A Salvage Pathway for Phytol Metabolism in Arabidopsis

Comprehensive metabolome characterization of leaves, internodes, and aerial roots of Vanilla planifolia by untargeted LC-MS and GC × GC-MS

INTRODUCTION

Untargeted metabolomics is a powerful tool that provides strategies for gaining a systematic understanding of quantitative changes in the levels of metabolites, especially when combining different metabolomic platforms. Vanilla is one of the world's most popular flavors originating from cured pods of the orchid Vanilla planifolia. However, only a few studies have investigated the metabolome of V. planifolia, and no LC-MS or GC-MS metabolomics studies with respect to leaves have been performed.

OBJECTIVE

The aim of the study was to comprehensively characterize the metabolome of different organs (leaves, internodes, and aerial roots) of V. planifolia.

MATERIAL AND METHODS

Characterization of the metabolome was achieved using two complementary platforms (GC × GC-MS, LC-QToF-MS), and metabolite identification was based on a comparison with in-house databases or curated external spectral libraries.

RESULTS

In total, 127 metabolites could be identified with high certainty (confidence level 1 or 2) including sugars, amino acids, fatty acids, organic acids, and amines/amides but also secondary metabolites such as vanillin-related metabolites, flavonoids, and terpenoids. Ninty-eight metabolites showed significantly different intensities between the plant organs. Most strikingly, aglycons of flavonoids and vanillin-related metabolites were elevated in aerial roots, whereas its O-glycoside forms tended to be higher in leaves and/or internodes. This suggests that the more bioactive aglycones may accumulate where preferably needed, e.g. for defense against pathogens.

CONCLUSION

The results derived from the study substantially expand the knowledge regarding the vanilla metabolome forming a valuable basis for more targeted investigations in future studies, e.g. towards an optimization of vanilla plant cultivation.

Tocopherol and phylloquinone biosynthesis in chloroplasts requires the phytol kinase VITAMIN E PATHWAY GENE5 (VTE5) and the farnesol kinase (FOLK)

Chlorophyll degradation causes the release of phytol, which is converted into phytyl diphosphate (phytyl-PP) by phytol kinase (VITAMIN E PATHWAY GENE5 [VTE5]) and phytyl phosphate (phytyl-P) kinase (VTE6). The kinase pathway is important for tocopherol synthesis, as the Arabidopsis (Arabidopsis thaliana) vte5 mutant contains reduced levels of tocopherol. Arabidopsis harbors one paralog of VTE5, farnesol kinase (FOLK) involved in farnesol phosphorylation. Here, we demonstrate that VTE5 and FOLK harbor kinase activities for phytol, geranylgeraniol, and farnesol with different specificities. While the tocopherol content of the folk mutant is unchanged, vte5-2 folk plants completely lack tocopherol. Tocopherol deficiency in vte5-2 plants can be complemented by overexpression of FOLK, indicating that FOLK is an authentic gene of tocopherol synthesis. The vte5-2 folk plants contain only ∼40% of wild-type amounts of phylloquinone, demonstrating that VTE5 and FOLK both contribute in part to phylloquinone synthesis. Tocotrienol and menaquinone-4 were produced in vte5-2 folk plants after supplementation with homogentisate or 1,4-dihydroxy-2-naphthoic acid, respectively, indicating that their synthesis is independent of the VTE5/FOLK pathway. These results show that phytyl moieties for tocopherol synthesis are completely but, for phylloquinone production, only partially derived from geranylgeranyl-chlorophyll and phytol phosphorylation by VTE5 and FOLK.

Vitamin E biofortification: enhancement of seed tocopherol concentrations by altered chlorophyll metabolism

Homogentisate Phytyltransferase (HPT) catalyzes condensation of homogentisate (HGA) and phytyl diphosphate (PDP) to produce tocopherols, but can also synthesize tocotrienols using geranylgeranyl diphosphate (GGDP) in plants engineered for deregulated HGA synthesis. In contrast to prior tocotrienol biofortification efforts, engineering enhanced tocopherol concentrations in green oilseeds has proven more challenging due to the integral role of chlorophyll metabolism in supplying the PDP substrate. This study show that RNAi suppression of CHLSYN coupled with HPT overexpression increases tocopherol concentrations by >two-fold in Arabidopsis seeds. We obtained additional increases in seed tocopherol concentrations by engineering increased HGA production via overexpression of bacterial TyrA that encodes chorismate mutase/prephenate dehydrogenase activities. In overexpression lines, seed tocopherol concentrations increased nearly three-fold, and resulted in modest tocotrienol accumulation. We further increased total tocochromanol concentrations by enhancing production of HGA and GGDP by overexpression of the gene for hydroxyphenylpyruvate dioxygenase (HPPD). This shifted metabolism towards increased amounts of tocotrienols relative to tocopherols, which was reflected in corresponding increases in ratios of GGDP/PDP in these seeds. Overall, our results provide a theoretical basis for genetic improvement of total tocopherol concentrations in green oilseeds (e.g., rapeseed, soybean) through strategies that include seed-suppression of CHLSYN coupled with increased HGA production.

Beyond the MEP Pathway: A novel kinase required for prenol utilization by malaria parasites

A proposed treatment for malaria is a combination of fosmidomycin and clindamycin. Both compounds inhibit the methylerythritol 4-phosphate (MEP) pathway, the parasitic source of farnesyl and geranylgeranyl pyrophosphate (FPP and GGPP, respectively). Both FPP and GGPP are crucial for the biosynthesis of several essential metabolites such as ubiquinone and dolichol, as well as for protein prenylation. Dietary prenols, such as farnesol (FOH) and geranylgeraniol (GGOH), can rescue parasites from MEP inhibitors, suggesting the existence of a missing pathway for prenol salvage via phosphorylation. In this study, we identified a gene in the genome of P. falciparum, encoding a transmembrane prenol kinase (PolK) involved in the salvage of FOH and GGOH. The enzyme was expressed in Saccharomyces cerevisiae, and its FOH/GGOH kinase activities were experimentally validated. Furthermore, conditional knockout parasites (Δ-PolK) were created to investigate the biological importance of the FOH/GGOH salvage pathway. Δ-PolK parasites were viable but displayed increased susceptibility to fosmidomycin. Their sensitivity to MEP inhibitors could not be rescued by adding prenols. Additionally, Δ-PolK parasites lost their capability to utilize prenols for protein prenylation. Experiments using culture medium supplemented with whole/delipidated human plasma in transgenic parasites revealed that human plasma has components that can diminish the effectiveness of fosmidomycin. Mass spectrometry tests indicated that both bovine supplements used in culture and human plasma contain GGOH. These findings suggest that the FOH/GGOH salvage pathway might offer an alternate source of isoprenoids for malaria parasites when de novo biosynthesis is inhibited. This study also identifies a novel kind of enzyme related to isoprenoid metabolism.

An Arabidopsis mutant line lacking the mitochondrial calcium transport regulator MICU shows an altered metabolite profile

Plant metabolism is constantly changing and requires input signals for efficient regulation. The mitochondrial calcium uniporter (MCU) couples organellar and cytoplasmic calcium oscillations leading to oxidative metabolism regulation in a vast array of species. In Arabidopsis thaliana, genetic deletion of AtMICU leads to altered mitochondrial calcium handling and ultrastructure. Here we aimed to further assess the consequences upon genetic deletion of AtMICU. Our results confirm that AtMICU safeguards intracellular calcium transport associated with carbohydrate, amino acid, and phytol metabolism modifications. The implications of such alterations are discussed.

Detrimental impact of sulfide on the seagrass Zostera marina in dark hypoxia

Sulfide poisoning, hypoxia events, and reduced light availability pose threats to marine ecosystems such as seagrass meadows. These threats are projected to intensify globally, largely due to accelerating eutrophication of estuaries and coastal environments. Despite the urgency, our current comprehension of the metabolic pathways that underlie the deleterious effects of sulfide toxicity and hypoxia on seagrasses remains inadequate. To address this knowledge gap, I conducted metabolomic analyses to investigate the impact of sulfide poisoning under dark-hypoxia in vitro conditions on Zostera marina, a vital habitat-forming marine plant. During the initial 45 minutes of dark-hypoxia exposure, I detected an acclimation phase characterized by the activation of anaerobic metabolic pathways and specific biochemical routes that mitigated hypoxia and sulfide toxicity. These pathways served to offset energy imbalances, cytosolic acidosis, and sulfide toxicity. Notably, one such route facilitated the transformation of toxic sulfide into non-toxic organic sulfur compounds, including cysteine and glutathione. However, this sulfide tolerance mechanism exhibited exhaustion post the initial 45-minute acclimation phase. Consequently, after 60 minutes of continuous sulfide exposure, the sulfide toxicity began to inhibit the hypoxia-mitigating pathways, culminating in leaf senescence and tissue degradation. Utilizing metabolomic approaches, I elucidated the intricate metabolic responses of seagrasses to sulfide toxicity under in vitro dark-hypoxic conditions. My findings suggest that future increases in coastal eutrophication will compromise the resilience of seagrass ecosystems to hypoxia, primarily due to the exacerbating influence of sulfide.

