Proteomic changes in response to low-light stress during cotton fiber elongation

Numerous studies have illustrated that low light is one of the major abiotic stresses limiting cotton (Gossypium hirsutum L.) fiber length, but studies addressing molecular mechanisms contributing to reduced fiber growth under low light are lacking. To investigate the molecular mechanisms of cotton fiber elongation in response to low light, an experiment of low light caused by shading was conducted with cotton cultivar NuCOTN 33B. The results showed that low light resulted in shorter fiber length. Proteomic analysis of four developmental stages (5, 10, 15 and 20 days post-anthesis) showed that 49 proteins were significantly responsive to low light. 39 differentially expressed proteins that included some known as well as some novel low-light stress-responsive proteins were identified. These differentially expressed proteins were involved in signal transduction, carbohydrate/energy metabolism, cell wall component synthesis, protein metabolism, cytoskeleton, nitrogen metabolism and stress responses. The results also showed that the decrease in fiber length might be because the levels of signal-related protein (phospholipase D), cytoskeletal proteins (two annexins isoforms), cell wall component-related proteins (sucrose synthase, UDP-d-glucuronic acid 4-epimerase and rhamnose synthase), carbohydrate metabolism-proteins (phosphofructokinase, dihydrolipoamide dehydrogenase, vacuolar H⁺-ATPase catalytic subunit, malate dehydrogenase and isocitrate dehydrogenase), and stress-related proteins (peroxisomal catalase, short chain alcohol dehydrogenase) were decreased under low light.     

Photosynthetic acclimation of Chlorella saccharophila to heat stress

Photosynthesis is one of the most important metabolic processes of algae; which is altered as a stress response. During mass cultivation of algae, temperature rise and high light are major factors that affect biomass productivity. High temperature affects photosystem II (PSII) complex irreversibly, damaging intermolecular interactions in it. However, the impact of high temperature on photosynthesis is highly variable among different algal species, depending on the prior acclimation to environmental conditions they were exposed to. The acclimation plays an important role in combating high temperature stress via regulation of photosynthetic responses. Chlorophyll a fluorescence is a highly sensitive, non‐destructive and reliable tool for such measurements of photosynthetic parameters, which provides information about algal photosynthetic performance under given conditions. To understand the effect of heat stress on the responses of high light acclimated alga Chlorella saccharophila, chlorophyll a fluorescence transients were measured after heat exposure at 40°C. Our study demonstrates that rise in temperature for short duration; during open field cultivation reversibly affects the efficiency of PSII in light acclimated alga C. saccharophila. The effects of heat stress on chlorophyll a fluorescence in this alga, grown under high light (max‐1600 μmol photons m^?2 s^?1) are presented here; they are used to infer changes in photosynthetic process during its exposure to heat, as well as their recovery after 72 h. We speculate that heat resistance may have been acquired due to prior exposures to high light.     

Physiological acclimation to drought stress in Solidago canadensis

Solidago canadensis is an invasive species from North America that is spreading across Europe, Australia and temperate Asia. We hypothesized that the species' wide ecological amplitude is also based on its potential in hydraulic acclimation, and analyzed hydraulic and anatomical properties along a transect with decreasing soil humidity. Stem hydraulic conductivity, vulnerability to drought‐induced embolism, stomatal closure during dehydration and xylem‐anatomical parameters were quantified at three sites. At the humid site, specific hydraulic conductivity of stems (1.0 ± 0.2 kg m^–1 MPa^–1 s^–1) was about twofold higher, and leaf‐specific conductivity about 1.5 times higher (3.1 ± 0.5 kg m^–1 MPa^–1 s^–1) than at the dry site. Water potential (Ψ) at 50% loss of conductivity was ?3.7 ± 0.1 MPa at the dry site and ?3.1 ± 0.2 MPa at the humid site (September). Vulnerability to drought‐induced embolism decreased along the transect and over the vegetation period. At drier sites, stomata started closing at lower Ψ while complete stomatal closure was reached at less negative Ψ (12% of maximum stomatal conductance: –2.5 ± 0.0 and ?3.0 ± 0.2 MPa at the dry and humid site). The safety margin between stomatal closure and 50% loss of conductivity was 1.2 and 0.2 MPa at the dry and humid sites. The observed variability indicated an efficient acclimation in hydraulic conductivity and safety: plants at dry sites exhibited lower specific hydraulic conductivity, higher embolism resistance and broader safety margins, signifying a trade‐off between the hydraulic safety and efficiency. The observed intraspecific plasticity in hydraulic and anatomical traits may help to explain the invasive potential of this species.     

