Link between light and fatty acid synthesis: thioredoxin-linked reductive activation of plastidic acetyl-CoA carboxylase

Arabidopsis trigalactosyldiacylglycerol1 mutants reveal a critical role for phosphtidylcholine remodeling in lipid homeostasis

Lipid remodeling plays a critical role in plant response to abiotic stress and metabolic perturbations. Key steps in this process involve modifications of phosphatidylcholine (PC) acyl chains mediated by lysophosphatidylcholine: acyl-CoA acyltransferases (LPCATs) and phosphatidylcholine: diacylglycerol cholinephosphotransferase (ROD1). To assess their importance in lipid homeostasis, we took advantage of the trigalactosyldiacylglycerol1 (tgd1) mutant that exhibits marked increases in fatty acid synthesis and fatty acid flux through PC due to a block in inter-organelle lipid trafficking. Here, we showed that the increased fatty acid synthesis in tgd1 is due to posttranslational activation of the plastidic acetyl-coenzyme A carboxylase. Genetic analysis showed that knockout of LPCAT1 and 2 resulted in a lethal phenotype in tgd1. In addition, plants homozygous for lpcat2 and heterozygous for lpcat1 in the tgd1 background showed reduced levels of PC and triacylglycerols (TAG) and alterations in their fatty acid profiles. We further showed that disruption of ROD1 in tgd1 resulted in changes in fatty acid composition of PC and TAG, decreased leaf TAG content and reduced seedling growth. Together, our results reveal a critical role of LPCATs and ROD1 in maintaining cellular lipid homeostasis under conditions, in which fatty acid production largely exceeds the cellular demand for membrane lipid synthesis.

The Molecular Frequency, Conservation and Role of Reactive Cysteines in Plant Lipid Metabolism

Cysteines (Cys) are chemically reactive amino acids containing sulfur that play diverse roles in plant biology. Recent proteomics investigations in Arabidopsis thaliana have revealed the presence of thiol post-translational modifications (PTMs) in several Cys residues. These PTMs are presumed to impact protein structure and function, yet mechanistic data regarding the specific Cys susceptible to modification and their biochemical relevance remain limited. To help address these limitations, we have conducted a wide-ranging analysis by integrating published datasets encompassing PTM proteomics (comparing S-sulfenylation, persulfidation, S-nitrosylation and S-acylation), genomics and protein structures, with a specific focus on proteins involved in plant lipid metabolism. The prevalence and distribution of modified Cys residues across all analyzed proteins is diverse and multifaceted. Nevertheless, by combining an evaluation of sequence conservation across 100+ plant genomes with AlphaFold-generated protein structures and physicochemical predictions, we have unveiled structural propensities associated with Cys modifications. Furthermore, we have identified discernible patterns in lipid biochemical pathways enriched with Cys PTMs, notably involving beta-oxidation, jasmonic acid biosynthesis, fatty acid biosynthesis and wax biosynthesis. These collective findings provide valuable insights for future investigations targeting the mechanistic foundations of Cys modifications and the regulation of modified proteins in lipid metabolism and other metabolic pathways.

m-Type thioredoxin regulates cytochrome b6f complex of photosynthesis

Depleting m-type thioredoxin disrupts plant cell redox, impacting cytochrome b6f in photosynthesis, hindering photosynthesis, and stunting growth.

Multi-omics-driven advances in the understanding of triacylglycerol biosynthesis in oil seeds

Vegetable oils are rich sources of polyunsaturated fatty acids and energy as well as valuable sources of human food, animal feed, and bioenergy. Triacylglycerols, which are comprised of three fatty acids attached to a glycerol backbone, are the main component of vegetable oils. Here, we review the development and application of multiple-level omics in major oilseeds and emphasize the progress in the analysis of the biological roles of key genes underlying seed oil content and quality in major oilseeds. Finally, we discuss future research directions in functional genomics research based on current omics and oil metabolic engineering strategies that aim to enhance seed oil content and quality, and specific fatty acids components according to either human health needs or industrial requirements.

