Continuous turnover of carotenes and chlorophyll a in mature leaves of Arabidopsis revealed by 14CO2 pulse-chase labeling

Chlorophyll Degradation and Its Physiological Function

Research on chlorophyll degradation has progressed significantly in recent decades. In the 1990s, the structure of linear tetrapyrrole, which is unambiguously a chlorophyll degradation product, was determined. From the 2000s until the 2010s, the major enzymes involved in chlorophyll degradation were identified, and the pheophorbide a oxygenase/phyllobilin pathway was established. This degradation pathway encompasses several steps: (i) initial conversion of chlorophyll b to 7-hydroxymethyl chlorophyll a, (ii) conversion of 7-hydroxymethyl chlorophyll a to chlorophyll a, (iii) dechelation of chlorophyll a to pheophytin a, (iv) dephytylation of pheophytin a to pheophorbide a, (v) opening of the macrocycle to yield a red chlorophyll catabolite (RCC) and (vi) conversion of RCC to phyllobilins. This pathway converts potentially harmful chlorophyll into safe molecules of phyllobilins, which are stored in the central vacuole of terrestrial plants. The expression of chlorophyll-degrading enzymes is mediated by various transcription factors and influenced by light conditions, stress and plant hormones. Chlorophyll degradation is differently regulated in different organs and developmental stages of plants. The initiation of chlorophyll degradation induces the further expression of chlorophyll-degrading enzymes, resulting in the acceleration of chlorophyll degradation. Chlorophyll degradation was initially considered the last reaction in senescence; however, chlorophyll degradation plays crucial roles in enhancing senescence, degrading chlorophyll-protein complexes, forming photosystem II and maintaining seed quality. Therefore, controlling chlorophyll degradation has important agricultural applications.

Carotenoid-carbohydrate crosstalk: evidence for genetic and physiological interactions in storage tissues across crop species

Carotenoids play essential roles in photosynthesis, photoprotection, and human health. Efforts to increase carotenoid content in several staple crops have been successful through both conventional selection and genetic engineering methods. Interestingly, in some cases, altering carotenoid content has had unexpected effects on other aspects of plant metabolism, impacting traits like sugar content, dry matter percentage, fatty acid content, stress tolerance, and phytohormone concentrations. Studies across several diverse crop species have identified negative correlations between carotenoid and starch contents, as well as positive correlations between carotenoids and soluble sugars. Collectively, these reports suggest a metabolic interaction between carotenoids and carbohydrates. We synthesize evidence pointing to four hypothesized mechanisms: (1) direct competition for precursors; (2) physical interactions in plastids; (3) influences of sugar or apocarotenoid signaling networks; and (4) nonmechanistic population or statistical sources of correlations. Though the carotenoid biosynthesis pathway is well understood, the regulation and interactions of carotenoids, especially in nonphotosynthetic tissues, remain unclear. This topic represents an underexplored interplay between primary and secondary metabolism where further research is needed.

Excessive composite pollution carbon sources enhance the bio-fertilizer efficiency of Tetradesmus obliquus: focused on cultivation period

Microalgae can use carbon sources in sludge extract prepared from sludge. Moreover, the high concentration of CO2 and the large number of carbon sources in the liquid phase will promote microalgae growth and metabolism. In this experiment, Tetradesmus obliquus was cultivated with sludge extract at 30% CO2. Algae liquid (the name used to describe the fertilizer made in this research) was further prepared as lettuce fertilizer. The effect of different times of microalgae culture (10, 15, 20, 25, and 30 days) on the fertilizer efficiency of the algae liquid was evaluated by lettuce hydroponic experiments. The findings indicate that lettuce cultivated in algae liquid collected on the 15th and 30th days exhibited superior performance in terms of growth, antioxidant capacity, and nutritional quality. We analyzed the experimental results in the context of microalgae metabolic mechanisms, aiming to contribute experience and data essential for the development of industrial microalgae fertilizer production.

Uncovering the secrets to vibrant flowers: the role of carotenoid esters and their interaction with plastoglobules in plant pigmentation

This article is a Commentary on Li et al. (2023), 240: 285–301.

Analysis of photosynthetic pigments pathway produced by CO2-toxicity-induced Scenedesmus obliquus

The coal-to-gas process produces carbon dioxide, which increases global warming, and its wastewater treatment generates sludge with high organic toxicity. Scenedesmus obliquus is a potential solution to such environmental problems, and photosynthetic pigments are the focus of this study. The optimal concentration of CO₂ for the growth of Scenedesmus obliquus was found to be 30 % after increasing the concentration of CO₂ (0.05 %-100 %). The accumulation of photosynthetic pigments during cultivation could reach 31.74 ± 1.33 mg/L, 11.21 ± 0.42 mg/L, and 5.59 ± 0.19 mg/L respectively, and the organic toxicity of sludge extract could be reduced by 44.97 %. Upregulation of A0A383VSL5, A0A383WMQ3, and A0A2Z4THB7 as photo systemic oxygen release proteins and propylene phosphate isomerase resulted in oxygen-evolving proteins in photosystem II, electron transport in photosystem I, and intermediates in carbon fixation. This is achieved by increasing the intracellular antennae protein and carbon fixation pathway, allowing Scenedesmus obliquus to both tolerate and fix CO₂ and reduce the organic toxicity of sludge. These findings provide insights into the innovative strategy underlining the fixation of CO₂, treatment and disposal of industrial residual sludge, and the enhancement of microalgal biomass production.

Review of Lipid Biomarkers and Signals of Photooxidative Stress in Plants

The degree of unsaturation of plant lipids is high, making them sensitive to oxidation. They thus constitute primary targets of reactive oxygen species and oxidative stress. Moreover, the hydroperoxides generated during lipid peroxidation decompose in a variety of secondary products which can propagate oxidative stress or trigger signaling mechanisms. Both primary and secondary products of lipid oxidation are helpful markers of oxidative stress in plants. This chapter describes a number of methods that have been developed to measure those biomarkers and signals, with special emphasis on the monitoring of photooxidative stress. Depending on their characteristics, those lipid markers provide information not only on the oxidation status of plant tissues but also on the origin of lipid peroxidation, the localization of the damage, or the type of reactive oxygen species involved.

Combination of long-term 13CO2 labeling and isotopolog profiling allows turnover analysis of photosynthetic pigments in Arabidopsis leaves

BACKGROUND

Living cells maintain and adjust structural and functional integrity by continual synthesis and degradation of metabolites and macromolecules. The maintenance and adjustment of thylakoid membrane involve turnover of photosynthetic pigments along with subunits of protein complexes. Quantifying their turnover is essential to understand the mechanisms of homeostasis and long-term acclimation of photosynthetic apparatus. Here we report methods combining whole-plant long-term ¹³CO₂ labeling and liquid chromatography - mass spectrometry (LC-MS) analysis to determine the size of non-labeled population (NLP) of carotenoids and chlorophylls (Chl) in leaf pigment extracts of partially ¹³C-labeled plants.

RESULTS

The labeling chamber enabled parallel ¹³CO₂ labeling of up to 15 plants of Arabidopsis thaliana with real-time environmental monitoring ([CO₂], light intensity, temperature, relative air humidity and pressure) and recording. No significant difference in growth or photosynthetic pigment composition was found in leaves after 7-d exposure to normal CO₂ (~ 400 ppm) or ¹³CO₂ in the labeling chamber, or in ambient air outside the labeling chamber (control). Following chromatographic separation of the pigments and mass peak assignment by high-resolution Fourier-transform ion cyclotron resonance MS, mass spectra of photosynthetic pigments were analyzed by triple quadrupole MS to calculate NLP. The size of NLP remaining after the 7-d ¹³CO₂ labeling was ~ 10.3% and ~ 11.5% for all-trans- and 9-cis-β-carotene, ~ 21.9% for lutein, ~ 18.8% for Chl a and 33.6% for Chl b, highlighting non-uniform turnover of these pigments in thylakoids. Comparable results were obtained in all replicate plants of the ¹³CO₂ labeling experiment except for three that were showing anthocyanin accumulation and growth impairment due to insufficient water supply (leading to stomatal closure and less ¹³C incorporation).