Tocopherols and associated derivatives track the phytoplanktonic response to evolving pelagic redox conditions spanning Oceanic Anoxic Event 2

Tocopherols serve a critical role as antioxidants inhibiting lipid peroxidation in photosynthetic organisms, yet are seldom used in geobiological investigations. The ubiquity of tocopherols in all photosynthetic lifeforms is often cited as an impediment to any diagnostic paleoenvironmental potential, while the inability to readily analyze these compounds via conventional methods, such as gas chromatography-mass spectrometry, further diminishes the capacity to serve as useful 'biomarkers'. Here, we analyzed an exceptionally preserved black shale sequence from the Demerara Rise that spans Oceanic Anoxic Event 2 (OAE-2) to reexamine the significance of tocopherols and associated derivatives (i.e. tocol derivatives) in ancient sediments. Tocol derivatives were analyzed via liquid chromatography-quadrupole time-of-flight-mass spectrometry and included tocopherols, a methyltrimethyltridecylchroman, and the first reported detection of tocopherol quinones and methylphytylbenzoquinones in the geologic record. Strong correlations between tocol derivatives were observed over the studied interval. Tocol derivative concentrations and ratios, which normalized tocopherols to potential derivatives, revealed absolute and relative increases in tocopherols as exclusive features of OAE-2 that can be explained by two possible mechanisms related to tocopherol production and preservation. The development of photic zone euxinia during OAE-2 likely forced an upward migration of oxygenic photoautotrophs, increasing oxidative stress that elicited heightened tocopherol biosynthesis. However, shoaling euxinic conditions may have simultaneously acted to enhance tocopherol preservation given the relatively high lability of tocopherols in the water column. Both scenarios could produce the observed stratigraphic distribution of tocol derivatives in this study, although the elevated tocopherol concentrations that define OAE-2 at the Demerara Rise are primarily attributed to enhanced tocopherol production by shoaling phytoplanktonic communities. Thus, the occurrence of tocopherols and associated derivatives in sediments and rocks of marine origin is likely indicative of shallow-water anoxia, tracking the phytoplanktonic response to the abiotic stresses associated with vertical fluctuations in pelagic redox.

Potential role of tocopherol in protecting crop plants against abiotic stresses

The changing global climate have given rise to abiotic stresses that adversely affect the metabolic activities of plants, limit their growth, and agricultural output posing a serious threat to food production. The abiotic stresses commonly lead to production of reactive oxygen species (ROS) that results in cellular oxidation. Over the course of evolution, plants have devised efficient enzymatic and non-enzymatic anti-oxidative strategies to counteract harmful effects of ROS. Among the emerging non-enzymatic anti-oxidative technologies, the chloroplast lipophilic antioxidant vitamin A (Tocopherol) shows great promise. Working in coordination with the other cellular antioxidant machinery, it scavenges ROS, prevents lipid peroxidation, regulates stable cellular redox conditions, simulates signal cascades, improves membrane stability, confers photoprotection and enhances resistance against abiotic stresses. The amount of tocopherol production varies based on the severity of stress and its proposed mechanism of action involves arresting lipid peroxidation while quenching singlet oxygen species and lipid peroxyl radicals. Additionally, studies have demonstrated its coordination with other cellular antioxidants and phytohormones. Despite its significance, the precise mechanism of tocopherol action and signaling coordination are not yet fully understood. To bridge this knowledge gap, the present review aims to explore and understand the biosynthesis and antioxidant functions of Vitamin E, along with its signal transduction and stress regulation capacities and responses. Furthermore, the review delves into the light harvesting and photoprotection capabilities of tocopherol. By providing insights into these domains, this review offers new opportunities and avenues for using tocopherol in the management of abiotic stresses in agriculture.

Evidence of increases of phytol and chlorophyllide by enzymatic dephytylation of chlorophylls in smoothie made from spinach leaves

Phytol is a diterpene alcohol found abundantly in nature as the phytyl side chain of chlorophylls. Free form of phytol and its metabolites have been attracting attention because they have a potential to improve the lipid and glucose metabolism. On the other hand, phytol is unfavorable for those who suffering from Refsum's disease. However, there is little information on the phytol contents in leafy vegetables rich in chlorophylls. This study indicated that raw spinach leaves contain phytol of 0.4-1.5 mg/100 g fresh weight. Furthermore, crude enzyme extracted from the leaves showed the enzyme activities involved in dephytylation of chlorophyll derivatives and they were high at mild alkaline pH and around 45°C, and lowered at 55°C or above. Under the optimum pH and temperature for such enzymes determined in the model reaction using the crude enzyme, phytol content in the smoothie made from raw spinach leaves increased with an increase of chlorophyllide, another reaction product. Comparison between the increased amounts of phytol and chlorophyllide showed that the enzymatic dephytylation of chlorophylls was critically responsible for the increase of phytol in the smoothie. PRACTICAL APPLICATION: Phytol, which is released by the enzymes related to chlorophyll metabolism in plants, has been investigated because of its potential abilities to improve the lipid metabolism and blood glucose level. In contrast to such health benefits, they are known to be toxic for patients suffering from Refsum's disease. This research for the first time reports the phytol content in raw spinach leaves and that phytol can be increased in the smoothie made from spinach leaves by the action of endogenous enzymes on chlorophyll derivatives under a certain condition. These results help control phytol content in the smoothies.

Identification of LncRNAs and Functional Analysis of ceRNA Related to Fatty Acid Synthesis during Flax Seed Development

Flax is a flowering plant cultivated for its oil and contains various unsaturated fatty acids. Linseed oil is known as the "deep-sea fish oil" of plants, and is beneficial to brain and blood lipids, among other positive effects. Long non-coding RNAs (lncRNAs) play an important role in plant growth and development. There are not many studies assessing how lncRNAs are related to the fatty acid synthesis of flax. The relative oil contents of the seeds of the variety Heiya NO.14 (for fiber) and the variety Macbeth (for oil) were determined at 5 day, 10 day, 20 day, and 30 day after flowering. We found that 10-20 day is an important period for ALA accumulation in the Macbeth variety. The strand-specific transcriptome data were analyzed at these four time points, and a series of lncRNAs related to flax seed development were screened. A competing endogenous RNA (ceRNA) network was constructed and the accuracy of the network was verified using qRT-PCR. MSTRG.20631.1 could act with miR156 on the same target, squamosa promoter-binding-like protein (SPL), to influence fatty acid biosynthesis through a gluconeogenesis-related pathway during flax seed development. This study provides a theoretical basis for future studies assessing the potential functions of lncRNAs during seed development.

The NAC transcription factor MdNAC4 positively regulates nitrogen deficiency-induced leaf senescence by enhancing ABA biosynthesis in apple

Although it is well established that nitrogen (N) deficiency induces leaf senescence, the molecular mechanism of N deficiency-induced leaf senescence remains largely unknown. Here, we show that an abscisic acid (ABA)-responsive NAC transcription factor (TF) is involved in N deficiency-induced leaf senescence. The overexpression of MdNAC4 led to increased ABA levels in apple calli by directly activating the transcription of the ABA biosynthesis gene MdNCED2. In addition, MdNAC4 overexpression promoted N deficiency-induced leaf senescence. Further investigation showed that MdNAC4 directly bound the promoter of the senescence-associated gene (SAG) MdSAG39 and upregulated its expression. Interestingly, the function of MdNAC4 in promoting N deficiency-induced leaf senescence was enhanced in the presence of ABA. Furthermore, we identified an interaction between the ABA receptor protein MdPYL4 and the MdNAC4 protein. Moreover, MdPYL4 showed a function similar to that of MdNAC4 in ABA-mediated N deficiency-induced leaf senescence. These findings suggest that ABA plays a central role in N deficiency-induced leaf senescence and that MdPYL4 interacts with MdNAC4 to enhance the response of the latter to N deficiency, thus promoting N deficiency-induced leaf senescence. In conclusion, our results provide new insight into how MdNAC4 regulates N deficiency-induced leaf senescence.

