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

Feedback regulation of the isoprenoid pathway by SsdTPS overexpression has the potential to enhance plant tolerance to drought stress

In order to maintain the dynamic physiological balance, plants are compelled to adjust their energy metabolism and signal transduction to cope with the abiotic stresses caused by complex and changeable environments. The diterpenoid natural compound and secondary metabolites, sclareol, derived from Salvia sclarea, has gained significant attention owing to its economic value as a spice material and diverse physiological activities. Here, we focused on the roles and regulatory mechanisms of the sclareol diterpene synthase gene SsdTPS in the resistance of S. sclarea to abiotic stresses. Our results suggested that abiotic stresses could induce the response and upregulation of SsdTPS expression and isoprenoid pathway in S. sclarea. Ectopic expression of SsdTPS conferred drought tolerance in transgenic Arabidopsis, compared with wild-type. Overexpression of SsdTPS enhanced the transcription of ABA signal transduction synthetic regulators and induced the positive feedback upregulating key regulatory genes in the MEP pathway, thereby promoting the increase of ABA content and improving drought tolerance in transgenic plants. In addition, SsdTPS-overexpressed transgenic Arabidopsis improved the responses of stomatal regulatory genes and ROS scavenging enzyme activities and gene expression to drought stress. This promoted the stomatal closure and ROS reduction, thus enhancing water retention capacity and reducing oxidative stress damage. These findings unveil the potentially positive role of SsdTPS in orchestrating multiple regulatory mechanisms and maintaining homeostasis for improved abiotic stress resistance in S. sclarea, providing a novel insight into strategies for promoting drought resistance and cultivating highly tolerant plants.

The Passage of H2O2 from Chloroplasts to Their Associated Nucleus during Retrograde Signalling: Reflections on the Role of the Nuclear Envelope

The response of chloroplasts to adverse environmental cues, principally increases in light intensity, stimulates chloroplast-to-nucleus retrograde signalling, which leads to the induction of immediate protective responses and longer-term acclimation. Hydrogen peroxide (H₂O₂), generated during photosynthesis, is proposed to both initiate and transduce a retrograde signal in response to photoinhibitory light intensities. Signalling specificity achieved by chloroplast-sourced H₂O₂ for signal transduction may be dependent upon the oft-observed close association of a proportion of these organelles with the nucleus. In this review, we consider more precisely the nature of the close association between a chloroplast appressed to the nucleus and the requirement for H₂O₂ to cross both the double membranes of the chloroplast and nuclear envelopes. Of particular relevance is that the endoplasmic reticulum (ER) has close physical contact with chloroplasts and is contiguous with the nuclear envelope. Therefore, the perinuclear space, which transducing H₂O₂ molecules would have to cross, may have an oxidising environment the same as the ER lumen. Based on studies in animal cells, the ER lumen may be a significant source of H₂O₂ in plant cells arising from the oxidative folding of proteins. If this is the case, then there is potential for the ER lumen/perinuclear space to be an important location to modify chloroplast-to-nucleus H₂O₂ signal transduction and thereby introduce modulation of it by additional different environmental cues. These would include for example, heat stress and pathogen infection, which induce the unfolded protein response characterised by an increased H₂O₂ level in the ER lumen.

RIR1 represses plant immunity by interacting with mitochondrial complex I subunit in rice

We previously observed decreased expression of rice OsmiR159a.1 on infection with the bacterial blight-causing pathogen Xanthomonas oryzae pv. oryzae (Xoo), and identified the OsLRR_RLK (leucine-rich repeat_ receptor like kinase) gene as an authentic target of OsmiR159a.1. Here, we found that a Tos17 insertion mutant of LRR_RLK displayed increasing temporal resistance to Xoo, whereas the LRR_RLK overexpression lines were susceptible to the pathogen early on in the infection, indicating that LRR_RLK encodes a repressor of rice resistance to Xoo infection, and it was renamed as RIR1 (Rice Immunity Repressor 1). RIR1 overexpression plants were more susceptible to Xoo at late growth stage, suggesting that RIR1 mRNA levels are negatively correlated with the resistance of rice against Xoo. We discovered that OsmiR159a.1 repression in Xoo-infected plants was largely dependent on the pathogen's type III secretion system. Co-immunoprecipitation, bimolecular fluoresence complementation, and pull-down assays indicated that RIR1 interacted with the NADH-ubiquinone oxidoreductase (NUO) 51-kDa subunit of the mitochondrial complex I through its kinase domain. Notably, impairment of RIR1 or overexpression of NUO resulted in reactive oxygen species accumulation and enhanced expression of pathogen-resistance genes, including jasmonic acid pathway genes. We propose that pathogens may inhibit OsmiR159 to interfere with the RIR1-NUO interaction, and subsequently depression of rice immune signalling pathways. The resistance genes manipulated by Xoo can be a probe to explore the regulatory network during host-pathogen interactions.