Photosynthetic acclimation: state transitions and adjustment of photosystem stoichiometry--functional relationships between short-term and long-term light quality acclimation in plants

In dense plant populations, individuals shade each other resulting in a low-light habitat that is enriched in far-red light. This light quality gradient decreases the efficiency of the photosynthetic light reaction as a result of imbalanced excitation of the two photosystems. Plants counteract such conditions by performing acclimation reactions. Two major mechanisms are known to assure efficient photosynthesis: state transitions, which act on a short-term timescale; and a long-term response, which enables the plant to re-adjust photosystem stoichiometry in favour of the rate-limiting photosystem. Both processes start with the perception of the imbalanced photosystem excitation via reduction/oxidation (redox) signals from the photosynthetic electron transport chain. Recent data in Arabidopsis indicate that initialization of the molecular processes in both cases involve the activity of the thylakoid membrane-associated kinase, STN7. Thus, redox-controlled phosphorylation events may not only adjust photosystem antenna structure but may also affect plastid, as well as nuclear, gene expression. Both state transitions and the long-term response have been described mainly in molecular terms, while the physiological relevance concerning plant survival and reproduction has been poorly investigated. Recent studies have shed more light on this topic. Here, we give an overview on the long-term response, its physiological effects, possible mechanisms and its relationship to state transitions as well as to nonphotochemical quenching, another important short-term mechanism that mediates high-light acclimation. Special emphasis is given to the functional roles and potential interactions between the different light acclimation strategies. A working model displays the various responses as an integrated molecular system that helps plants to acclimate to the changing light environment.     

Changes in plasma-membrane lipid composition: a strategy for acclimation to copper stress

Wheat seedlings were grown hydroponically in the presence of 50 microM Cu2+. The copper stress resulted in plasma-membrane (PM) changes of the root cells as altered lipid composition, a decreased phosphatidylcholine (PC)/phosphatidylethanolamine (PE) ratio from 0.7 to 0.3, a decreased fatty acyl unsaturation and a decrease in the lipid/protein ratio. Membrane vesicles made from total lipid extracts of isolated PMs of wheat grown under copper excess showed a remarkably low permeability to polar molecules like glucose, as compared with the control, and no difference in proton permeability. Permeability studies of vesicles of plasma-membrane lipids, which were selectively modified by addition of specific lipids such as PC and PE, were also performed. The results are discussed with emphasis on the role of the increased PE proportion.     

Metabolic and hormonal acclimation to heat stress in domesticated ruminants

Environmentally induced periods of heat stress decrease productivity with devastating economic consequences to global animal agriculture. Heat stress can be defined as a physiological condition when the core body temperature of a given species exceeds its range specified for normal activity, which results from a total heat load (internal production and environment) exceeding the capacity for heat dissipation and this prompts physiological and behavioral responses to reduce the strain. The ability of ruminants to regulate body temperature is species- and breed-dependent. Dairy breeds are typically more sensitive to heat stress than meat breeds, and higher-producing animals are more susceptible to heat stress because they generate more metabolic heat. During heat stress, ruminants, like other homeothermic animals, increase avenues of heat loss and reduce heat production in an attempt to maintain euthermia. The immediate responses to heat load are increased respiration rates, decreased feed intake and increased water intake. Acclimatization is a process by which animals adapt to environmental conditions and engage behavioral, hormonal and metabolic changes that are characteristics of either acclimatory homeostasis or homeorhetic mechanisms used by the animals to survive in a new 'physiological state'. For example, alterations in the hormonal profile are mainly characterized by a decline and increase in anabolic and catabolic hormones, respectively. The response to heat load and the heat-induced change in homeorhetic modifiers alters post-absorptive energy, lipid and protein metabolism, impairs liver function, causes oxidative stress, jeopardizes the immune response and decreases reproductive performance. These physiological modifications alter nutrient partitioning and may prevent heat-stressed lactating cows from recruiting glucose-sparing mechanisms (despite the reduced nutrient intake). This might explain, in large part, why decreased feed intake only accounts for a minor portion of the reduced milk yield from environmentally induced hyperthermic cows. How these metabolic changes are initiated and regulated is not known. It also remains unclear how these changes differ between short-term v. long-term heat acclimation to impact animal productivity and well-being. A better understanding of the adaptations enlisted by ruminants during heat stress is necessary to enhance the likelihood of developing strategies to simultaneously improve heat tolerance and increase productivity.     