Nutritional Enrichment of Plant Leaves by Combining Genes Promoting Tocopherol Biosynthesis and Storage

The enrichment of plant tissues in tocochromanols (tocopherols and tocotrienols) is an important biotechnological goal due to their vitamin E and antioxidant properties. Improvements based on stimulating tocochromanol biosynthesis have repeatedly been achieved, however, enhancing sequestering and storage in plant plastids remains virtually unexplored. We previously showed that leaf chloroplasts can be converted into artificial chromoplasts with a proliferation of plastoglobules by overexpression of the bacterial crtB gene. Here we combined coexpression of crtB with genes involved in tocopherol biosynthesis to investigate the potential of artificial leaf chromoplasts for vitamin E accumulation in Nicotiana benthamiana leaves. We show that this combination improves tocopherol levels compared to controls without crtB and confirm that VTE1, VTE5, VTE6 and tyrA genes are useful to increase the total tocopherol levels, while VTE4 further leads to enrichment in α-tocopherol (the tocochromanol showing highest vitamin E activity). Additionally, we show that treatments that further promote plastoglobule formation (e.g., exposure to intense light or dark-induced senescence) result in even higher improvements in the tocopherol content of the leaves. An added advantage of our strategy is that it also results in increased levels of other related plastidial isoprenoids such as carotenoids (provitamin A) and phylloquinones (vitamin K1).

The Biomedical Importance of the Missing Pathway for Farnesol and Geranylgeraniol Salvage

Isoprenoids are the output of the polymerization of five-carbon, branched isoprenic chains derived from isopentenyl pyrophosphate (IPP) and its isomer, dimethylallyl pyrophosphate (DMAPP). Isoprene units are consecutively condensed to form longer structures such as farnesyl and geranylgeranyl pyrophosphate (FPP and GGPP, respectively), necessary for the biosynthesis of several metabolites. Polyprenyl transferases and synthases use polyprenyl pyrophosphates as their natural substrates; however, it is known that free polyprenols, such as farnesol (FOH), and geranylgeraniol (GGOH) can be incorporated into prenylated proteins, ubiquinone, cholesterol, and dolichols. Furthermore, FOH and GGOH have been shown to block the effects of isoprenoid biosynthesis inhibitors such as fosmidomycin, bisphosphonates, or statins in several organisms. This phenomenon is the consequence of a short pathway, which was observed for the first time more than 25 years ago: the polyprenol salvage pathway, which works via the phosphorylation of FOH and GGOH. Biochemical studies in bacteria, animals, and plants suggest that this pathway can be carried out by two enzymes: a polyprenol kinase and a polyprenyl-phosphate kinase. However, to date, only a few genes have been unequivocally identified to encode these enzymes in photosynthetic organisms. Nevertheless, pieces of evidence for the importance of this pathway abound in studies related to infectious diseases, cancer, dyslipidemias, and nutrition, and to the mitigation of the secondary effects of several drugs. Furthermore, nowadays it is known that both FOH and GGOH can be incorporated via dietary sources that produce various biological effects. This review presents, in a simplified but comprehensive manner, the most important data on the FOH and GGOH salvage pathway, stressing its biomedical importance The main objective of this review is to bring to light the need to discover and characterize the kinases associated with the isoprenoid salvage pathway in animals and pathogens.

Isoprenoid alcohols utilization by malaria parasites

Plasmodium falciparum is the etiological agent of human malaria, one of the most widespread diseases in tropical and subtropical regions. Drug resistance is one of the biggest problems in controlling the disease, which leads to the need to discover new antimalarial compounds. One of the most promissory drugs purposed is fosmidomycin, an inhibitor of the biosynthesis of isoprene units by the methylerythritol 4-phosphate (MEP) pathway, which in some cases failed in clinical studies. Once formed, isoprene units are condensed to form longer structures such as farnesyl and geranylgeranyl pyrophosphate, which are necessary for Heme O and A formation, ubiquinone, and dolichyl phosphate biosynthesis as well as for protein isoprenylation. Even though the natural substrates of polyprenyl transferases and synthases are polyprenyl pyrophosphates, it was already demonstrated that isoprenoid alcohols (polyprenols) such as farnesol (FOH) and geranylgeraniol (GGOH) can rescue parasites from fosmidomycin. This study better investigated how this rescue phenomenon occurs by performing drug-rescue assays. Similarly, to FOH and GGOH, it was observed that phytol (POH), a 20-carbon plant isoprenoid, as well as unsaponifiable lipid extracts from foods rescue parasites from the antimalarial effect of fosmidomycin. Contrarily, neither dolichols nor nonaprenol rescue parasites from fosmidomycin. Considering this, here we characterized the transport of FOH, GGOH, and POH. Once incorporated, it was observed that these substances are phosphorylated, condensed into longer isoprenoid alcohols, and incorporated into proteins and dolichyl phosphates. Through proteomic and radiolabelling approaches, it was found that prenylated proteins are naturally attached to several isoprenoids, derived from GGOH, dolichol, and POH if exogenously added. Furthermore, the results suggest the presence of at least two promiscuous protein prenyltransferases in the parasite: one enzyme which can use FPP among other unidentified substrates and another enzyme that can use GGPP, phytyl pyrophosphate (PPP), and dolichols, among other substrates not identified here. Thus, further evidence was obtained for dolichols and other isoprenoid products attached to proteins. This study helps to better understand the apicoplast-targeting antimalarial mechanism of action and a novel post-translational modification of proteins in P. falciparum.

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.

Chlorophyll dephytylase 1 and chlorophyll synthase: a chlorophyll salvage pathway for the turnover of photosystems I and II

Chlorophyll (Chl) is made up of the tetrapyrrole chlorophyllide and phytol, a diterpenoid alcohol. The photosynthetic protein complexes utilize Chl for light harvesting to produce biochemical energy for plant development. However, excess light and adverse environmental conditions facilitate generation of reactive oxygen species, which damage photosystems I and II (PSI and PSII) and induce their turnover. During this process, Chl is released, and is thought to be recycled via dephytylation and rephytylation. We previously demonstrated that Chl recycling in Arabidopsis under heat stress is mediated by the enzymes chlorophyll dephytylase 1 (CLD1) and chlorophyll synthase (CHLG) using chlg and cld1 mutants. Here, we show that the mutants with high CLD1/CHLG ratio, by different combinations of chlg-1 (a knock-down mutant) and the hyperactive cld1-1 alleles, develop necrotic leaves when grown under long- and short-day, but not continuous light conditions, owing to the accumulation of chlorophyllide in the dark. Combination of chlg-1 with cld1-4 (a knock-out mutant) leads to reduced chlorophyllide accumulation and necrosis. The operation of CLD1 and CHLG as a Chl salvage pathway was also explored in the context of Chl recycling during the turnover of Chl-binding proteins of the two photosystems. CLD1 was found to interact with CHLG and the light-harvesting complex-like proteins OHP1 and LIL3, implying that auxiliary factors are required for this process.

Analysis of Physiological and Transcriptomic Differences between a Premature Senescence Mutant (GSm) and Its Wild-Type in Common Wheat (Triticum aestivum L.)

Simple Summary

Early leaf senescence is an important agronomic trait that affects crop yield and quality. To understand the molecular mechanism of early leaf senescence, a wheat (Triticum aestivum L.) premature leaf senescence mutant (GSm) and its wild type were employed in this study. We compared the physiological characteristics and transcriptome of wheat leaves between the wild type (WT) and the mutant at two-time points. Physiological characteristics and differentially expressed gene (DEG) analysis revealed many genes and metabolic pathways that were closely related to senescence. These results will not only support further gene cloning and functional analysis of GSm, but also facilitate the study of leaf senescence in wheat.

Abstract

Premature leaf senescence has a profound influence on crop yield and quality. Here, a stable premature senescence mutant (GSm) was obtained from the common wheat (Triticum aestivum L.) cultivar Chang 6878 by mutagenesis with ethyl methanesulfonate. The differences between the GSm mutant and its wild-type (WT) were analyzed in terms of yield characteristics, photosynthetic fluorescence indices, and senescence-related physiological parameters. RNA sequencing was used to reveal gene expression differences between GSm and WT. The results showed that the yield of GSm was considerably lower than that of WT. The net photosynthetic rate, transpiration rate, maximum quantum yield, non-photochemical quenching coefficient, photosynthetic electron transport rate, soluble protein, peroxidase activity, and catalase activity all remarkably decreased in flag leaves of GSm, whereas malondialdehyde content distinctively increased compared with those of WT. The analysis of differentially expressed genes indicated blockade of chlorophyll and carotenoid biosynthesis, accelerated degradation of chlorophyll, and diminished photosynthetic capacity in mutant leaves; brassinolide might facilitate chlorophyll breakdown and consequently accelerate leaf senescence. NAC genes positively regulated the senescence process. Compared with NAC genes, expression of WRKY and MYB genes was induced earlier in the mutant possibly due to increased levels of reactive oxygen species and plant hormones (e.g., brassinolide, salicylic acid, and jasmonic acid), thereby accelerating leaf senescence. Furthermore, the antioxidant system played a role in minimizing oxidative damage in the mutant. These results provides novel insight into the molecular mechanisms of premature leaf senescence in crops.