Green light attenuates blue-light-induced chloroplast avoidance movement in Arabidopsis and Landoltia punctata

PREMISE

Chloroplast movement to the anticlinal walls in excess light, referred to as chloroplast avoidance movement, is one strategy to prevent high light damage. Chloroplast avoidance movement is mediated by the blue-light photoreceptor phototropin. Since some blue-light effects are reversed by green light, we investigated the effect of green wavelengths on chloroplast avoidance.

METHODS

Chloroplast position was visualized via microscopy and by transmission of red light through the leaves of Arabidopsis thaliana and Landoltia punctata (duckweed).

RESULTS

Green light reduced blue-light-induced chloroplast avoidance movement but only when green light was presented simultaneously with blue light. Green light alone had no effect on chloroplast position. An action spectrum for green-light attenuation of chloroplast avoidance in duckweed revealed peaks at 510, 550, and 590 nm. Blue-light-induced chloroplast avoidance movement in three Arabidopsis mutants with reduced nonphotochemical quenching, npq1, npq4, and npq7 was not affected by green light.

CONCLUSIONS

The action spectrum does not conform to any known photoreceptor. The lack of a green-light response in the npq mutants of Arabidopsis suggests a possible role for the xanthophyll cycle or a signal from the chloroplast in control of chloroplast avoidance movement.

Heat stress in Marchantia polymorpha: Sensing and mechanisms underlying a dynamic response

Sensing and response to high temperatures are crucial to prevent heat-related damage and to preserve cellular and metabolic functions. The response to heat stress is a complex and coordinated process that involves several subcellular compartments and multi-level regulatory networks that are synchronized to avoid cell damage while maintaining cellular homeostasis. In this review, we provide an insight into the most recent advances in elucidating the molecular mechanisms involved in heat stress sensing and response in Marchantia polymorpha. Based on the signaling pathways and genes that were identified in Marchantia, our analyses indicate that although with specific particularities, the core components of the heat stress response seem conserved in bryophytes and angiosperms. Liverworts not only constitute a powerful tool to study heat stress response and signaling pathways during plant evolution, but also provide key and simple mechanisms to cope with extreme temperatures. Given the increasing prevalence of high temperatures around the world as a result of global warming, this knowledge provides a new set of molecular tools with potential agronomical applications.

Effects of graphene oxide on tomato growth in different stages

The nano-carbon graphene has unique structural and physicochemical properties, which are conducive to various biomedical applications. We assessed the effect of graphene oxide (GO) on tomato plants at the seedling and mature stages in terms of morphological and biochemical indices. GO treatment significantly improved the shoot/stem growth of tomato in a dose-dependent manner by increasing the cortical cells number, cross-sectional area, diameter and vascular-column area. In addition, GO also promoted the morphological development of the root system and increased biomass accumulation. The surface area of root tips and hairs of tomato plants treated with 50 mg/L and 100 mg/L GO were significantly greater compared to the untreated control. At the molecular level, GO induced the expression of root development-related genes (SlExt1 and LeCTR1) and inhibited the auxin-responsive gene (SlIAA3). However, 50 mg/L and 100 mg/L GO significantly increased the root auxin content, which in turn increased the number of fruits and hastened fruit ripening compared to the control plants. Taken together, GO can improve the tomato growth when used at the appropriate concentration, and is a promising nano-carbon material for agricultural use.

Blue Light Mediates Chloroplast Avoidance and Enhances Photoprotection of Vanilla Orchid

Vanilla orchid, which is well-known for its flavor and fragrance, is cultivated in tropical and subtropical regions. This shade-loving plant is very sensitive to high irradiance. In this study, we show that vanilla chloroplasts started to have avoidance movement when blue light (BL) was higher than 20 μmol m-2s-1 and significant avoidance movement was observed under BL irradiation at 100 μmol m-2s-1 (BL100). The light response curve indicated that when vanilla was exposed to 1000 μmol m-2s-1, the electron transport rate (ETR) and photochemical quenching of fluorescence (qP) were significantly reduced to a negligible amount. We found that if a vanilla orchid was irradiated with BL100 for 12 days, it acquired BL-acclimation. Chloroplasts moved to the side of cells in order to reduce light-harvesting antenna size, and chloroplast photodamage was eliminated. Therefore, BL-acclimation enhanced vanilla orchid growth and tolerance to moderate (500 μmol m-2s-1) and high light (1000 μmol m-2s-1) stress conditions. It was found that under high irradiation, BL-acclimatized vanilla maintained higher ETR and qP capacity than the control without BL-acclimation. BL-acclimation induced antioxidant enzyme activities, reduced ROS accumulation, and accumulated more carbohydrates. Moreover, BL-acclimatized orchids upregulated photosystem-II-associated marker genes (D1 and PetC), Rubisco and PEPC transcripts and sustained expression levels thereof, and also maximized the photosynthesis rate. Consequently, BL-acclimatized orchids had higher biomass. In short, this study found that acclimating vanilla orchid with BL before transplantation to the field might eliminate photoinhibition and enhance vanilla growth and production.

