Age-dependent changes in the functions and compositions of photosynthetic complexes in the thylakoid membranes of Arabidopsis thaliana

The contrasting photosynthesis and growth response of young test species irrigated with electro-chemical modified water

The study evaluated the effects of treating irrigation water with a coaxial flow variator (CFV) on the morpho-physiology of pot-cultivated test species, including cucumber (Cucumis sativus, CU), lettuce (Lactuca sativa, LE), and sorghum (Sorghum vulgare, SO), in early stages of growth. CFV caused a lower oxidation reduction potential (ORP), increased pH and flow resistance and inductance. It induced changes in the absorbance characteristics of water in specific spectral regions, likely associated with greater stretching and reduced bending vibrations compared to untreated water. While assimilation rate and photosynthetic efficiency were not significantly affected at 60 days after sowing, treated water increased the stomatal conductance to water vapour gsw (+79%) and the electron transport rate ETR (+10%) in CU, as well as the non-photochemical quenching NPQ (+33%) in SO. Treated water also reduced leaf temperature in all species (-0.86 °C on average). This translated into improved plant biomass (leaves: +34%; roots: +140%) and reduced leaf-to-root biomass ratio (-42%) in SO, allowing both faster aerial growth and soil colonization, which can be exploited to improve plant tolerance against abiotic stresses. In the C3 species CU and LE, plant biomass was instead reduced, although significantly in LE only, while the leaf-to-root biomass ratio was generally enhanced, a result likely profitable in the cultivation of leafy vegetables. This is a preliminary trial on the effects of functionalized water and much remains to be investigated in other physiological processes, plant species, and growth stages for the full exploitation of this water treatment in agronomy.

Untangling the role of leaf age specific osmoprotectant and antioxidant responses of two poplar clones under increasing ozone concentrations

Plants possess different degrees of tolerance to abiotic stress, which can mitigate the detrimental effect of environmental inputs affecting carbon balance. Less is known about the functions of osmoprotectants in scavenging of reactive oxygen species (ROS), generated at different sites depending on leaf age. This study aimed to clarify the osmotic adjustments adopted by old and young leaves of Oxford and I-214 poplar clones [differing in ozone (O3) sensitivity] to cope with three levels of O3 [ambient (AA), and two elevated O3 levels]. In both clones, the impact of intermediate O3 concentrations (1.5 × AA) on ROS production appeared to be leaf age-specific, given the accumulation of hydrogen peroxide (H2O2) observed only in old leaves of the Oxford plants and in young leaves of the I-214 ones (2- fold higher than AA and +79%, respectively). The induction of an oxidative burst was associated with membrane injury, indicating an inadequate response of the antioxidative systems [decrease of lutein and β-carotene (-37 and -85% in the old leaves of the Oxford plants), accumulation of proline and tocopherols (+60 and +12% in the young leaves of the I-214 ones)]. Intermediate O3 concentrations reacted with unsaturated lipids of the plasma membrane in old and young leaves of the Oxford plants, leading to an increase of malondialdehyde by-products (more than 2- fold higher than AA), while no effect was recorded for I-214. The impact of the highest O3 concentrations (2.0 × AA) on ROS production did not appear clone-specific, which may react with cell wall components by leading to oxidative pressure. Outcomes demonstrated the ability of young leaves of I-214 plants in contain O3 phytotoxic effects.

The Arabidopsis leaf quantitative atlas: a cellular and subcellular mapping through unified data integration

Quantitative analyses and models are required to connect a plant's cellular organisation with its metabolism. However, quantitative data are often scattered over multiple studies, and finding such data and converting them into useful information is time-consuming. Consequently, there is a need to centralise the available data and to highlight the remaining knowledge gaps. Here, we present a step-by-step approach to manually extract quantitative data from various information sources, and to unify the data format. First, data from Arabidopsis leaf were collated, checked for consistency and correctness and curated by cross-checking sources. Second, quantitative data were combined by applying calculation rules. They were then integrated into a unique comprehensive, referenced, modifiable and reusable data compendium representing an Arabidopsis reference leaf. This atlas contains the metrics of the 15 cell types found in leaves at the cellular and subcellular levels.

Biophysical and molecular characteristics of senescing leaves of two Norway maple varieties differing in anthocyanin content

Disassembly and degradation of the photosynthetic protein complexes during autumn senescence, a vital step to ensure efficient nutrient relocalization for winter storage, is poorly understood. Concomitantly with the degradation, anthocyanins are often synthesized. However, as to why leaves accumulate red pigments, no consensus exists. One possibility is that anthocyanins protect senescing leaves from excess light. In this study, we investigated the pigment composition, photosynthetic performance, radical production, and degradation of the photosynthetic protein complexes in Norway maple (Acer platanoides) and in its highly pigmented, purple-colored variety (Faassen's black) during autumn senescence, to dissect the possible roles of anthocyanins in photoprotection. Our findings show that senescing Faassen's black was indeed more resistant to Photosystem II (PSII) photoinhibition, presumably due to its high anthocyanin content, than the green maple. However, senescing Faassen's black exhibited low photosynthetic performance, probably due to a poor capacity to repair PSII. Furthermore, an analysis of photosynthetic protein complexes demonstrated that in both maple varieties, the supercomplexes consisting of PSII and its antenna were disassembled first, followed by the degradation of the PSII core, Photosystem I, Cytochrome b6 f, and ATP synthase. Strikingly, the degradation process appeared to proceed faster in Faassen's black, possibly explaining its poor PSII repair capacity. The results suggest that tolerance against PSII photoinhibition may not necessarily translate to a better fitness. Finally, thylakoids isolated from senescing and non-senescing leaves of both maple varieties accumulated very little carbon-centered radicals, suggesting that thylakoids may not be a major source of reactive oxygen species in senescing leaves.