Transcriptomic comparison of seeds and silique walls from two rapeseed genotypes with contrasting seed oil content

Silique walls play pivotal roles in contributing photoassimilates and nutrients to fuel seed growth. However, the interaction between seeds and silique walls impacting oil biosynthesis is not clear during silique development. Changes in sugar, fatty acid and gene expression during Brassica napus silique development of L192 with high oil content and A260 with low oil content were investigated to identify key factors affecting difference of their seed oil content. During the silique development, silique walls contained more hexose and less sucrose than seeds, and glucose and fructose contents in seeds and silique walls of L192 were higher than that of A260 at 15 DAF, and sucrose content in the silique walls of L192 were lower than that of A260 at three time points. Genes related to fatty acid biosynthesis were activated over time, and differences on fatty acid content between the two genotypes occurred after 25 DAF. Genes related to photosynthesis expressed more highly in silique walls than in contemporaneous seeds, and were inhibited over time. Gene set enrichment analysis suggested photosynthesis were activated in L192 at 25 and 35 DAF in silique walls and at both 15 and 35 DAF in the seed. Expressions of sugar transporter genes in L192 was higher than that in A260, especially at 35 DAF. Expressions of genes related to fatty acid biosynthesis, such as BCCP2s, bZIP67 and LEC1s were higher in L192 than in A260, especially at 35 DAF. Meanwhile, genes related to oil body proteins were expressed at much lower levels in L192 than in A260. According to the WGCNA results, hub modules, such as ME.turquoise relative to photosynthesis, ME.green relative to embryo development and ME.yellow relative to lipid biosynthesis, were identified and synergistically regulated seed development and oil accumulation. Our results are helpful for understanding the mechanism of oil accumulation of seeds in oilseed rape for seed oil content improvement.

The PPR protein RARE1-mediated editing of chloroplast accD transcripts is required for fatty acid biosynthesis and heat tolerance in Arabidopsis

It has been reported that Arabidopsis chloroplast accD transcripts undergo RNA editing and that loss of accD-C794 RNA editing does not affect plant growth under normal conditions. To date, the exact biological role of accD-C794 editing has remained elusive. Here, we reveal an unexpected role for accD-C794 editing in response to heat stress. Loss of accD-C794 editing results in a yellow and dwarf phenotype with decreased chloroplast gene expression under heat stress, and artificial improvement of C794-edited accD gene expression enhances heat tolerance in Arabidopsis. These data suggest that accD-C794 editing confers heat tolerance in planta. We also found that treatment with the product of acetyl coenzyme A carboxylase (ACCase) could allay mutant phenotypic characteristics and showed that a mutation in the CAC3 gene for the α-subunit of ACCase was associated with dwarfism under heat stress. These observations indicate that defective accD-C794 editing may be intrinsic to reduced ACCase activity, thereby contributing to heat sensitivity. ACCase catalyzes the committed step of de novo fatty acid (FA) biosynthesis. FA content analysis revealed that unsaturated oleic (C18:1) and linoleic acids (C18:2) were low in the accD-C794 editing-defective mutant but high in the C794-edited accD-overexpressing plants compared with the wild type. Supplying exogenous C18:1 and C18:2 could rescue the mutant phenotype, suggesting that these FAs play an essential role in tolerance to heat stress. Transmission electron microscopy observations showed that heat stress seriously affected the membrane architecture in accD editing-defective mutants but not in accD-overexpressing plants. These results provide the first evidence that accD-C794 editing regulates FA biosynthesis for maintenance of membrane structural homeostasis under heat stress.

Overexpression of pea α-carboxyltransferase in Arabidopsis and camelina increases fatty acid synthesis leading to improved seed oil content

Heteromeric acetyl‐CoA carboxylase (htACCase) catalyzes the committed step of de novo fatty acid biosynthesis in most plant plastids. Plant htACCase is comprised of four subunits: α‐ and β‐carboxyltransferase (α‐ and β‐CT), biotin carboxylase, and biotin carboxyl carrier protein. Based on in vivo absolute quantification of htACCase subunits, α‐CT is 3‐ to 10‐fold less abundant than its partner subunit β‐CT in developing Arabidopsis seeds [Wilson and Thelen, J. Proteome Res., 2018, 17 (5)]. To test the hypothesis that low expression of α‐CT limits htACCase activity and flux through fatty acid synthesis in planta, we overexpressed Pisum sativum α‐CT, either with or without its C‐terminal non‐catalytic domain, in both Arabidopsis thaliana and Camelina sativa. First‐generation Arabidopsis seed of 35S::Ps α‐CT (n = 25) and 35S::Ps α‐CTΔ406‐875 (n = 47) were on average 14% higher in oil content (% dry weight) than wild type co‐cultivated in a growth chamber. First‐generation camelina seed showed an average 8% increase compared to co‐cultivated wild type. Biochemical analyses confirmed the accumulation of Ps α‐CT and Ps α‐CTΔ406‐875 protein and higher htACCase activity in overexpression lines during early seed development. Overexpressed Ps α‐CT co‐migrated with native At β‐CT during anion exchange chromatography, indicating co‐association. By successfully increasing seed oil content upon heterologous overexpression of α‐CT, we demonstrate how absolute quantitation of in vivo protein complex stoichiometry can be used to guide rational metabolic engineering.

