as in other plant phototropins, to activation of a C-terminal serine/threonine protein kinase domain, which is tightly coupled with phototropic responsiveness. Of the five phot1-interacting proteins identified to date, only one, NPH3 (non-phototropic hypocotyl 3), is essential for all phot1-dependent phototropic responses, yet little is known about how phot1 signals through NPH3. Here, we show that, in dark-grown seedlings, NPH3 exists as a phosphorylated protein and that BL stimulates its dephosphorylation. phot1 is necessary for this response and appears to regulate the activity of a type 1 protein phosphatase that catalyzes the reaction. The abrogation of both BL-dependent dephosphorylation of NPH3 and development of phototropic curvatures by protein phosphatase inhibitors further suggests that this post-translational modification represents a crucial event in phot1-dependent phototropism. Given that NPH3 may represent a core component of a CUL3-based ubiquitin-protein ligase (E3), we hypothesize that the phosphorylation state of NPH3 determines the functional status of such an E3 and that differential regulation of this E3 is required for normal phototropic responsiveness.

Nitric oxide inhibits blue light-specific stomatal opening via abscisic acid signaling pathways in Vicia guard cells

Recent evidence suggests that nitric oxide (NO) acts as an intermediate of ABA signal transduction for stomatal closure. However, NO's effect on stomatal opening is poorly understood even though both opening and closing activities determine stomatal aperture. Here we show that NO inhibits stomatal opening specific to blue light, thereby stimulating stomatal closure. NO inhibited blue light-specific stomatal opening but not red light-induced opening. NO inhibited both blue light-induced H(+) pumping and H(+)-ATPase phosphorylation. The NO scavenger 2-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO) restored all these inhibitory effects. ABA and hydrogen peroxide (H(2)O(2)) inhibited all of these blue light-specific responses in a manner similar to NO. c-PTIO partially restored the ABA-induced inhibition of all of these opening responses but did not restore inhibition of the responses by H(2)O(2). ABA, H(2)O(2) and NO had slight inhibitory effects on the phosphorylation of phototropins, which are blue light receptors in guard cells. NO inhibited neither fusicoccin-induced H(+) pumping in guard cells nor H(+) transport by H(+)-ATPase in the isolated membranes. From these results, we conclude that both NO and H(2)O(2) inhibit blue light-induced activation of H(+)-ATPase by inhibiting the component(s) between phototropins and H(+)-ATPase in guard cells and stimulate stomatal closure by ABA.