Stimulation of the blue light phototropic receptor NPH1 causes a transient increase in cytosolic Ca2+

SlCPK27 cross-links SlHY5 and SlPIF4 in brassinosteroid-dependent photo- and thermo-morphogenesis in tomato

ELONGATED HYPOCOTOYL5 (HY5) and PHYTOCHROME INTERACTING FACTORs (PIFs) are two types of important light-related regulators of plant growth, however, their interplay remains elusive. Here, we report that the activated tomato (Solanum lycopersicum) HY5 (SlHY5) triggers the transcription of a Calcium-dependent Protein Kinase SlCPK27. SlCPK27 interacts with and phosphorylates SlPIF4 at Ser-252 and Ser-308 phosphosites to promote its degradation. SlPIF4 promotes hypocotyl elongation mainly by activating the transcription of SlDWF, a key gene in brassinosteroid (BR) biosynthesis. Such a SlHY5-SlCPK27-SlPIF4-BR cascade not only plays a crucial role in photomorphogenesis but also regulates thermomorphogenesis. Our results uncover a previously unidentified mechanism that integrates Ca2+ signaling with the light signaling pathways to regulate plant growth by modulating BR biosynthesis in response to changes in ambient light and temperature.

Kinase CPK10 regulates low light-induced tomato flower drop downstream of IDL6 in a calcium-dependent manner

Flower drop is a major cause for yield loss in many crops. Previously, we found that tomato (Solanum lycopersicum) INFLORESCENCE DEFICIENT IN ABSCISSION-Like (SlIDL6) contributes to flower drop induced by low light. However, the molecular mechanisms by which SlIDL6 acts as a signal to regulate low light-induced abscission remain unclear. In this study, SlIDL6 was found to elevate cytosolic Ca2+ concentrations ([Ca2+]cyt) in the abscission zone (AZ), which was required for SlIDL6-induced flower drop under low light. We further identified that one calcium-dependent protein kinase gene (SlCPK10) was highly expressed in the AZ and up-regulated by SlIDL6-triggered [Ca2+]cyt. Over-expression and knockout of SlCPK10 in tomato resulted in accelerated and delayed abscission, respectively. Genetic evidence further indicated that knockout of SlCPK10 significantly impaired the function of SlIDL6 in accelerating abscission. Furthermore, Ser-371 phosphorylation in SlCPK10 dependent on SlIDL6 was necessary and sufficient for its function in regulating flower drop, probably by stabilizing the SlCPK10 proteins. Taken together, our findings reveal that SlCPK10, as a downstream component of the IDL6 signaling pathway, regulates flower drop in tomato under low light stress.

Genome-Wide Identification and Expression Analysis of BraGLRs Reveal Their Potential Roles in Abiotic Stress Tolerance and Sexual Reproduction

Glutamate receptors (GLRs) are involved in multiple functions during the plant life cycle through affecting the Ca²⁺ concentration. However, GLRs in Brassica species have not yet been reported. In this study, 16 glutamate receptor-like channels (GLR) belonged to two groups were identified in the Brassica rapa (B. rapa) genome by bioinformatic analysis. Most members contain domains of ANF_receptor, Peripla_BP_6, Lig_chan, SBP_bac_3, and Lig_chan_Glu_bd that are closely related to glutamate receptor channels. This gene family contains many elements associated with drought stress, low temperature stress, methyl jasmonate (MeJA), salicylic acid (SA), and other stress resistance. Gene expression profiles showed that BraGLR genes were expressed in roots, stems, leaves, flowers, and siliques. BraGLR5 expression was elevated after drought stress in drought-sensitive plants. BraGLR1, BraGLR8, and BraGLR11 expression were significantly upregulated after salt stress. BraGLR3 expression is higher in the female sterile-line mutants than in the wild type. The expression levels of BraGLR6, BraGLR9, BraGLR12, and BraGLR13 were significantly higher in the male sterile-line mutants than in the wild type. The expression of most BraGLRs increased after self-pollination, with BraGLR9 exhibiting the greatest increase. These results suggest that BraGLRs play an important role in abiotic stress tolerance and sexual reproduction.

Application of Green-enhanced Nano-lantern as a bioluminescent ratiometric indicator for measurement of Arabidopsis thaliana root apoplastic fluid pH

Plant root absorbs water and nutrients from the soil, and the root apoplastic fluid (AF) is an important intermediate between cells and the surrounding environment. The acid growth theory suggests that an acidic AF is needed for cell wall expansion during root growth. However, technical limitations have precluded the quantification of root apoplastic fluid pH (AF-pH). Here, we used Green-enhanced Nano-lantern (GeNL), a chimeric protein of the luciferase NanoLuc (Nluc) and the green fluorescent protein mNeonGreen (mNG), as a ratiometric pH indicator based on the pH dependency of bioluminescence resonance energy transfer efficiency from Nluc to mNG. Luminescence spectrum of GeNL changed reciprocally from pH 4.5 to 7.5, with a pK_a of 5.5. By fusing GeNL to a novel signal peptide from Arabidopsis thaliana Cellulase 1, we localised GeNL in A. thaliana AF. We visualised AF dynamics at subcellular resolution over 30 min and determined flow velocity in the maturation zone to be 0.97± 0.06 μm/s. We confirmed that the developing root AF is acidic in the pH range of 5.1-5.7, suggesting that the AF-pH is tightly regulated during root elongation. These results support the acid growth theory and provide evidence for AF-pH maintenance despite changes in ambient pH.

Calcium-dependent protein kinase 16 phosphorylates and activates the aquaporin PIP2;2 to regulate reversible flower opening in Gentiana scabra

Flower opening is important for successful pollination in many plant species, and some species repeatedly open and close their flowers. This is thought to be due to turgor pressure changes caused by water influx/efflux, which depends on osmotic oscillations in the cells. In some ornamental plants, water-transporting aquaporins, also known as plasma membrane intrinsic proteins (PIPs), may play an important role in flower opening. However, the molecular mechanism(s) involved in corolla movement are largely unknown. Gentian (Gentiana spp.) flowers undergo reversible movement in response to temperature and light stimuli; using gentian as a model, we showed that the Gentiana scabra aquaporins GsPIP2;2 and GsPIP2;7 regulate repeated flower opening. In particular, phosphorylation of a C-terminal serine residue of GsPIP2;2 is important for its transport activity and relates closely to the flower re-opening rate. Furthermore, GsPIP2;2 is phosphorylated and activated by the calcium (Ca2+)-dependent protein kinase GsCPK16, which is activated by elevated cytosolic Ca2+ levels in response to temperature and light stimuli. We propose that GsCPK16-dependent phosphorylation and activation of GsPIP2;2 regulate gentian flower re-opening, with stimulus-induced Ca2+ signals acting as triggers.

Can Light Spectrum Composition Increase Growth and Nutritional Quality of Linum usitatissimum L. Sprouts and Microgreens?