Presence of Phylloquinone in the Intraerythrocytic Stages of Plasmodium falciparum

Malaria is one of the most widespread parasitic diseases, especially in Africa, Southeast Asia and South America. One of the greatest problems for control of the disease is the emergence of drug resistance, which leads to a need for the development of new antimalarial compounds. The biosynthesis of isoprenoids has been investigated as part of a strategy to identify new targets to obtain new antimalarial drugs. Several isoprenoid quinones, including menaquinone-4 (MK-4/vitamin K2), α- and γ-tocopherol and ubiquinone (UQ) homologs UQ-8 and UQ-9, were previously detected in in vitro cultures of Plasmodium falciparum in asexual stages. Herein, we described for the first time the presence of phylloquinone (PK/vitamin K1) in P. falciparum and discuss the possible origins of this prenylquinone. While our results in metabolic labeling experiments suggest a biosynthesis of PK prenylation via phytyl pyrophosphate (phytyl-PP) with phytol being phosphorylated, on the other hand, exogenous PK attenuated atovaquone effects on parasitic growth and respiration, showing that this metabolite can be transported from extracellular environment and that the mitochondrial electron transport system (ETS) of P. falciparum is capable to interact with PK. Although the natural role and origin of PK remains elusive, this work highlights the PK importance in plasmodial metabolism and future studies will be important to elucidate in seeking new targets for antimalarial drugs.

Role of Nitric Oxide in Plant Senescence

In plants senescence is the final stage of plant growth and development that ultimately leads to death. Plants experience age-related as well as stress-induced developmental ageing. Senescence involves significant changes at the transcriptional, post-translational and metabolomic levels. Furthermore, phytohormones also play a critical role in the programmed senescence of plants. Nitric oxide (NO) is a gaseous signalling molecule that regulates a plethora of physiological processes in plants. Its role in the control of ageing and senescence has just started to be elucidated. Here, we review the role of NO in the regulation of programmed cell death, seed ageing, fruit ripening and senescence. We also discuss the role of NO in the modulation of phytohormones during senescence and the significance of NO-ROS cross-talk during programmed cell death and senescence.

Chlorophyll and Pheophytin Dephytylating Enzymes Required for Efficient Repair of PSII in Synechococcus elongatus PCC 7942

The Chlorophyll Dephytylase1 (CLD1) and pheophytinase (PPH) proteins of Arabidopsis thaliana are homologous proteins characterized respectively as a dephytylase for chlorophylls (Chls) and pheophytin a (Phein a) and a Phein a-specific dephytylase. Three genes encoding CLD1/PPH homologs (dphA1, dphA2 and dphA3) were found in the genome of the cyanobacterium Synechococcus elongatus PCC 7942 and shown to be conserved in most cyanobacteria. His6-tagged DphA1, DphA2 and DphA3 proteins were expressed in Escherichia coli, purified to near homogeneity, and shown to exhibit significant levels of dephytylase activity for Chl a and Phein a. Each DphA protein showed similar dephytylase activities for Chl a and Phein a, but the three proteins were distinct in their kinetic properties, with DphA3 showing the highest and lowest Vmax and Km values, respectively, among the three. Transcription of dphA1 and dphA3 was enhanced under high-light conditions, whereas that of dphA2 was not affected by the light conditions. None of the dphA single mutants of S. elongatus showed profound growth defects under low (50 µmol photons m-2 s-1) or high (400 µmol photons m-2 s-1) light conditions. The triple dphA mutant did not show obvious growth defects under these conditions, either, but under illumination of 1,000 µmol photons m-2 s-1, the mutant showed more profound growth retardation compared with wild type (WT). The repair of photodamaged photosystem II (PSII) was much slower in the triple mutant than in WT. These results revealed that dephytylation of Chl a or Phein a or of both is required for efficient repair of photodamaged PSII.

2-Hydroxy-phytanoyl-CoA lyase (AtHPCL) is involved in phytol metabolism in Arabidopsis

During chlorophyll degradation, large amounts of the isoprenoid alcohol phytol are released. The pathway of phytol catabolism has been studied in humans, because chlorophyll is part of the human diet, but little is known for plants. In humans, phytanoyl-CoA derived from phytol is degraded via α-oxidation by phytanoyl-CoA hydroxylase (PAHX) and 2-hydroxy-phytanoyl-CoA lyase (HPCL). Arabidopsis contains two sequences homologous to the human proteins AtPAHX and AtHPCL. Insertional mutants of Arabidopsis (pahx, hpcl) were grown under N deprivation to stimulate chlorophyll breakdown or supplemented with phytol to increase the endogenous amount of phytol. During N deprivation, chlorophyll, phytol, phytenal, upstream metabolites of phytol breakdown, and tocopherol and fatty acid phytyl esters, alternative phytol-derived lipids, accumulated in pahx and hpcl mutants, in line with the scenario that the mutations interfere with phytol degradation. AtHPCL was localized to the peroxisomes. Expression analysis of the AtHPCL sequence in the yeast Δpxp1 or Δmpo1 mutants followed by supplementation with 2-hydroxy-palmitic acid and enzyme assays of peroxisomal proteins from Col-0 and hpcl plants with 2-hydroxy-stearoyl-CoA revealed that AtHPCL harbors 2-hydroxy-acyl-CoA lyase activity. The α-dioxygenases αDOX1 and αDOX2 are involved in α-oxidation of fatty acids and could be involved in an alternative pathway of phytol degradation. However, phytol-related lipids in the αdox1, αdox2, or αdox1 αdox2 mutants were not altered compared with Col-0, indicating that αDOX1 and αDOX2 are not involved in phytol degradation. These results demonstrate that phytol degradation in Arabidopsis involves α-oxidation by AtPAHX and AtHPCL, but that it is independent of αDOX1/αDOX2.

ROS-Scavengers, Osmoprotectants and Violaxanthin De-Epoxidation in Salt-Stressed Arabidopsis thaliana with Different Tocopherol Composition

To determine the role of α- and γ-tocopherol (TC), this study compared the response to salt stress (200 mM NaCl) in wild type (WT) Arabidopsis thaliana (L.) Heynh. And its two mutants: (1) totally TC-deficient vte1; (2) vte4 accumulating γ-TC instead of α-TC; and (3) tmt transgenic line overaccumulating α-TC. Raman spectra revealed that salt-exposed α-TC accumulating plants were more flexible in regulating chlorophyll, carotenoid and polysaccharide levels than TC deficient mutants, while the plants overaccumulating γ-TC had the lowest levels of these biocompounds. Tocopherol composition and NaCl concentration affected xanthophyll cycle by changing the rate of violaxanthin de-epoxidation and zeaxanthin formation. NaCl treated plants with altered TC composition accumulated less oligosaccharides than WT plants. α-TC deficient plants increased their oligosaccharide levels and reduced maltose amount, while excessive accumulation of α-TC corresponded with enhanced amounts of maltose. Salt-stressed TC-deficient mutants and tmt transgenic line exhibited greater proline levels than WT plants, lower chlorogenic acid levels, and lower activity of catalase and peroxidases. α-TC accumulating plants produced more methylated proline- and glycine- betaines, and showed greater activity of superoxide dismutase than γ-TC deficient plants. Under salt stress, α-TC demonstrated a stronger regulatory effect on carbon- and nitrogen-related metabolites reorganization and modulation of antioxidant patterns than γ-TC. This suggested different links of α- and γ-TCs with various metabolic pathways via various functions and metabolic loops.