Spatial chloroplast-to-nucleus signalling involving plastid-nuclear complexes and stromules

Communication between chloroplasts and the nucleus in response to various environmental cues may be mediated by various small molecules. Signalling specificity could be enhanced if the physical contact between these organelles facilitates direct transfer and prevents interference from other subcellular sources of the same molecules. Plant cells have plastid-nuclear complexes, which provide close physical contact between these organelles. Plastid-nuclear complexes have been proposed to facilitate transfer of photosynthesis-derived H₂O₂ to the nucleus in high light. Stromules (stroma filled tubular plastid extensions) may provide an additional conduit for transfer of a wider range of signalling molecules, including proteins. However, plastid-nuclear complexes and stromules have been hitherto treated as distinct phenomena. We suggest that plastid-nuclear complexes and stromules work in a coordinated manner so that, according to environmental conditions or developmental state, the two modes of connection contribute to varying extents. We hypothesize that this association is dynamic and that there may be a link between plastid-nuclear complexes and the development of stromules. Furthermore, the changes in contact could alter signalling specificity by allowing an extended or different range of signalling molecules to be delivered to the nucleus. This article is part of the theme issue 'Retrograde signalling from endosymbiotic organelles'.

Chloroplast avoidance movement: a novel paradigm of ROS signalling

The damaging effects of supra-optimal irradiance on plants, often turning to be lethal, may be circumvented by chloroplast avoidance movement which realigns chloroplasts to the anticlinal surfaces of cells (parallel to the incident light), essentially minimizing photon absorption. In angiosperms and many other groups of plants, chloroplast avoidance movement has been identified to be a strong blue light (BL)-dependent process being mediated by actin filaments wherein phototropins are identified as the photoreceptor involved. Studies through the last few decades have identified key molecular mechanisms involving Chloroplast Unusual Positioning 1 (CHUP1) protein and specific chloroplast-actin (cp-actin) filaments. However, the signal transduction pathway from strong BL absorption down to directional re-localization of chloroplasts by actin filaments is complex and ambiguous. Being the immediate cellular products of high irradiance absorption and having properties of remodelling actin as well as phototropin, reactive oxygen species (ROS) deemed to be more able and prompt than any other signalling agent in mediating chloroplast avoidance movement. Although ROS are presently being identified as fundamental component for regulating different plant processes ranging from growth, development and immunity, its role in avoidance movement have hardly been explored in depth. However, few recent reports have demonstrated the direct stimulatory involvement of ROS, especially H₂O₂, in chloroplast avoidance movement with Ca²⁺ playing a pivotal role. With this perspective, the present review discusses the mechanisms of ROS-mediated chloroplast avoidance movement involving ROS-Ca²⁺-actin communication system and NADPH oxidase (NOX)-plasma membrane (PM) H⁺-ATPase positive feed-forward loop. A possible working model is proposed.

Time-Course Transcriptome Study Reveals Mode of bZIP Transcription Factors on Light Exposure in Arabidopsis

The etiolation process, which occurs after germination, is terminated once light is perceived and then de-etiolation commences. During the de-etiolation period, monochromatic lights (blue, red and far-red) induce differences in gene expression profiles and plant behavior through their respective photoreceptors. ELONGATED HYPOCOTYL 5 (HY5), a bZIP-type transcription factor (TF), regulates gene expression in the de-etiolation process, and other bZIP TFs are also involved in this regulation. However, transcriptomic changes that occur in etiolated seedlings upon monochromatic light irradiation and the relationship with the bZIP TFs still remain to be elucidated. Here, we track changes in the transcriptome after exposure to white, blue, red and far-red light following darkness and reveal both shared and non-shared trends of transcriptomic change between the four kinds of light. Interestingly, after exposure to light, HY5 expression synchronized with those of the related bZIP TF genes, GBF2 and GBF3, rather than HY5 HOMOLOG (HYH). To speculate on the redundancy of target genes between the bZIP TFs, we inspected the genome-wide physical binding sites of homodimers of seven bZIP TFs, HY5, HYH, GBF1, GBF2, GBF3, GBF4 and EEL, using an in vitro binding assay. The results reveal large overlaps of target gene candidates, indicating a complicated regulatory literature among TFs. This work provides novel insight into understanding the regulation of gene expression of the plant response to monochromatic light irradiation.