Flaxseed could be suitable for obtaining high-quality sprouts and microgreens thanks to high amounts of nutrients and antioxidant, antidiabetic, and anticancer compound content in its seeds. Recent studies highlighted that seedling growth, nutritional compound, and secondary metabolite content can be strongly managed by regulation of the light spectrum used during germination. The present study intended to shed light on flaxseed as emerging and novel species for sprouts and microgreens and to evaluate the effect of light, with different spectrum compositions (100% blue, 100% red, 100% green, and red:green:blue—1:1:1) on the performance of flax microgreens and sprouts grown indoors under controlled conditions. Microgreens showed, compared to sprouts, a higher chlorophyll (+62.6%), carotenoid (+24.4%), and phenol content (+37.8%), antioxidant capacity (+25.1%) and a lower dry matter content (−30.7%). Besides, microgreens treated with 100% blue light were characterized by the highest content of flavonoids (2.48 mg CAE g−1 FW), total phenols (3.76 mg GAE g−1 FW), chlorogenic acid (1.10 mg g−1 FW), and antioxidant capacity (8.06 µmol TEAC g−1 FW). The paper demonstrates the feasibility of obtaining flax sprouts and microgreens indoors with a considerable antioxidant capacity and health-promoting compounds by modulating the light spectrum.

Electrical Signaling of Plants under Abiotic Stressors: Transmission of Stimulus-Specific Information

Plants have developed complex systems of perception and signaling to adapt to changing environmental conditions. Electrical signaling is one of the most promising candidates for the regulatory mechanisms of the systemic functional response under the local action of various stimuli. Long-distance electrical signals of plants, such as action potential (AP), variation potential (VP), and systemic potential (SP), show specificities to types of inducing stimuli. The systemic response induced by a long-distance electrical signal, representing a change in the activity of a complex of molecular-physiological processes, includes a nonspecific component and a stimulus-specific component. This review discusses possible mechanisms for transmitting information about the nature of the stimulus and the formation of a specific systemic response with the participation of electrical signals induced by various abiotic factors.

Ca2+talyzing Initial Responses to Environmental Stresses

Plants have evolved stress-sensing machineries that initiate rapid adaptive environmental stress responses. Cytosolic calcium ion (Ca²⁺) is the most prominent second messenger that couples extracellular signals with specific intracellular responses. Essential early events that generate a cytosolic Ca²⁺ spike in response to environmental stress are starting to emerge. We review sensory machineries, including ion channels and transporters, which perceive various stress stimuli and allow cytosolic Ca²⁺ influx. We highlight integrative roles of Ca²⁺ channels in plant responses to various environmental stresses, as well as possible interplay of Ca²⁺ with other early signaling components, which facilitates signal propagation for systemic spread and spatiotemporal variations in respect to external cues. The early Ca²⁺ signaling schemes inspire the identification of additional stress sensors.

Screening for Arabidopsis mutants with altered Ca2+ signal response using aequorin-based Ca2+ reporter system

Environmental stimuli evoke transient increases of the cytosolic Ca²⁺ level. To identify upstream components of Ca²⁺ signaling, we have optimized two forward genetic screening systems based on Ca²⁺ reporter aequorin. AEQsig6 and AEQub plants were used for generating ethyl methanesulfonate (EMS)-mutagenized libraries. The AEQsig6 EMS-mutagenized library was preferably used to screen the mutants with reduced Ca²⁺ signal response due to its high effectiveness, while the AEQub EMS-mutagenized library was used for screening of the mutants with altered Ca²⁺ signal response.

For complete details on the use and execution of this protocol, please refer to Chen et al. (2020) and Zhu et al. (2013).

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Highly efficient systems for screening of Ca²⁺ signal mutants in Arabidopsis

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Step-by-step instructions to analyze Ca²⁺ signal response using Ca²⁺ reporter aequorin

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AEQsig6 system for identifying mutants impaired in shoot-based Ca²⁺ signaling

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FAS system for isolating tissue- or stimuli-specific Ca²⁺ responsive mutants

Arabidopsis Ca2+-ATPases 1, 2, and 7 in the endoplasmic reticulum contribute to growth and pollen fitness

Generating cellular Ca2+ signals requires coordinated transport activities from both Ca2+ influx and efflux pathways. In Arabidopsis (Arabidopsis thaliana), multiple efflux pathways exist, some of which involve Ca2+-pumps belonging to the Autoinhibited Ca2+-ATPase (ACA) family. Here, we show that ACA1, 2, and 7 localize to the endoplasmic reticulum (ER) and are important for plant growth and pollen fertility. While phenotypes for plants harboring single-gene knockouts (KOs) were weak or undetected, a triple KO of aca1/2/7 displayed a 2.6-fold decrease in pollen transmission efficiency, whereas inheritance through female gametes was normal. The triple KO also resulted in smaller rosettes showing a high frequency of lesions. Both vegetative and reproductive phenotypes were rescued by transgenes encoding either ACA1, 2, or 7, suggesting that all three isoforms are biochemically redundant. Lesions were suppressed by expression of a transgene encoding NahG, an enzyme that degrades salicylic acid (SA). Triple KO mutants showed elevated mRNA expression for two SA-inducible marker genes, Pathogenesis-related1 (PR1) and PR2. The aca1/2/7 lesion phenotype was similar but less severe than SA-dependent lesions associated with a double KO of vacuolar pumps aca4 and 11. Imaging of Ca2+ dynamics triggered by blue light or the pathogen elicitor flg22 revealed that aca1/2/7 mutants display Ca2+ transients with increased magnitudes and durations. Together, these results indicate that ER-localized ACAs play important roles in regulating Ca2+ signals, and that the loss of these pumps results in male fertility and vegetative growth deficiencies.

Effect of high light on canopy-level photosynthesis and leaf mesophyll ion flux in tomato

This study highlights the potential link between high light-induced canopy-level photosynthesis and mesophyll cell K⁺, Cl^-, Ca²⁺, and H⁺ homeostasis in tomato. Light is a primary energy source for photosynthesis and a vital regulator of mineral nutrient uptake and distribution in plants. Plants need to optimize photosynthesis and nutrient balance in leaves for performance in fluctuating light conditions that are partially regulated by light-induced ion homeostatsis in the mesophyll cells. It is still elusive whether high light-induced leaf mesophyll ion fluxes affect leaf photosynthesis at different canopy levels in Solanum lycopersicum L. Leaf gas exchange and microelectrode ion flux (MIFE) measurements were employed to study the effects of prolonged light-induced canopy-level leaf physiological responses of tomato plants. High light resulted in a significant lowering in photosynthesis in the fully-exposed top canopy leaves of tomato, but not to mid- or low-canopy leaves. Leaf mesophyll K⁺ effluxes of all canopies were significantly decreased after three weeks of high light treatment. However, high light-induced leaf mesophyll Ca²⁺ effluxes were significantly enhanced only in the top and mid canopies. Moreover, we found that photosynthetic parameters were significantly correlated with leaf mesophyll ion fluxes. We thus propose that canopy-level significant Ca²⁺ efflux and K⁺ efflux of leaf mesophyll may serve as early indicators for light-induced regulation on photosynthesis. We conclude that light-induced differential photosynthetic performance and ion fluxes in leaves may implicate a requirement of more uniform light irradiance and spectra at different canopy levels of tall greenhouse tomato plants. This can be achieved through new innovative greenhouse lighting technologies and covering materials towards the enhancement of crop photosynthesis and yield.