Tocochromanols and Chlorophylls Accumulation in Young Pomelo (Citrus maxima) during Early Fruit Development

Pomelo is an important cultivar of the genus Citrus that contains a variety of beneficial nutrients, and its young fruit is an agricultural by-product that is currently not fully utilized because it is often thrown away during cultivation and management. In this study, the dynamics of tocochromanol during young pomelo development were investigated by measuring chlorophyll content, tocochromanol accumulation, and expression levels of related genes during early fruit development. The results showed that chlorophyll content decreased overall during these four developmental stages and had some synergism with tocochromanol. Four tocochromanol components were detected in pomelo of both genotypes, and α-tocopherol was the main component. The tocochromanol content of honey pomelo was highest in the first period, reaching 70 ± 5 μg/g in dry weight (DW), and golden pomelo peaked in the second period at 86.10 ± 0.18 μg/g DW, with an overall decreasing trend in both genotypes. The different gene expression patterns of the tocochromanol biosynthesis pathway could partially explain the changes in these components and further elucidate the regulatory mechanisms of tocochromanol accumulation during early fruit development. As a natural product, young pomelo fruit is an attractive source of tocochromanol and has potential application in industrial production. The results of this study may provide directions for the high additional value utilization of young pomelo fruit.

Soybean Inoculated With One Bradyrhizobium Strain Isolated at Elevated [CO2] Show an Impaired C and N Metabolism When Grown at Ambient [CO2]

Soybean (Glycine max L.) future response to elevated [CO₂] has been shown to differ when inoculated with B. japonicum strains isolated at ambient or elevated [CO₂]. Plants, inoculated with three Bradyrhizobium strains isolated at different [CO₂], were grown in chambers at current and elevated [CO₂] (400 vs. 700 ppm). Together with nodule and leaf metabolomic profile, characterization of nodule N-fixation and exchange between organs were tested through ¹⁵N₂-labeling analysis. Soybeans inoculated with SFJ14-36 strain (isolated at elevated [CO₂]) showed a strong metabolic imbalance, at nodule and leaf levels when grown at ambient [CO₂], probably due to an insufficient supply of N by nodules, as shown by ¹⁵N₂-labeling. In nodules, due to shortage of photoassimilate, C may be diverted to aspartic acid instead of malate in order to improve the efficiency of the C source sustaining N₂-fixation. In leaves, photorespiration and respiration were boosted at ambient [CO₂] in plants inoculated with this strain. Additionally, free phytol, antioxidants, and fatty acid content could be indicate induced senescence due to oxidative stress and lack of nitrogen. Therefore, plants inoculated with Bradyrhizobium strain isolated at elevated [CO₂] may have lost their capacity to form effective symbiosis at ambient [CO₂] and that was translated at whole plant level through metabolic impairment.

Plasma membrane phylloquinone biosynthesis in nonphotosynthetic parasitic plants

Nonphotosynthetic holoparasites exploit flexible targeting of phylloquinone biosynthesis to facilitate plasma membrane redox signaling. Phylloquinone is a lipophilic naphthoquinone found predominantly in chloroplasts and best known for its function in photosystem I electron transport and disulfide bridge formation of photosystem II subunits. Phylloquinone has also been detected in plasma membrane (PM) preparations of heterotrophic tissues with potential transmembrane redox function, but the molecular basis for this noncanonical pathway is unknown. Here, we provide evidence of PM phylloquinone biosynthesis in a nonphotosynthetic holoparasite Phelipanche aegyptiaca. A nonphotosynthetic and nonplastidial role for phylloquinone is supported by transcription of phylloquinone biosynthetic genes during seed germination and haustorium development, by PM-localization of alternative terminal enzymes, and by detection of phylloquinone in germinated seeds. Comparative gene network analysis with photosynthetically competent parasites revealed a bias of P. aegyptiaca phylloquinone genes toward coexpression with oxidoreductases involved in PM electron transport. Genes encoding the PM phylloquinone pathway are also present in several photoautotrophic taxa of Asterids, suggesting an ancient origin of multifunctionality. Our findings suggest that nonphotosynthetic holoparasites exploit alternative targeting of phylloquinone for transmembrane redox signaling associated with parasitism.

Phytol derived from chlorophyll hydrolysis in plants is metabolized via phytenal

Phytol is the isoprenoid alcohol bound in ester linkage to chlorophyll, the most abundant photosynthetic pigment in plants. During leaf senescence, large amounts of phytol are released by chlorophyll degradation. However, the pathway of phytol catabolism in plants is unknown. We hypothesized that phytol degradation in plants might involve its oxidation into the long-chain aldehyde phytenal. Using GC-MS for aldehyde quantification after derivatization with methylhydroxylamine, phytenal was identified in leaves, whereas other long-chain aldehydes (phytanal and pristanal) were barely detectable. We found that phytenal accumulates during chlorotic stresses, for example, salt stress, dark-induced senescence, and nitrogen deprivation. The increase in the phytenal content is mediated at least in part independently of enzyme activities, and it is independent of light. Characterization of phytenal accumulation in the pao1 mutant affected in chlorophyll degradation revealed that phytenal is an authentic phytol metabolite derived from chlorophyll breakdown. The increase in phytenal was even stronger in mutants affected in the production of other phytol metabolites including vte5-2 (tocopherol deficient) and pes1 pes2 (fatty acid phytyl ester deficient). Therefore, phytenal accumulation is controlled by competing, alternative pathways of phosphorylation (leading to tocopherol production) or esterification (fatty acid phytyl ester production). As a consequence, the content of phytenal is maintained at low levels, presumably to minimize its toxic effects caused by its highly reactive aldehyde group that can form covalent bonds with and inactivate the amino groups of proteins.

Phytol, a Constituent of Chlorophyll, Induces Root-Knot Nematode Resistance in Arabidopsis via the Ethylene Signaling Pathway

Root-knot nematodes (RKNs; Meloidogyne spp.) parasitize the roots or stems of a wide range of plant species, resulting in severe damage to the parasitized plant. The phytohormone ethylene (ET) plays an important role in signal transduction pathways leading to resistance against RKNs. However, little is currently known about the induction mechanisms of ET-dependent RKN resistance. Inoculation of Arabidopsis thaliana roots with RKNs decreased chlorophyll contents in aerial parts of the plant. We observed accumulation of phytol, a constituent of chlorophyll and a precursor of tocopherols, in RKN-parasitized roots. Application of sclareol, a diterpene that has been shown to induce ET-dependent RKN resistance, to the roots of Arabidopsis plants increased phytol contents in roots accompanied by a decrease in chlorophyll in aerial parts. Exogenously applied phytol inhibited RKN penetration of roots without exhibiting nematicidal activity. This phytol-induced inhibition of RKN penetration was attenuated in the ET-insensitive Arabidopsis mutant ein2-1. Exogenously applied phytol enhanced the production of α-tocopherol and expression of VTE5, a gene involved in tocopherol production, in Arabidopsis roots. α-Tocopherol exerted induction of RKN resistance similar to that of phytol and showed increased accumulation in roots inoculated with RKNs. Furthermore, the Arabidopsis vte5 mutant displayed no inhibition of RKN penetration in response to phytol. These results suggest that exogenously applied phytol induces EIN2-dependent RKN resistance, possibly via tocopherol production.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Chemical Profile and Phytotoxic Action of Hibiscus trionum Essential Oil

The chemical profile and phytotoxic action of Hibiscus trionum essential oil (EO) was studied. In total 17 compounds were identified via GC/MS, representing 94.18 % of the entire oil, with phytol (40.37 %) being the dominant constituent. Bioassay revealed that the EO inhibited root elongation of Medicago sativa and Amaranthus retroflexus by 32.66 % and 61.86 % at 5 mg/mL, respectively; meanwhile, the major component phytol also exhibited significant phytotoxic activity, suppressing radical elongation of Pennisetum alopecuroides, M. sativa and A. retroflexus by 26.08 %, 27.55 % and 43.96 % at 1 mg/mL, respectively. The fact that the EO showed weaker activity than phytol implied that some constituents might trigger antagonistic action to decrease the oil's activity. Our study is the first on the chemical profile and phytotoxic effect of H. trionum EO.

Lipid Analysis by Gas Chromatography and Gas Chromatography-Mass Spectrometry

Gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) represent powerful tools for the quantitative and structural analysis of plant lipids. Here, we outline protocols for the isolation, separation, and derivatization of plant lipids for subsequent GC and GC-MS analysis. Plant lipids are extracted with organic solvents and separated according to their polarity by thin-layer chromatography or solid phase extraction. As most lipids are not volatile, the analytes are derivatized by transmethylation or trimethylsilylation to enable the transition of the molecules into the gas phase. After separation on the polymer matrix of the GC column, the analytes are detected by flame ionization or mass spectrometry. This chapter includes methods suitable for the analysis of lipid-bound or free fatty acids, long chain alcohols, and monoacylglycerols and for the determination of double bond positions in fatty acids.

