Cell survival after UV radiation stress in the unicellular chlorophyte Dunaliella tertiolecta is mediated by DNA repair and MAPK phosphorylation

Changes in Carbon Partitioning and Pattern of Antioxidant Enzyme Activity Induced by Arginine Treatment in the Green Microalga Dunaliella salina Under Long-Term Salinity

In this work, the effects of arginine (Arg) on biochemical responses and antioxidant enzyme activity in the green microalga Dunaliella salina grown at different salt concentrations were investigated. Suspensions adapted with the concentrations of 1, 2, and 3 M NaCl were treated at the exponential growth phase with a concentration of 5 mM Arg. Salt stress was associated with a large decrease in the number of cells and non-reducing sugar levels but accumulated higher amounts of chlorophyll, β-carotene, reducing sugar, starch, total protein, free amino acid, and glycerol. Increased levels of protein carbonylation, lipid peroxidation, proteolysis, hydrogen peroxide, and antioxidant enzyme activity also occurred during salinity. Arg treatment changed the pattern of biochemical responses in the cells grown at high salinity by directing carbon flow to the biosynthesis of non-reducing sugars instead of starch, lowering levels of hydrogen peroxide, and downregulating antioxidant enzyme activity, but the levels of lipid peroxidation, glycerol, and β-carotene remained nearly unchanged. These results suggest that Arg treatment alleviates salinity-induced oxidative stress in D. salina cells by modifying carbon partitioning and inducing signaling molecules rather than antioxidant enzymes.

Comparative Study of Algal Responses and Adaptation Capability to Ultraviolet Radiation with Different Nutrient Regimes

Frequent outbreaks of harmful algal blooms (HABs) represent one of the most serious outcomes of eutrophication, and light radiation plays a critical role in the succession of species. Therefore, a better understanding of the impact of light radiation is essential for mitigating HABs. In this study, Chlorella pyrenoidosa and non-toxic and toxic Microcystis aeruginosa were mono-cultured and co-cultured to explore algal responses under different nutrient regimes. Comparisons were made according to photosynthetically active radiation (PAR), UV-B radiation exerted oxidative stresses, and negative effects on the photosynthesis and growth of three species under normal growth conditions, and algal adaptive responses included extracellular polymeric substance (EPS) production, the regulation of superoxide dismutase (SOD) activity, photosynthetic pigments synthesis, etc. Three species had strain-specific responses to UV-B radiation and toxic M. aeruginosa was more tolerant and showed a higher adaptation capability to UV-B in the mono-cultures, including the lower sensitivity and better self-repair efficiency. In addition to stable μmax in PAR ad UV-B treatments, higher EPS production and enhanced production of photosynthetic pigments under UV-B radiation, toxic M. aeruginosa showed a better recovery of its photosynthetic efficiency. Nutrient enrichment alleviated the negative effects of UV-B radiation on three species, and the growth of toxic M. aeruginosa was comparable between PAR and UV-B treatment. In the co-cultures with nutrient enrichment, M. aeruginosa gradually outcompeted C. pyrenoidosa in the PAR treatment and UV-B treatment enhanced the growth advantages of M. aeruginosa, when toxic M. aeruginosa showed a greater competitiveness. Overall, our study indicated the adaptation of typical algal species to ambient UV-B radiation and the stronger competitive ability of toxic M. aeruginosa in the UV-radiated waters with severer eutrophication.

Seasonal cellular stress responses of commercially important invertebrates at different habitats of the North Aegean Sea