Metabolic flux analysis of the non-transitory starch tradeoff for lipid production in mature tobacco leaves

The metabolic plasticity of tobacco leaves has been demonstrated via the generation of transgenic plants that can accumulate over 30% dry weight as triacylglycerols. In investigating the changes in carbon partitioning in these high lipid-producing (HLP) leaves, foliar lipids accumulated stepwise over development. Interestingly, non-transient starch was observed to accumulate with plant age in WT but not HLP leaves, with a drop in foliar starch concurrent with an increase in lipid content. The metabolic carbon tradeoff between starch and lipid was studied using 13CO2-labeling experiments and isotopically nonstationary metabolic flux analysis, not previously applied to the mature leaves of a crop. Fatty acid synthesis was investigated through assessment of acyl-acyl carrier proteins using a recently derived quantification method that was extended to accommodate isotopic labeling. Analysis of labeling patterns and flux modeling indicated the continued production of unlabeled starch, sucrose cycling, and a significant contribution of NADP-malic enzyme to plastidic pyruvate production for the production of lipids in HLP leaves, with the latter verified by enzyme activity assays. The results suggest an inherent capacity for a developmentally regulated carbon sink in tobacco leaves and may in part explain the uniquely successful leaf lipid engineering efforts in this crop.

Arabidopsis thaliana 2,3-bisphosphoglycerate-independent phosphoglycerate mutase 2 activity requires serine 82 phosphorylation

Phosphoglycerate mutases (PGAMs) catalyse the reversible isomerisation of 3-phosphoglycerate and 2-phosphoglycerate, a step of glycolysis. PGAMs can be sub-divided into 2,3-bisphosphoglycerate-dependent (dPGAM) and -independent (iPGAM) enzymes. In plants, phosphoglycerate isomerisation is carried out by cytosolic iPGAM. Despite its crucial role in catabolism, little is known about post-translational modifications of plant iPGAM. In Arabidopsis thaliana, phosphoproteomics analyses have previously identified an iPGAM phosphopeptide where serine 82 is phosphorylated. Here, we show that this phosphopeptide is less abundant in dark-adapted compared to illuminated Arabidopsis leaves. In silico comparison of iPGAM protein sequences and 3D structural modelling of AtiPGAM2 based on non-plant iPGAM enzymes suggest a role for phosphorylated serine in the catalytic reaction mechanism. This is confirmed by the activity (or the lack thereof) of mutated recombinant Arabidopsis iPGAM2 forms, affected in different steps of the reaction mechanism. We thus propose that the occurrence of the S82-phosphopeptide reflects iPGAM2 steady-state catalysis. Based on this assumption, the metabolic consequences of a higher iPGAM activity in illuminated versus darkened leaves are discussed.

Chloroplast Lipids Metabolism and Function. A Redox Perspective

Plant productivity is determined by the conversion of solar energy into biomass through oxygenic photosynthesis, a process performed by protein-cofactor complexes including photosystems (PS) II and I, and ATP synthase. These complexes are embedded in chloroplast thylakoid membrane lipids, which thus function as structural support of the photosynthetic machinery and provide the lipid matrix to avoid free ion diffusion. The lipid and fatty acid composition of thylakoid membranes are unique in chloroplasts and cyanobacteria, which implies that these molecules are specifically required in oxygenic photosynthesis. Indeed, there is extensive evidence supporting a relevant function of glycerolipids in chloroplast biogenesis and photosynthetic efficiency in response to environmental stimuli, such as light and temperature. The rapid acclimation of higher plants to environmental changes is largely based on thiol-based redox regulation and the disulphide reductase activity thioredoxins (Trxs), which are reduced by ferredoxin (Fdx) via an Fdx-dependent Trx reductase. In addition, chloroplasts harbour an NADPH-dependent Trx reductase C, which allows the use of NADPH to maintain the redox homeostasis of the organelle. Here, we summarise the current knowledge of chloroplast lipid metabolism and the function of these molecules as structural basis of the complex membrane network of the organelle. Furthermore, we discuss evidence supporting the relevant role of lipids in chloroplast biogenesis and photosynthetic performance in response to environmental cues in which the redox state of the organelle plays a relevant role.