Gibberellin Induces an Increase in Cytosolic Ca2+ via a DELLA-Independent Signaling Pathway

DELLA proteins play a central role in gibberellin (GA) signaling. GA triggers DELLA degradation via the ubiquitin-proteasome pathway, thereby promoting plant growth. An increase in cytosolic Ca²⁺ ([Ca²⁺]_cyt) was observed previously after several hours of GA application. Recent studies also suggest the existence of a DELLA-independent GA response. However, the effect of DELLA on the GA-induced increase in [Ca²⁺]_cyt remains unknown. This study reexamined the effects of GAs on [Ca²⁺]_cyt using the Ca²⁺ sensor protein aequorin in Arabidopsis (Arabidopsis thaliana). [Ca²⁺]_cyt increased within a few minutes of GA treatment, even in transgenic plants expressing a mutated degradation-resistant version of REPRESSOR OF ga1-3 and in della pentuple mutant plants. In addition, it was also revealed that Ca²⁺ is not involved in DELLA degradation. These results suggest that the GA-induced increase in [Ca²⁺]_cyt occurs via a DELLA-independent pathway, providing important information on the GA signaling network.

Are plants sentient?

Feelings in humans are mental states representing groups of physiological functions that usually have defined behavioural purposes. Feelings, being evolutionarily ancient, are thought to be coordinated in the brain stem of animals. One function of the brain is to prioritise between competing mental states and, thus, groups of physiological functions and in turn behaviour. Plants use groups of coordinated physiological activities to deal with defined environmental situations but currently have no known mental state to prioritise any order of response. Plants do have a nervous system based on action potentials transmitted along phloem conduits but which in addition, through anastomoses and other cross-links, forms a complex network. The emergent potential for this excitable network to form a mental state is unknown, but it might be used to distinguish between different and even contradictory signals to the individual plant and thus determine a priority of response. This plant nervous system stretches throughout the whole plant providing the potential for assessment in all parts and commensurate with its self-organising, phenotypically plastic behaviour. Plasticity may, in turn, depend heavily on the instructive capabilities of local bioelectric fields enabling both a degree of behavioural independence but influenced by the condition of the whole plant.

Real-time In Vivo Recording of Arabidopsis Calcium Signals During Insect Feeding Using a Fluorescent Biosensor

Calcium ions are predicted to be key signaling entities during biotic interactions, with calcium signaling forming an established part of the plant defense response to microbial elicitors and to wounding caused by chewing insects, eliciting systemic calcium signals in plants. However, the role of calcium in vivo during biotic stress is still unclear. This protocol describes the use of a genetically-encoded calcium sensor to detect calcium signals in plants during feeding by a hemipteran pest. Hemipterans such as aphids pierce a small number of cells with specialized, elongated sucking mouthparts, making them the ideal tool to study calcium dynamics when a plant is faced with a biotic stress, which is distinct from a wounding response. In addition, fluorescent biosensors are revolutionizing the measurement of signaling molecules in vivo in both animals and plants. Expressing a GFP-based calcium biosensor, GCaMP3, in the model plant Arabidopsis thaliana allows for the real-time imaging of plant calcium dynamics during insect feeding, with a high spatial and temporal resolution. A repeatable and robust assay has been developed using the fluorescence microscopy of detached GCaMP3 leaves, allowing for the continuous measurement of cytosolic calcium dynamics before, during, and after insect feeding. This reveals a highly-localized rapid calcium elevation around the aphid feeding site that occurs within a few minutes. The protocol can be adapted to other biotic stresses, such as additional insect species, while the use of Arabidopsis thaliana allows for the rapid generation of mutants to facilitate the molecular analysis of the phenomenon.

Temperature-dependent signal transmission in chloroplast accumulation response

Chloroplast photorelocation movement, well-characterized light-induced response found in various plant species from alga to higher plants, is an important phenomenon for plants to increase photosynthesis efficiency and avoid photodamage. The signal for chloroplast accumulation movement connecting the blue light receptor, phototropin, and chloroplasts remains to be identified, although the photoreceptors and the mechanism of movement via chloroplast actin filaments have now been revealed in land plants. The characteristics of the signal have been found; the speed of signal transfer is about 1 µm min^-1 and that the signal for the accumulation response has a longer life and is transferred a longer distance than that of the avoidance response. Here, to collect the clues of the unknown signal substances, we studied the effect of temperature on the speed of signal transmission using the fern Adiantum capillus-veneris and found the possibility that the mechanism of signal transfer was not dependent on the simple diffusion of a substance; thus, some chemical reaction must also be involved. We also found new insights of signaling substances, such that microtubules are not involved in the signal transmission, and that the signal could even be transmitted through the narrow space between chloroplasts and the plasma membrane.

Interaction of light quality and fertility on biomass, shoot pigmentation and xanthophyll cycle flux in Chinese kale

BACKGROUND

Nutritionally important carotenoids in 21-day-old brassica microgreens increase following short and long-term exposure to narrow-band wavelengths from light-emitting diodes (LED). The present study aimed to measure the impact of: (1) fluorescent/incandescent light and different percentages of blue/red LED light and (2) different levels of nutrient fertility on biomass and pigment concentrations in 30-day-old 'Green Lance' Chinese kale (Brassica oleracea var. alboglabra). Kale plants were exposed to four light treatments and two fertility levels and were harvested 30 days after seeding and analyzed for nutritionally important shoot pigments.

RESULTS

Kale under the fluorescent/incandescent light treatment had a significantly higher shoot fresh and dry mass. The shoot tissue concentrations of most pigment were significantly higher under blue/red LED light treatments. The higher fertility level resulted in higher concentrations for most pigments. Interestingly, the pool of xanthophyll cycle pigments and de-epoxidized xanthophylls was higher under all LED treatments.

CONCLUSION

The results obtained in the present study support previous data demonstrating the stimulation of nutritionally important shoot tissue pigment concentrations following exposure to sole source blue/red LEDs compared to traditional lighting. Xanthophyll cycle flux was impacted by LEDs and this may support the role of zeaxanthin in blue light perception in leafy specialty crops. © 2016 Society of Chemical Industry.

Blue light-dependent changes in loosely bound calcium in Arabidopsis mesophyll cells: an X-ray microanalysis study

Calcium is involved in the signal transduction pathway from phototropins, the blue light photoreceptor kinases which mediate chloroplast movements. The chloroplast accumulation response in low light is controlled by both phot1 and phot2, while only phot2 is involved in avoidance movement induced by strong light. Phototropins elevate cytosolic Ca(2+) after activation by blue light. In higher plants, both types of chloroplast responses depend on Ca(2+), and internal calcium stores seem to be crucial for these processes. Yet, the calcium signatures generated after the perception of blue light by phototropins are not well understood. To characterize the localization of calcium in Arabidopsis mesophyll cells, loosely bound (exchangeable) Ca(2+) was precipitated with potassium pyroantimonate and analyzed by transmission electron microscopy followed by energy-dispersive X-ray microanalysis. In dark-adapted wild-type Arabidopsis leaves, calcium precipitates were observed at the cell wall, where they formed spherical structures. After strong blue light irradiation, calcium at the apoplast prevailed, and bigger, multilayer precipitates were found. Spherical calcium precipitates were also detected at the tonoplast. After red light treatment as a control, the precipitates at the cell wall were smaller and less numerous. In the phot2 and phot1phot2 mutants, calcium patterns were different from those of wild-type plants. In both mutants, no elevation of calcium after blue light treatment was observed at the cell periphery (including the cell wall and a fragment of cytoplasm). This result confirms the involvement of phototropin2 in the regulation of Ca(2+) homeostasis in mesophyll cells.