CONCLUSIONS

Our methods allow ¹³CO₂ labeling and estimation of NLP for photosynthetic pigments with high reproducibility despite potential variations in [¹³CO₂] between the experiments. The results indicate distinct turnover rates of carotenoids and Chls in thylakoid membrane, which can be investigated in the future by time course experiments. Since ¹³C enrichment can be measured in a range of compounds, long-term ¹³CO₂ labeling chamber, in combination with appropriate MS methods, facilitates turnover analysis of various metabolites and macromolecules in plants on a time scale of hours to days.

Inhibition of antioxidant enzyme activities enhances carotenogenesis in microalga Dactylococcus dissociatus MT1

Microalgal biotechnology has become a promising field of research for the production of valuable, sustainable and environmentally friendly byproducts, especially for carotenoids. Bulk accumulation of secondary carotenoids in microalgae are mostly induced by oxidative stress of cells. In this research, we investigated the effects of antioxidant enzyme activity inhibition on carotenogenesis in a microalga Dactylococcus dissociatus MT1. The activities of four major antioxidant enzyme families, namely superoxide dismutase (SOD), catalases (CAT), glutathione peroxydases (GPX) and ascorbate perxodases (APX), were inhibited by relevant inhibitors during the stressed cultivation of D. dissociatus to observe the effects on carotenogensis. A 91% decrease in activity was observed for CAT, comparing with controls without any inhibitors added, followed by 65%, 61%, and 47% for the enzymes SOD, APX, and GPX, respectively. Concomitantly, it was found that this partial inhibition had substantial influences on the accumulation of carotenoids, with the highest production levels obtained in CAT inhibition conditions and an increase of 2.6 times of carotenoid concentration observed, comparing with control cultivation conditions. We conclude that the modulation of antioxidant enzyme activities could lead to the overproduction of carotenoids in this microalgal cell culture, and we expect that this novel approach of optimizing carotenogenesis processes for D. dissociatus cell cultures could be transferrable to other cell culture systems and might have an important impact on the carotenoid production industry.

Mutation in Mg-Protoporphyrin IX Monomethyl Ester (Oxidative) Cyclase Gene ZmCRD1 Causes Chlorophyll-Deficiency in Maize

Chlorophyll molecules are non-covalently associated with chlorophyll-binding proteins to harvest light and perform charge separation vital for energy conservation during photosynthetic electron transfer in photosynthesis for photosynthetic organisms. The present study characterized a pale-green leaf (pgl) maize mutant controlled by a single recessive gene causing chlorophyll reduction throughout the whole life cycle. Through positional mapping and complementation allelic test, Zm00001d008230 (ZmCRD1) with two missense mutations (p.A44T and p.T326M) was identified as the causal gene encoding magnesium-protoporphyrin IX monomethyl ester cyclase (MgPEC). Phylogenetic analysis of ZmCRD1 within and among species revealed that the p.T326M mutation was more likely to be causal. Subcellular localization showed that ZmCRD1 was targeted to chloroplasts. The pgl mutant showed a malformed chloroplast morphology and reduced number of starch grains in bundle sheath cells. The ZmCRD1 gene was mainly expressed in WT and mutant leaves, but the expression was reduced in the mutant. Most of the genes involved in chlorophyll biosynthesis, chlorophyll degradation, chloroplast development and photosynthesis were down-regulated in pgl. The photosynthetic capacity was limited along with developmental retardation and production reduction in pgl. These results confirmed the crucial role of ZmCRD1 in chlorophyll biosynthesis, chloroplast development and photosynthesis in maize.

Functional characterization and comparison of lycopene epsilon-cyclase genes in Nicotiana tabacum

BACKGROUND

Lycopene epsilon-cyclase (ε-LCY) is a key enzyme in the carotenoid biosynthetic pathway (CBP) of higher plants. In previous work, we cloned two Ntε-LCY genes from allotetraploid tobacco (Nicotiana tabacum), Ntε-LCY2 and Ntε-LCY1, and demonstrated the overall effect of Ntε-LCY genes on carotenoid biosynthesis and stress resistance. However, their genetic and functional characteristics require further research in polyploid plants.

RESULTS

Here, we used CRISPR/Cas9 to obtain Ntε-LCY2 and Ntε-LCY1 mutants in allotetraploid N.tabacum K326. Ntε-LCY2 and Ntε-LCY1 had similar promoter cis-acting elements, including light-responsive elements. The Ntε-LCY genes were expressed in roots, stems, leaves, flowers, and young fruit, and their highest expression levels were found in leaves. Ntε-LCY2 and Ntε-LCY1 genes responded differently to normal light and high light stress. Both the Ntε-LCY2 and the Ntε-LCY1 mutants had a more rapid leaf growth rate, especially ntε-lcy2-1. The expression levels of CBP genes were increased in the ntε-lcy mutants, and their total carotenoid content was higher. Under both normal light and high light stress, the ntε-lcy mutants had higher photosynthetic capacities and heat dissipation levels than the wild type, and this was especially true of ntε-lcy2-1. The reactive oxygen species content was lower in leaves of the ntε-lcy mutants.

CONCLUSION

In summary, the expression patterns and biological functions of the Ntε-LCY genes Ntε-LCY1 and Ntε-LCY2 differed in several respects. The mutation of Ntε-LCY2 was associated with a greater increase in the content of chlorophyll and various carotenoid components, and it enhanced the stress resistance of tobacco plants under high light.

Triplet-driven chemical reactivity of β-carotene and its biological implications

The endoperoxides of β-carotene (βCar-EPOs) are regarded as main products of the chemical deactivation of ¹O₂ by β-carotene, one of the most important antioxidants, following a concerted singlet-singlet reaction. Here we challenge this view by showing that βCar-EPOs are formed in the absence of ¹O₂ in a non-concerted triplet-triplet reaction: ³O₂ + ³β-carotene → βCar-EPOs, in which ³β-carotene manifests a strong biradical character. Thus, the reactivity of β-carotene towards oxygen is governed by its excited triplet state. βCar-EPOs, while being stable in the dark, are photochemically labile, and are a rare example of nonaromatic endoperoxides that release ¹O₂, again not in a concerted reaction. Their light-induced breakdown triggers an avalanche of free radicals, which accounts for the pro-oxidant activity of β-carotene and the puzzling swap from its anti- to pro-oxidant features. Furthermore, we show that βCar-EPOs, and carotenoids in general, weakly sensitize ¹O₂. These findings underlie the key role of the triplet state in determining the chemical and photophysical features of β-carotene. They shake up the prevailing models of carotenoid photophysics, the anti-oxidant functioning of β-carotene, and the role of ¹O₂ in chemical signaling in biological photosynthetic systems. βCar-EPOs and their degradation products are not markers of ¹O₂ and oxidative stress but of the overproduction of extremely hazardous chlorophyll triplets in photosystems. Hence, the chemical signaling of overexcitation of the photosynthetic apparatus is based on a ³chlorophyll-³β-carotene relay, rather than on extremely short-lived ¹O₂.

The terminal enzymes of (bacterio)chlorophyll biosynthesis

(Bacterio)chlorophylls are modified tetrapyrroles that are used by phototrophic organisms to harvest solar energy, powering the metabolic processes that sustain most of the life on Earth. Biosynthesis of these pigments involves enzymatic modification of the side chains and oxidation state of a porphyrin precursor, modifications that differ by species and alter the absorption properties of the pigments. (Bacterio)chlorophylls are coordinated by proteins that form macromolecular assemblies to absorb light and transfer excitation energy to a special pair of redox-active (bacterio)chlorophyll molecules in the photosynthetic reaction centre. Assembly of these pigment-protein complexes is aided by an isoprenoid moiety esterified to the (bacterio)chlorin macrocycle, which anchors and stabilizes the pigments within their protein scaffolds. The reduction of the isoprenoid 'tail' and its addition to the macrocycle are the final stages in (bacterio)chlorophyll biosynthesis and are catalysed by two enzymes, geranylgeranyl reductase and (bacterio)chlorophyll synthase. These enzymes work in conjunction with photosynthetic complex assembly factors and the membrane biogenesis machinery to synchronize delivery of the pigments to the proteins that coordinate them. In this review, we summarize current understanding of the catalytic mechanism, substrate recognition and regulation of these crucial enzymes and their involvement in thylakoid biogenesis and photosystem repair in oxygenic phototrophs.