In many aquatic species, the negative effect of temperature variations has a significant impact on physiological performance since beyond Tp (upper pejus) and Tc (critical temperatures), according to the oxygen- and capacity-limited thermal tolerance (OCLTT), transition to hypoxemia and mitochondrial metabolism triggers the increase in reactive oxygen species (ROS) production. However, climate change may have different spatial impact, and as a result, areas with more favorable climatic conditions (refugia) can be identified. The aim of the present study, based on cellular stress responses, is the demarcation of these areas and the preservation of commercially important marine species. Under this prism, individuals of the species Callinectes sapidus (blue crab), Sepia officinalis (common cuttlefish), Holothuria tubulosa (sea cucumber) and Venus verrucosa (clam) from Thermaikos, Pagasitikos and Vistonikos gulf were collected seasonally. The results showed an increase in the levels of several stress indicators exhibiting the triggering of Heat Shock Response, MAPK activation, apoptotic phenomena and increased ubiquitilination during the summer sampling in relation to the spring and autumn samplings concerning blue crab and clam, while no changes were observed for common cuttlefish and sea cucumber. It seems that these cellular responses consist a cytoprotective mechanism against environmental thermal stress. Regarding collection sites, for all examined species, higher cellular stress levels were observed in Pagasitikos, and lower in Vistonikos gulf. This analysis of biochemical and molecular markers is expected to provide a clearer picture for the definition of "refugia" for the above species.

Microalgae Nannochloropsis oceanica as a future new natural source of vitamin D3

For the last two decades there has been a rise in awareness about the general low dietary intake of vitamin D₃. Fish have the highest natural content of vitamin D₃, which is suggested to originate from zooplankton and microalgae. However there are no studies reporting which microalgal species may be the source of vitamin D₃. In this study, four selected microalgal species were cultivated during exposure of artificial UVB. The effect of UVB dose on the growth and biochemical composition of the cells (vitamin D₃, PUFAs and carotenoids) was evaluated. Of the four species, exclusively Nannochloropsis oceanica was able to produce vitamin D₃ (up to 1 ± 0.3 µg/g DM), and production was significantly enhanced by increasing the dose of the UVB. These findings suggest that N.oceanica exposed to artificial UVB could be used as a new natural source of vitamin D_3, either as direct source or through animal feed.

Metabolic responses of the green microalga Dunaliella salina to silver nanoparticles-induced oxidative stress in the presence of salicylic acid treatment

In the present study, the biochemical responses and antioxidant enzymes activity of the Dunaliella salina, a green microalga, to the interaction of silver nanoparticles (AgNPs) and salicylic acid (SA) were investigated. Algal suspensions in the phase of logarithmic growth were subjected to the concentrations of 0, 5, 15, and 25 pM AgNPs with or without 1 mM SA. AgNPs level of 25 pM declined cell division but highly accumulated levels of chlorophyll, β-carotene, proteins, free amino acid, carbohydrates, and hydrogen peroxide, which was associated with enhanced the activity of proteolysis, lipid peroxidation, and antioxidant enzymes. SA-treated cells at 25 pM AgNPs improved cell growth but declined the activities of antioxidant enzymes and proteolytic along with a lower accumulation of metabolites except β-carotene relative to untreated controls. These results suggest that AgNPs treatment induce oxidative stress in D. salina cells, which tolerated by alga through the metabolic modifications and accumulating β-carotene, while SA induces AgNPs tolerance by the mechanisms that direct carbon flux to growth and β-carotene biosynthesis rather than the antioxidant enzymes or osmoprotectant metabolites.

Type II-Metacaspases are involved in cell stress but not in cell death in the unicellular green alga Dunaliella tertiolecta

Ultraviolet radiation (UVR; 280–400 nm) has a great impact on aquatic ecosystems by affecting ecophysiological and biogeochemical processes as a consequence of the global change scenario generated by anthropogenic activities. We studied the effect of PAR (P)+UVA (A)+UVB (B) i.e. PAB, on the molecular physiology of the unicellular green alga Dunaliella tertiolecta for six days. We assessed the relationship between the triggered UVR stress response and metacaspases and caspase-like (CL)activities, which are proteases denoted to participate in cell death (CD) in phytoplankton. UVR inhibited cell growth and in vivo chlorophyll a fluorescence but did not cause cell death. Western blot analyses reflected that Type-II metacaspases (MCs) are present and appear to be involved in UVR induced-cell stress but not in dark-induced CD in D. tertiolecta. Enzyme kinetics revealed that cleavage of the MCs-reporter substrates RVRR, QRR, GRR, LKR, HEK, and VLK was 10-fold higher than WEHD, DEVD, IETD, and LETD CLs-substrates. The lowest apparent Michaelis-Menten constants (K_M^ap) corresponded to RVRRase (37.5 μM) indicating a high affinity by the RVRR substrate. The inhibition of enzymatic activities by using inhibitors with different target sites for hydrolyses demonstrated that from all of the R/ Kase activities only RVRRase was a potential candidate for being a metacaspase. In parallel, zymograms and peptide-mass fingerprinting analyses revealed the identities of such Rase activities suggesting an indirect evidence of possible natural physiological substrates of MCs. We present evidence of type II-MCs not being involved in CD in D. tertiolecta, but rather in survival strategies under the stressful irradiance conditions applied in this study.