AtCNGC2 is involved in jasmonic acid-induced calcium mobilization

Calcium (Ca(2+)) mobilization is a central theme in various plant signal transduction pathways. We demonstrate that Arabidopsis thaliana cyclic nucleotide-gated channel 2 (AtCNGC2) is involved in jasmonic acid (JA)-induced apoplastic Ca(2+) influx in Arabidopsis epidermal cells. Ca(2+) imaging results showed that JA can induce an elevation in the cytosolic cAMP concentration ([cAMP]cyt), reaching a maximum within 3 min. Dibutyryl cAMP (db-cAMP), a cell membrane-permeable analogue of cAMP, induced an increase in the cytosolic Ca(2+) concentration ([Ca(2+)]cyt), with a peak at 4 min. This [Ca(2+)]cyt increase was triggered by the JA-induced increase in [cAMP]cyt. W-7[N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide], an antagonist of calmodulin, positively modulated the JA-induced increase in [Ca(2+)]cyt, while W-5[N-(6-aminohexyl)-1-naphthalenesulfonamide], an inactive antagonist of calmodulin, had no apparent effect. db-cAMP and JA positively induced the expression of primary (i.e. JAZ1 and MYC2) and secondary (i.e. VSP1) response genes in the JA signalling pathway in wild-type Arabidopsis thaliana, whereas they had no significant effect in the AtCNGC2 mutant 'defense, no death (dnd1) plants. These data provide evidence that JA first induces the elevation of cAMP, and cAMP, as an activating ligand, activates the AtCNGC2 channel, resulting in apoplastic Ca(2+) influx through AtCNGC2.

The Cytoskeleton and Its Regulation by Calcium and Protons

Calcium and protons exert control over the formation and activity of the cytoskeleton, usually by modulating an associated motor protein or one that affects the structural organization of the polymer.

Dissecting blue light signal transduction pathway in leaf epidermis using a pharmacological approach

Blue light signalling pathway in broad bean leaf epidermal cells includes key membrane transporters: plasma- and endomembrane channels and pumps of H (+) , Ca (2+) and K (+) ions, and plasma membrane redox system. Blue light signalling pathway in epidermal tissue isolated from the abaxial side of fully developed Vicia faba leaves was dissected by measuring the effect of inhibitors of second messengers on net K(+), Ca(2+) and H(+) fluxes using non-invasive ion-selective microelectrodes (the MIFE system). Switching the blue light on-off caused transient changes of the ion fluxes. The effects of seven groups of inhibitors were tested in this study: CaM antagonists, ATPase inhibitors, Ca(2+) anatagonists or chelators, agents affecting IP3 formation, redox system inhibitors, inhibitors of endomembrane Ca(2+) transport systems and an inhibitor of plasma membrane Ca(2+)-permeable channels. Most of the inhibitors had a significant effect on steady-state (basal) net fluxes, as well as on the magnitude of the transient ion flux responses to blue light fluctuations. The data presented in this study suggest that redox signalling and, specifically, plasma membrane NADPH oxidase and coupled Ca(2+) and K(+) fluxes play an essential role in blue light signal transduction.

NITRIC OXIDE-ASSOCIATED PROTEIN1 (AtNOA1) is essential for salicylic acid-induced root waving in Arabidopsis thaliana

Root waving responses have been attributed to both environmental and genetics factors, but the potential inducers and transducers of root waving remain elusive. Thus, the identification of novel signal elements related to root waving is an intriguing field of research. Genetic, physiological, cytological, live cell imaging, and pharmacological approaches provide strong evidence for the involvement of Arabidopsis thaliana NITRIC OXIDE-ASSOCIATED PROTEIN1 (AtNOA1) in salicylic acid (SA)-induced root waving. SA specially induced root waving, with an overall decrease in root elongation in A. thaliana, and this SA-induced response was disrupted in the Atnoa1 mutant, as well as in nonexpresser of pathogenesis-related genes 1 (npr1), which is defective in SA-mediated plant defense signal transduction, but not in npr3/4 single and double mutants. The expression assays revealed that the abundance of AtNOA1 was significantly increased by application of SA. Genetic and pharmacological analyses showed that SA-induced root waving involved an AtNOA1-dependent Ca(2+) signal transduction pathway, and PIN-FORMED2 (PIN2) -based polar auxin transport possibly plays a crucial role in this process. Our work suggests that SA signaling through NPR1 and AtNOA1 is involved in the control of root waving, which provides new insights into the mechanisms that control root growth behavior on a hard agar surface.

Ca²⁺ signal contributing to the synthesis and emission of monoterpenes regulated by light intensity in Lilium 'siberia'

The floral scent is an important part of plant volatile compounds, and is influenced by environmental factors. The emission of monoterpenes of Lilium 'siberia' is regulated by light intensity, but the mechanism is large unknown. In this study, the expression of Li-mTPS, a monoterpene synthase gene in the tepals of Lilium 'siberia', and net Ca(2+) flux were investigated after exposure to different levels of light intensity (0, 100, 300, 600, 1000, and 1500 μmol m(-2) s(-1)). Moreover the effect of LaCl3 and ethylene glycol-bis-(2-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA) on the Li-mTPS expression, monoterpene emission, and net Ca(2+) flux were examined at 600 μmol m(-2) s(-1). The results showed that along with the enhancement of light intensity, the expression level of Li-mTPS increased gradually, and the net Ca(2+) influx was also enhanced showing a similar pattern. It was found that LaCl3 and EGTA effectively inhibited the increase in expression of Li-mTPS and the net Ca(2+) influx induced by light treatment. Moreover, the release amounts of monoterpenes decreased significantly after treatment with LaCl3 and EGTA. So it can be concluded that Ca(2+) signal contributed to the biosynthesis and emission of monoterpenes regulated by light intensity in Lilium 'siberia' tepals. The increased light intensity firstly triggered the Ca(2+) influx to cytoplasm, and then the gene expression of monoterpene synthases downstream was activated to regulate the biosynthesis and emission of monoterpenes. But in the signaling pathway other mechanisms were thought to be involved in the emission of monoterpenes regulated by light intensity, which need to be investigated in future research.

Strategies of seedlings to overcome their sessile nature: auxin in mobility control

Plants are sessile organisms that are permanently restricted to their site of germination. To compensate for their lack of mobility, plants evolved unique mechanisms enabling them to rapidly react to ever changing environmental conditions and flexibly adapt their postembryonic developmental program. A prominent demonstration of this developmental plasticity is their ability to bend organs in order to reach the position most optimal for growth and utilization of light, nutrients, and other resources. Shortly after germination, dicotyledonous seedlings form a bended structure, the so-called apical hook, to protect the delicate shoot meristem and cotyledons from damage when penetrating through the soil. Upon perception of a light stimulus, the apical hook rapidly opens and the photomorphogenic developmental program is activated. After germination, plant organs are able to align their growth with the light source and adopt the most favorable orientation through bending, in a process named phototropism. On the other hand, when roots and shoots are diverted from their upright orientation, they immediately detect a change in the gravity vector and bend to maintain a vertical growth direction. Noteworthy, despite the diversity of external stimuli perceived by different plant organs, all plant tropic movements share a common mechanistic basis: differential cell growth. In our review, we will discuss the molecular principles underlying various tropic responses with the focus on mechanisms mediating the perception of external signals, transduction cascades and downstream responses that regulate differential cell growth and consequently, organ bending. In particular, we highlight common and specific features of regulatory pathways in control of the bending of organs and a role for the plant hormone auxin as a key regulatory component.