Community Composition and Structure Affect Ecosystem and Canopy Water Use Efficiency Across Three Typical Alpine Ecosystems

Unique ecosystems distributed in alpine areas of the Qinghai-Tibetan Plateau play important roles in climate change mitigation, local food supply, and conservation of species diversity. To understand the water use efficiency (WUE) of this fragile and sensitive region, this study combined observed data from the eddy covariance system and the Shuttleworth-Wallace (S-W) model to measure the continuous mass exchange, including gross primary productivity (GPP), evapotranspiration (ET), and canopy transpiration (T) throughout 2 or 3 years (2016-2018) in three common alpine ecosystems (i.e., alpine steppe, alpine meadow, and alpine swamp). These ecosystems represent a water availability gradient and thus provide the opportunity to quantify environmental and biological controls on WUE at various spatiotemporal scales. We analyzed the ecosystem WUE (WUEe; defined as the ratio of GPP to ET) and canopy WUE (WUEc; defined as the ratio of GPP and canopy T). It was found that the yearly WUEe was 1.40, 1.63, and 2.16 g C kg-1 H2O, and the yearly WUEc was 8.93, 2.46, and 5.19 g C kg-1 H2O in the three typical ecosystems, respectively. The controlling factors of yearly WUE diverged between WUEe and WUEc. We found that plant functional group proportion (e.g., gramineous and Cyperaceae) highly explained the yearly WUEe variation across sites, and a good correlation was observed between community species diversity and WUEc. These findings suggest that community composition and trait change are critical in regulating WUEe and WUEc across different alpine ecosystems and that the regulation mechanisms may differ fundamentally between WUEe and WUEc.

Long-term survival of Chlamydomonas reinhardtii during conditional senescence

Chlamydomonas reinhardtii undergoes conditional senescence when grown in batch culture due to nutrient limitation. Here, we explored plastid and photo-physiological adaptations in Chlamydomonas reinhardtii during a long-term ageing experiment by methodically sampling them over 22 weeks. Following exponential growth, Chlamydomonas entered an extended declining growth phase where cells continued to divide, although at a lower rate. Ultimately, this ongoing division was fueled by the recycling of macromolecules that was obvious in the rapidly declining protein and chlorophyll content in the cell during this phase. This process was sufficient to maintain a high level of cell viability as the culture entered stationary phase. Beyond that the cell viability starts to plummet. During the turnover of macromolecules after exponential growth that saw RuBisCO levels drop, the LHCII antenna was relatively stable. This, along with the upregulation of the light stress-related proteins (LHCSR), contributes to an efficient energy dissipation mechanism to protect the ageing cells from photooxidative stress during the senescence process. Ultimately, viability dropped to about 7% at 22 weeks in a batch culture. We anticipate that this research will help further understand the various acclimation strategies carried out by Chlamydomonas to maximize survival under conditional senescence.

Chloroplast dismantling in leaf senescence

In photosynthetic plant cells, chloroplasts act as factories of metabolic intermediates that support plant growth. Chloroplast performance is highly influenced by environmental cues. Thus, these organelles have the additional function of sensing ever changing environmental conditions, thereby playing a key role in harmonizing the growth and development of different organs and in plant acclimation to the environment. Moreover, chloroplasts constitute an excellent source of metabolic intermediates that are remobilized to sink tissues during senescence so that chloroplast dismantling is a tightly regulated process that plays a key role in plant development. Stressful environmental conditions enhance the generation of reactive oxygen species (ROS) by chloroplasts, which may lead to oxidative stress causing damage to the organelle. These environmental conditions trigger mechanisms that allow the rapid dismantling of damaged chloroplasts, which is crucial to avoid deleterious effects of toxic by-products of the degradative process. In this review, we discuss the effect of redox homeostasis and ROS generation in the process of chloroplast dismantling. Furthermore, we summarize the structural and biochemical events, both intra- and extraplastid, that characterize the process of chloroplast dismantling in senescence and in response to environmental stresses.