Resistance to thermal stress in adult Drosophila buzzatii: acclimation and variation among populations

Survival after heat stress was examined in adult Drosophila buzzatii from seven localities. Males and females were conditioned by a non-lethal high temperature before exposure to a severe thermal stress. Variable times elapsed between conditioning, either as adults or pupae, and exposure to the stress were used, and experimental times to the stress ranged from 2 to 96 hours. Survival after stress varied among populations, and differences generally were consistent across conditioning treatments and across experiments. Resistance to thermal stress was much higher following one conditioning bout 2–4 hours before exposure to a severe stress than when the time elapsed between conditioning and exposure was increased to 24 or more hours. Significantly more adults survived the stress if conditioned 4 days before exposure to the thermal stress, either as adults or as pupae, than if not conditioned. The rank order of resistance roughly followed that predicted from the climatic conditions of the localities of origin.     

Applications of low-light imaging to life sciences

Photon imaging is a new technique for the quantitative analysis of bioluminescence and chemiluminescence and can be performed both at the macro and micro levels. The high sensitivity and spatial resolution of photon-counting cameras have resulted in the development of new applications in the life sciences. At the macro level, imaging is a valuable tool for the rapid identification of biological samples emitting long-lasting glows in assays using microtitre plates or filter formats (immunoassays, DNA probes, phagocytosis, gene expression, metabolite and drug analysis) and also for in vivo studies of promoter activity. At the micro level, low-light imaging can be used for analysing multiple analytes on micro sensors and for advanced cell analysis (immunocytology, in situ hybridization, identification of cells or tissues expressing the luciferase gene, intracellular or intercellular protein traffic, metabolite analysis and imaging of Ca²⁺ flux and phosphorylation reactions). Two-dimensional photon-counting instrumentation is a versatile and powerful research tool for imaging and is complementary to conventional luminometers. The main applications to the life sciences involve many types of luminescence assays and can be performed on multiple samples in standard and non-standard formats. Photon-counting coupled to imaging is very helpful in selecting microorganisms or cells expressing bioluminescent genes. Measurements can be made in vitro and in vivo with a sensitivity comparable to that of phototube luminometers.     

Optimizing nitrogen economy under drought: increased leaf nitrogen is an acclimation to water stress in willow (Salix spp.)

Background and Aims

The major objective was to identify plant traits functionally important for optimization of shoot growth and nitrogen (N) economy under drought. Although increased leaf N content (area basis) has been observed in dry environments and theory predicts increased leaf N to be an acclimation to drought, experimental evidence for the prediction is rare.

Methods

A pedigree of 200 full-sibling hybrid willows was pot-grown in a glasshouse in three replicate blocks and exposed to two water regimes for 3 weeks. Drought conditions were simulated as repeated periods of water shortage. The total leaf mass and area, leaf area efficiency (shoot growth per unit leaf area, E_A), area-based leaf N content (N_A), total leaf N pool (N_L) and leaf N efficiency (shoot growth per unit leaf N, E_N) were assessed.

Key Results

In the water-stress treatment, shoot biomass growth was N limited in the genotypes with low N_L, but increasingly limited by other factors in the genotypes with greatest N_L. The N_A was increased by drought, and drought-induced shift in N_A varied between genotypes (significant G × E). Judged from the E_A–N_A relationship, optimal N_A was 16 % higher in the water-stress compared with the well-watered treatment. Biomass allocation to leaves and shoots varied between treatments, but the treatment response of the leaf : shoot ratio was similar across all genotypes.

Conclusions

It is concluded that N-uptake efficiency and leaf N efficiency are important traits to improve growth under drought. Increased leaf N content (area basis) is an acclimation to optimize N economy under drought. The leaf N content is an interesting trait for breeding of willow bioenergy crops in a climate change future. In contrast, leaf biomass allocation is a less interesting breeding target to improve yield under drought.     

Heat stress in grapevine: the pros and cons of acclimation

Heat stress is a major limiting factor of grapevine production and quality. Acclimation and recovery are essential to ensure plant survival, and the recovery mechanisms can be independent of the heat response mechanisms. An experimental set up with and without acclimation to heat followed by recovery [stepwise acclimation and recovery (SAR) and stepwise recovery (SR), respectively] was applied to two grapevine varieties, Touriga Nacional (TN), and Trincadeira (TR), with different tolerance to abiotic stress. Major differences were found between leaves of SAR and SR, especially after recovery; in SAR, almost all parameters returned to basal levels while in SR they remained altered. Acclimation led to a swifter and short‐term antioxidative response, affecting the plant to a lesser extent than SR. Significant differences were found among varieties: upon stress, TN significantly increased ascorbate and glutathione reduction levels, boosting the cell's redox‐buffering capacity, while TR needed to synthesize both metabolites, its response being insufficient to keep the redox state at working levels. TR was affected by stress for a longer period and the up‐regulation pattern of antioxidative stress genes was more obvious. In TN, heat shock proteins were significantly induced, but the canonical heat‐stress gene signature was not evident probably because no shutdown of the housekeeping metabolism was needed.     