Clues to the signals for chloroplast photo-relocation from the lifetimes of accumulation and avoidance responses

Chloroplast photo-relocation movement is crucial for plant survival; however, the mechanism of this phenomenon is still poorly understood. Especially, the signal that goes from photoreceptor to chloroplast is unknown, although the photoreceptors (phototropin 1 and 2) have been identified and an actin structure (chloroplast actin filaments) has been characterized that is specific for chloroplast movement. Here, in gametophytes of the fern Adiantum capillus-veneris, gametophores of the moss Physcomiterella patens, and leaves of the seed plant Arabidopsis thaliana, we sought to characterize the signaling system by measuring the lifetime of the induced response. Chloroplast movements were induced by microbeam irradiation with high-intensity blue light and recorded. The lifetime of the avoidance state was measured as a lag time between switching off the beam and the loss of avoidance behavior, and that of the accumulation state was measured as the duration of accumulation behavior following the extinction of the beam. The lifetime for the avoidance response state is approximately 3-4 min and that for the accumulation response is 19-28 min. These data suggest that the two responses are based on distinct signals.

Blue-light-induced rapid chloroplast de-anchoring in Vallisneria epidermal cells

In the outer periclinal cytoplasm of leaf epidermal cells of an aquatic angiosperm Vallisneria, blue light induces "chloroplast de-anchoring", a rapid decline in the resistance of chloroplasts against centrifugal force. Chloroplast de-anchoring is known induced within 1 min of irradiation with high-fluence-rate blue light specifically, preceding the commencement of chloroplasts migration toward the anticlinal cytoplasm. However, its regulatory mechanism has remained elusive, although pharmacological analysis suggested that a calcium release from intracellular calcium stores is necessary for the response. In search of the responsible photoreceptors, immunoblotting analysis using antibodies against phototropins demonstrated that cross-reactive polypeptides of 120-kDa exist in the plasma-membrane fraction prepared from the leaves. In vitro phosphorylation analysis revealed that 120-kDa polypeptides were phosphorylated by exposure to blue light in a fluence-dependent manner. The blue-light-induced phosphorylation activity was sensitive to a Ser/Thr kinase inhibitor, staurosporine, and unusually was retained at a high level for a long time in darkness. Furthermore, phototropin gene homologs (Vallisneria PHOTOTROPIN1 and PHOTOTROPIN2) expressed in leaves were isolated. We propose that calcium-regulated chloroplast de-anchoring, possibly mediated by phototropins, is an initial process of the blue-light-induced avoidance response of chloroplasts in Vallisneria.

Multi-level modeling of light-induced stomatal opening offers new insights into its regulation by drought

Plant guard cells gate CO2 uptake and transpirational water loss through stomatal pores. As a result of decades of experimental investigation, there is an abundance of information on the involvement of specific proteins and secondary messengers in the regulation of stomatal movements and on the pairwise relationships between guard cell components. We constructed a multi-level dynamic model of guard cell signal transduction during light-induced stomatal opening and of the effect of the plant hormone abscisic acid (ABA) on this process. The model integrates into a coherent network the direct and indirect biological evidence regarding the regulation of seventy components implicated in stomatal opening. Analysis of this signal transduction network identified robust cross-talk between blue light and ABA, in which [Ca2+]c plays a key role, and indicated an absence of cross-talk between red light and ABA. The dynamic model captured more than 10(31) distinct states for the system and yielded outcomes that were in qualitative agreement with a wide variety of previous experimental results. We obtained novel model predictions by simulating single component knockout phenotypes. We found that under white light or blue light, over 60%, and under red light, over 90% of all simulated knockouts had similar opening responses as wild type, showing that the system is robust against single node loss. The model revealed an open question concerning the effect of ABA on red light-induced stomatal opening. We experimentally showed that ABA is able to inhibit red light-induced stomatal opening, and our model offers possible hypotheses for the underlying mechanism, which point to potential future experiments. Our modelling methodology combines simplicity and flexibility with dynamic richness, making it well suited for a wide class of biological regulatory systems.

Phosphoproteomic analysis provides novel insights into stress responses in Phaeodactylum tricornutum, a model diatom

Protein phosphorylation on serine, threonine, and tyrosine (Ser/Thr/Tyr) is well established as a key regulatory posttranslational modification used in signal transduction to control cell growth, proliferation, and stress responses. However, little is known about its extent and function in diatoms. Phaeodactylum tricornutum is a unicellular marine diatom that has been used as a model organism for research on diatom molecular biology. Although more than 1000 protein kinases and phosphatases with specificity for Ser/Thr/Tyr residues have been predicted in P. tricornutum, no phosphorylation event has so far been revealed by classical biochemical approaches. Here, we performed a global phosphoproteomic analysis combining protein/peptide fractionation, TiO(2) enrichment, and LC-MS/MS analyses. In total, we identified 264 unique phosphopeptides, including 434 in vivo phosphorylated sites on 245 phosphoproteins. The phosphorylated proteins were implicated in the regulation of diverse biological processes, including signaling, metabolic pathways, and stress responses. Six identified phosphoproteins were further validated by Western blotting using phospho-specific antibodies. The functions of these proteins are discussed in the context of signal transduction networks in P. tricornutum. Our results advance the current understanding of diatom biology and will be useful for elucidating the phosphor-relay signaling networks in this model diatom.

Cell- and stimulus type-specific intracellular free Ca2+ signals in Arabidopsis

Appropriate stimulus-response coupling requires that each signal induces a characteristic response, distinct from that induced by other signals, and that there is the potential for individual signals to initiate different downstream responses dependent on cell type. How such specificity is encoded in plant signaling is not known. One possibility is that information is encoded in signal transduction pathways to ensure stimulus- and cell type-specific responses. The calcium ion acts as a second messenger in response to mechanical stimulation, hydrogen peroxide, NaCl, and cold in plants and also in circadian timing. We use GAL4 transactivation of aequorin in enhancer trap lines of Arabidopsis (Arabidopsis thaliana) to test the hypothesis that stimulus- and cell-specific information can be encoded in the pattern of dynamic alterations in the concentration of intracellular free Ca(2+) ([Ca(2+)]i). We demonstrate that mechanically induced increases in [Ca(2+)]i are largely restricted to the epidermal pavement cells of leaves, that NaCl induces oscillatory [Ca(2+)]i signals in spongy mesophyll and vascular bundle cells, but not other cell types, and detect circadian rhythms of [Ca(2+)]i only in the spongy mesophyll. We demonstrate stimulus-specific [Ca(2+)]i dynamics in response to touch, cold, and hydrogen peroxide, which in the case of the latter two signals are common to all cell types tested. GAL4 transactivation of aequorin in specific leaf cell types has allowed us to bypass the technical limitations associated with fluorescent Ca(2+) reporter dyes in chlorophyll-containing tissues to identify the cell- and stimulus-specific complexity of [Ca(2+)]i dynamics in leaves of Arabidopsis and to determine from which tissues stress- and circadian-regulated [Ca(2+)]i signals arise.