Color recycling: metabolization of apocarotenoid degradation products suggests carbon regeneration via primary metabolic pathways

Analysis of carotenoid-accumulating roots revealed that oxidative carotenoid degradation yields glyoxal and methylglyoxal. Our data suggest that these compounds are detoxified via the glyoxalase system and re-enter primary metabolic pathways. Carotenoid levels in plant tissues depend on the relative rates of synthesis and degradation. We recently identified redox enzymes previously known to be involved in the detoxification of fatty acid-derived reactive carbonyl species which were able to convert apocarotenoids into corresponding alcohols and carboxylic acids. However, their subsequent metabolization pathways remain unresolved. Interestingly, we found that carotenoid-accumulating roots have increased levels of glutathione, suggesting apocarotenoid glutathionylation to occur. In vitro and in planta investigations did not, however, support the occurrence of non-enzymatic or enzymatic glutathionylation of β-apocarotenoids. An alternative breakdown pathway is the continued oxidative degradation of primary apocarotenoids or their derivatives into the shortest possible oxidation products, namely glyoxal and methylglyoxal, which also accumulated in carotenoid-accumulating roots. In fact, combined transcriptome and metabolome analysis suggest that the high levels of glutathione are most probably required for detoxifying apocarotenoid-derived glyoxal and methylglyoxal via the glyoxalase pathway, yielding glycolate and D-lactate, respectively. Further transcriptome analysis suggested subsequent reactions involving activities associated with photorespiration and the peroxisome-specific glycolate/glyoxylate transporter. Finally, detoxified primary apocarotenoid degradation products might be converted into pyruvate which is possibly re-used for the synthesis of carotenoid biosynthesis precursors. Our findings allow to envision carbon recycling during carotenoid biosynthesis, degradation and re-synthesis which consumes energy, but partially maintains initially fixed carbon via re-introducing reactive carotenoid degradation products into primary metabolic pathways.

A foliar pigment-based bioassay for interrogating chloroplast signalling revealed that carotenoid isomerisation regulates chlorophyll abundance

BACKGROUND

Some plastid-derived metabolites can control nuclear gene expression, chloroplast biogenesis, and chlorophyll biosynthesis. For example, norflurazon (NFZ) induced inhibition of carotenoid biosynthesis in leaves elicits a protoporphyrin IX (Mg-ProtoIX) retrograde signal that controls chlorophyll biosynthesis and chloroplast development. Carotenoid cleavage products, known as apocarotenoids, also regulate plastid development. The key steps in carotenoid biosynthesis or catabolism that can regulate chlorophyll biosynthesis in leaf tissues remain unclear. Here, we established a foliar pigment-based bioassay using Arabidopsis rosette leaves to investigate plastid signalling processes in young expanding leaves comprising rapidly dividing and expanding cells containing active chloroplast biogenesis.

RESULTS

We demonstrate that environmental treatments (extended darkness and cold exposure) as well as chemical (norflurazon; NFZ) inhibition of carotenoid biosynthesis, reduce chlorophyll levels in young, but not older leaves of Arabidopsis. Mutants with disrupted xanthophyll accumulation, apocarotenoid phytohormone biosynthesis (abscisic acid and strigolactone), or enzymatic carotenoid cleavage, did not alter chlorophyll levels in young or old leaves. However, perturbations in acyclic cis-carotene biosynthesis revealed that disruption of CAROTENOID ISOMERASE (CRTISO), but not ZETA-CAROTENE ISOMERASE (Z-ISO) activity, reduced chlorophyll levels in young leaves of Arabidopsis plants. NFZ-induced inhibition of PHYTOENE DESATURASE (PDS) activity caused higher phytoene accumulation in younger crtiso leaves compared to WT indicating a continued substrate supply from the methylerythritol 4-phosphate (MEP) pathway.

CONCLUSION

The Arabidopsis foliar pigment-based bioassay can be used to differentiate signalling events elicited by environmental change, chemical treatment, and/or genetic perturbation, and determine how they control chloroplast biogenesis and chlorophyll biosynthesis. Genetic perturbations that impaired xanthophyll biosynthesis and/or carotenoid catabolism did not affect chlorophyll biosynthesis. The lack of CAROTENOID ISOMERISATION reduced chlorophyll accumulation, but not phytoene biosynthesis in young leaves of Arabidopsis plants growing under a long photoperiod. Findings generated using the newly customised foliar pigment-based bioassay implicate that carotenoid isomerase activity and NFZ-induced inhibition of PDS activity elicit different signalling pathways to control chlorophyll homeostasis in young leaves of Arabidopsis.

Plant carotenoids: recent advances and future perspectives

Carotenoids are isoprenoid metabolites synthesized de novo in all photosynthetic organisms. Carotenoids are essential for plants with diverse functions in photosynthesis, photoprotection, pigmentation, phytohormone synthesis, and signaling. They are also critically important for humans as precursors of vitamin A synthesis and as dietary antioxidants. The vital roles of carotenoids to plants and humans have prompted significant progress toward our understanding of carotenoid metabolism and regulation. New regulators and novel roles of carotenoid metabolites are continuously revealed. This review focuses on current status of carotenoid metabolism and highlights recent advances in comprehension of the intrinsic and multi-dimensional regulation of carotenoid accumulation. We also discuss the functional evolution of carotenoids, the agricultural and horticultural application, and some key areas for future research.

Research Progress in the Interconversion, Turnover and Degradation of Chlorophyll

Chlorophylls (Chls, Chl a and Chl b) are tetrapyrrole molecules essential for photosynthetic light harvesting and energy transduction in plants. Once formed, Chls are noncovalently bound to photosynthetic proteins on the thylakoid membrane. In contrast, they are dismantled from photosystems in response to environmental changes or developmental processes; thus, they undergo interconversion, turnover, and degradation. In the last twenty years, fruitful research progress has been achieved on these Chl metabolic processes. The discovery of new metabolic pathways has been accompanied by the identification of enzymes associated with biochemical steps. This article reviews recent progress in the analysis of the Chl cycle, turnover and degradation pathways and the involved enzymes. In addition, open questions regarding these pathways that require further investigation are also suggested.

Rapid Detection and Quantification of Plant Innate Immunity Response Using Raman Spectroscopy

We have developed a rapid Raman spectroscopy-based method for the detection and quantification of early innate immunity responses in Arabidopsis and Choy Sum plants. Arabidopsis plants challenged with flg22 and elf18 elicitors could be differentiated from mock-treated plants by their Raman spectral fingerprints. From the difference Raman spectrum and the value of p at each Raman shift, we derived the Elicitor Response Index (ERI) as a quantitative measure of the response whereby a higher ERI value indicates a more significant elicitor-induced immune response. Among various Raman spectral bands contributing toward the ERI value, the most significant changes were observed in those associated with carotenoids and proteins. To validate these results, we investigated several characterized Arabidopsis pattern-triggered immunity (PTI) mutants. Compared to wild type (WT), positive regulatory mutants had ERI values close to zero, whereas negative regulatory mutants at early time points had higher ERI values. Similar to elicitor treatments, we derived an analogous Infection Response Index (IRI) as a quantitative measure to detect the early PTI response in Arabidopsis and Choy Sum plants infected with bacterial pathogens. The Raman spectral bands contributing toward a high IRI value were largely identical to the ERI Raman spectral bands. Raman spectroscopy is a convenient tool for rapid screening for Arabidopsis PTI mutants and may be suitable for the noninvasive and early diagnosis of pathogen-infected crop plants.