Phosphoproteomics Reveals the Biosynthesis of Secondary Metabolites in Catharanthus roseus under Ultraviolet-B Radiation

Ultraviolet (UV)-B radiation acts as an elicitor to enhance the production of secondary metabolites in medicinal plants. To investigate the mechanisms, which lead to secondary metabolites in Catharanthus roseus under UVB radiation, a phosphoproteomic technique was used. ATP content increased in the leaves of C. roseus under UVB radiation. Phosphoproteins related to calcium such as calmodulin, calcium-dependent kinase, and heat shock proteins increased. Phosphoproteins related to protein synthesis/modification/degradation and signaling intensively changed. Metabolomic analysis indicated that the metabolites classified with pentoses, aromatic amino acids, and phenylpropanoids accumulated under UVB radiation. Phosphoproteomic and immunoblot analyses indicated that proteins related to glycolysis and the reactive-oxygen species scavenging system were changed under UVB radiation. These results suggest that UVB radiation activates the calcium-related pathway and reactive-oxygen species scavenging system in C. roseus. These changes lead to the upregulation of proteins, which are responsible for the redox reactions in secondary metabolism and are important for the accumulation of secondary metabolites in C. roseus under UVB radiation.

Plant chemical genetics reveals colistin sulphate as a SA and NPR1-independent PR1 inducer functioning via a p38-like kinase pathway

In plants, low-dose of exogenous bacterial cyclic lipopeptides (CLPs) trigger transient membrane changes leading to activation of early and late defence responses. Here, a forward chemical genetics approach identifies colistin sulphate (CS) CLP as a novel plant defence inducer. CS uniquely triggers activation of the PATHOGENESIS-RELATED 1 (PR1) gene and resistance against Pseudomonas syringae pv. tomato DC3000 (Pst DC3000) in Arabidopsis thaliana (Arabidopsis) independently of the PR1 classical inducer, salicylic acid (SA) and the key SA-signalling protein, NON-EXPRESSOR OF PR1 (NPR1). Low bioactive concentration of CS does not trigger activation of early defence markers such as reactive oxygen species (ROS) and mitogen activated protein kinase (MAPK). However, it strongly suppresses primary root length elongation. Structure activity relationship (SAR) assays and mode-of-action (MoA) studies show the acyl chain and activation of a ∼46 kDa p38-like kinase pathway to be crucial for CS' bioactivity. Selective pharmacological inhibition of the active p38-like kinase pathway by SB203580 reverses CS' effects on PR1 activation and root length suppression. Our results with CS as a chemical probe highlight the existence of a novel SA- and NPR1-independent branch of PR1 activation functioning via a membrane-sensitive p38-like kinase pathway.

Oral administration of oyster (Crassostrea gigas) hydrolysates protects against wrinkle formation by regulating the MAPK pathway in UVB-irradiated hairless mice