Phototropins function in high-intensity blue light-induced hypocotyl phototropism in Arabidopsis by altering cytosolic calcium

Phototropins (phot1 and phot2), the blue light receptors in plants, regulate hypocotyl phototropism in a fluence-dependent manner. Especially under high fluence rates of blue light (HBL), the redundant function mediated by both phot1 and phot2 drastically restricts the understanding of the roles of phot2. Here, systematic analysis of phototropin-related mutants and overexpression transgenic lines revealed that HBL specifically induced a transient increase in cytosolic Ca(2+) concentration ([Ca(2+)]cyt) in Arabidopsis (Arabidopsis thaliana) hypocotyls and that the increase in [Ca(2+)]cyt was primarily attributed to phot2. Pharmacological and genetic experiments illustrated that HBL-induced Ca(2+) increases were modulated differently by phot1 and phot2. Phot2 mediated the HBL-induced increase in [Ca(2+)]cyt mainly by an inner store-dependent Ca(2+)-release pathway, not by activating plasma membrane Ca(2+) channels. Further analysis showed that the increase in [Ca(2+)]cyt was possibly responsible for HBL-induced hypocotyl phototropism. An inhibitor of auxin efflux carrier exhibited significant inhibitions of both phototropism and increases in [Ca(2+)]cyt, which indicates that polar auxin transport is possibly involved in HBL-induced responses. Moreover, PHYTOCHROME KINASE SUBSTRATE1 (PKS1), the phototropin-related signaling element identified, interacted physically with phototropins, auxin efflux carrier PIN-FORMED1 and calcium-binding protein CALMODULIN4, in vitro and in vivo, respectively, and HBL-induced phototropism was impaired in pks multiple mutants, indicating the role of the PKS family in HBL-induced phototropism. Together, these results provide new insights into the functions of phototropins and highlight a potential integration point through which Ca(2+) signaling-related HBL modulates hypocotyl phototropic responses.

Nitric oxide blocks blue light-induced K+ influx by elevating the cytosolic Ca2+ concentration in Vicia faba L. guard cells

Ca(2+) plays a pivotal role in nitric oxide (NO)-promoted stomatal closure. However, the function of Ca(2+) in NO inhibition of blue light (BL)-induced stomatal opening remains largely unknown. Here, we analyzed the role of Ca(2+) in the crosstalk between BL and NO signaling in Vicia faba L. guard cells. Extracellular Ca(2+) modulated the BL-induced stomatal opening in a dose-dependent manner, and an application of 5 μM Ca(2+) in the pipette solution significantly inhibited BL-activated K(+) influx. Sodium nitroprusside (SNP), a NO donor, showed little effect on BL-induced K(+) influx and stomatal opening response in the absence of extracellular Ca(2+), but K(+) influx and stomatal opening were inhibited by SNP when Ca(2+) was added to the bath solution. Interestingly, although both SNP and BL could activate the plasma membrane Ca(2+) channels and induce the rise of cytosolic Ca(2+), the change in levels of Ca(2+) channel activity and cytosolic Ca(2+) concentration were different between SNP and BL treatments. SNP at 100 μM obviously activated the plasma membrane Ca(2+) channels and induced cytosolic Ca(2+) rise by 102.4%. In contrast, a BL pulse (100 μmol/m(2) per s for 30 s) slightly activated the Ca(2+) channels and resulted in a Ca(2+) rise of only 20.8%. Consistently, cytosolic Ca(2+) promoted K(+) influx at 0.5 μM or below, and significantly inhibited K(+) influx at 5 μM or above. Taken together, our findings indicate that Ca(2+) plays dual and distinctive roles in the crosstalk between BL and NO signaling in guard cells, mediating both the BL-induced K(+) influx as an activator at a lower concentration and the NO-blocked K(+) influx as an inhibitor at a higher concentration.

Phosphoinositides play differential roles in regulating phototropin1- and phototropin2-mediated chloroplast movements in Arabidopsis

Phototropins are UVA/blue-light receptors involved in controlling the light-dependent physiological responses which serve to optimize the photosynthetic activity of plants and promote growth. The phototropin-induced phosphoinositide (PI) metabolism has been shown to be essential for stomatal opening and phototropism. However, the role of PIs in phototropin-induced chloroplast movements remains poorly understood. The aim of this work is to determine which PI species are involved in the control of chloroplast movements in Arabidopsis and the nature of their involvement. We present the effects of the inactivation of phospholipase C (PLC), PI3-kinase (PI3K) and PI4-kinase (PI4K) on chloroplast relocations in Arabidopsis. The inhibition of the phosphatidylinositol 4,5-bisphospahte [PI(4,5)P2]-PLC pathway, using neomycin and U73122, suppressed the phot2-mediated chloroplast accumulation and avoidance responses, without affecting movement responses controlled by phot1. On the other hand, PI3K and PI4K activities are more restricted to phot1- and phot2-induced weak-light responses. The inactivation of PI3K and PI4K by wortmannin and LY294002 severely affected the weak blue-light-activated accumulation response but had little effect on the strong blue-light-activated avoidance response. The inhibitory effect observed with PI metabolism inhibitors is, at least partly, due to a disturbance in Ca²⁺_(c) signaling. Using the transgenic aequorin system, we show that the application of these inhibitors suppresses the blue-light-induced transient Ca²⁺_(c) rise. These results demonstrate the importance of PIs in chloroplast movements, with the PI(4,5)P2-PLC pathway involved in phot2 signaling while PI3K and PI4K are required for the phot1- and phot2-induced accumulation response. Our results suggest that these PIs modulate cytosolic Ca²⁺ signaling during movements.

Aequorin luminescence-based functional calcium assay for heterotrimeric G-proteins in Arabidopsis

Heterotrimeric GTP-binding proteins (G-proteins) and G-protein-coupled receptors are important signaling components in eukaryotes. In plants, the G-proteins are involved in diverse physiological processes, some of which are exerted via changes in the level of cytosolic free calcium concentration ([Ca(2+)]cyt). Various techniques have been developed to measure the change of [Ca(2+)]cyt, e.g., calcium-sensitive microelectrodes, chemical fluorescent dyes, and biosensors based on luminescent or fluorescent indicators. In this chapter, we describe a protocol for in vivo [Ca(2+)]cyt measurement in G-protein mutants expressing aequorin, a luminescent-based calcium biosensor, to extend our knowledge about G-protein mediated Ca(2+) signaling. This method is also applicable to other early signaling events that are mediated by changes in [Ca(2+)]cyt levels.

Molecular genetic analysis of phototropism in Arabidopsis

Plant life is strongly dependent on the environment, and plants regulate their growth and development in response to many different environmental stimuli. One of the regulatory mechanisms involved in these responses is phototropism, which allows plants to change their growth direction in response to the location of the light source. Since the study of phototropism by Darwin, many physiological studies of this phenomenon have been published. Recently, molecular genetic analyses of Arabidopsis have begun to shed light on the molecular mechanisms underlying this response system, including phototropin blue light photoreceptors, phototropin signaling components, auxin transporters, auxin action mechanisms and others. This review highlights some of the recent progress that has been made in further elucidating the phototropic response, with particular emphasis on mutant phenotypes.

Chloroplasts move towards the nearest anticlinal walls under dark condition

Chloroplasts change their intracellular positions in response to their light environment. Under darkness, chloroplasts assume special positions that are different from those under light conditions. Here, we analyzed chloroplast dark positioning using Adiantum capillus-veneris gametophyte cells. When chloroplasts were transferred into darkness, during the first 1-5 h, they moved towards the anticlinal cell walls bordering the adjacent cells rather rapidly. Then, they slowed down and accumulated at the anticlinal walls gradually over the following 24-36 h. The chloroplast movements could be roughly classified into two different categories: initial rapid straight movement and later, slow staggering movement. When the chloroplast accumulation response was induced in dark-adapted cells by partial cell irradiation with a microbeam targeted to the center of the cells, chloroplasts moved towards the beam spot from the anticlinal walls. However, when the microbeam was switched off, they moved to the nearest anticlinal walls and not to their original positions if they were not the closest, indicating that they know the direction of the nearest anticlinal wall and do not have particular areas that they migrate to during dark positioning.