Physiology of acclimation to salinity stress in pea (Pisum sativum)

Pea (Pisum sativum L.) seedlings were grown in half strength Hoagland solution and exposed to 0, 10, 25 mM NaCl and 2.5% PEG 6000 for 1 week (pre-treatment). Thereafter plants were exposed to 0 and 80 mM NaCl for 2 weeks (main treatment). The control plants were maintained in half strength Hoagland solution without NaCl. Various physiological parameters were recorded from control, pretreated and non-pretreated plants. There was no negative effect of the pre-treatments on growth (total fresh and dry matter production), and plants pre-treated with 10 mM NaCl had biomass accumulation equal to control plants. The beneficial effect of salt acclimation was also evident in the prevention of K⁺ leakage and Na⁺ accumulation, primary in roots, suggesting that here the physiological processes play the major role. 2.5% PEG 6000 was not as efficient as salt in enhancing salt tolerance and acclimation appears to be more related to ion-specific rather than osmotic component of stress. We also recorded an increase of the xylem K/Na in the salt acclimated plants. Therefore, the present study reveals that short-term exposure of the glycophyte P. sativum species activates a set of physiological adjustments enabling the plants to withstand severe saline conditions, and while acclimation takes place primary in the root tissues, control of xylem ion loading and efficient Na⁺ sequestration in mesophyll cells are also important components of this process.     

The time course of acclimation to the stress of triose phosphate use limitation

Triose phosphate utilisation (TPU) limits the maximum rate at which plants can photosynthesise. However, TPU is almost never found to be limiting photosynthesis under ambient conditions for plants. This, along with previous results showing adaptability of TPU at low temperature, suggest that TPU capacity is regulated to be just above the photosynthetic rate achievable under the prevailing conditions. A set of experiments were performed to study the adaptability of TPU capacity when plants are acclimated to elevated CO₂ concentrations. Plants held at 1500 ppm CO₂ were initially TPU limited. After 30 h they no longer exhibited TPU limitations but they did not elevate their TPU capacity. Instead, the maximum rates of carboxylation and electron transport declined. A timecourse of regulatory responses was established. A step increase of CO₂ first caused PSI to be oxidised but after 40 s both PSI and PSII had excess electrons as a result of acceptor-side limitations. Electron flow to PSI slowed and the proton motive force increased. Eventually, non-photochemical quenching reduced electron flow sufficiently to balance the TPU limitation. Over several minutes rubisco deactivated contributing to regulation of metabolism to overcome the TPU limitation.     

Cold acclimation allows Drosophila flies to maintain mitochondrial functioning under cold stress

Environmental stress generally disturbs cellular homeostasis. Researchers have hypothesized that chilling injury is linked to a shortage of ATP. However, previous studies conducted on insects exposed to nonfreezing low temperatures presented conflicting results. In this study, we investigated the mitochondrial bioenergetics of Drosophila melanogaster flies exposed to chronic cold stress (4 °C). We assessed mitochondrial oxygen consumption while monitoring the rate of ATP synthesis at various times (0, 1, 2, and 3 days) during prolonged cold stress and at two assay temperatures (25 and 4 °C). We compared organelle responses between cold-susceptible and cold-acclimated phenotypes. Continuous exposure to low temperature provoked temporal declines in the rates of mitochondrial respiration and ATP synthesis. Respiratory control ratios (RCRs) suggested that mitochondria were not critically uncoupled. Nevertheless, after 3 days of continuous cold stress, a sharp decline in the mitochondrial ATP synthesis rate was observed in control flies when they were assayed at low temperature. This change was associated with reduced survival capacity in control flies. In contrast, cold-acclimated flies exhibited high survival and maintained higher rates of mitochondrial ATP synthesis and coupling (i.e., higher RCRs). Adaptive changes due to cold acclimation observed in the whole organism were thus manifested in isolated mitochondria. Our observations suggest that cold tolerance is linked to the ability to maintain bioenergetics capacity under cold stress.     