Dissipation of Light Energy Absorbed in Excess: The Molecular Mechanisms

Light is essential for photosynthesis. Nevertheless, its intensity widely changes depending on time of day, weather, season, and localization of individual leaves within canopies. This variability means that light collected by the light-harvesting system is often in excess with respect to photon fluence or spectral quality in the context of the capacity of photosynthetic metabolism to use ATP and reductants produced from the light reactions. Absorption of excess light can lead to increased production of excited, highly reactive intermediates, which expose photosynthetic organisms to serious risks of oxidative damage. Prevention and management of such stress are performed by photoprotective mechanisms, which operate by cutting down light absorption, limiting the generation of redox-active molecules, or scavenging reactive oxygen species that are released despite the operation of preventive mechanisms. Here, we describe the major physiological and molecular mechanisms of photoprotection involved in the harmless removal of the excess light energy absorbed by green algae and land plants. In vivo analyses of mutants targeting photosynthetic components and the enhanced resolution of spectroscopic techniques have highlighted specific mechanisms protecting the photosynthetic apparatus from overexcitation. Recent findings unveil a network of multiple interacting elements, the reaction times of which vary from a millisecond to weeks, that continuously maintain photosynthetic organisms within the narrow safety range between efficient light harvesting and photoprotection.

Combined Acute Ozone and Water Stress Alters the Quantitative Relationships between O3 Uptake, Photosynthetic Characteristics and Volatile Emissions in Brassica nigra

Ozone (O₃) entry into plant leaves depends on atmospheric O₃ concentration, exposure time and openness of stomata. O₃ negatively impacts photosynthesis rate (A) and might induce the release of reactive volatile organic compounds (VOCs) that can quench O₃, and thereby partly ameliorate O₃ stress. Water stress reduces stomatal conductance (g_s) and O₃ uptake and can affect VOC release and O₃ quenching by VOC, but the interactive effects of O₃ exposure and water stress, as possibly mediated by VOC, are poorly understood. Well-watered (WW) and water-stressed (WS) Brassica nigra plants were exposed to 250 and 550 ppb O₃ for 1 h, and O₃ uptake rates, photosynthetic characteristics and VOC emissions were measured through 22 h recovery. The highest O₃ uptake was observed in WW plants exposed to 550 ppb O₃ with the greatest reduction and poorest recovery of g_s and A, and elicitation of lipoxygenase (LOX) pathway volatiles 10 min-1.5 h after exposure indicating cellular damage. Ozone uptake was similar in 250 ppb WW and 550 ppb WS plants and, in both treatments, O₃-dependent reduction in photosynthetic characteristics was moderate and fully reversible, and VOC emissions were little affected. Water stress alone did not affect the total amount and composition of VOC emissions. The results indicate that drought ameliorated O₃ stress by reducing O₃ uptake through stomatal closure and the two stresses operated in an antagonistic manner in B. nigra.

Connecting Chlorophyll Metabolism with Accumulation of the Photosynthetic Apparatus

Chlorophyll (Chl) is indispensable for photosynthesis. In association with Chl-binding proteins (CBPs), it is responsible for light absorption, excitation energy transfer, and charge separation within the photosynthetic complexes. By contrast, photoexcitation of free Chl and its metabolic intermediates generates hazardous reactive oxygen species (ROS). While antagonistic activities of Chl synthesis and catabolism have been mostly elucidated, the tight synchronization of these metabolic activities with the formation and dismantling of the photosynthetic complexes is poorly understood. Recently, a set of auxiliary factors were identified to adjust metabolic activities and provide accurate amounts of Chl for pigment-protein complexes. Here, we review current knowledge of post-translational coordination of Chl formation, breakdown, and turnover with the assembly and disassembly of various CBPs and highlight future research perspectives.

Factors Differentiating the Antioxidant Activity of Macular Xanthophylls in the Human Eye Retina

Macular xanthophylls, which are absorbed from the human diet, accumulate in high concentrations in the human retina, where they efficiently protect against oxidative stress that may lead to retinal damage. In addition, macular xanthophylls are uniquely spatially distributed in the retina. The zeaxanthin concentration (including the lutein metabolite meso-zeaxanthin) is ~9-fold greater than lutein concentration in the central fovea. These numbers do not correlate at all with the dietary intake of xanthophylls, for which there is a dietary zeaxanthin-to-lutein molar ratio of 1:12 to 1:5. The unique spatial distributions of macular xanthophylls-lutein, zeaxanthin, and meso-zeaxanthin-in the retina, which developed during evolution, maximize the protection of the retina provided by these xanthophylls. We will correlate the differences in the spatial distributions of macular xanthophylls with their different antioxidant activities in the retina. Can the major protective function of macular xanthophylls in the retina, namely antioxidant actions, explain their evolutionarily determined, unique spatial distributions? In this review, we will address this question.

Isotope ratio-based quantification of carbon assimilation highlights the role of plastidial isoprenoid precursor availability in photosynthesis

BACKGROUND

We report a method to estimate carbon assimilation based on isotope ratio-mass spectrometry (IRMS) of ¹³CO₂ labeled plant tissue. Photosynthetic carbon assimilation is the principal experimental observable which integrates important aspects of primary plant metabolism. It is traditionally measured through gas exchange. Despite its centrality in plant research, gas exchange performs poorly with rosette growth habits typical of Arabidopsis thaliana, mutant lines with limited biomass, and accounts poorly for leaf shading.

RESULTS

IRMS-based carbon assimilation values from plants labeled at different light intensities were compared to those obtained by gas exchange, and the two methods yielded similar values. Using this method, we observed a strong correlation between ¹³C content and labeling time (R² = 0.999) for 158 wild-type plants labeled for 6 to 42 min. Plants cultivated under different light regimes showed a linear response with respect to carbon assimilation, varying from 7.38 nmol ¹³C mg^-1 leaf tissue min^-1 at 80 PAR to 19.27 nmol ¹³C mg^-1 leaf tissue min^-1 at 500 PAR. We applied this method to examine the link between inhibition of the 2C-methyl-D-erythritol-4-phosphate (MEP) pathway and suppression of photosynthesis. A significant decrease in carbon assimilation was observed when metabolic activity in the MEP pathway was compromised by mutation or herbicides targeting the MEP pathway. Mutants affected in MEP pathway genes 1-DEOXY-D-XYLULOSE 5-PHOSPHATE SYNTHASE (DXS) or 1-HYDROXY-2-METHYL-2-(E)-BUTENYL 4-DIPHOSPHATE SYNTHASE (HDS) showed assimilation rates 36% and 61% lower than wild type. Similarly, wild type plants treated with the MEP pathway inhibitors clomazone or fosmidomycin showed reductions of 52% and 43%, respectively, while inhibition of the analogous mevalonic acid pathway, which supplies the same isoprenoid intermediates in the cytosol, did not, suggesting inhibition of photosynthesis was specific to disruption of the MEP pathway.

CONCLUSIONS

This method provides an alternative to gas exchange that offers several advantages: resilience to differences in leaf overlap, measurements based on tissue mass rather than leaf surface area, and compatibility with mutant Arabidopsis lines which are not amenable to gas exchange measurements due to low biomass and limited leaf surface area. It is suitable for screening large numbers of replicates simultaneously as well as post-hoc analysis of previously labeled plant tissue and is complementary to downstream detection of isotopic label in targeted metabolite pools.