Chronic ultraviolet (UV) irradiation induces wrinkle formation. UV exposure increases reactive oxygen species (ROS) and upregulates the expression of matrix metalloproteinases (MMPs), which results in skin photoaging. Oyster (Crassostrea gigas), which is an abundant food resource in Asia and Europe, contains various sources of biological compounds and has several effects. Also, oyster hydrolysate (OH) has many biological activities. We investigated the inhibitory effects of OH on wrinkle formation in UVB-irradiated hairless mice. We induced UVB irradiation in hairless mice for 18 weeks and administered OH orally from the 9th week to the 18th week. We performed skin replicas and histological analyses in UVB-irradiated hairless mice dorsal skins. To determine the inhibitory mechanism of OH on wrinkle formation, we measured gene and protein expressions in dorsal skin using RT-qPCR and western blot analyses respectively. In our study, OH decreases wrinkle formation, epidermal thickness and collagen degradation in UVB-irradiated hairless mice. The gene expressions of MMPs were decreased and the gene expressions of collagen type I and TIMP-1 were increased in OH administered groups. Like gene expression tendencies, the protein expressions of MMPs were reduced and that of collagen type I was increased. Furthermore, the phosphorylation levels of ERK, JNK, and p38 were reduced in OH administered groups. We found that OH inhibits wrinkle formation, skin thickening, and collagen degradation by downregulating the MMP expression via the regulation of phosphorylation of MAPK. The results showed that OH significantly prevents UVB-induced photoaging in dorsal skin. Consistent with in vivo data, OH has potential as an anti-wrinkle agent.

The p38-like MAP kinase modulated H2O2 accumulation in wounding signaling pathways of sweet potato

In sweet potato (Ipomoea batatas cv Tainung 57), MAPK cascades are involved in the regulation of Ipomoelin (IPO) expression upon wounding. p38 MAPK plays an important role in plant's responses to various environmental stresses. However, the role of p38-like MAPK in wounding response is still unknown. In this study, the levels of phosphorylated-p38-like MAPK (pp38-like MAPK) in sweet potato were noticeably reduced after wounding. In addition, SB203580 (SB), a specific inhibitor blocking p38 MAPK phosphorylation, considerably decreased the accumulation of pp38-like MAPK. Expression of a wound-inducible gene IPO was elevated by SB. Moreover, it stimulated hydrogen peroxide (H₂O₂) production rather than cytosolic Ca²⁺ elevation in sweet potato leaves. However, NADPH oxidase (NOX) inhibitor diphenyleneiodonium could not inhibit IPO induction stimulated by SB. These results indicated a p38-like MAPK mechanism was involved in the regulation of IPO expression through NOX-independent H₂O₂ generation. In addition, the presence of the protein phosphatase inhibitor okadaic acid or the MEK1/ERK inhibitor PD98059 repressed the H₂O₂- or SB-induced IPO expression, demonstrating phosphatase(s) and MEK1/ERK functioning in the downstream of H₂O₂ and pp38-like MAPK in the signal transduction pathway stimulating IPO. Conclusively, wounding decreased the amount of pp38-like MAPK, stimulated H₂O₂ production, and then induced IPO expression.

The induction of salt stress tolerance by propyl gallate treatment in green microalga Dunaliella bardawil, through enhancing ascorbate pool and antioxidant enzymes activity

The effect of propyl gallate (PG), a synthetic antioxidant, on antioxidant responses and salinity tolerance was investigated in the cells of the green microalga, Dunaliella bardawil. Algal suspensions grown at three salinity levels of 1, 2, and 3 M NaCl were incubated with 1 mM of PG. The number of cells was significantly lower in all PG-treated cells compared to untreated controls. Despite PG-induced cell death, the fresh weight of all PG-treated cells was considerably higher than controls. PG-treated cells had enhanced antioxidant capacity because of increased levels of Chlorophyll a, β-carotene, reduced ascorbate, protein, and enzymatic activities, but accumulated lower levels of malonyldialdehyde and hydrogen peroxide compared to untreated cells. The results suggest that PG acts as a signal molecule both directly by reducing of free radical oxidants and indirectly by augmenting ascorbate pool levels, β-carotene production, and antioxidant enzymes activity to boost the capacity of antioxidant systems and radical oxygen species scavenging. Therefore, induction of salt stress tolerance by PG in D. bardawil is associated with metabolic adjustments through activation or synthesis of both enzymatic and non-enzymatic molecules involved in antioxidant systems.