Growth-phase dependent morphological alteration in higher plant thylakoid is accompanied by changes in both photodamage and repair rates

Thylakoid membranes of young leaves consist of grana and stroma lamellae (stroma-grana [SG] structure). The SG thylakoid is gradually converted into isolated grana (IG), almost lacking the stroma lamellae during growth. This morphological alteration was found to cause a reduction in maximum photosynthetic rate and an enhancement of photoinhibition in photosystem II (PSII). In situ microspectrometric measurements of chlorophyll fluorescence in individual chloroplasts suggested an increase of the PSII/PSI ratio in IG thylakoids of mature leaves. Western blot analysis of isolated IG thylakoids showed relative increases in some PSII components, including the core protein (D1) and light-harvesting components CP24 and Lhcb2. Notably, a nonphotochemical quenching-related factor in the PSII supercomplex, PsbS, decreased by 40%. Changes in the high light response of PSII were detected through parameters of pulse-amplitude modulation fluorometry. Chlorophyll fluorescence lifetime indicated an increase of fluorescence quantum yield in IG. A minimal photodamage-repair rate analysis on a lincomycin treatment of the leaves indicated that repair rate constant of IG is slower than that of SG, while photodamage rate of IG is higher than that of SG. These results suggest that IG thylakoids are relatively sensitive to high light, which is not only due to a higher photodamage rate caused by some rearrangements of PS complexes, but also to the retarded PSII repair that may result from the lack of stroma lamellae. The IG thylakoids found among many plant species thus seem to be an adaptive form to low light environments, although their physiological roles still remain unclear.

Degradation of the photosystem II core complex is independent of chlorophyll degradation mediated by Stay-Green Mg2+ dechelatase in Arabidopsis

During leaf senescence, the degradation of photosystems and photosynthetic pigments proceeds in a coordinated manner, which would minimize the potential photodamage to cells. Both photosystem I and II are composed of core complexes and peripheral antenna complexes, with the former binding chlorophyll a and the latter binding chlorophyll a and b. Although the degradation of peripheral antenna complexes is initiated by chlorophyll degradation, it remains unclear whether the degradation of core complexes and chlorophyll is coordinated. In this study, we examined the degradation of peripheral antenna and core complexes in the Arabidopsis sgr1/sgr2/sgrl triple mutant, lacking all the isoforms of chlorophyll a:Mg²⁺ dechelatase. In this mutant, the degradation of peripheral antenna complexes and photosystem I core complexes was substantially retarded, but the core complexes of photosystem II were rapidly degraded during leaf senescence. On the contrary, the photosynthetic activity declined at a similar rate as in the wild type plants. These results suggest that the degradation of photosystem II core complexes is regulated independently of the major chlorophyll degradation pathway mediated by the dechelatase. The study should contribute to the understanding of the complex molecular mechanisms underlying the degradation of photosystems, which is an essential step during leaf senescence.

Leaf Phenological Stages of Winter Oilseed Rape (Brassica napus L.) Have Conserved Photosynthetic Efficiencies but Contrasted Intrinsic Water Use Efficiencies at High Light Intensities

Leaf senescence in source leaves leads to the active degradation of chloroplast components [photosystems, chlorophylls, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)] and plays a key role in the efficient remobilization of nutrients toward sink tissues. However, the progression of leaf senescence can differentially modify the photosynthetic properties of source leaves depending on plant species. In this study, the photosynthetic and respiratory properties of four leaf ranks of oilseed rape describing leaf phenological stages having different sink-source activities were analyzed. To achieve this, photosynthetic pigments, total soluble proteins, Rubisco amounts, and the light response of chlorophyll fluorescence parameters coupled to leaf gas exchanges and leaf water content were measured. Photosynthetic CO₂ assimilation and electron transfer rates, Rubisco and chlorophyll levels per leaf area were gradually decreased between young, mature and senescent leaves but they remained highly correlated at saturating light intensities. However, senescent leaves of oilseed rape had a lower intrinsic water use efficiency compared to young and mature leaves at saturating light intensities that was mainly due to higher stomatal conductance and transpiration rate with respect to stomatal density and net CO₂ assimilation. The results are in favor of a concerted degradation of chloroplast components but a contrasted regulation of water status between leaves of different phenological stages of winter oilseed rape.

Pigments, ascorbic acid, total polyphenols and antioxidant capacities in deetiolated barley (Hordeum vulgare) and wheat (Triticum aestivum) microgreens

The study was aimed to evaluate if deetiolation of barley and wheat microgreens after cultivaton in dark (for 5, 7 and 9 days) can enhance the contents of pigments, ascorbic acid, polyphenols, and equivalent antioxidant capacities (EAC) (measured by DPPH and FRAP assay) in correlation to other. Chlorophylls and carotenoids were higher in microgreens that were exposed more to daylight. In contrast, ascorbic acid, polyphenols and EAC of microgreens could be enhanced by 5-7 days of etiolation. However, prolonged etiolation reduced overall antioxidant capacities of microgreens. All evaluated parameters could be satisfactorily represented by regression expressions for the given number of days of etiolation and growth. The ascorbic acid and total carotenoids content had higher correlations with total chlorophyll contents, while the antioxidant capacities were highly correlated to total polyphenols content. The study confirms the potential of deetiolated cultivation of microgreens to enhance selective phytochemicals content and EAC of microgreens.