Plant apocarotenoid metabolism utilizes defense mechanisms against reactive carbonyl species and xenobiotics

Carotenoid levels in plant tissues depend on the relative rates of synthesis and degradation of the molecules in the pathway. While plant carotenoid biosynthesis has been extensively characterized, research on carotenoid degradation and catabolism into apocarotenoids is a relatively novel field. To identify apocarotenoid metabolic processes, we characterized the transcriptome of transgenic Arabidopsis (Arabidopsis thaliana) roots accumulating high levels of β-carotene and, consequently, β-apocarotenoids. Transcriptome analysis revealed feedback regulation on carotenogenic gene transcripts suitable for reducing β-carotene levels, suggesting involvement of specific apocarotenoid signaling molecules originating directly from β-carotene degradation or after secondary enzymatic derivatizations. Enzymes implicated in apocarotenoid modification reactions overlapped with detoxification enzymes of xenobiotics and reactive carbonyl species (RCS), while metabolite analysis excluded lipid stress response, a potential secondary effect of carotenoid accumulation. In agreement with structural similarities between RCS and β-apocarotenoids, RCS detoxification enzymes also converted apocarotenoids derived from β-carotene and from xanthophylls into apocarotenols and apocarotenoic acids in vitro. Moreover, glycosylation and glutathionylation-related processes and translocators were induced. In view of similarities to mechanisms found in crocin biosynthesis and cellular deposition in saffron (Crocus sativus), our data suggest apocarotenoid metabolization, derivatization and compartmentalization as key processes in (apo)carotenoid metabolism in plants.

Too rigid to fold: Carotenoid-dependent decrease in thylakoid fluidity hampers the formation of chloroplast grana

In chloroplasts of land plants, the thylakoid network is organized into appressed regions called grana stacks and loosely arranged parallel stroma thylakoids. Many factors determining such intricate structural arrangements have been identified so far, including various thylakoid-embedded proteins, and polar lipids that build the thylakoid matrix. Although carotenoids are important components of proteins and the lipid phase of chloroplast membranes, their role in determining the thylakoid network structure remains elusive. We studied 2D and 3D thylakoid network organization in carotenoid-deficient mutants (ccr1-1, lut5-1, szl1-1, and szl1-1npq1-2) of Arabidopsis (Arabidopsis thaliana) to reveal the structural role of carotenoids in the formation and dynamics of the internal chloroplast membrane system. The most significant structural aberrations took place in chloroplasts of the szl1-1 and szl1-1npq1-2 plants. Increased lutein/carotene ratio in these mutants impaired the formation of grana, resulting in a significant decrease in the number of thylakoids used to build a particular stack. Further, combined biochemical and biophysical analyses revealed that hampered grana folding was related to decreased thylakoid membrane fluidity and significant changes in the amount, organization, and phosphorylation status of photosystem (PS) II (PSII) supercomplexes in the szl1-1 and szl1-1npq1-2 plants. Such changes resulted from a synergistic effect of lutein overaccumulation in the lipid matrix and a decreased level of carotenes bound with PS core complexes. Moreover, more rigid membrane in the lutein overaccumulating plants led to binding of Rubisco to the thylakoid surface, additionally providing steric hindrance for the dynamic changes in the level of membrane folding.

A new function for the xanthophyll zeaxanthin: glueing chlorophyll biosynthesis to thylakoid protein assembly

Xanthophylls are coloured isoprenoid metabolites synthesized in many organisms with a variety of functions from the attraction of animals for impollination to absorption of light energy for photosynthesis to photoprotection against photooxidative stress. The finding by Proctor and co-workers makes a new addition to the last type of functions by showing that zeaxanthin is instrumental in coordinating chlorophyll biosynthesis with the insertion of pigment-binding proteins into the photosynthetic membrane by glueing the protein components catalyzing these functions into a supercomplex and regulating its activity.

Quantification of Carotenoid Pathway Flux in Green and Nongreen Systems

Metabolite accumulation in plant tissues represents the transient net result of their constant biosynthesis and degradation. For carotenoids, degradation might occur enzymatically by carotenoid cleavage producing plant hormones and volatiles or by nonenzymatic oxidation, both depending on environmental and developmental conditions. Carotenoid biosynthesis is therefore constantly regulated at various levels to attain sufficient carotenoid accumulation, mainly for photosynthesis and photoprotection. Due to the plenitude of carotenoids and their degradation products, it is not feasible to investigate overall carotenoid biosynthetic activity and its regulation by the quantification of all carotenoids including their derivatives. This is an issue encountered in investigations on many other highly branched pathways. We therefore present protocols to determine carotenoid biosynthesis flux in a given plant tissue by HPLC quantification of phytoene, the first pathway-specific intermediate and precursor of all carotenoids synthesized by phytoene synthase (PSY). For this purpose, enzymatic metabolization of phytoene in the tissue under investigation is prevented by treatment with the bleaching herbicide norflurazon, blocking the carotenogenic pathway downstream of PSY. As phytoene is more resistant to oxidation than desaturated carotenoids, the rate of phytoene biosynthesis serves as a good measure for total carotenogenic flux in a given tissue. The method is described for Arabidopsis for two photosynthetically active sample types, namely, seedlings and leaves, as well as for seed-derived callus as nongreen tissue. It should be realizable using only a relatively simple experimental setup and is applicable to other plant tissues as well as to different plant species. Additionally, similar experimental setups could be a useful tool to investigate total flux and turnover rates in other biosynthetic pathways.

Overexpression of the Golden SNP-Carrying Orange Gene Enhances Carotenoid Accumulation and Heat Stress Tolerance in Sweetpotato Plants

Carotenoids function as photosynthetic accessory pigments, antioxidants, and vitamin A precursors. We recently showed that transgenic sweetpotato calli overexpressing the mutant sweetpotato (Ipomoea batatas [L.] Lam) Orange gene (IbOr-R96H), which carries a single nucleotide polymorphism responsible for Arg to His substitution at amino acid position 96, exhibited dramatically higher carotenoid content and abiotic stress tolerance than calli overexpressing the wild-type IbOr gene (IbOr-WT). In this study, we generated transgenic sweetpotato plants overexpressing IbOr-R96H under the control of the cauliflower mosaic virus (CaMV) 35S promoter via Agrobacterium-mediated transformation. The total carotenoid contents of IbOr-R96H storage roots (light-orange flesh) and IbOr-WT storage roots (light-yellow flesh) were 5.4-19.6 and 3.2-fold higher, respectively, than those of non-transgenic (NT) storage roots (white flesh). The β-carotene content of IbOr-R96H storage roots was up to 186.2-fold higher than that of NT storage roots. In addition, IbOr-R96H plants showed greater tolerance to heat stress (47 °C) than NT and IbOr-WT plants, possibly because of higher DPPH radical scavenging activity and ABA contents. These results indicate that IbOr-R96H is a promising strategy for developing new sweetpotato cultivars with improved carotenoid contents and heat stress tolerance.

Responses of isoprene emission and photochemical efficiency to severe drought combined with prolonged hot weather in hybrid Populus

Isoprene emissions have been considered as a protective response of plants to heat stress, but there is limited information of how prolonged heat spells affect isoprene emission capacity, particularly under the drought conditions that often accompany hot weather. Under combined long-term stresses, presence of isoprene emission could contribute to the maintenance of the precursor pool for rapid synthesis of essential isoprenoids to repair damaged components of leaf photosynthetic apparatus. We studied changes in leaf isoprene emission rate, photosynthetic characteristics, and antioxidant enzyme activities in two hybrid Populus clones, Nanlin 1388 (relatively high drought tolerance) and Nanlin 895 (relatively high thermotolerance) that were subjected to long-term (30 d) soil water stress (25% versus 90% soil field capacity) combined with a natural heat spell (day-time temperatures of 35-40 °C) that affected both control and water-stressed plants. Unexpectedly, isoprene emissions from both the clones were similar and the overall effects of drought on the emission characteristics were initially minor; however, treatment effects and clonal differences increased with time. In particular, the isoprene emission rate only increased slightly in the Nanlin 895 control plants after 15 d of treatment, whereas it decreased by more than 5-fold in all treatment × clone combinations after 30 d. The reduction in isoprene emission rate was associated with a decrease in the pool size of the isoprene precursor dimethylallyl diphosphate in all cases at 30 d after the start of treatment. Net assimilation rate, stomatal conductance, the openness of PSII centers, and the effective quantum yield all decreased, and non-photochemical quenching and catalase activity increased in both control and water-stressed plants. Contrary to the hypothesis of protection of leaf photosynthetic apparatus by isoprene, the data collectively indicated that prolonged stress affected isoprene emissions more strongly than leaf photosynthetic characteristics. This primarily reflected the depletion of isoprene precursor pools under long-term severe stress.