Chlorophyll dephytylation in chlorophyll metabolism: a simple reaction catalyzed by various enzymes

Chlorophyll (Chl) is composed of a tetrapyrrole ring and a phytol tail, which facilitate light energy absorbance and assembly with photosynthetic protein complexes, respectively. Chl dephytylation, the hydrolytic removal of the phytol tail, is considered a pivotal step in diverse physiological processes, such as Chl salvage during repair of the photosystem, the Chl cycle in the adjustment of antenna size, and Chl breakdown in leaf senescence and fruit maturation. Moreover, phytol is a component of the tocopherols, a major form of vitamin E that is essential in the human diet. This phytol mostly comes from Chl hydrolysis. However, the authentic enzyme responsible for Chl dephytylation has proved elusive. CHLOROPHYLLASE (CLH) which was discovered over a century ago, was the first enzyme found to have dephytylation activity in vitro, but its role in Chl metabolism has been questioned and remains under debate. Recently, novel dephytylases, i.e., PHEOPHYTINASE (PPH) and CHLOROPHYLL DEPHYTYLASE1 (CLD1) have emerged from genetic studies, indicating that dephytylation in Chl catabolism involves different players and is more complicated than previously thought. Based on sequence homology, substrate specificity, and subcellular localization, CLH, PPH, and CLD1 belong to different types of dephytylase, which prompted us to re-examine the dilemmas and missing links that still exist in Chl metabolism. This review thus focuses on the hitherto unanswered questions involving the Chl dephytylation reaction by highlighting relevant literature, updating recent progress, and synthesizing ideas.

Benefiting others and self: Production of vitamins in plants

Vitamins maintain growth and development in humans, animals, and plants. Because plants serve as essential producers of vitamins, increasing the vitamin contents in plants has become a goal of crop breeding worldwide. Here, we begin with a summary of the functions of vitamins. We then review the achievements to date in elucidating the molecular mechanisms underlying how vitamins are synthesized, transported, and regulated in plants. We also stress the exploration of variation in vitamins by the use of forward genetic approaches, such as quantitative trait locus mapping and genome-wide association studies. Overall, we conclude that exploring the diversity of vitamins could provide new insights into plant metabolism and crop breeding.

Probing the Response of the Amphibious Plant Butomus umbellatus to Nutrient Enrichment and Shading by Integrating Eco-Physiological With Metabolomic Analyses

Amphibious plants, living in land-water ecotones, have to cope with challenging and continuously changing growth conditions in their habitats with respect to nutrient and light availability. They have thus evolved a variety of mechanisms to tolerate and adapt to these changes. Therefore, the study of these plants is a major area of ecophysiology and environmental ecological research. However, our understanding of their capacity for physiological adaptation and tolerance remains limited and requires systemic approaches for comprehensive analyses. To this end, in this study, we have conducted a mesocosm experiment to analyze the response of Butomus umbellatus, a common amphibious species in Denmark, to nutrient enrichment and shading. Our study follows a systematic integration of morphological (including plant height, leaf number, and biomass accumulation), ecophysiological (photosynthesis-irradiance responses, leaf pigment content, and C and N content in plant organs), and leaf metabolomic measurements using gas chromatography-mass spectrometry (39 mainly primary metabolites), based on bioinformatic methods. No studies of this type have been previously reported for this plant species. We observed that B. umbellatus responds to nutrient enrichment and light reduction through different mechanisms and were able to identify its nutrient enrichment acclimation threshold within the applied nutrient gradient. Up to that threshold, the morpho-physiological response to nutrient enrichment was profound, indicating fast-growing trends (higher growth rates and biomass accumulation), but only few parameters changed significantly from light to shade [specific leaf area (SLA); quantum yield (φ)]. Metabolomic analysis supported the morpho-physiological results regarding nutrient overloading, indicating also subtle changes due to shading not directly apparent in the other measurements. The combined profile analysis revealed leaf metabolite and morpho-physiological parameter associations. In this context, leaf lactate, currently of uncertain role in higher plants, emerged as a shading acclimation biomarker, along with SLA and φ. The study enhances both the ecophysiology methodological toolbox and our knowledge of the adaptive capacity of amphibious species. It demonstrates that the educated combination of physiological with metabolomic measurements using bioinformatic approaches is a promising approach for ecophysiology research, enabling the elucidation of discriminatory metabolic shifts to be used for early diagnosis and even prognosis of natural ecosystem responses to climate change.

Fate of free and bound phytol and tocopherols during fruit ripening of two Capsicum cultivars

Phytol and tocopherols and their fatty acid esters (PFAE and TFAE) are isoprenoid lipid components which can be found for instance in vegetables. Their behavior during maturation of fruits and vegetables could reveal valuable information on their biosynthetic formation and biological function. As pods of the genus Capsicum contain considerable amounts of both PFAE and TFAE, two cultivars (i.e. Capsicum annuum var. Forajido and Capsicum chinense var. Habanero) were grown in a greenhouse project. The date of flowering and fruit formation of each blossom was noted and fruits were harvested in four specific periods which corresponded with different stages of ripening, i.e. unripe, semi-ripe, ripe and overripe. Quantification by means of gas chromatography mass spectrometry and creation of development profiles strongly supported the suggestion that PFAE and TFAE were formed as storage molecules during fruit ripening and parallel degradation of chlorophyll. Additionally, compound-specific carbon isotope ratios (δ¹³C values (‰)) of originally in PFAE and chlorophyll bound phytol ultimately proved that PFAE, besides tocopherols, serve as sink for the cytotoxic phytol moiety released from chlorophyll degradation during fruit ripening. Furthermore, color measurements were successfully implemented to simplify the usually cumbersome separation of chili fruits into different ripening degrees.

Lipid droplets throughout the evolutionary tree

Intracellular lipid droplets are utilized for lipid storage and metabolism in organisms as evolutionarily diverse as animals, fungi, plants, bacteria, and archaea. These lipid droplets demonstrate great diversity in biological functions and protein and lipid compositions, yet fundamentally share common molecular and ultrastructural characteristics. Lipid droplet research has been largely fragmented across the diversity of lipid droplet classes and sub-classes. However, we suggest that there is great potential benefit to the lipid community in better integrating the lipid droplet research fields. To facilitate such integration, we survey the protein and lipid compositions, functional roles, and mechanisms of biogenesis across the breadth of lipid droplets studied throughout the natural world. We depict the big picture of lipid droplet biology, emphasizing shared characteristics and unique differences seen between different classes. In presenting the known diversity of lipid droplets side-by-side it becomes necessary to offer for the first time a consistent system of categorization and nomenclature. We propose a division into three primary classes that reflect their sub-cellular location: i) cytoplasmic lipid droplets (CYTO-LDs), that are present in the eukaryotic cytoplasm, ii) prokaryotic lipid droplets (PRO-LDs), that exist in the prokaryotic cytoplasm, and iii) plastid lipid droplets (PL-LDs), that are found in plant plastids, organelles of photosynthetic eukaryotes. Within each class there is a remarkable array of sub-classes displaying various sizes, shapes and compositions. A more integrated lipid droplet research field will provide opportunities to better build on discoveries and accelerate the pace of research in ways that have not been possible.

A Synergistic Genetic Engineering Strategy Induced Triacylglycerol Accumulation in Potato (Solanum tuberosum) Leaf

Potato is the 4th largest staple food in the world currently. As a high biomass crop, potato harbors excellent potential to produce energy-rich compounds such as triacylglycerol as a valuable co-product. We have previously reported that transgenic potato tubers overexpressing WRINKLED1, DIACYLGLYCEROL ACYLTRANSFERASE 1, and OLEOSIN genes produced considerable levels of triacylglycerol. In this study, the same genetic engineering strategy was employed on potato leaves. The overexpression of Arabidopsis thaliana WRINKED1 under the transcriptional control of a senescence-inducible promoter together with Arabidopsis thaliana DIACYLGLYCEROL ACYLTRANSFERASE 1 and Sesamum indicum OLEOSIN driven by the Cauliflower Mosaic Virus 35S promoter and small subunit of Rubisco promoter respectively, resulted in an approximately 30- fold enhancement of triacylglycerols in the senescent transgenic potato leaves compared to the wild type. The increase of triacylglycerol in the transgenic potato leaves was accompanied by perturbations of carbohydrate accumulation, apparent in a reduction in starch content and increased total soluble sugars, as well as changes of polar membrane lipids at different developmental stages. Microscopic and biochemical analysis further indicated that triacylglycerols and lipid droplets could not be produced in chloroplasts, despite the increase and enlargement of plastoglobuli at the senescent stage. Possibly enhanced accumulation of fatty acid phytyl esters in the plastoglobuli were reflected in transgenic potato leaves relative to wild type. It is likely that the plastoglobuli may have hijacked some of the carbon as the result of WRINKED1 expression, which could be a potential factor restricting the effective accumulation of triacylglycerols in potato leaves. Increased lipid production was also observed in potato tubers, which may have affected the tuberization to a certain extent. The expression of transgenes in potato leaf not only altered the carbon partitioning in the photosynthetic source tissue, but also the underground sink organs which highly relies on the leaves in development and energy deposition.