Changes in Photosynthetic Electron Transport during Leaf Senescence in Two Barley Varieties Grown in Contrasting Growth Regimes

Leaf senescence is an important process for plants to remobilize a variety of metabolites and nutrients to sink tissues, such as developing leaves, fruits and seeds. It has been suggested that reactive oxygen species (ROS) play an important role in the initiation of leaf senescence. Flag leaves of two different barley varieties, cv. Lomerit and cv. Carina, showed differences in the loss of photosystems and in the production of ROS at a late stage of senescence after significant loss of chlorophyll (Krieger-Liszkay et al. 2015). Here, we investigated photosynthetic electron transport and ROS production in primary leaves of these two varieties at earlier stages of senescence. Comparisons were made between plants grown outside in natural light and temperatures and plants grown in temperature-controlled growth chambers under low light intensity. Alterations in the content of photoactive P700, ferredoxin and plastocyanin (PC) photosynthetic electron transport were analyzed using in vivo near-infrared absorbance changes and chlorophyll fluorescence, while ROS were measured with spin-trapping electron paramagnetic resonance spectroscopy. Differences in ROS production between the two varieties were only observed in outdoor plants, whereas a loss of PC was common in both barley varieties regardless of growth conditions. We conclude that the loss of PC is the earliest detectable photosynthetic parameter of leaf senescence while differences in the production of individual ROS species occur later and depend on environmental factors.

Combinatory actions of CP29 phosphorylation by STN7 and stability regulate leaf age-dependent disassembly of photosynthetic complexes

A predominant physiological change that occurs during leaf senescence is a decrease in photosynthetic efficiency. An optimal organization of photosynthesis complexes in plant leaves is critical for efficient photosynthesis. However, molecular mechanisms for regulating photosynthesis complexes during leaf senescence remain largely unknown. Here we tracked photosynthesis complexes alterations during leaf senescence in Arabidopsis thaliana. Grana stack is significantly thickened and photosynthesis complexes were disassembled in senescing leaves. Defects in STN7 and CP29 led to an altered chloroplast ultrastructure and a malformation of photosynthesis complex organization in stroma lamella. Both CP29 phosphorylation by STN7 and CP29 fragmentation are highly associated with the photosynthesis complex disassembly. In turn, CP29 functions as a molecular glue to facilitate protein complex formation leading phosphorylation cascade and to maintain photosynthetic efficiency during leaf senescence. These data suggest a novel molecular mechanism to modulate leaf senescence via CP29 phosphorylation and fragmentation, serving as an efficient strategy to control photosynthesis complexes.

Effects of amendments of PCB-containing Hudson River sediment on soil quality and biochemical and growth response of cucumber (Cucumis sativus L. cv 'Wisconsin SMR 58')

Approximately 200 million m³ of sediments are dredged every year in the United States. Of this amount, 2.3-9 million m³ are contaminated to the extent that they require special, and often costly, handling. Therefore, there is a pressing need to develop appropriate technology for the safe utilization of these sediments, especially in the case of the Hudson River, which is well known to demonstrate significant polychlorinated biphenyls (PCBs) contamination. Hence, the aim of the present study was to examine the influence of different doses of Hudson River sediments (10%, 25%, 50%, 75% and 100% admixtures) on soil quality and on the biochemical and growth response of cucumber (Cucumis sativus L. cv 'Wisconsin SMR 58'), used as potential phytoremediation tool for sediment-borne PCBs. A sediment/soil admixture was found to significantly decrease the nitrogen (N) content in the substratum; in addition, phosphorus (P) content was significantly increased by 50-100% sediment, while potassium (K) content was significantly increased by 10% sediment, and significantly decreased by >50% sediment. Although sediment treatment resulted in a gradual increase in PCB content in the soil-sediment substratum, exceeding the threshold effect concentration (TEC) for the ≥50% sediment admixture, the Microtox assay did not suggest toxicity to microorganisms. The results demonstrated also that admixture of 10-25% Hudson River sediment increased cucumber growth; however, higher doses led to growth inhibition, manifested as lower biomass and smaller leaves. Also, chlorophyll a and b content decreased with increasing doses of sediment. Phenylpropanoid and flavonol contents were significantly higher in plants grown in soil amended with 10% of sediment, but significantly lower in soil treated with a 100% sediment admixture. The anthocyanin content in plants was lower at admixtures of 50% and higher. The obtained results corresponded with the decreasing content of N and K.