Modelling photosystem I as a complex interacting network

In this paper, we model the excitation energy transfer (EET) of photosystem I (PSI) of the common pea plant Pisum sativum as a complex interacting network. The magnitude of the link energy transfer between nodes/chromophores is computed by Forster resonant energy transfer (FRET) using the pairwise physical distances between chromophores from the PDB 5L8R (Protein Data Bank). We measure the global PSI network EET efficiency adopting well-known network theory indicators: the network efficiency (Eff) and the largest connected component (LCC). We also account the number of connected nodes/chromophores to P700 (CN), a new ad hoc measure we introduce here to indicate how many nodes in the network can actually transfer energy to the P700 reaction centre. We find that when progressively removing the weak links of lower EET, the Eff decreases, while the EET paths integrity (LCC and CN) is still preserved. This finding would show that the PSI is a resilient system owning a large window of functioning feasibility and it is completely impaired only when removing most of the network links. From the study of different types of chromophore, we propose different primary functions within the PSI system: chlorophyll a (CLA) molecules are the central nodes in the EET process, while other chromophore types have different primary functions. Furthermore, we perform nodes removal simulations to understand how the nodes/chromophores malfunctioning may affect PSI functioning. We discover that the removal of the CLA triggers the fastest decrease in the Eff, confirming that CAL is the main contributors to the high EET efficiency. Our outcomes open new perspectives of research, such comparing the PSI energy transfer efficiency of different natural and agricultural plant species and investigating the light-harvesting mechanisms of artificial photosynthesis both in plant agriculture and in the field of solar energy applications.

Unexplored nutritive potential of tomato to combat global malnutrition

Tomato, a widely consumed vegetable crop, offers a real potential to combat human nutritional deficiencies. Tomatoes are rich in micronutrients and other bioactive compounds (including vitamins, carotenoids, and minerals) that are known to be essential or beneficial for human health. This review highlights the current state of the art in the molecular understanding of the nutritional aspects, conventional and molecular breeding efforts, and biofortification studies undertaken to improve the nutritional content and quality of tomato. Transcriptomics and metabolomics studies, which offer a deeper understanding of the molecular regulation of the tomato's nutrients, are discussed. The potential uses of the wastes from the tomato processing industry (i.e., the peels and seed extracts) that are particularly rich in oils and proteins are also discussed. Recent advancements with CRISPR/Cas mediated gene-editing technology provide enormous opportunities to enhance the nutritional content of agricultural produces, including tomatoes. In this regard, genome editing efforts with respect to biofortification in the tomato plant are also discussed. The recent technological advancements and knowledge gaps described herein aim to help explore the unexplored nutritional potential of the tomato.

Chloroplast Sec14-like 1 (CPSFL1) is essential for normal chloroplast development and affects carotenoid accumulation in Chlamydomonas

Carotenoids are essential molecules in oxygenic photoautotrophs, and they fulfill essential requirements for human and animal nutrition. How carotenoid accumulation is regulated in the chloroplast, a cyanobacterium-derived organelle, remains poorly understood, despite significant advancements in identifying enzymes of the carotenoid biosynthetic pathway. This study identifies a role of chloroplast Sec14-like 1 (CPSFL1), a CRAL-TRIO protein of eukaryotic origin, in modulation of carotenoid biosynthesis and accumulation in the chloroplast. The CPSFL1 protein represents an isoprenoid- and carotenoid-binding protein that associates with membranes through interactions with the phospholipid phosphatidic acid. These findings have implications for understanding carotenoid biosynthesis and optimizing algal carotenoid nutritional quality.

Plastid isoprenoid-derived carotenoids serve essential roles in chloroplast development and photosynthesis. Although nearly all enzymes that participate in the biosynthesis of carotenoids in plants have been identified, the complement of auxiliary proteins that regulate synthesis, transport, sequestration, and degradation of these molecules and their isoprenoid precursors have not been fully described. To identify such proteins that are necessary for the optimal functioning of oxygenic photosynthesis, we screened a large collection of nonphotosynthetic (acetate-requiring) DNA insertional mutants of Chlamydomonas reinhardtii and isolated cpsfl1. The cpsfl1 mutant is extremely light-sensitive and susceptible to photoinhibition and photobleaching. The CPSFL1 gene encodes a CRAL-TRIO hydrophobic ligand-binding (Sec14) domain protein. Proteins containing this domain are limited to eukaryotes, but some may have been retargeted to function in organelles of endosymbiotic origin. The cpsfl1 mutant showed decreased accumulation of plastidial isoprenoid-derived pigments, especially carotenoids, and whole-cell focused ion-beam scanning-electron microscopy revealed a deficiency of carotenoid-rich chloroplast structures (e.g., eyespot and plastoglobules). The low carotenoid content resulted from impaired biosynthesis at a step prior to phytoene, the committed precursor to carotenoids. The CPSFL1 protein bound phytoene and β-carotene when expressed in Escherichia coli and phosphatidic acid in vitro. We suggest that CPSFL1 is involved in the regulation of phytoene synthesis and carotenoid transport and thereby modulates carotenoid accumulation in the chloroplast.

Molecular dynamics simulations in photosynthesis

Photosynthesis is regulated by a dynamic interplay between proteins, enzymes, pigments, lipids, and cofactors that takes place on a large spatio-temporal scale. Molecular dynamics (MD) simulations provide a powerful toolkit to investigate dynamical processes in (bio)molecular ensembles from the (sub)picosecond to the (sub)millisecond regime and from the Å to hundreds of nm length scale. Therefore, MD is well suited to address a variety of questions arising in the field of photosynthesis research. In this review, we provide an introduction to the basic concepts of MD simulations, at atomistic and coarse-grained level of resolution. Furthermore, we discuss applications of MD simulations to model photosynthetic systems of different sizes and complexity and their connection to experimental observables. Finally, we provide a brief glance on which methods provide opportunities to capture phenomena beyond the applicability of classical MD.

Influence of Brevibacterium linens RS16 on foliage photosynthetic and volatile emission characteristics upon heat stress in Eucalyptus grandis

Heat stress induces secondary metabolic changes in plants, channeling photosynthetic carbon and energy, away from primary metabolic processes, including, growth. Use of ACC (1-aminocyclopropane-1-carboxylate) deaminase containing plant growth promoting bacteria (PGPB) in conferring heat resistance in plants and the role of PGPB, in altering net carbon assimilation, constitutive and stress volatile emissions has not been studied yet. We exposed leaves of Eucalyptus grandis inoculated and non-inoculated with PGPB Brevibacterium linens RS16 to two levels of heat stress (37 °C and 41 °C for 5 min) and quantified temporal changes in foliage photosynthetic characteristics and volatile emission rates at 0.5 h, day 1 and day 5 after the stress application. Heat stress resulted in immediate reductions in dark-adapted photosystem II (PSII) quantum yield (F_v/F_m), net assimilation rate (A), stomatal conductance to water vapor (g_s), and enhancement of stress volatile emissions, including enhanced emissions of green leaf volatiles (GLV), mono- and sesquiterpenes, light weight oxygenated volatile organic compounds (LOC), geranyl-geranyl diphosphate pathway volatiles (GGDP), saturated aldehydes, and benzenoids, with partial recovery by day 5. Changes in stress-induced volatiles were always less in leaves inoculated with B. linens RS16. However, net assimilation rate was enhanced by bacterial inoculation only in the 37 °C treatment and overall reduction of isoprene emissions was observed in bacterially-treated leaves. Principal component analysis (PCA), correlation analysis and partial least squares discriminant analysis (PLS-DA) indicated that different stress applications influenced specific volatile organic compounds. In addition, changes in the expression analysis of heat shock protein 70 gene (DnaK) gene in B. linens RS16 upon exposure to higher temperatures further indicated that B. linens RS16 has developed its own heat resistance mechanism to survive under higher temperature regimes. Taken together, this study demonstrates that foliar application of ACC deaminase containing PGPB can ameliorate heat stress effects in realistic biological settings.