Nitric oxide inhibits blue light-induced stomatal opening by regulating the K+ influx in guard cells

Blue light (BL)-induced stomatal opening and nitric oxide (NO)-promoted stomatal closure comprise two main aspects of stomatal regulation. Stomatal movement depends on ion fluxion in guard cells, whereas the physiological roles of BL or NO in regulating ion channel activities remain largely unknown. For gaining further insights into NO function in mediating BL-induced stomatal opening, guard cell protoplasts (GCPs) were patch-clamped in a whole-cell configuration. The results showed that twice BL pulses (100 μmol m⁻² s⁻¹ for 30s) effectively activated inward rectifying K⁺ channels by 67% and 20% in Vicia GCPs, respectively. In contrast, Red light (RL) showed little effect on inward rectifying K⁺ channels. In accord with this, BL also increased inward K⁺ currents by 54% in Arabidopsis thaliana wild type gl1, but not in phot1-5 phot2-1 (BL receptor phototropin deletion mutant). Sodium nitroprusside (SNP, a NO donor), at 100 μM, inhibited BL-dependent K⁺ influx and stomatal opening, which were abolished by c-PTIO (a specific NO scavenger). These results indicated that NO inhibits BL-induced stomatal opening maybe through restricting the K⁺ influx across plasma membrane in guard cells.

Blue light signalling in chloroplast movements

Chloroplast movements are among the mechanisms allowing plants to cope with changes in their environment. Chloroplasts accumulate at illuminated cell areas under weak light while they avoid areas exposed to strong light. These directional responses may be controlled by blue and/or red light, depending on the plant group. In terrestrial angiosperms only the blue light perceived by phototropins is active. The last decade has seen a rapid development of studies on the mechanism of directional chloroplast movements, which started with an identification of the photoreceptors. A forward genetic approach has been used to identify the components which control chloroplast movements. This review summarizes the current state of research into the signalling pathways which lead to chloroplast responses. First, the molecular properties of phototropins are presented, followed by a characterization both of proteins which are active downstream of phototropins and of secondary messengers. Finally, cross-talk between light signalling involved in chloroplast movements and other signalling pathways is discussed.

Evolutionary Adaptations of Plant AGC Kinases: From Light Signaling to Cell Polarity Regulation

Signaling and trafficking over membranes involves a plethora of transmembrane proteins that control the flow of compounds or relay specific signaling events. Next to external cues, internal stimuli can modify the activity or abundance of these proteins at the plasma membrane (PM). One such regulatory mechanism is protein phosphorylation by membrane-associated kinases, several of which are AGC kinases. The AGC kinase family is one of seven kinase families that are conserved in all eukaryotic genomes. In plants evolutionary adaptations introduced specific structural changes within the AGC kinases that most likely allow modulation of kinase activity by external stimuli (e.g., light). Starting from the well-defined structural basis common to all AGC kinases we review the current knowledge on the structure-function relationship in plant AGC kinases. Nine of the 39 Arabidopsis AGC kinases have now been shown to be involved in the regulation of auxin transport. In particular, AGC kinase-mediated phosphorylation of the auxin transporters ABCB1 and ABCB19 has been shown to regulate their activity, while auxin transporters of the PIN family are located to different positions at the PM depending on their phosphorylation status, which is a result of counteracting AGC kinase and PP6 phosphatase activities. We therefore focus on regulation of AGC kinase activity in this context. Identified structural adaptations of the involved AGC kinases may provide new insight into AGC kinase functionality and demonstrate their position as central hubs in the cellular network controlling plant development and growth.

MDP25, a novel calcium regulatory protein, mediates hypocotyl cell elongation by destabilizing cortical microtubules in Arabidopsis

The regulation of hypocotyl elongation is important for plant growth. Microtubules play a crucial role during hypocotyl cell elongation. However, the molecular mechanism underlying this process is not well understood. In this study, we describe a novel Arabidopsis thaliana microtubule-destabilizing protein 25 (MDP25) as a negative regulator of hypocotyl cell elongation. We found that MDP25 directly bound to and destabilized microtubules to enhance microtubule depolymerization in vitro. The seedlings of mdp25 mutant Arabidopsis lines had longer etiolated hypocotyls. In addition, MDP25 overexpression resulted in significant overall shortening of hypocotyl cells, which exhibited destabilized cortical microtubules and abnormal cortical microtubule orientation, suggesting that MDP25 plays a crucial role in the negative regulation of hypocotyl cell elongation. Although MDP25 localized to the plasma membrane under normal conditions, increased calcium levels in cells caused MDP25 to partially dissociate from the plasma membrane and move into the cytosol. Cellular MDP25 bound to and destabilized cortical microtubules, resulting in their reorientation, and subsequently inhibited hypocotyl cell elongation. Our results suggest that MDP25 exerts its function on cortical microtubules by responding to cytoplasmic calcium levels to mediate hypocotyl cell elongation.

Analysis of calcium signaling pathways in plants

BACKGROUND

Calcium serves as a versatile messenger in many adaptation and developmental processes in plants. Ca2+ signals are represented by stimulus-specific spatially and temporally defined Ca2+ signatures. These Ca2+ signatures are detected, decoded and transmitted to downstream responses by a complex toolkit of Ca2+ binding proteins that function as Ca2+ sensors.

SCOPE OF REVIEW

This review will reflect on advancements in monitoring Ca2+ dynamics in plants. Moreover, it will provide insights in the extensive and complex toolkit of plant Ca2+ sensor proteins that relay the information presented in the Ca2+ signatures into phosphorylation events, changes in protein-protein interaction or regulation of gene expression.

MAJOR CONCLUSIONS

Plants' response to signals is encoded by different Ca2+ signatures. The plant decoding Ca2+ toolkit encompasses different families of Ca2+ sensors like Calmodulins (CaM), Calmodulin-like proteins (CMLs), Ca2+-dependent protein kinases (CDPKs), Calcineurin B-like proteins (CBLs) and their interacting kinases (CIPKs). These Ca2+ sensors are encoded by complex gene families and form intricate signaling networks in plants that enable specific, robust and flexible information processing.

GENERAL SIGNIFICANCE

This review provides new insights about the biochemical regulation, physiological functions and of newly identified target proteins of the major plant Ca2+ sensor families. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signaling.

New insights into dynamic actin-based chloroplast photorelocation movement

Chloroplast movement is essential for plants to survive under various environmental light conditions. Phototropins-plant-specific blue-light-activated receptor kinases-mediate the response by perceiving light intensity and direction. Recently, novel chloroplast actin (cp-actin) filaments have been identified as playing a pivotal role in the directional chloroplast photorelocation movement. Encouraging progress has recently been made in this field of research through molecular genetics and cell biological analyses. This review describes factors that have been identified as being involved in chloroplast movement and their roles in the regulation of cp-actin filaments, thus providing a basis for reflection on their biochemical activities and functions.