Effects of soil amendment with PCB-contaminated sediment on the growth of two cucurbit species

The aim of the study was to evaluate the influence of the application of increasing proportions (0%, 10%, 25%, 50%, 75%, and 100%) of an admixture of PCB-contaminated Hudson River sediment collected from the Upper Hudson River, near Waterford, Saratoga county (New York, USA) on soil properties, phytotoxicity, and biometric and physiological responses of cucumber (Cucumis sativus L. cv 'Wisconsin SMR 58') and zucchini (Cucurbita pepo L. cv 'Black Beauty') grown as potential phyto- and rhizoremediators. The experiment was performed for 4 weeks in a growth chamber under controlled conditions. Amendment of Hudson River sediment to soil led to a gradual increase in PCB content of the substratum from 13.7 μg/kg (with 10% sediment) to 255 μg/kg (with 100% sediment). Sediment amendment showed no phytotoxic effects during the initial stages, even Lepidium sativum root growth was stimulated; however, this positive response diminished following a 4-week growth period, with the greatest inhibition observed in unplanted soil and zucchini-planted soil. The stimulatory effect remained high for cucumber treatments. The sediment admixture also increased cucurbit fresh biomass as compared to control samples, especially at lower doses of sediment admixture, even though PCB content of the soil amended with sediment increased. Cucurbits' leaf surface area, in turn, demonstrated an increase for zucchini, however only for 50% and 75% sediment admixture, while cucumber showed no changes when lower doses were applied and decrease for 75% and 100% sediment admixture. Chlorophyll a + b decreased significantly in sediment-amended soils, with greater inhibition observed for cucumber than zucchini. Our results suggest that admixture of riverine sediment from relatively less-contaminated locations may be used as soil amendments under controlled conditions; however, further detailed investigation on the fate of pollutants is required, especially in terms of the bioaccumulation and biomagnification properties of PCBs, before contaminated sediment can be applied in an open environment.

The Application of Different Biological Remediation Strategies to PCDDs/PCDFs Contaminated Urban Sediments

Our aim was to assess the efficacy of four different bioremediation strategies applied to soil treated with urban sediments for alleviating soil phytotoxicity (examined using Lepidium sativum), by removing polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), and mitigating the toxic effect on plants by the applied sediment: (1) Natural attenuation, (2) phytoremediation with the use of two plants Tagetes patula L. and Festuca arundinacea, (3) rhizobacterial inoculation with Massilia niastensis p87 and Streptomyces costaricanus RP92 strains, (4) rhizobacteria-assisted phytoremediation with both plants and strains. The applied sediment had a positive influence on L. sativum growth (90% higher than in the unamended soil), mostly due to its high content of nutrients, mainly Ca and Fe, which immobilize pollutants. The positive effect of sediments continued for up to 10-week duration of the experiment; however, the rhizobacterial inoculated samples were characterized by higher growth of L. sativum. The application of rhizobacteria-assisted phytoremediation further increased the growth of L. sativum, and was also found to improve the efficiency of PCDD/PCDF removal, resulting in a maximum 44% reduction of its content. This strategy also alleviated the negative impact of urban sediments on T. patula and F. arundinacea biomass, and had a beneficial effect on protein and chlorophyll content in the studied plants.

The Influence of Bottom Sediments and Inoculation with Rhizobacterial Inoculants on the Physiological State of Plants Used in Urban Plantings

Bottom sediments accumulate rapidly in urban reservoirs and should be periodically removed. Their high organic matter content makes them valuable fertilizers, but they often contain toxic substances. The present study compares the responses of the dicotyledonous Tagetes patula and monocotyledon Festuca arundinacea to the presence of such sediments in soil and to soil inoculation with two rhizobacterial strains (Massilia niastensis p87 and Streptomyces costaricanus RP92) isolated from contaminated soil. Total soluble protein, total chlorophyll content, as well as chlorophyll a/b ratio, degree of lipid peroxidation (TBARS), α-tocopherol content, total phenolic compounds (TPC) content and anthocyanins content were examined in the leaves of investigated plants. T. patula was more sensitive to the toxic substances in the sediments than F. arundinacea. Rhizobacterial inoculation reduced the toxic effect of the sediment. RP92 has a more favorable effect on the condition of T. patula than p87. F. arundinacea was not adversely affected by the addition of sediments or inoculation with the p87 or RP92 strains. Both tested plant species are suitable for planting on soils enriched with urban sediments, and the addition of bacterial inoculums promote plant growth and reduce the damage caused by the xenobiotics contained in the sediments.

The response of cucumber plants (Cucumis sativus L.) to the application of PCB-contaminated sewage sludge and urban sediment

Background

The increasing production of sewage sludge (SS) engenders the problem of its responsible utilization and disposal. Likewise, urban sediments (SED) are deposited at the bottom of urban reservoirs and sedimentation ponds, and these require periodical dredging and utilization. However, while the SS and SED deposits often contain nutrients such as nitrogen and phosphorus; however, they also contain a variety of hazardous compounds including heavy metals, Persistent Organic Pollutants (POPs) and microbial pollutants. Fortunately, some species of Cucurbitaceae can accumulate high levels of POPs, such as polychlorinated dibenzo-p-dioxins (PCDD), polychlorinated dibenzofurans (PCDF) and polychlorinated biphenyls (PCB), in their tissues.

Methods

SS was collected from the Lodz Municipal Wastewater Treatment Plant and SED from the Sokołówka Sequential Biofiltration System. The SS and SED samples were added to soil in flower pots at three concentrations (1.8 g, 5.4 g and 10.8 g per flower pot), and one pot was left as an unamended control (C). Soil PCB concentrations were determined before cucumber planting, and after five weeks of growth. Also, total soluble protein, total chlorophyll content, chlorophyll a/b ratio and degree of lipid peroxidation (TBARS) were examined in the leaves of the cucumber plants (Cucumis sativus L.) cv. Cezar after five weeks. Antioxidative response was assessed by ascorbate peroxidase (APx) and catalase (CAT) assay.