Pathways for Carotenoid Biosynthesis, Degradation, and Storage

Carotenoids are isoprenoid compounds synthesized de novo in all photosynthetic organisms as well as in some nonphotosynthetic bacteria and fungi. In plants, carotenoids are essential for light harvesting and photoprotection. They contribute to the vivid color found in many plant organs. The cleavage of carotenoids produces small molecules (apocarotenoids) that serve as aroma compounds, as well as phytohormones and signals to affect plant growth and development. Since carotenoids provide valuable nutrition and health benefits for humans, understanding of carotenoid biosynthesis, catabolism and storage is important for biofortification of crops with improved nutritional quality. This chapter primarily introduces our current knowledge about carotenoid biosynthesis and degradation pathways as well as carotenoid storage in plants.

Carotenoid Isotopolog Profiling in 13C-Labeled Leaf Extracts by LC-MS and LC-FTICR-MS

Mass spectrometry (MS)-based metabolite analysis combined with stable isotope labeling offers a powerful tool to study dynamic regulation of metabolic pathways and metabolite fluxes in biological systems. Here we describe a method to analyze the composition of carotenoid isotopologs in ¹³C-labeled leaf extracts by using liquid chromatography (LC)-MS and LC-Fourier transform ion cyclotron resonance (FTICR)-MS. High mass resolution of the latter enables unambiguous assignment of observed mass to a unique chemical formula. Based on peak intensity the relative abundance and the degree of ¹³C labeling are calculated for individual carotenoid isotopologs.

Chlorophyll Oxidative Metabolism During the Phototrophic and Heterotrophic Growth of Scenedesmus obliquus

Different cultivation strategies have been developed with the aim of increasing the production rate of microalgal pigments. Specifically, biotechnological approaches are designed to increase antioxidant metabolites as chlorophyll and carotenoids. However, although significant advances have been built up, available information regarding both the chlorophyll metabolism and their oxidative reactions in photobioreactors is scarce. To unravel such processes, the detailed chlorophyll and carotenoid fraction of Scenedesmus obliquus has been studied by HPLC-ESI/APCI-hrTOF-MS from phototrophic and heterotrophic cultures. Scenedesmus is provided with a controlled strategy of interconversion between chlorophyll a and b to avoid the formation of reactive oxygen species (ROS) at high irradiances in addition to the photoacclimation of carotenoids. Indeed, precise kinetics of 132-hydroxy- and 151-hydroxy-lactone chlorophyll metabolites shows the existence of a chlorophyll oxidative metabolism as a tool to manage the excess of energy at high light conditions. Unexpectedly, the oxidation under phototrophy favored chlorophyll b metabolites over the chlorophyll a series, while the heterotrophic conditions exclusively induced the formation of 132-hydroxy-chlorophyll a. In parallel, during the first 48 h of growth in the dark, the chlorophyll fraction maintained a promising steady state. Although future studies are required to resolve the biochemical reactions implied in the chlorophyll oxidative metabolism, the present results agree with phytoplankton metabolism.

Foliage inoculation by Burkholderia vietnamiensis CBMB40 antagonizes methyl jasmonate-mediated stress in Eucalyptus grandis

Methyl jasmonate (MeJA) is widely used as a model chemical to study hypersensitive responses to biotic stress impacts in plants. Elevated levels of methyl jasmonate induce jasmonate-dependent defense responses, associated with a decline in primary metabolism and enhancement of secondary metabolism of plants. However, there is no information of how stress resistance of plants, and accordingly the sensitivity to exogenous MeJA can be decreased by endophytic plant growth promoting rhizobacteria (PGPR) harboring ACC (1-aminocyclopropane-1-carboxylate) deaminase. In this study, we estimated stress alleviating potential of endophytic PGPR against MeJA-induced plant perturbations through assessing photosynthetic traits and stress volatile emissions. We used mild (5 mM) to severe (20 mM) MeJA and endophytic plant growth promoting rhizobacteria Burkholderia vietnamiensis CBMB40 and studied how MeJA and B. vietnamiensis treatments influenced temporal changes in photosynthetic characteristics and stress volatile emissions. Separate application of MeJA markedly decreased photosynthetic characteristics and increased lipoxygenase pathway (LOX) volatiles, volatile isoprenoids, saturated aldehydes, lightweight oxygenated compounds (LOC), geranyl-geranyl diphosphate pathway (GGDP) volatiles, and benzenoids. However, MeJA-treated leaves inoculated by endophytic bacteria B. vietnamiensis had substantially increased photosynthetic characteristics and decreased emissions of LOX, volatile isoprenoids and other stress volatiles compared with non-inoculated MeJA treatments, especially at later stages of recovery. In addition, analysis of leaf terpenoid contents demonstrated that several mono- and sesquiterpenes were de novo synthesized upon MeJA and B. vietnamiensis applications. This study demonstrates that foliar application of endophytic bacteria B. vietnamiensis can potentially enhance resistance to biotic stresses and contribute to the maintenance of the integrity of plant metabolic activity.

An allele of ZmPORB2 encoding a protochlorophyllide oxidoreductase promotes tocopherol accumulation in both leaves and kernels of maize

Phytol is one of the key precursors for tocopherol synthesis in plants, however, the underlying mechanisms concerning the accumulation of tocopherol remain poorly understood. In this study, qVE5, a major QTL affecting tocopherol accumulation in maize kernels was identified via a positional cloning approach. qVE5 encodes a protochlorophyllide oxidoreductase (ZmPORB2), which localizes to the chloroplast. Overexpression of ZmPORB2 increased tocopherol content in both leaves and kernels. Candidate gene association analysis identified a 5/8-bp insertion/deletion (InDel058) in the 5' untranslated region (UTR) as the causal polymorphism in affecting ZmPORB2 expression and being highly associated with tocopherol content. We showed that higher expression of ZmPORB2 correlated with more chlorophyll metabolites in the leaf following pollination. RNA-sequencing and metabolic analysis in near isogenic lines (NILs) support that ZmPORB2 participates in chlorophyll metabolism enabling the production of phytol, an important precursor of tocopherol. We also found that the tocopherol content in the kernel is mainly determined by the maternal genotype, a fact that was further confirmed by in vitro culture experiments. Finally, a PCR-based marker based on Indel058 was developed in order to facilitate the high tocopherol (vitamin E) maize breeding.

Cryptic chlorophyll breakdown in non-senescent green Arabidopsis thaliana leaves

Chlorophyll (Chl) breakdown is a diagnostic visual process of leaf senescence, which furnishes phyllobilins (PBs) by the PAO/phyllobilin pathway. As Chl breakdown disables photosynthesis, it appears to have no role in photoactive green leaves. Here, colorless PBs were detected in green, non-senescent leaves of Arabidopsis thaliana. The PBs from the green leaves had structures entirely consistent with the PAO/phyllobilin pathway and the mutation of a single Chl catabolic enzyme completely abolished PBs with the particular modification. Hence, the PAO/phyllobilin pathway was active in the absence of visible senescence and expression of genes encoding Chl catabolic enzymes was observed in green Arabidopsis leaves. PBs accumulated to only sub-% amounts compared to the Chls present in the green leaves, excluding a substantial contribution of Chl breakdown from rapid Chl turnover associated with photosystem II repair. Indeed, Chl turnover was shown to involve a Chl a dephytylation and Chl a reconstitution cycle. However, non-recyclable pheophytin a is also liberated in the course of photosystem II repair, and is proposed here to be scavenged and degraded to the observed PBs. Hence, a cryptic form of the established pathway of Chl breakdown is indicated to play a constitutive role in photoactive leaves.