Cytosolic calcium rises and related events in ergosterol-treated Nicotiana cells

The typical fungal membrane component ergosterol was previously shown to trigger defence responses and protect plants against pathogens. Most of the elicitors mobilize the second messenger calcium, to trigger plant defences. We checked the involvement of calcium in response to ergosterol using Nicotiana plumbaginifolia and Nicotiana tabacum cv Xanthi cells expressing apoaequorin in the cytosol. First, it was verified if ergosterol was efficient in these cells inducing modifications of proton fluxes and increased expression of defence-related genes. Then, it was shown that ergosterol induced a rapid and transient biphasic increase of free [Ca²⁺](cyt) which intensity depends on ergosterol concentration in the range 0.002-10 μM. Among sterols, this calcium mobilization was specific for ergosterol and, ergosterol-induced pH and [Ca²⁺](cyt) changes were specifically desensitized after two subsequent applications of ergosterol. Specific modulators allowed elucidating some events in the signalling pathway triggered by ergosterol. The action of BAPTA, LaCl₃, nifedipine, verapamil, neomycin, U73122 and ruthenium red suggested that the first phase was linked to calcium influx from external medium which subsequently triggered the second phase linked to calcium release from internal stores. The calcium influx and the [Ca²⁺](cyt) increase depended on upstream protein phosphorylation. The extracellular alkalinization and ROS production depended on calcium influx but, the ergosterol-induced MAPK activation was calcium-independent. ROS were not involved in cytosolic calcium rise as described in other models, indicating that ROS do not systematically participate in the amplification of calcium signalling. Interestingly, ergosterol-induced ROS production is not linked to cell death and ergosterol does not induce any calcium elevation in the nucleus.

A negative effector of blue light-induced and gravitropic bending in Arabidopsis

Although sessile, plants are able to grow toward or away from an environmental stimulus. Important examples are stem or leaf orientation of higher plants in response to the direction of the incident light. The responsible photoreceptors belong to the phototropin photoreceptor family. Although the mode of phototropin action is quite well understood, much less is known of how the light signal is transformed into a bending response. Several lines of evidence indicate that a lateral auxin gradient is responsible for asymmetric cell elongation along the light gradient within the stem. However, some of the molecular key players leading to this asymmetric auxin distribution are, as yet, unidentified. Previously, it was shown that phototropin gets autophosphorylated upon illumination and binds to a scaffold protein termed NPH3 (for nonphototropic hypocotyl 3). Using a yeast three-hybrid approach with phototropin and NPH3 as a bait complex, we isolated a protein, termed EHB1 (for enhanced bending 1), with a so far unknown function, which binds to this binary complex. This novel interacting factor negatively affects hypocotyl bending under blue light conditions in Arabidopsis (Arabidopsis thaliana) and thus seems to be an important component regulating phototropism. Interestingly, it could be shown that the gravitropic response was also affected. Thus, it cannot be ruled out that this protein might also have a more general role in auxin-mediated bending toward an environmental stimulus.

Transitions of gene expression induced by short-term blue light

Blue light modulates many processes in plants and plant cells. It influences global and long-term responses, such as seedling development and phototropism, and induces short-term adaptations like stomatal opening and chloroplast movement. Three genes were identified as important for the latter process, namely PHOT1, PHOT2 and CHUP1. The former two phototropin blue light receptors act in perception of the blue light signal. The protein CHUP1 is localised to the outer envelope membrane of chloroplasts and is involved in chloroplast movement. To explore whether short-term reactions required for chloroplast movement are under transcriptional control, we analysed the transcriptome in wild-type Arabidopsis thaliana, phot1, phot2 and chup1 with different blue light treatments for 5 or 30 min. Blue light-induced changes in transcription depended on illumination time and intensity. Illumination with 100 μmol·m(-2) · s(-1) blue light induced down-regulation of several genes and might point to cascades that could be important for sensing low levels of blue light. Analysis of the transcriptome of the mutants in response to the different light regimes suggests that the transcriptional response to blue light in the wild-type can be attributed to phot1 rather than phot2, suggesting that blue light-induced alteration of expression is a function of phot1. In contrast, the blue light response at the transcriptional level of chup1 plants was unique, and confirmed the higher light sensitivity of this mutant.

Role of inositol 1,4,5-triphosphate signalling in gravitropic and phototropic gene expression

Plants sense light and gravity to orient their direction of growth. One common component in the early events of both phototropic and gravitropic signal transduction is activation of phospholipase C (PLC), which leads to an increase in inositol 1,4,5-triphosphate (InsP(3)) levels. The InsP(3) signal is terminated by hydrolysis of InsP(3) through inositolpolyphosphate-5-phosphatases (InsP 5-ptases). Arabidopsis plants expressing a heterologous InsP 5-ptase have low basal InsP(3) levels and exhibit reduced gravitropic and phototropic bending. Downstream effects of InsP(3)-mediated signalling are not understood. We used comparative transcript profiling to characterize gene expression changes in gravity- or light-stimulated Arabidopsis root apices that were manipulated in their InsP(3) metabolism either through inhibition of PLC activity or expression of InsP 5-ptase. We identified InsP(3)-dependent and InsP(3)-independent co-regulated gene sets in response to gravity or light stimulation. Inhibition of PLC activity in wild-type plants caused similar changes in transcript abundance in response to gravitropic and phototropic stimulation as in the transgenic lines. Therefore, we conclude that changes in gene expression in response to gravitropic and phototropic stimulation are mediated by two signal transduction pathways that vary in their dependence on changes in InsP(3).

Light-induced modification of plant plasma membrane ion transport

Light is not only the driving force for electron and ion transport in the thylakoid membrane, but also regulates ion transport in various other membranes of plant cells. Light-dependent changes in ion transport at the plasma membrane and associated membrane potential changes have been studied intensively over the last century. These studies, with various species and cell types, revealed that apart from regulation by chloroplasts, plasma membrane transport can be controlled by phytochromes, phototropins or channel rhodopsins. In this review, we compare light-dependent plasma membrane responses of unicellular algae (Eremosphaera and Chlamydomonas), with those of a multicellular alga (Chara), liverworts (Conocephalum), mosses (Physcomitrella) and several angiosperm cell types. Light-dependent plasma membrane responses of Eremosphaera and Chara are characterised by the dominant role of K(+) channels during membrane potential changes. In most other species, the Ca(2+)-dependent activation of plasma membrane anion channels represents a general light-triggered event. Cell type-specific responses are likely to have evolved by modification of this general response or through the development of additional light-dependent signalling pathways. Future research to elucidate these light-activated signalling chains is likely to benefit from the recent identification of S-type anion channel genes and proteins capable of regulating these channels.

Cross-talk between ROS and calcium in regulation of nuclear activities

Calcium and Reactive Oxygen Species (ROS) are acknowledged as crucial second messengers involved in the response to various biotic and abiotic stresses. However, it is still not clear how these two compounds can play a role in different signaling pathways leading the plant to a variety of processes such as root development or defense against pathogens. Recently, it has been shown that the concept of calcium and ROS signatures, initially discovered in the cytoplasm, can also be extended to the nucleus of plant cells. In addition, it has been clearly proved that both ROS and calcium signals are intimately interconnected. How this cross-talk can finally modulate the translocation and/or the activity of nuclear proteins leading to the control of specific genes expression is the main focus of this review. We will especially focus on how calcium and ROS interact at the molecular level to modify their targets.

Calcium-regulated transcription in plants

The past two decades revealed a plethora of Ca2+-responsive proteins and downstream targets in plants, of which several are unique to plants. More recent high-throughput 'omics' approaches and bioinformatics are exposing Ca2+-responsive cis-elements and the corresponding Ca2+-responsive genes. Here, we review the current knowledge on Ca2+-signaling pathways that regulate gene expression in plants, and we link these to mechanisms by which plants respond to biotic and abiotic stresses.