The effect of light quality on the pro-/antioxidant balance, activity of photosystem II, and expression of light-dependent genes in Eutrema salsugineum callus cells

The antioxidant balance, photochemical activity of photosystem II (PSII), and photosynthetic pigment content, as well as the expression of genes involved in the light signalling of callus lines of Eutrema salsugineum plants (earlier Thellungiella salsuginea) under different spectral light compositions were studied. Growth of callus in red light (RL, maximum 660 nm), in contrast to blue light (BL, maximum 450 nm), resulted in a lower H₂O₂ content and thiobarbituric acid reactive substances (TBARS). The BL increased the activities of key antioxidant enzymes in comparison with the white light (WL) and RL and demonstrated the minimum level of PSII photochemical activity. The activities of catalase (CAT) and peroxidase (POD) had the highest values in BL, which, along with the increased H₂O₂ and TBARS content, indicate a higher level of oxidative stress in the cells. The expression levels of the main chloroplast protein genes of PSII (PSBA and PSBD), the NADPH-dependent oxidase gene of the plasma membrane (RbohD), the protochlorophyllide oxidoreductase genes (POR B, C) involved in the biosynthesis of chlorophyll, and the key photoreceptor signalling genes (CIB1, CRY2, PhyB, PhyA, and PIF3) were determined. Possible mechanisms of light quality effects on the physiological parameters of callus cells are discussed.

Ferns, mosses and liverworts as model systems for light-mediated chloroplast movements

Light-induced chloroplast movement is found in most plant species, including algae and land plants. In land plants with multiple small chloroplasts, under weak light conditions, the chloroplasts move towards the light and accumulate on the periclinal cell walls to efficiently perceive light for photosynthesis (the accumulation response). Under strong light conditions, chloroplasts escape from light to avoid photodamage (the avoidance response). In most plant species, blue light induces chloroplast movement, and phototropin receptor kinases are the blue light receptors. Molecular mechanisms for photoreceptors, signal transduction and chloroplast motility systems are being studied using the model plant Arabidopsis thaliana. However, to further understand the molecular mechanisms and evolutionary history of chloroplast movement in green plants, analyses using other plant systems are required. Here, we review recent works on chloroplast movement in green algae, liverwort, mosses and ferns that provide new insights on chloroplast movement.

Effects of catalase on chloroplast arrangement in Opuntia streptacantha chlorenchyma cells under salt stress

In arid and semiarid regions, low precipitation rates lead to soil salinity problems, which may limit plant establishment, growth, and survival. Herein, we investigated the NaCl stress effect on chlorophyll fluorescence, photosynthetic-pigments, movement and chloroplasts ultrastructure in chlorenchyma cells of Opuntia streptacantha cladodes. Cladodes segments were exposed to salt stress at 0, 100, 200, and 300 mM NaCl for 8, 16, and 24 h. The results showed that salt stress reduced chlorophyll content, F _v /F _m , ΦPSII, and qP values. Under the highest salt stress treatments, the chloroplasts were densely clumped toward the cell center and thylakoid membranes were notably affected. We analyzed the effect of exogenous catalase in salt-stressed cladode segments during 8, 16, and 24 h. The catalase application to salt-stressed cladodes counteracted the NaCl adverse effects, increasing the chlorophyll fluorescence parameters, photosynthetic-pigments, and avoided chloroplast clustering. Our results indicate that salt stress triggered the chloroplast clumping and affected the photosynthesis in O. streptacantha chlorenchyma cells. The exogenous catalase reverted the H₂O₂ accumulation and clustering of chloroplast, which led to an improvement of the photosynthetic efficiency. These data suggest that H₂O₂ detoxification by catalase is important to protect the chloroplast, thus conserving the photosynthetic activity in O. streptacantha under stress.

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.

Histone Deacetylase HDA-2 Regulates Trichoderma atroviride Growth, Conidiation, Blue Light Perception, and Oxidative Stress Responses

Fungal blue-light photoreceptors have been proposed as integrators of light and oxidative stress. However, additional elements participating in the integrative pathway remain to be identified. In Trichoderma atroviride, the blue-light regulator (BLR) proteins BLR-1 and -2 are known to regulate gene transcription, mycelial growth, and asexual development upon illumination, and recent global transcriptional analysis revealed that the histone deacetylase-encoding gene hda-2 is induced by light. Here, by assessing responses to stimuli in wild-type and Δhda-2 backgrounds, we evaluate the role of HDA-2 in the regulation of genes responsive to light and oxidative stress. Δhda-2 strains present reduced growth, misregulation of the con-1 gene, and absence of conidia in response to light and mechanical injury. We found that the expression of hda-2 is BLR-1 dependent and HDA-2 in turn is essential for the transcription of early and late light-responsive genes that include blr-1, indicating a regulatory feedback loop. When subjected to reactive oxygen species (ROS), Δhda-2 mutants display high sensitivity whereas Δblr strains exhibit the opposite phenotype. Consistently, in the presence of ROS, ROS-related genes show high transcription levels in wild-type and Δblr strains but misregulation in Δhda-2 mutants. Finally, chromatin immunoprecipitations of histone H3 acetylated at Lys9/Lys14 on cat-3 and gst-1 promoters display low accumulation of H3K9K14ac in Δblr and Δhda-2 strains, suggesting indirect regulation of ROS-related genes by HDA-2. Our results point to a mutual dependence between HDA-2 and BLR proteins and reveal the role of these proteins in an intricate gene regulation landscape in response to blue light and ROS.