Sensing β-carotene oxidation in photosystem II to master plant stress tolerance

Stressful environmental conditions lead to the production of reactive oxygen species in the chloroplasts, due to limited photosynthesis and enhanced excitation pressure on the photosystems. Among these reactive species, singlet oxygen (¹ O₂ ), which is generated at the level of the PSII reaction center, is very reactive, readily oxidizing macromolecules in its immediate surroundings, and it has been identified as the principal cause of photooxidative damage in plant leaves. The two β-carotene molecules present in the PSII reaction center are prime targets of ¹ O₂ oxidation, leading to the formation of various oxidized derivatives. Plants have evolved sensing mechanisms for those PSII-generated metabolites, which regulate gene expression, putting in place defense mechanisms and alleviating the effects of PSII-damaging conditions. A new picture is thus emerging which places PSII as a sensor and transducer in plant stress resilience through its capacity to generate signaling metabolites under excess light energy. This review summarizes new advances in the characterization of the apocarotenoids involved in the PSII-mediated stress response and of the pathways elicited by these molecules, among which is the xenobiotic detoxification.

Oxidative post-translational modification of EXECUTER1 is required for singlet oxygen sensing in plastids

Environmental information perceived by chloroplasts can be translated into retrograde signals that alter the expression of nuclear genes. Singlet oxygen (¹O₂) generated by photosystem II (PSII) can cause photo-oxidative damage of PSII but has also been implicated in retrograde signaling. We previously reported that a nuclear-encoded chloroplast FtsH2 metalloprotease coordinates ¹O₂-triggered retrograde signaling by promoting the degradation of the EXECUTER1 (EX1) protein, a putative ¹O₂ sensor. Here, we show that a ¹O₂-mediated oxidative post-translational modification of EX1 is essential for initiating ¹O₂-derived signaling. Specifically, the Trp643 residue in DUF3506 domain of EX1 is prone to oxidation by ¹O₂. Both the substitution of Trp643 with ¹O₂-insensitive amino acids and the deletion of the DUF3506 domain abolish the EX1-mediated ¹O₂ signaling. We thus provide mechanistic insight into how EX1 senses ¹O₂ via Trp643 located in the DUF3506 domain.

Fluctuating Light Interacts with Time of Day and Leaf Development Stage to Reprogram Gene Expression

Natural light environments are highly variable. Flexible adjustment between light energy utilization and photoprotection is therefore of vital importance for plant performance and fitness in the field. Short-term reactions to changing light intensity are triggered inside chloroplasts and leaves within seconds to minutes, whereas long-term adjustments proceed over hours and days, integrating multiple signals. While the mechanisms of long-term acclimation to light intensity have been studied by changing constant growth light intensity during the day, responses to fluctuating growth light intensity have rarely been inspected in detail. We performed transcriptome profiling in Arabidopsis (Arabidopsis thaliana) leaves to investigate long-term gene expression responses to fluctuating light (FL). In particular, we examined whether responses differ between young and mature leaves or between morning and the end of the day. Our results highlight global reprogramming of gene expression under FL, including that of genes related to photoprotection, photosynthesis, and photorespiration and to pigment, prenylquinone, and vitamin metabolism. The FL-induced changes in gene expression varied between young and mature leaves at the same time point and between the same leaves in the morning and at the end of the day, indicating interactions of FL acclimation with leaf development stage and time of day. Only 46 genes were up- or down-regulated in both young and mature leaves at both time points. Combined analyses of gene coexpression and cis-elements pointed to a role of the circadian clock and light in coordinating the acclimatory responses of functionally related genes. Our results also suggest a possible cross talk between FL acclimation and systemic acquired resistance-like gene expression in young leaves.

Phytol metabolism in plants

Phytol, the prenyl side chain of chlorophyll, is derived from geranylgeraniol by reduction of three double bonds. Recent results demonstrated that the conversion of geranylgeraniol to phytol is linked to chlorophyll synthesis, which is catalyzed by protein complexes associated with the thylakoid membranes. One of these complexes contains light harvesting chlorophyll binding like proteins (LIL3), enzymes of chlorophyll synthesis (protoporphyrinogen oxidoreductase, POR; chlorophyll synthase, CHLG) and geranylgeranyl reductase (GGR). Phytol is not only employed for the synthesis of chlorophyll, but also for tocopherol (vitamin E), phylloquinol (vitamin K) and fatty acid phytyl ester production. Previously, it was believed that phytol is derived from reduction of geranylgeranyl-diphosphate originating from the 4-methylerythritol-5-phosphate (MEP) pathway. The identification and characterization of two kinases, VTE5 and VTE6, involved in phytol and phytyl-phosphate phosphorylation, respectively, indicated that most phytol employed for tocopherol synthesis is derived from reduction of geranylgeranylated chlorophyll to (phytol-) chlorophyll. After hydrolysis from chlorophyll, free phytol is phosphorylated by the two kinases, and phytyl-diphosphate employed for the synthesis of tocopherol and phylloquinol. The reason why some chloroplast lipids, i.e. chlorophyll, tocopherol and phylloquinol, are derived from phytol, while others, i.e. carotenoids and tocotrienols (in some plant species) are synthesized from geranylgeraniol, remains unclear.

Copper and iron metabolism in Ostreococcus tauri – the role of phytotransferrin, plastocyanin and a chloroplast copper-transporting ATPase

We have identified Ostreococcus tauri major iron uptake mediating protein, phytotransferrin (Ot-FEA1), whose expression and binding of iron is copper dependent.

Degradation of chlorophyll and synthesis of flavonols during autumn senescence-the story told by individual leaves

Autumn senescence of deciduous trees is characterized by chlorophyll degradation and flavonoid synthesis. In the present study, chlorophyll and flavonol contents were measured every morning and evening during the whole autumn with a non-destructive method from individual leaves of Sorbus aucuparia, Acer platanoides, Betula pendula and Prunus padus. In most of the studied trees, the chlorophyll content of each individual leaf remained constant until a phase of rapid degradation commenced. The fast phase lasted only ~1 week and ended with abscission. In S. aucuparia, contrary to the other species, the chlorophyll content of leaflets slowly but steadily decreased during the whole autumn, but rapid chlorophyll degradation commenced only prior to leaflet abscission also in this species. An increase in flavonols commonly accompanied the rapid degradation of chlorophyll. The results may suggest that each individual tree leaf retains its photosynthetic activity, reflected by a high chlorophyll content, until a rapid phase of chlorophyll degradation and flavonoid synthesis begins. Therefore, in studies of autumn senescence, leaves whose chlorophyll content is decreasing and leaves with summertime chlorophyll content (i.e. the leaves that have not yet started to degrade chlorophyll) should be treated separately.

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.

Establishment of an Arabidopsis callus system to study the interrelations of biosynthesis, degradation and accumulation of carotenoids

The net amounts of carotenoids accumulating in plant tissues are determined by the rates of biosynthesis and degradation. While biosynthesis is rate-limited by the activity of PHYTOENE SYNTHASE (PSY), carotenoid losses are caused by catabolic enzymatic and non-enzymatic degradation. We established a system based on non-green Arabidopsis callus which allowed investigating major determinants for high steady-state levels of β-carotene. Wild-type callus development was characterized by strong carotenoid degradation which was only marginally caused by the activity of carotenoid cleavage oxygenases. In contrast, carotenoid degradation occurred mostly non-enzymatically and selectively affected carotenoids in a molecule-dependent manner. Using carotenogenic pathway mutants, we found that linear carotenes such as phytoene, phytofluene and pro-lycopene resisted degradation and accumulated while β-carotene was highly susceptible towards degradation. Moderately increased pathway activity through PSY overexpression was compensated by degradation revealing no net increase in β-carotene. However, higher pathway activities outcompeted carotenoid degradation and efficiently increased steady-state β-carotene amounts to up to 500 μg g-1 dry mass. Furthermore, we identified oxidative β-carotene degradation products which correlated with pathway activities, yielding β-apocarotenals of different chain length and various apocarotene-dialdehydes. The latter included methylglyoxal and glyoxal as putative oxidative end products suggesting a potential recovery of carotenoid-derived carbon for primary metabolic pathways. Moreover, we investigated the site of β-carotene sequestration by co-localization experiments which revealed that β-carotene accumulated as intra-plastid crystals which was confirmed by electron microscopy with carotenoid-accumulating roots. The results are discussed in the context of using the non-green calli carotenoid assay system for approaches targeting high steady-state β-carotene levels prior to their application in crops.