The c-Jun N-terminal kinase prevents oxidative stress induced by UV and thermal stresses in corals and human cells

Coral reefs are of major ecological and socio-economic interest. They are threatened by global warming and natural pressures such as solar ultraviolet radiation. While great efforts have been made to understand the physiological response of corals to these stresses, the signalling pathways involved in the immediate cellular response exhibited by corals remain largely unknown. Here, we demonstrate that c-Jun N-terminal kinase (JNK) activation is involved in the early response of corals to thermal and UV stress. Furthermore, we found that JNK activity is required to repress stress-induced reactive oxygen species (ROS) accumulation in both the coral Stylophora pistillata and human skin cells. We also show that inhibiting JNK activation under stress conditions leads to ROS accumulation, subsequent coral bleaching and cell death. Taken together, our results suggest that an ancestral response, involving the JNK pathway, is remarkably conserved from corals to human, protecting cells from the adverse environmental effects.

Mapping out p38MAPK

To generate new hypotheses, sometimes a "systems" approach is needed. In this review, I focus on the mitogen-activated kinase p38 because it has been recently shown to play an important role in the developmental programing and senescence of normal and stressed reproductive tissues. What follows is an overview of (i) pathways of p38 activation and their involvement in basic biological processes, (ii) evidence that p38 is involved in the homeostasis of reproductive tissues, (iii) how focus on p38 can be incorporated into investigation of normal and stressed pregnancies. Existence of excellent reviews will be mentioned as well as relevant animal models.

Comparative Transcriptomic Analysis of the Response of Dunaliella acidophila (Chlorophyta) to Short-Term Cadmium and Chronic Natural Metal-Rich Water Exposures

Heavy metals are toxic compounds known to cause multiple and severe cellular damage. However, acidophilic extremophiles are able to cope with very high concentrations of heavy metals. This study investigated the stress response under natural environmental heavy metal concentrations in an acidophilic Dunaliella acidophila. We employed Illumina sequencing for a de novo transcriptome assembly and to identify changes in response to high cadmium concentrations and natural metal-rich water. The photosynthetic performance was also estimated by pulse amplitude-modulated (PAM) fluorescence. Transcriptomic analysis highlights a number of processes mainly related to a high constitutive expression of genes involved in oxidative stress and response to reactive oxygen species (ROS), even in the absence of heavy metals. Photosynthetic activity seems to be unaltered under short-term exposition to Cd and chronic exposure to natural metal-rich water, probably due to an increase in the synthesis of structural photosynthetic components preserving their functional integrity. An overrepresentation of Gene Ontology (GO) terms related to metabolic activities, transcription, and proteosomal catabolic process was observed when D. acidophila grew under chronic exposure to natural metal-rich water. GO terms involved in carbohydrate metabolic process, reticulum endoplasmic and Golgi bodies, were also specifically overrepresented in natural metal-rich water library suggesting an endoplasmic reticulum stress response.

The Role of Phospholipase D and MAPK Signaling Cascades in the Adaption of Lichen Microalgae to Desiccation: Changes in Membrane Lipids and Phosphoproteome

Classically, lichen phycobionts are described as poikilohydric organisms able to undergo desiccation due to the constitutive presence of molecular protection mechanisms. However, little is known about the induction of cellular responses in lichen phycobionts during drying. The analysis of the lipid composition of the desiccated lichen microalga Asterochloris erici revealed the unusual accumulation of highly polar lipids (oligogalactolipids and phosphatidylinositol), which prevents the fusion of membranes during stress, but also the active degradation of cone-shaped lipids (monogalactosyldiacylglycerol and phosphatidylethanolamine) to stabilize membranes in desiccated cells. The level of phosphatidic acid increased 7-fold during desiccation, implicating a possible role for phospholipase D (PLD) in the response to osmotic stress. Inhibition of PLD with 1-butanol markedly impaired the recovery of photosynthesis activity in A. erici upon desiccation and salt stress (2 M NaCl). These two hyperosmotic stresses caused the phosphorylation of c-Jun N-terminal kinase (JNK) and p38-like mitogen-activated protein kinase (MAPK) and the dephosphorylation of extracellular signal-regulated kinase (ERK). The incubation with 1-butanol reduced the phosphorylation of JNK-like proteins and increased the dephosphorylation of ERK-like proteins, which indicates an upstream control of MAPK cascades by PLD. The phosphoproteome showed that desiccation caused the phosphorylation of several proteins in A. erici, most of them involved in protein turnover. The results demonstrate that lichen phycobionts possess both constitutive and inducible protective mechanisms to acquire desiccation tolerance. Among others, these responses are controlled by the PLD pathway through the activation of MAPK cascades.