IMPORTANCE

Trichoderma atroviride is a free-living fungus commonly found in soil or colonizing plant roots and is widely used as an agent in biocontrol as it parasitizes other fungi, stimulates plant growth, and induces the plant defense system. To survive in various environments, fungi constantly sense and respond to potentially threatening external factors, such as light. In particular, UV light can damage biomolecules by producing free-radical reactions, in most cases involving reactive oxygen species (ROS). In T. atroviride, conidiation is essential for its survival, which is induced by light and mechanical injury. Notably, conidia are typically used as the inoculum in the field during biocontrol. Therefore, understanding the linkages between responses to light and exposure to ROS in T. atroviride is of major basic and practical relevance. Here, the histone deacetylase-encoding gene hda-2 is induced by light and ROS, and its product regulates growth, conidiation, blue light perception, and oxidative stress responses.

Phytotoxicity and oxidative stress perspective of two selected nanoparticles in Brassica juncea

This study elaborates the consequences of oxidative stress caused by copper oxide (CuO) and titanium dioxide (TiO₂) nanoparticles (NPs) in Brassica juncea. Effect of these two NPs on plant physiology, reactive oxygen scavenging enzyme system (ascorbate peroxidase, catalase, superoxide dismutase), proline content and lipid peroxidation has been estimated in leaves as well as root tissues. Bioaccumulation of NPs has also been evaluated in the current study and the interrelated cascade of the enzymatic system with H₂O₂ production was identified. The uptake of NPs in plant leaves was confirmed by scanning electron microscopy, X-ray diffraction, and Fourier Transform Infrared Spectroscopy. Plant growth was found to be diminished with elevated levels of CuO NPs whereas TiO₂ NPs had shown an opposite effect. The plant species accumulated lower concentration of NPs and displayed considerable tolerance against stress, probably due to well-organized and coordinated defense system at the root and shoot level by the intonation of antioxidative enzymes.

Integrated role of ROS and Ca+2 in blue light-induced chloroplast avoidance movement in leaves of Hydrilla verticillata (L.f.) Royle

Directional chloroplast photorelocation is a major physio-biochemical mechanism that allows these organelles to realign themselves intracellularly in response to the intensity of the incident light as an adaptive response. Signaling processes involved in blue light (BL)-dependent chloroplast movements were investigated in Hydrilla verticillata (L.f.) Royle leaves. Treatments with antagonists of actin filaments [2,3,5-triiodobenzoic acid (TIBA)] and microtubules (oryzalin) revealed that actin filaments, but not microtubules, play a pivotal role in chloroplast movement. Involvement of reactive oxygen species (ROS) in controlling chloroplast avoidance movement has been demonstrated, as exogenous H₂O₂ not only accelerated chloroplast avoidance but also could induce chloroplast avoidance even in weak blue light (WBL). Further support came from experiments with different ROS scavengers, i.e., dimethylthiourea (DMTU), KI, and CuCl₂, which inhibited chloroplast avoidance, and from ROS localization using specific stains. Such avoidance was also partially inhibited by ZnCl₂, an inhibitor of NADPH oxidase (NOX) as well as 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), a photosynthetic electron transport chain (ETC) inhibitor at PS II. However, methyl viologen (MV), a PS I ETC inhibitor, rather accelerated avoidance response. Exogenous calcium (Ca⁺²) induced avoidance even in WBL while inhibited chloroplast accumulation partially. On the other hand, chloroplast movements (both accumulation and avoidance) were blocked by Ca⁺² antagonists, La³⁺ (inhibitor of plasma membrane Ca⁺² channel) and ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA, Ca⁺² chelator) while LiCl that affects Ca⁺² release from endosomal compartments did not show any effect. A model on integrated role of ROS and Ca⁺² (influx from apolastic space) in actin-mediated chloroplast avoidance has been proposed.

Reactive oxygen species and plant resistance to fungal pathogens

Reactive oxygen species (ROS) have been studied for their role in plant development as well as in plant immunity. ROS were consistently observed to accumulate in the plant after the perception of pathogens and microbes and over the years, ROS were postulated to be an integral part of the defence response of the plant. In this article we will focus on recent findings about ROS involved in the interaction of plants with pathogenic fungi. We will describe the ways to detect ROS, their modes of action and their importance in relation to resistance to fungal pathogens. In addition we include some results from works focussing on the fungal interactor and from studies investigating roots during pathogen attack.