Results

The initial PCB concentration in soil after application of SS or SED was dependent on the applied dose. After five weeks, PCB concentration fell significantly for all samples and confirmed that the dose of SS/SED had a strong effect. Soil remediation was found to be more effective after SS application. Total soluble protein content in the cucumber leaf tissues was dependent on both the type and the dose of the applied amendments, and increased with greater SS doses in the soil. The total chlorophyll content remained unchanged, and the chlorophyll a/b ratio was slightly elevated only after the application of the highest SS and SED dose. The use of SS and SED did not significantly affect TBARS content. APx activity fell after SS or SED application; however, CAT activity tended to increase, but only in the leaves of plants grown in SS-amended soil.

Discussion

The cultivation of cucumber plants reduces PCB concentration in soil amended with SS or SED; however, this effect is more evident in the case of SS. SS application also induced more intensive changes in the activity of enzymes engaged in antioxidative response and oxidative stress markers in plant tissues than SED. The levels of PCB in the SS may have triggered a more severe imbalance between pro- and antioxidative reactions in plants. Cucumber plants appear to be resistant to the presence of toxic substances in SS and SED, and the addition of SS and SED not only acts as a fertilizer, but also protects against accelerated aging.

The impact of photosynthesis on initiation of leaf senescence

Senescence is the last stage of leaf development preceding the death of the organ, and it is important for nutrient remobilization and for feeding sink tissues. There are many reports on leaf senescence, but the mechanisms initiating leaf senescence are still poorly understood. Leaf senescence is affected by many environmental factors and seems to vary in different species and even varieties of plants, which makes it difficult to generalize the mechanism. Here, we give an overview on studies reporting about alterations in the composition of the photosynthetic electron transport chain in chloroplasts during senescence. We hypothesize that alternative electron flow and related generation of the proton motive force required for ATP synthesis become increasingly important during progression of senescence. We address the generation of reactive oxygen species (ROS) in chloroplasts in the initiation of senescence, retrograde signaling from the chloroplast to the nucleus and ROS-dependent signaling associated with leaf senescence. Finally, a few ideas for increasing crop yields by increasing the chloroplast lifespan are presented.

Exogenous application of cytokinin during dark senescence eliminates the acceleration of photosystem II impairment caused by chlorophyll b deficiency in barley

Recent studies have shown that chlorophyll (Chl) b has an important role in the regulation of leaf senescence. However, there is only limited information about senescence of plants lacking Chl b and senescence-induced decrease in photosystem II (PSII) and photosystem I (PSI) function has not even been investigated in such plants. We have studied senescence-induced changes in photosynthetic pigment content and PSII and PSI activities in detached leaves of Chl b-deficient barley mutant, chlorina f2^f2 (clo). After 4 days in the dark, the senescence-induced decrease in PSI activity was smaller in clo compared to WT leaves. On the contrary, the senescence-induced impairment in PSII function (estimated from Chl fluorescence parameters) was much more pronounced in clo leaves, even though the relative decrease in Chl content was similar to wild type (WT) leaves (Hordeum vulgare L., cv. Bonus). The stronger impairment of PSII function seems to be related to more pronounced damage of reaction centers of PSII. Interestingly, exogenously applied plant hormone cytokinin 6-benzylaminopurine (BA) was able to maintain PSII function in the dark senescing clo leaves to a similar extent as in WT. Thus, considering the fact that without BA the senescence-induced decrease in PSII photochemistry in clo was more pronounced than in WT, the relative protective effect of BA was higher in Chl b-deficient mutant than in WT.

Photosystem I-LHCII megacomplexes respond to high light and aging in plants

Photosystem II is known to be a highly dynamic multi-protein complex that participates in a variety of regulatory and repair processes. In contrast, photosystem I (PSI) has, until quite recently, been thought of as relatively static. We report the discovery of plant PSI-LHCII megacomplexes containing multiple LHCII trimers per PSI reaction center. These PSI-LHCII megacomplexes respond rapidly to changes in light intensity, as visualized by native gel electrophoresis. PSI-LHCII megacomplex formation was found to require thylakoid stacking, and to depend upon growth light intensity and leaf age. These factors were, in turn, correlated with changes in PSI/PSII ratios and, intriguingly, PSI-LHCII megacomplex dynamics appeared to depend upon PSII core phosphorylation. These findings suggest new functions for PSI and a new level of regulation involving specialized subpopulations of photosystem I which have profound implications for current models of thylakoid dynamics.