Elevated Expression of Hepatoma Up-Regulated Protein Inhibits γ-Irradiation-Induced Apoptosis of Prostate Cancer Cells

Despite progression in diagnosis and treatment, prostate cancer (PCa) still represents the main cause of cancer-related mortality and morbidity in men. Although radiation therapy offers clinical benefit over other therapeutic modalities, the success of this therapeutic modality is commonly hampered by the resistance of advanced tumors. So far, the mechanisms governing tumor resistance to radiotherapy are not discussed in detail. Here, we demonstrate for the first time that the resistance of PCa to radiation therapy is attributed to elevated expression of Hepatoma Up-Regulated Protein (HURP). In PCa cells, the induction of HURP expression suppresses γ-irradiation-induced apoptosis. γ-irradiation-induced apoptosis of PCa cells is associated with expression of E2F1, p53, p21 proteins together with the phosphorylation of apoptosis signal-regulating kinase1 (ASK1), c-jun-N-terminal kinase (JNK) and Ataxia-telangiectasia mutated (ATM) and histone family member X (H2AX). Whereas, the induction of HURP expression is able to suppress γ-irradiation-induced effects on E2F1, p53, p21, ATM, ASK1, JNK and ATM, and H2AX. Also, inhibition of γ-irradiation-induced- cytochrome c release, cleavage of caspase-9, caspase-3, PARP, and reactive oxygen species (ROS) were noted in PCa cells induced for HURP expression. The observed radio-resistance of PCa is thought to be the consequence of HURP-mediated destabilization of p53 and ATM proteins that are essential for the modulation of γ-irradiation-induced apoptosis. Thus, based on our findings, PCa resistance to radiation therapy results from the deregulation of ASK1/ JNK; ATM/ H2AX; ATM/p53 and checkpoint kinase 2 (Chk2)/ E2F-1 in response to the elevated expression of HURP.

Elevated CO2 alleviates high PAR and UV stress in the unicellular chlorophyte Dunaliella tertiolecta

The effects of increased CO2 and irradiance on the physiological performance of the chlorophyte Dunaliella tertiolecta were studied at different PAR and UVR (UVA + UVB) irradiances, simulating the solar radiation at different depths, at present (390 ppmv, LC) and predicted CO2 levels for the year 2100 (1000 ppmv, HC). Elevated CO2 resulted in higher optimum and effective quantum yields (F(v)/F(m) and ϕPSII, respectively), electron transport rates (ETR) and specific growth rates (μ). Cell stress was alleviated in HC with respect to LC as evidenced by a decrease in reactive oxygen species (ROS) accumulation. DNA damage showed a 42-fold increase in cyclobutane-pyrimidine dimer (CPD) formation under the highest irradiance (1100 μmol quanta m(-2) s(-1)) in LC with respect to the lowest irradiance (200 μmol quanta m(-2) s(-1)). Photolyase (CII-PCD-PL) gene expression was upregulated under HC resulting in a drastic decrease in CPD accumulation to only 25% with respect to LC. Proliferating cell nuclear antigen (PCNA) accumulation was always higher in HC and the accumulation pattern indicated its involvement in repair or growth depending on the irradiance dose. The repressor of silencing (ROS1) was only marginally involved in the response, suggesting that photoreactivation was the most relevant mechanism to overcome UVR damage. Our results demonstrate that future scenarios of global change result in alleviation of irradiance stress by CO2-induced photoprotection in D. tertiolecta.