Reducing self-shading effects in Botryococcus braunii cultures: effect of Mg2+ deficiency on optical and biochemical properties, photosynthesis and lipidomic profile

Microalgae biomass exploitation as a carbon-neutral energy source is currently limited by several factors, productivity being one of the most relevant. Due to the high absorption properties of light-harvesting antenna, photosynthetic cells tend to capture an excessive amount of energy that cannot be entirely channeled through the electron transfer chain that ends up dissipated as heat and fluorescence, reducing the overall light use efficiency. Aiming to minimize this hurdle, in this work we studied the effect of decreasing concentrations of Magnesium (Mg2+) on the chlorophyll a content, photosynthetic performance, biomass and lipid production of autotrophic cultures of Botryococcus braunii LB 572. We also performed, for the first time, a comparative lipidomic analysis to identify the influence of limited Mg2+ supply on the lipid profile of this algae. The results indicated that a level of 0.0037 g L-1 MgSO4 caused a significant decline on chlorophyll a content with a concomitant 2.3-fold reduction in the biomass absorption coefficient. In addition, the Mg2+ limitation caused a decrease in the total carbohydrate content and triggered lipid accumulation, achieving levels of up to 53% DCW, whereas the biomass productivity remained similar for all tested conditions. The lipidome analysis revealed that the lowest Mg2+ concentrations also caused a differential lipid profile distribution, with an enrichment of neutral lipids and an increase of structural lipids. In that sense, we showed that Mg2+ limitation represents an alternative optimization approach that not only enhances accumulation of neutral lipids in B. braunii cells but also may potentially lead to a better areal biomass productivity due to the reduction in the cellular light absorption properties of the cells.

Knockdown of the plastid-encoded acetyl-CoA carboxylase gene uncovers functions in metabolism and development

De novo fatty acid biosynthesis in plants relies on a prokaryotic-type acetyl-CoA carboxylase (ACCase) that resides in the plastid compartment. The enzyme is composed of four subunits, one of which is encoded in the plastid genome, whereas the other three subunits are encoded by nuclear genes. The plastid gene (accD) encodes the β-carboxyltransferase subunit of ACCase and is essential for cell viability. To facilitate the functional analysis of accD, we pursued a transplastomic knockdown strategy in tobacco (Nicotiana tabacum). By introducing point mutations into the translational start codon of accD, we obtained stable transplastomic lines with altered ACCase activity. Replacement of the standard initiator codon AUG with UUG strongly reduced AccD expression, whereas replacement with GUG had no detectable effects. AccD knockdown mutants displayed reduced ACCase activity, which resulted in changes in the levels of many but not all species of cellular lipids. Limiting fatty acid availability caused a wide range of macroscopic, microscopic, and biochemical phenotypes, including impaired chloroplast division, reduced seed set, and altered storage metabolism. Finally, while the mutants displayed reduced growth under photoautotrophic conditions, they showed exaggerated growth under heterotrophic conditions, thus uncovering an unexpected antagonistic role of AccD activity in autotrophic and heterotrophic growth.

Abundant synthesis of long-chain polyunsaturated fatty acids in Eutreptiella sp. (Euglenozoa) revealed by chromatographic and transcriptomic analyses

Algal lipids are important molecules to store energy in algae and transfer energy in the marine food chain, and are potential materials for high value nutraceuticals (e.g., omega-3 fatty acids) or biofuel production. However, how lipid biosynthesis is regulated is not well understood in many species including Eutreptiella from the phylum of Euglenozoa. Here, we characterized the fatty acid (FA) profile of an Eutreptiella species isolated from Long Island Sound, USA, using gas chromatography-tandem mass spectrometry (GC/MS/MS) and investigated their biosynthesis pathways by transcriptome sequencing. We discovered 24 types of FAs including a relatively high proportion of long-chain unsaturated FAs. The abundances of C16, C18, and saturated FAs decreased when phosphate in the culture medium was depleted. Among the 24 FAs, docosahexaenoic acid (C22:6^{∆4,7,10,13,16,19} ) was most abundant, suggesting that Eutreptiella sp. preferentially invests in the synthesis of long-chain polyunsaturated fatty acids (LC-PFAs). Further transcriptomic analysis revealed that Eutreptiella sp. likely synthesizes LC-PFAs via ∆8 pathway and uses type I and II fatty acid synthases. Using RT-qPCR, we found that some of the lipid synthesis genes, such as β-ketoacyl-ACP reductase, fatty acid desaturase, acetyl-CoA carboxylase, acyl carrier protein, ∆8 desaturase, and Acyl-ACP thioesterase, were more actively expressed during light period, and two carbon fixation genes were up-regulated in the high-lipid illuminated cultures, suggesting a linkage between photosynthesis and lipid production. The lipid profile renders Eutreptiella sp. a nutritional prey and valuable source for nutraceuticals, and the biosynthesis pathway documented here will be useful for future research and applications.