Triacylglycerol and phytyl ester synthesis in Synechocystis sp. PCC6803

Cyanobacteria are unicellular prokaryotic algae that perform oxygenic photosynthesis, similar to plants. The cells harbor thylakoid membranes composed of lipids related to those of chloroplasts in plants to accommodate the complexes of photosynthesis. The occurrence of storage lipids, including triacylglycerol or wax esters, which are found in plants, animals, and some bacteria, nevertheless remained unclear in cyanobacteria. We show here that the cyanobacterium Synechocystis sp. PCC6803 accumulates both triacylglycerol and wax esters (fatty acid phytyl esters). Phytyl esters accumulate in higher levels under abiotic stress conditions. The analysis of an insertional mutant revealed that the acyltransferase slr2103, with sequence similarity to plant esterase/lipase/thioesterase (ELT) proteins, is essential for triacylglycerol and phytyl ester synthesis in Synechocystis The recombinant slr2103 enzyme showed acyltransferase activity with phytol and diacylglycerol, thus producing phytyl esters and triacylglycerol. Acyl-CoA thioesters were the preferred acyl donors, while acyl-ACP (acyl carrier protein), free fatty acids, or galactolipid-bound fatty acids were poor substrates. The slr2103 protein sequence is unrelated to acyltransferases from bacteria (AtfA) or plants (DGAT1, DGAT2, PDAT), and therefore establishes an independent group of bacterial acyltransferases involved in triacylglycerol and wax ester synthesis. The identification of the gene slr2103 responsible for triacylglycerol synthesis in cyanobacteria opens the possibility of using prokaryotic photosynthetic cells in biotechnological applications.

Interaction Between Induced and Natural Variation at oil yellow1 Delays Reproductive Maturity in Maize

We previously demonstrated that maize (Zea mays) locus very oil yellow1 (vey1) encodes a putative cis-regulatory expression polymorphism at the magnesium chelatase subunit I gene (aka oil yellow1) that strongly modifies the chlorophyll content of the semi-dominant Oy1-N1989 mutants. The vey1 allele of Mo17 inbred line reduces chlorophyll content in the mutants leading to reduced photosynthetic output. Oy1-N1989 mutants in B73 reached reproductive maturity four days later than wild-type siblings. Enhancement of Oy1-N1989 by the Mo17 allele at the vey1 QTL delayed maturity further, resulting in detection of a flowering time QTL in two bi-parental mapping populations crossed to Oy1-N1989. The near isogenic lines of B73 harboring the vey1 allele from Mo17 delayed flowering of Oy1-N1989 mutants by twelve days. Just as previously observed for chlorophyll content, vey1 had no effect on reproductive maturity in the absence of the Oy1-N1989 allele. Loss of chlorophyll biosynthesis in Oy1-N1989 mutants and enhancement by vey1 reduced CO₂ assimilation. We attempted to separate the effects of photosynthesis on the induction of flowering from a possible impact of chlorophyll metabolites and retrograde signaling by manually reducing leaf area. Removal of leaves, independent of the Oy1-N1989 mutant, delayed flowering but surprisingly reduced chlorophyll contents of emerging leaves. Thus, defoliation did not completely separate the identity of the signal(s) that regulates flowering time from changes in chlorophyll content in the foliage. These findings illustrate the necessity to explore the linkage between metabolism and the mechanisms that connect it to flowering time regulation.

A lil3 chlp double mutant with exclusive accumulation of geranylgeranyl chlorophyll displays a lethal phenotype in rice

BACKGROUND

Phytyl residues are the common side chains of chlorophyll (Chl) and tocopherols. Geranylgeranyl reductase (GGR), which is encoded by CHLP gene, is responsible for phytyl biosynthesis. The light-harvesting like protein LIL3 was suggested to be required for stability of GGR and protochlorophyllide oxidoreductase in Arabidopsis.

RESULTS

In this study, we isolated a yellow-green leaf mutant, 637ys, in rice (Oryza sativa). The mutant accumulated majority of Chls with unsaturated geranylgeraniol side chains and displayed a yellow-green leaf phenotype through the whole growth period. The development of chloroplasts was suppressed, and the major agronomic traits, especially No. of productive panicles per plant and of spikelets per panicle, dramatically decreased in 637ys. Besides, the mutant exhibited to be sensitive to light intensity and deficiency of tocopherols without obvious alteration in tocotrienols in leaves and grains. Map-based cloning and complementation experiment demonstrated that a point mutation on the OsLIL3 gene accounted for the mutant phenotype of 637ys. OsLIL3 is mainly expressed in green tissues, and its encoded protein is targeted to the chloroplast. Furthermore, the 637ys 502ys (lil3 chlp) double mutant exclusively accumulated geranylgeranyl Chl and exhibited lethality at the three-leaf stage.

CONCLUSIONS

We identified the OsLIL3 gene through a map-based cloning approach. Meanwhile, we demonstrated that OsLIL3 is of extreme importance to the function of OsGGR, and that the complete replacement of phytyl side chain of chlorophyll by geranylgeranyl chain could be fatal to plant survival in rice.

Prenylquinones in Human Parasitic Protozoa: Biosynthesis, Physiological Functions, and Potential as Chemotherapeutic Targets

Human parasitic protozoa cause a large number of diseases worldwide and, for some of these diseases, there are no effective treatments to date, and drug resistance has been observed. For these reasons, the discovery of new etiological treatments is necessary. In this sense, parasitic metabolic pathways that are absent in vertebrate hosts would be interesting research candidates for the identification of new drug targets. Most likely due to the protozoa variability, uncertain phylogenetic origin, endosymbiotic events, and evolutionary pressure for adaptation to adverse environments, a surprising variety of prenylquinones can be found within these organisms. These compounds are involved in essential metabolic reactions in organisms, for example, prevention of lipoperoxidation, participation in the mitochondrial respiratory chain or as enzymatic cofactors. This review will describe several prenylquinones that have been previously characterized in human pathogenic protozoa. Among all existing prenylquinones, this review is focused on ubiquinone, menaquinone, tocopherols, chlorobiumquinone, and thermoplasmaquinone. This review will also discuss the biosynthesis of prenylquinones, starting from the isoprenic side chains to the aromatic head group precursors. The isoprenic side chain biosynthesis maybe come from mevalonate or non-mevalonate pathways as well as leucine dependent pathways for isoprenoid biosynthesis. Finally, the isoprenic chains elongation and prenylquinone aromatic precursors origins from amino acid degradation or the shikimate pathway is reviewed. The phylogenetic distribution and what is known about the biological functions of these compounds among species will be described, as will the therapeutic strategies associated with prenylquinone metabolism in protozoan parasites.

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.

Role of Ca2+ as protectant under heat stress by regulation of photosynthesis and membrane saturation in Anabaena PCC 7120

The present study was aimed at understanding the effects of heat stress on selected physiological and biochemical parameters of a model cyanobacterium, Anabaena PCC 7120 in addition to amelioration strategy using exogenous Ca²⁺. A comparison of the cells exposed to heat stress (0-24 h) in the presence or absence of Ca²⁺ clearly showed reduction in colony-forming ability and increase in reactive oxygen species (ROS) leading to loss in the viability of cells of Ca²⁺-deficient cultures. There was higher level of saturation in membrane lipids of the cells supplemented with Ca²⁺ along with higher accumulation of proline. Similarly, higher quantum yield (7.8-fold) in Ca²⁺-supplemented cultures indicated role of Ca²⁺ in regulation of photosynthesis. Relative electron transport rate (rETR) decreased in both the sets with the difference in the rate of decrease (slow) in Ca²⁺-supplemented cultures. The Ca²⁺-supplemented sets also maintained high levels of open reaction centers of PS II in comparison to Ca²⁺-deprived cells. Increase in transcripts of both subunits ((rbcL and rbcS) of RubisCO from Ca²⁺-supplemented Anabaena cultures pointed out the role of Ca²⁺ in sustenance of photosynthesis of cells via CO₂ fixation, thus, playing an important role in maintaining metabolic status of the heat-stressed cyanobacterium.