Metabolic switching: synergistic induction of carotenogenesis in the aerial microalga, Vischeria helvetica, under environmental stress conditions by inhibitors of fatty acid biosynthesis

OBJECTIVES

To investigate the roles of fatty acid biosynthesis in carotenogenesis in the high-lipid accumulating aerial microalga Vischeria helvetica KGU-Y001, we cultured algal cells with fatty acid biosynthesis inhibitors.

RESULTS

Under nitrogen-deficient, high-light (200 µmol photons m(-2) s(-1)) conditions, the alga accumulated 6.2 mg carotenoids g(-1) dry weight cells (DWC) after 1 week of culture. The total fatty acid content increased gradually, and reached 290 mg g(-1) DWC after 9 weeks. When algal cells were cultured with a fatty acid biosynthesis inhibitor (molinate) under nitrogen-deficient, high-light conditions for 1 week, carotenoid accumulation was synergistically increased to 2.4 times that in algal cells cultured without the inhibitor in nitrogen-deficient, low-light conditions (40 µmol photons m(-2) s(-1)). The synergistic induction of carotenogenesis was suppressed by an inhibitor of c-jun N-terminal kinase, a mitogen-activated protein kinase-like protein.

CONCLUSION

In a commercial context, carotenoid production could be increased by using fatty acid biosynthesis inhibitors to redirect metabolic flux to carotenoid biosynthesis instead of fatty acid synthesis.

The induction of menadione stress tolerance in the marine microalga, Dunaliella viridis, through cold pretreatment and modulation of the ascorbate and glutathione pools

The effect of cold pretreatment on menadione tolerance was investigated in the cells of the marine microalga, Dunaliella viridis. In addition, the involvement of ascorbate and glutathione in the response to menadione stress was tested by treating cell suspensions with l-galactono-1,4-lactone, an ascorbate precursor, and buthionine sulfoximine, an inhibitor of glutathione synthesis. Menadione was highly toxic to non cold-pretreated cells, and caused a large decrease in cell number. Cold pretreatment alleviated menadione toxicity and cold pretreated cells accumulated lower levels of reactive oxygen species, and had enhanced antioxidant capacity due to increased levels of β-carotene, reduced ascorbate and total glutathione compared to non cold-pretreated cells. Cold pretreatment also altered the response to l-galactono-1,4-lactone and buthionine sulfoximine treatments. Combined l-galactono-1,4-lactone and menadione treatment was lethal in non-cold pretreated cells, but in cold-pretreated cells it had a positive effect on cell numbers compared to menadione alone. Overall, exposure of Dunaliella cells to cold stress enhanced tolerance to subsequent oxidative stress induced by menadione.

Phosphorylation of a p38-like MAPK is involved in sensing cellular redox state and drives atypical tubulin polymer assembly in angiosperms

Reactive oxygen species (ROS) imbalance is a stressful condition for plant cells accompanied by dramatic changes in tubulin cytoskeleton. Here, evidence is provided that alterations in ROS levels directly interfere with the phosphorylation state of a p38-like MAPK in the angiosperms Triticum turgidum and Arabidopsis thaliana. Both oxidative stress generators and chemicals inducing ROS scavenging or decreasing ROS production resulted in the accumulation of a phospho-p46 protein similar to p38-MAPK. Importantly, the rhd2 A. thaliana mutants exhibited a remarkable increase in levels of phospho-p46. The presence of the p38-MAPK inhibitor SB203580 attenuated the response to ROS disturbance, prevented microtubule disappearance and resulted in a dramatic decrease in the number of atypical tubulin polymers. Moreover, in roots treated simultaneously with substances inducing ROS overproduction and others resulting in low ROS levels, phospho-p46 levels and the organization of tubulin cytoskeleton were similar to controls. Collectively, our experimental data suggest, for the first time in plants, that p46 functions as a putative sensor of redox state, the activation of which initiates downstream signalling events leading to microtubule disruption and subsequent assembly of atypical tubulin polymers. Thus, p46 seems to participate in perception of ROS homeostasis disturbance as well as in cellular responses to redox imbalance.