Leaf-age dependent response of carotenoid accumulation to elevated CO2 in Arabidopsis

Carotenoids contribute to photosynthesis, photoprotection, phytohormone and apocarotenoid biosynthesis in plants. Carotenoid-derived metabolites control plant growth, development and signalling processes and their accumulation can depend upon changes in the environment. Elevated carbon dioxide (eCO₂) often enhances carbon assimilation, early growth patterns and overall plant biomass, and may increase carotenoid accumulation due to higher levels of precursors from isoprenoid biosynthesis. Variable effects of eCO₂ on carotenoid accumulation in leaves have been observed for different plant species. Here, we determined whether the variable response of carotenoids to eCO₂ was potentially a function of leaf age and the impact of eCO₂ on leaf development by growing Arabidopsis in ambient CO₂ (400 ppm) and eCO₂ (800 ppm). eCO₂ increased plant leaf number, rosette area, biomass, seed yield and net photosynthesis. In addition, eCO₂ increased carotenoid content by 10-20% in younger emerging leaves, but not in older mature leaves. Older leaves contained approximately 60% less total carotenoids compared to younger leaves. The age-dependent effect on carotenoid content was observed for cotyledon, juvenile and adult phase leaves. We conclude that younger leaves utilize additional carbon from enhanced photosynthesis in eCO₂ to increase carotenoid content, yet older leaves have less capacity to store additional carbon into carotenoids.

Expression of a Plastid-Targeted Flavodoxin Decreases Chloroplast Reactive Oxygen Species Accumulation and Delays Senescence in Aging Tobacco Leaves

Leaf senescence is a concerted physiological process involving controlled degradation of cellular structures and reallocation of breakdown products to other plant organs. It is accompanied by increased production of reactive oxygen species (ROS) that are proposed to signal cell death, although both the origin and the precise role of ROS in the execution of this developmental program are still poorly understood. To investigate the contribution of chloroplast-associated ROS to natural leaf senescence, we used tobacco plants expressing a plastid-targeted flavodoxin, an electron shuttle flavoprotein present in prokaryotes and algae. When expressed in plants, flavodoxin specifically prevents ROS formation in chloroplasts during stress situations. Senescence symptoms were significantly mitigated in these transformants, with decreased accumulation of chloroplastic ROS and differential preservation of chlorophylls, carotenoids, protein contents, cell and chloroplast structures, membrane integrity and cell viability. Flavodoxin also improved maintenance of chlorophyll-protein complexes, photosynthetic electron flow, CO₂ assimilation, central metabolic routes and levels of bioactive cytokinins and auxins in aging leaves. Delayed induction of senescence-associated genes indicates that the entire genetic program of senescence was affected by flavodoxin. The results suggest that ROS generated in chloroplasts are involved in the regulation of natural leaf senescence.

Root-level exposure reveals multiple physiological toxicity of triazine xenobiotics in Arabidopsis thaliana

Herbicides are pollutants of great concern due to environmental ubiquity resulting from extensive use in modern agriculture and persistence in soil and water. Studies at various spatial scales have also highlighted frequent occurrences of major herbicide breakdown products in the environment. Analysis of plant behavior toward such molecules and their metabolites under conditions of transient or persistent soil pollution is important for toxicity evaluation in the context of environmental risk assessment. In order to understand the mechanisms underlying the action of such environmental contaminants, the model plant Arabidopsis thaliana, which has been shown to be highly responsive to pesticides and other xenobiotics, was confronted with varying levels of the widely-used herbicide atrazine and of two of its metabolites, desethylatrazine and hydroxyatrazine, which are both frequently detected in water streams of agriculturally-intensive areas. After 24h of exposure to varying concentrations covering the range of triazine concentrations detected in the environment, root-level contaminations of atrazine, desethylatrazine and hydroxyatrazine were found to affect early growth and development in various dose-dependent and differential manners. Moreover, these differential effects of atrazine, desethylatrazine and hydroxyatrazine pointed to the involvement of distinct mechanisms directly affecting respiration and root development. The consequences of the identification of additional targets, in addition to the canonical photosystem II target, are discussed in relation with the ecotoxicological assessment of environmental xenobiotic contamination.

Significance of structural variation in thylakoid membranes in maintaining functional photosystems during reproductive growth

Structural variation in the stroma-grana (SG) arrangement of the thylakoid membranes, such as changes in the thickness of the grana stacks and in the ratio between grana and inter-grana thylakoid, is often observed. Broadly, such alterations are considered acclimation to changes in growth and the environment. However, the relation of thylakoid morphology to plant growth and photosynthesis remains obscure. Here, we report changes in the thylakoid during leaf development under a fixed light condition. Histological studies on the chloroplasts of fresh green Arabidopsis leaves have shown that characteristically shaped thylakoid membranes lacking the inter-grana region, referred to hereafter as isolated-grana (IG), occurred adjacent to highly ordered, large grana layers. This morphology was restored to conventional SG thylakoid membranes with the removal of bolting stems from reproductive plants. Statistical analysis showed a negative correlation between the incidences of IG-type chloroplasts in mesophyll cells and the rates of leaf growth. Fluorescence parameters calculated from pulse-amplitude modulated fluorometry measurements and CO₂ assimilation data showed that the IG thylakoids had a photosynthetic ability that was equivalent to that of the SG thylakoids under moderate light. However, clear differences were observed in the chlorophyll a/b ratio. The IG thylakoids were apparently an acclimated phenotype to the internal condition of source leaves. The idea is supported by the fact that the life span of the IG thylakoids increased significantly in the later developing leaves. In conclusion, the heterogeneous state of thylakoid membranes is likely important in maintaining photosynthesis during the reproductive phase of growth.