Non-photochemical quenching-dependent acclimation and thylakoid organization of Chlamydomonas reinhardtii to high light stress

Light is essential for all photosynthetic organisms while an excess of it can lead to damage mainly the photosystems of the thylakoid membrane. In this study, we have grown Chlamydomonas reinhardtii cells in different intensities of high light to understand the photosynthetic process with reference to thylakoid membrane organization during its acclimation process. We observed, the cells acclimatized to long-term response to high light intensities of 500 and 1000 µmol m^?2 s^?1 with faster growth and more biomass production when compared to cells at 50 µmol m^?2 s^?1 light intensity. The ratio of Chl a/b was marginally decreased from the mid-log phase of growth at the high light intensity. Increased level of zeaxanthin and LHCSR3 expression was also found which is known to play a key role in non-photochemical quenching (NPQ) mechanism for photoprotection. Changes in photosynthetic parameters were observed such as increased levels of NPQ, marginal change in electron transport rate, and many other changes which demonstrate that cells were acclimatized to high light which is an adaptive mechanism. Surprisingly, PSII core protein contents have marginally reduced when compared to peripherally arranged LHCII in high light-grown cells. Further, we also observed alterations in stromal subunits of PSI and low levels of PsaG, probably due to disruption of PSI assembly and also its association with LHCI. During the process of acclimation, changes in thylakoid organization occurred in high light intensities with reduction of PSII supercomplex formation. This change may be attributed to alteration of protein–pigment complexes which are in agreement with circular dichoism spectra of high light-acclimatized cells, where decrease in the magnitude of psi-type bands indicates changes in ordered arrays of PSII–LHCII supercomplexes. These results specify that acclimation to high light stress through NPQ mechanism by expression of LHCSR3 and also observed changes in thylakoid protein profile/supercomplex formation lead to low photochemical yield and more biomass production in high light condition.

     

Banana (Musa spp.) as a model to study the meristem proteome: acclimation to osmotic stress

Banana (Musa spp.) multiple shoot meristems are an excellent model to study the meristem proteome. Using a 2-DE protocol developed for small amounts of tissue and MS-based cross species polypeptide identification, we have revealed the meristem proteome and investigated the influence of sucrose-mediated osmotic stress in a dehydration-tolerant variety. Proteins that were significantly up- or down-regulated due to the high-sucrose treatment were classified using non-parametric univariate statistics. Our results suggest that the maintenance of an osmoprotective intracellular sucrose concentration, the enhanced expression of particular genes of the energy-conserving glycolysis and the conservation of the cell wall integrity are essential to maintain homeostasis, to acclimate and to survive dehydration. By comparing the dehydration-tolerant variety with a dehydration-sensitive variety, we were able to distinguish several genotype-specific proteins (isoforms), and could associate the dehydration-tolerant variety with proteins involved in energy metabolism (e.g., phosphoglycerate kinase, phosphoglucomutase, UDP-glucose pyrophosphorylase) and proteins that are associated with stress adaptation (e.g., OSR40-like protein, abscisic stress ripening protein-like protein). This work shows that proteome analysis can be used successfully to perform quantitative difference analysis and to characterize genetic variations in a recalcitrant crop.     

Towards an understanding of photosynthetic acclimation

It has long been recognized that higher plants vary the composition and organization of the photosynthetic apparatus in response to the prevailing environmental conditions, with particular attention being paid to the responses to incident light. Under high light conditions there are increases in the amounts of photosystems, electron transport and ATP synthase complexes, and enzymes of the Calvin-Benson cycle; conversely, under low light there is an increase in the relative amounts of light-harvesting complexes (LHC) and in the stacking of thylakoid membranes to form grana. It is believed that these changes are of adaptive significance, and in a few instances evidence has been provided that this is indeed the case; an increase in photosynthetic capacity reduces susceptibility to photodamage, while changes in photosystem stoichiometry serve to optimize light utilization. By contrast, the potential benefit to the plant of other changes in chloroplast composition, such as in the levels of LHC, is far less clear. It is also believed that redox signals derived from photosynthetic electron transport play an important regulatory role in acclimation. However, while there is convincing evidence that such redox signals modulate the expression of many plastidic and nuclear genes encoding photosynthetic components, there is little to demonstrate that such changes are responsible for regulating chloroplast composition. This review discusses the evidence that particular aspects of acclimation are advantageous to the plant, and highlights the significant gaps in our understanding of the mechanisms underlying acclimation.