PHYTOCHROME-INTERACTING FACTOR 3 mediates light-dependent induction of tocopherol biosynthesis during tomato fruit ripening

Tocopherols are important antioxidants exclusively produced in plastids that protect the photosynthetic apparatus from oxidative stress. These compounds with vitamin E activity are also essential dietary nutrients for humans. Although the tocopherol biosynthetic pathway has been elucidated, the mechanisms that regulate tocopherol production and accumulation remain elusive. Here, we investigated the regulatory mechanism underlying tocopherol biosynthesis during ripening in tomato fruits, which are an important source of vitamin E. Our results show that ripening under light conditions increases tocopherol fruit content in a phytochrome-dependent manner by the transcriptional regulation of biosynthetic genes. Moreover, we show that light-controlled expression of the GERANYLGERANYL DIPHOSPHATE REDUCTASE (SlGGDR) gene, responsible for the synthesis of the central tocopherol precursor phytyl diphosphate, is mediated by PHYTOCHROME-INTERACTING FACTOR 3 (SlPIF3). In the absence of light, SlPIF3 physically interacts with the promoter of SlGGDR, down-regulating its expression. By contrast, light activation of phytochromes prevents the interaction between SlPIF3 and the SlGGDR promoter, leading to transcriptional derepression and higher availability of the PDP precursor for tocopherol biosynthesis. The unraveled mechanism provides a new strategy to manipulate fruit metabolism towards improving tomato nutritional quality.

Tocopherol biosynthesis in Leishmania (L.) amazonensis promastigotes

Leishmaniasis is a neglected disease caused by a trypanosomatid protozoan of the genus Leishmania. Most drugs used to treat leishmaniasis are highly toxic, and the emergence of drug‐resistant strains has been observed. Therefore, new therapeutic targets against leishmaniasis are required. Several isoprenoid compounds, including dolichols or ubiquinones, have been shown to be important for cell viability and proliferation in various trypanosomatid species. Here, we detected the biosynthesis of tocopherol in Leishmania (L.) amazonensis promastigotes in vitro through metabolic labelling with [1‐(n)‐3H]‐phytol. Subsequently, we confirmed the presence of vitamin E in the parasite by gas chromatography–mass spectrometry. Treatment with usnic acid or nitisinone, inhibitors of precursors of vitamin E synthesis, inhibited growth of the parasite in a concentration‐dependent manner. This study provides the first evidence of tocopherol biosynthesis in a trypanosomatid and suggests that inhibitors of the enzyme 4‐hydroxyphenylpyruvate dioxygenase may be suitable for use as antileishmanial compounds.

Database

The amino acid sequence of a conserved hypothetical protein [Leishmania mexicana MHOM/GT/2001/U1103] has been deposited in GenBank (CBZ28005.1)

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.

Comparative physiological and proteomic analyses of the chloroplasts in halophyte Sesuvium portulacastrum under differential salt conditions

Sesuvium portulacastrum, an important mangrove-associated true halophyte belongs to the family Aizoaceae, has excellent salt tolerance. Chloroplasts are the most sensitive organelles involved in the response to salinity. However, the regulation mechanism of chloroplasts of S. portulacastrum under salinity stress has not been reported. In this study, morphological and physiological analyses of leaves and comparative proteomics of chloroplasts isolated from the leaves of S. portulacastrum under different NaCl treatments were performed. Our results showed that the thickness of the palisade tissue, the leaf area, the maximum photochemical efficiency of photosystem II, and the electron transport rate increased remarkably after the plants were subjected to differential saline environments, indicating that salinity can increase photosynthetic efficiency and improve the growth of S. portulacastrum. Subsequently, 55 differentially expressed protein species (DEPs) from the chloroplasts of S. portulacastrum under differential salt conditions were positively identified by mass spectrometry. These DEPs were involved in multiple metabolic pathways, such as photosynthesis, carbon metabolism, ATP synthesis and the cell structure. Among these DEPs, the abundance of most proteins was induced by salt stress. Based on a combination of the morphological and physiological data, as well as the chloroplast proteome results, we speculated that S. portulacastrum can maintain photosynthetic efficiency and growth by maintaining the stability of the photosystem II complex, promoting the photochemical reaction rate, enhancing carbon fixation, developing plastoglobules, and preserving the biomembrane system of chloroplasts under salt stress.

From in planta Function to Vitamin-Rich Food Crops: The ACE of Biofortification

Humans are highly dependent on plants to reach their dietary requirements, as plant products contribute both to energy and essential nutrients. For many decades, plant breeders have been able to gradually increase yields of several staple crops, thereby alleviating nutritional needs with varying degrees of success. However, many staple crops such as rice, wheat and corn, although delivering sufficient calories, fail to satisfy micronutrient demands, causing the so called 'hidden hunger.' Biofortification, the process of augmenting nutritional quality of food through the use of agricultural methodologies, is a pivotal asset in the fight against micronutrient malnutrition, mainly due to vitamin and mineral deficiencies. Several technical advances have led to recent breakthroughs. Nutritional genomics has come to fruition based on marker-assisted breeding enabling rapid identification of micronutrient related quantitative trait loci (QTL) in the germplasm of interest. As a complement to these breeding techniques, metabolic engineering approaches, relying on a continuously growing fundamental knowledge of plant metabolism, are able to overcome some of the inevitable pitfalls of breeding. Alteration of micronutrient levels does also require fundamental knowledge about their role and influence on plant growth and development. This review focuses on our knowledge about provitamin A (beta-carotene), vitamin C (ascorbate) and the vitamin E group (tocochromanols). We begin by providing an overview of the functions of these vitamins in planta, followed by highlighting some of the achievements in the nutritional enhancement of food crops via conventional breeding and genetic modification, concluding with an evaluation of the need for such biofortification interventions. The review further elaborates on the vast potential of creating nutritionally enhanced crops through multi-pathway engineering and the synergistic potential of conventional breeding in combination with genetic engineering, including the impact of novel genome editing technologies.

Lipid profiles reveal different responses to brown planthopper infestation for pest susceptible and resistant rice plants

INTRODUCTION

Brown planthopper (BPH) is the most destructive insect pest for rice, causing major reductions in rice yield and large economic losses. More than 31 BPH-resistance genes have been located, and several of them have been isolated. Nevertheless, the metabolic mechanism related to BPH-resistance genes remain uncharacterized.

OBJECTIVES

To elucidate the resistance mechanism of the BPH-resistance gene Bph6 at the metabolic level, a Bph6-transgenic line R6 (BPH-resistant) and the wild-type Nipponbare (BPH-susceptible) were used to investigate their lipid profiles under control and BPH treatments.

METHODS

In conjunction with multivariate statistical analysis and quantitative real-time PCR, BPH-induced lipid changes in leaf blade and leaf sheath were investigated by GC-MS-based lipidomics.

RESULTS

Forty-five lipids were identified in leaf sheath extracts. Leaf sheath lipidomics analysis results show that BPH infestation induces significant differences in the lipid profiles of Nipponbare and R6. The levels of hexadecanoic acid, methyl ester, linoleic acid, methyl ester, linolenic acid, methyl ester, glycidyl palmitate, eicosanoic acid, methyl ester, docosanoic acid, methyl ester, beta-monolinolein, campesterol, beta-sitosterol, cycloartenol, phytol and phytyl acetate had undergone enormous changes after BPH feeding. These results illustrate that BPH feeding enhances sterol biosynthetic pathway in Nipponbare plants, and strengthens wax biosynthesis and phytol metabolism in R6 plants. The results of quantitative real-time PCR of 5 relevant genes were consistent with the changes in metabolic level. Forty-five lipids were identified in the leaf blade extracts. BPH infestation induces distinct changes in the lipid profiles of the leaf blade samples of Nipponbare and R6. Although the lipid changes in Nipponbare are more drastic, the changes within the two varieties are similar. Lipid profiles in leaf sheath brought out significant differences than in leaf blade within Nipponbare and R6. We propose that Bph6 mainly affects the levels of lipids in leaf sheath, and mediates resistance by deploying metabolic re-programming during BPH feeding.

CONCLUSION

The results indicate that wax biosynthesis, sterol biosynthetic pathway and phytol metabolism play vital roles in rice response to BPH infestation. This finding demonstrated that the combination of lipidomics and quantitative real-time PCR is an effective approach to elucidating the interactions between brown planthopper and rice mediated by resistance genes.