Quantifying the dynamics of light tolerance in Arabidopsis plants during ontogenesis

The amount of light plants can tolerate during different phases of ontogenesis remains largely unknown. This was addressed here employing a novel methodology that uses the coefficient of photochemical quenching (qP) to assess the intactness of photosystem II reaction centres. Fluorescence quenching coefficients, total chlorophyll content and concentration of anthocyanins were determined weekly during the juvenile, adult, reproductive and senescent phases of plant ontogenesis. This enabled quantification of the protective effectiveness of non-photochemical fluorescence quenching (NPQ) and determination of light tolerance. The light intensity that caused photoinhibition in 50% of leaf population increased from ∼70 μmol m(-2)  s(-1) , for 1-week-old seedlings, to a maximum of 1385 μmol m(-2)  s(-1) for 8-week-old plants. After 8 weeks, the tolerated light intensity started to gradually decline, becoming only 332 μmol m(-2)  s(-1) for 13-week-old plants. The dependency of light tolerance on plant age was well-related to the amplitude of protective NPQ (pNPQ) and the electron transport rates (ETRs). Light tolerance did not, however, show a similar trend to chlorophyll a/b ratios and content of anthocyanins. Our data suggest that pNPQ is crucial in defining the capability of high light tolerance by Arabidopsis plants during ontogenesis.

Photobiological hydrogen production and artificial photosynthesis for clean energy: from bio to nanotechnologies

Global energy demand is increasing rapidly and due to intensive consumption of different forms of fuels, there are increasing concerns over the reduction in readily available conventional energy resources. Because of the deleterious atmospheric effects of fossil fuels and the uncertainties of future energy supplies, there is a surge of interest to find environmentally friendly alternative energy sources. Hydrogen (H2) has attracted worldwide attention as a secondary energy carrier, since it is the lightest carbon-neutral fuel rich in energy per unit mass and easy to store. Several methods and technologies have been developed for H2 production, but none of them are able to replace the traditional combustion fuel used in automobiles so far. Extensively modified and renovated methods and technologies are required to introduce H2 as an alternative efficient, clean, and cost-effective future fuel. Among several emerging renewable energy technologies, photobiological H2 production by oxygenic photosynthetic microbes such as green algae and cyanobacteria or by artificial photosynthesis has attracted significant interest. In this short review, we summarize the recent progress and challenges in H2-based energy production by means of biological and artificial photosynthesis routes.

Photosynthetic lesions can trigger accelerated senescence in Arabidopsis thaliana

Senescence is a highly regulated process characterized by the active breakdown of cells, which ultimately leads to the death of plant organs or whole plants. In annual plants such as Arabidopsis thaliana senescence can be observed in each individual leaf. Whether deficiencies in photosynthesis promote the induction of senescence was investigated by monitoring chlorophyll degradation, photosynthetic parameters, and reactive oxygen species accumulation in photosynthetic mutants. Several mutations affecting components of the photosynthetic apparatus, including psal-2, psan-2, and psbs, were found to lead to premature or faster senescence, as did simultaneous inactivation of the STN7 and STN8 kinases. Premature senescence is apparently not directly linked to an overall reduction in photosynthesis but to perturbations in specific aspects of the process. Dark-induced senescence is accelerated in mutants affected in linear electron flow, especially psad2-1, psan-2, and pete2-1, as well as in stn7 and stn8 mutants and STN7 and STN8 overexpressor lines. Interestingly, no direct link with ROS production could be observed.

Dissecting the subcellular compartmentation of proteins and metabolites in arabidopsis leaves using non-aqueous fractionation

Non-aqueous fractionation is a technique for the enrichment of different subcellular compartments derived from lyophilized material. It was developed to study the subcellular distribution of metabolites. Here we analyzed the distribution of about 1,000 proteins and 70 metabolites, including 22 phosphorylated intermediates in wild-type Arabidopsis rosette leaves, using non-aqueous gradients divided into 12 fractions. Good separation of plastidial, cytosolic, and vacuolar metabolites and proteins was achieved, but cytosolic, mitochondrial, and peroxisomal proteins clustered together. There was considerable heterogeneity in the fractional distribution of transcription factors, ribosomal proteins, and subunits of the vacuolar-ATPase, indicating diverse compartmental location. Within the plastid, sub-organellar separation of thylakoids and stromal proteins was observed. Metabolites from the Calvin-Benson cycle, photorespiration, starch and sucrose synthesis, glycolysis, and the tricarboxylic acid cycle grouped with their associated proteins of the respective compartment. Non-aqueous fractionation thus proved to be a powerful method for the study of the organellar, and in some cases sub-organellar, distribution of proteins and their association with metabolites. It remains the technique of choice for the assignment of subcellular location to metabolites in intact plant tissues, and thus the technique of choice for doing combined metabolite-protein analysis on a single tissue sample.