Pb-induced avoidance-like chloroplast movements in fronds of Lemna trisulca L

Lead ions are particularly dangerous to the photosynthetic apparatus, but little is known about the effects of trace metals, including Pb, on regulation of chloroplast redistribution. In this study a new effect of lead on chloroplast distribution patterns and movements was demonstrated in mesophyll cells of a small-sized aquatic angiosperm Lemna trisulca L. (star duckweed). An analysis of confocal microscopy images of L. trisulca fronds treated with lead (15 μM Pb2+, 24 h) in darkness or in weak white light revealed an enhanced accumulation of chloroplasts in the profile position along the anticlinal cell walls, in comparison to untreated plants. The rearrangement of chloroplasts in their response to lead ions in darkness was similar to the avoidance response of chloroplasts in plants treated with strong white light. Transmission electron microscopy X-ray microanalysis showed that intracellular chloroplast arrangement was independent of the location of Pb deposits, suggesting that lead causes redistribution of chloroplasts, which looks like a light-induced avoidance response, but is not a real avoidance response to the metal. Furthermore, a similar redistribution of chloroplasts in L. trisulca cells in darkness was observed also under the influence of exogenously applied hydrogen peroxide (H2O2). In addition, we detected an enhanced accumulation of endogenous H2O2 after treatment of plants with lead. Interestingly, H2O2-specific scavenger catalase partly abolished the Pb-induced chloroplast response. These results suggest that H2O2 can be involved in the avoidance-like movement of chloroplasts induced by lead. Analysis of photometric measurements revealed also strong inhibition (but not complete) of blue-light-induced chloroplast movements by lead. This inhibition may result from disturbances in the actin cytoskeleton, as we observed fragmentation and disappearance of actin filaments around chloroplasts. Results of this study show that the mechanisms of the toxic effect of lead on chloroplasts can include disturbances in their movement and distribution pattern.

The interplay between ROS and tubulin cytoskeleton in plants

Plants have to deal with reactive oxygen species (ROS) production, since it could potentially cause severe damages to different cellular components. On the other hand, ROS functioning as important second messengers are implicated in various developmental processes and are transiently produced during biotic or abiotic stresses. Furthermore, the microtubules (MTs) play a primary role in plant development and appear as potent players in sensing stressful situations and in the subsequent cellular responses. Emerging evidence suggests that ROS affect MTs in multiple ways. The cellular redox status seems to be tightly coupled with MTs. ROS signals regulate the organization of tubulin cytoskeleton and induce tubulin modifications. This review aims at summarizing the signaling mechanisms and the key operators orchestrating the crosstalk between ROS and tubulin cytoskeleton in plant cells. The contribution of several molecules, including microtubule associated proteins, oxidases, kinases, phospholipases, and transcription factors, is highlighted.

A protein phosphatase 2A catalytic subunit modulates blue light-induced chloroplast avoidance movements through regulating actin cytoskeleton in Arabidopsis

Chloroplast avoidance movements mediated by phototropin 2 (phot2) are one of most important physiological events in the response to high-fluence blue light (BL), which reduces damage to the photosynthetic machinery under excess light. Protein phosphatase 2A-2 (PP2A-2) is an isoform of the catalytic subunit of PP2A, which regulates a number of developmental processes. To investigate whether PP2A-2 was involved in high-fluence BL-induced chloroplast avoidance movements, we first analyzed chloroplast migration in the leaves of the pp2a-2 mutant in response to BL. The data showed that PP2A-2 might act as a positive regulator in phot2-mediated chloroplast avoidance movements, but not in phot1-mediated chloroplast accumulation movements. Then, the effect of okadaic acid (OA) and cantharidin (selective PP2A inhibitors) on high-fluence BL response was further investigated in Arabidopsis thaliana mesophyll cells. Within a certain concentration range, exogenously applied OA or cantharidin inhibited the high-fluence BL-induced chloroplast movements in a concentration-dependent manner. Actin depolymerizing factor (ADF)/cofilin phosphorylation assays demonstrated that PP2A-2 can activate/dephosphorylate ADF/cofilin, an actin-binding protein, in Arabidopsis mesophyll cells. Consistent with this observation, the experiments showed that OA could inhibit ADF1 binding to the actin and suppress the reorganization of the actin cytoskeleton after high-fluence BL irradiation. The adf1 and adf3 mutants also exhibited reduced high-fluence BL-induced chloroplast avoidance movements. In conclusion, we identified that PP2A-2 regulated the activation of ADF/cofilin, which, in turn, regulated actin cytoskeleton remodeling and was involved in phot2-mediated chloroplast avoidance movements.

Over-expression of Arabidopsis Bax inhibitor-1 delays methyl jasmonate-induced leaf senescence by suppressing the activation of MAP kinase 6

Methyl jasmonate (MeJA) is an important signalling molecule that has been reported to be able to promote plant senescence. The cell death suppressor Bax inhibitor-1 (BI1) has been found to suppress stress factor-mediated cell death in yeast and Arabidopsis. However, the effect and the genetic mechanism of Arabidopsis thaliana BI1 (AtBI1) on leaf senescence remain unclear. It was found here that the AtBI1 mutant, atbi1-2 (a gene knock-out), showed accelerated progression of MeJA-induced leaf senescence, while the AtBI1 complementation lines displayed similar symptoms as the WT during the senescence process. In addition, over-expression of the AtBI1 gene delayed the onset of MeJA-induced leaf senescence. Further analyses showed that during the process of MeJA-induced senescence, the activity of MPK6, a mitogen-activated protein kinase (MAPK), increased in WT plants, whereas it was significantly suppressed in AtBI1-overexpressing plants. Under the MeJA treatment, cytosolic calcium ([Ca(2+)](cyt)) functioned upstream of MPK6 activation and the elevation of [Ca(2+)](cyt) was reduced in AtBI1-overexpressing leaves. These results suggested a role of AtBI1 over-expression in delaying MeJA-induced leaf senescence by suppressing the [Ca(2+)](cyt)-dependent activation of MPK6, thus providing a new insight into the function and mechanism of AtBI1 in plant senescence.

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.

Neurospora crassa Light Signal Transduction Is Affected by ROS

In the ascomycete fungus Neurospora crassa blue-violet light controls the expression of genes responsible for differentiation of reproductive structures, synthesis of secondary metabolites, and the circadian oscillator activity. A major photoreceptor in Neurospora cells is WCC, a heterodimeric complex formed by the PAS-domain-containing polypeptides WC-1 and WC-2, the products of genes white collar-1 and white collar-2. The photosignal transduction is started by photochemical activity of an excited FAD molecule noncovalently bound by the LOV domain (a specialized variant of the PAS domain). The presence of zinc fingers (the GATA-recognizing sequences) in both WC-1 and WC-2 proteins suggests that they might function as transcription factors. However, a critical analysis of the phototransduction mechanism considers the existence of residual light responses upon absence of WCC or its homologs in fungi. The data presented point at endogenous ROS generated by a photon stimulus as an alternative input to pass on light signals to downstream targets.

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.

Differential positioning of chloroplasts in C4 mesophyll and bundle sheath cells

Chloroplast photorelocation movement is extensively studied in C3 but not C4 plants. C4 plants have 2 types of photosynthetic cells: mesophyll and bundle sheath cells. Mesophyll chloroplasts are randomly distributed along cell walls, whereas bundle sheath chloroplasts are located close to the vascular tissues or mesophyll cells depending on the plant species. The cell-specific C 4 chloroplast arrangement is established during cell maturation, and is maintained throughout the life of the cell. However, only mesophyll chloroplasts can change their positions in response to environmental stresses. The migration pattern is unique to C4 plants and differs from that of C3 chloroplasts. In this mini-review, we highlight the cell-specific disposition of chloroplasts in C4 plants and discuss the possible physiological significances.

SA and ROS are involved in methyl salicylate-induced programmed cell death in Arabidopsis thaliana

Programmed cell death (PCD) is a genetically encoded, active process that results in the death of individual cells, tissues, or whole organs, which plays an important role in the life cycles of plants and animals. Previous studies show that methyl salicylate (MeSA) is a defense signal molecular associated with systemic acquired resistance and hypersensitive reaction; however, whether MeSA can induce PCD in plant is still unknown. The morphological changes of Arabidopsis thaliana protoplasts exposed to MeSA were observed under fluorescence microscopy and transmission electron microscopy, and the induction of PCD was clearly distinguished by intense perinuclear chromatin margination, condensation of nuclear chromatin and DNA laddering after 3-h exposure of 100 μM MeSA. Our results also showed that salicylic acid (SA) was involved in MeSA-induced PCD by using a transgenic nahG Arabidopsis thaliana line, and the process was mediated by reactive oxygen species, which functioned with SA by making an amplification loop. Our study showed that MeSA could induce PCD in plant cell for the first time.

The avoidance and aggregative movements of mesophyll chloroplasts in C(4) monocots in response to blue light and abscisic acid

In C(4) plants, mesophyll (M) chloroplasts are randomly distributed along the cell walls, whereas bundle sheath chloroplasts are located in either a centripetal or centrifugal position. It was reported previously that only M chloroplasts aggregatively redistribute to the bundle sheath side in response to extremely strong light or environmental stresses. The aggregative movement of M chloroplasts is also induced in a light-dependent fashion upon incubation with abscisic acid (ABA). The involvement of reactive oxygen species (ROS) and red/blue light in the aggregative movement of M chloroplasts are examined here in two distinct subtypes of C(4) plants, finger millet and maize. Exogenously applied hydrogen peroxide or ROS scavengers could not change the response patterns of M chloroplast movement to light and ABA. Blue light irradiation essentially induced the rearrangement of M chloroplasts along the sides of anticlinal walls, parallel to the direction of the incident light, which is analogous to the avoidance movement of C(3) chloroplasts. In the presence of ABA, most of the M chloroplasts showed the aggregative movement in response to blue light but not red light. Together these results suggest that ROS are not involved in signal transduction for the aggregative movement, and ABA can shift the blue light-induced avoidance movement of C(4)-M chloroplasts to the aggregative movement.

In vivo imaging of Ca2+, pH, and reactive oxygen species using fluorescent probes in plants

Changes in the levels of Ca(2+), pH, and reactive oxygen species (ROS) are recognized as key cellular regulators involved in diverse physiological and developmental processes in plants. Critical to understanding how they exert such widespread control is an appreciation of their spatial and temporal dynamics at levels from organ to organelle and from seconds to many hours. With appropriate controls, fluorescent sensors can provide a robust approach with which to quantify such changes in Ca(2+), pH, and ROS in real time, in vivo. The fluorescent cellular probes available for visualization split into two broad classes: (a) dyes and (b) an increasingly diverse set of genetically encoded sensors based around green fluorescent proteins (GFPs). The GFP probes in particular can be targeted to well-defined subcellular locales, offering the possibility of high-resolution mapping of these signals within the cell.

Light, genotype, and abscisic acid affect chloroplast positioning in guard cells of Arabidopsis thaliana leaves in distinct ways

The goal of this study was to investigate the effects of light intensity, genotype, and various chemical treatments on chloroplast movement in guard cells of Arabidopsis thaliana leaves. After treatment at various light intensities (dark, low, and high light), leaf discs were fixed with glutaraldehyde, and imaged using confocal laser microscopy. Each chloroplast was assigned a horizontal (close to pore, center, or epidermal side) and vertical (outer, middle, inner) position. White light had a distinct effect on chloroplast positioning, most notably under high light (HL) when chloroplasts on the upper leaf surface of wild-type (WT) moved from epidermal and center positions toward the pore. This was not the case for phot1-5/phot2-1 or phot2-1 plants, thus phototropins are essential for chloroplast positioning in guard cells. In npq1-2 mutants, fewer chloroplasts moved to the pore position under HL than in WT plants, indicating that white light can affect chloroplast positioning also in a zeaxanthin-dependent way. Cytochalasin B inhibited the movement of chloroplasts to the pore under HL, while oryzalin did not, supporting the idea that actin plays a role in the movement. The movement along actin cables is dependent on CHUP1 since chloroplast positioning in chup1 was significantly altered. Abscisic acid (ABA) caused most chloroplasts in WT and phot1-5/phot2-1 to be localized in the center, middle part of the guard cells irrespective of light treatment. This indicates that not only light but also water stress influences chloroplast positioning.

Hydrogen peroxide mediates the expression of ascorbate-related genes in response to methanol stimulation in Oncidium

We investigated the signaling role of hydrogen peroxide (H(2)O(2)) in regulating the ascorbate (AsA) level after exogenous methanol (MeOH) application. The endogenous H(2)O(2) and AsA levels as well as the expression of related genes were monitored after MeOH treatment of cultures of Oncidium protocorm-like bodies (PLB). A high MeOH concentration was deleterious and caused irreversible consumption of endogenous AsA. However, a low MeOH concentration (50mM) triggered the synthesis of H(2)O(2) and was effective in enhancing the expression of AsA-biosynthetic genes of the Smirnoff-Wheeler and galacturonate (GalUA) pathways. The increased expression of these genes could be blocked by the addition of hydroxylamine, an inhibitor of alcohol oxidase (EC: 1.1.3.13), and diphenyleneiodonium chloride (DPI), an inhibitor of NADPH oxidase (EC: 1.6.3.1). Thus, the H(2)O(2) generated by MeOH application is a product of MeOH detoxification through alcohol oxidase and NADPH oxidase activation. In this chain, H(2)O(2) acts as a secondary messenger for the activation of AsA-related genes. Our results reveal the signaling function of H(2)O(2) and cellular AsA homeostasis in Oncidium orchids in response to MeOH stimulation. A mechanism for the MeOH effect on AsA production is suggested.

Differential positioning of C4 mesophyll and bundle sheath chloroplasts: aggregative movement of C4 mesophyll chloroplasts in response to environmental stresses

In C(4) plants, mesophyll (M) chloroplasts are randomly distributed along the cell walls, while bundle sheath (BS) chloroplasts are typically located in either a centripetal or centrifugal position. We investigated whether these intracellular positions are affected by environmental stresses. When mature leaves of finger millet (Eleusine coracana) were exposed to extremely high intensity light, most M chloroplasts aggregatively re-distributed to the BS side, whereas the intracellular arrangement of BS chloroplasts was unaffected. Compared with the homologous light-avoidance movement of M chloroplasts in C(3) plants, it requires extremely high light (3,000-4,000 micromol m(-2) s(-1)) and responds more slowly (distinctive movement observed in 1 h). The high light-induced movement of M chloroplasts was also observed in maize (Zea mays), another C(4) species, but with a distinct pattern of redistribution along the sides of anticlinal walls, analogous to C(3) plants. The aggregative movement of M chloroplasts occurred at normal light intensities (250-500 micromol m(-2) s(-1)) in response to environmental stresses, such as drought, salinity and hyperosmosis. Moreover, the re-arrangement of M chloroplasts was observed in field-grown C(4) plants when exposed to mid-day sunlight, but also under midsummer drought conditions. The migration of M chloroplasts was controlled by actin filaments and also induced in a light-dependent fashion upon incubation with ABA, which may be the physiological signal transducer. Together these results suggest that M and BS cells of C(4) plants have different mechanisms controlling intracellular chloroplast positioning, and that the aggregative movement of C(4) M chloroplasts is thought to be a protective response under environmental stress conditions.