Docking of acetyl-CoA carboxylase to the plastid envelope membrane attenuates fatty acid production in plants

In plants, light-dependent activation of de novo fatty acid synthesis (FAS) is partially mediated by acetyl-CoA carboxylase (ACCase), the first committed step for this pathway. However, it is not fully understood how plants control light-dependent FAS regulation to meet the cellular demand for acyl chains. We report here the identification of a gene family encoding for three small plastidial proteins of the envelope membrane that interact with the α-carboxyltransferase (α-CT) subunit of ACCase and participate in an original mechanism restraining FAS in the light. Light enhances the interaction between carboxyltransferase interactors (CTIs) and α-CT, which in turn attenuates carbon flux into FAS. Knockouts for CTI exhibit higher rates of FAS and marked increase in absolute triacylglycerol levels in leaves, more than 4-fold higher than in wild-type plants. Furthermore, WRINKLED1, a master transcriptional regulator of FAS, positively regulates CTI1 expression by direct binding to its promoter. This study reveals that in addition to light-dependent activation, “envelope docking” of ACCase permits fine-tuning of fatty acid supply during the plant life cycle.

In plants, light-dependent activation fatty acid synthesis (FAS) is mediated in part by acetyl-CoA carboxylase (ACCase). Here the authors identify a family of genes encoding carboxyltransferase interactors that attenuate FAS in the light by docking acetyl-CoA carboxylase to the plastid envelope.

The BADC and BCCP subunits of chloroplast acetyl-CoA carboxylase sense the pH changes of the light-dark cycle

Acetyl-CoA carboxylase (ACCase) catalyzes the first committed step in the de novo synthesis of fatty acids. The multisubunit ACCase in the chloroplast is activated by a shift to pH 8 upon light adaptation and is inhibited by a shift to pH 7 upon dark adaptation. Here, titrations with the purified ACCase biotin attachment domain-containing (BADC) and biotin carboxyl carrier protein (BCCP) subunits from Arabidopsis indicated that they can competently and independently bind biotin carboxylase (BC) but differ in responses to pH changes representing those in the plastid stroma during light or dark conditions. At pH 7 in phosphate buffer, BADC1 and BADC2 gain an advantage over BCCP1 and BCCP2 in affinity for BC. At pH 8 in KCl solution, however, BCCP1 and BCCP2 had more than 10-fold higher affinity for BC than did BADC1. The pH-modulated shifts in BC preferences for BCCP and BADC partners suggest they contribute to light-dependent regulation of heteromeric ACCase. Using NMR spectroscopy, we found evidence for increased intrinsic disorder of the BADC and BCCP subunits at pH 7. We propose that this intrinsic disorder potentially promotes fast association with BC through a "fly-casting mechanism." We hypothesize that the pH effects on the BADC and BCCP subunits attenuate ACCase activity by night and enhance it by day. Consistent with this hypothesis, Arabidopsis badc1 badc3 mutant lines grown in a light-dark cycle synthesized more fatty acids in their seeds. In summary, our findings provide evidence that the BADC and BCCP subunits function as pH sensors required for light-dependent switching of heteromeric ACCase activity.

Towards model-driven characterization and manipulation of plant lipid metabolism

Plant lipids have versatile applications and provide essential fatty acids in human diet. Therefore, there has been a growing interest to better characterize the genetic basis, regulatory networks, and metabolic pathways that shape lipid quantity and composition. Addressing these issues is challenging due to context-specificity of lipid metabolism integrating environmental, developmental, and tissue-specific cues. Here we systematically review the known metabolic pathways and regulatory interactions that modulate the levels of storage lipids in oilseeds. We argue that the current understanding of lipid metabolism provides the basis for its study in the context of genome-wide plant metabolic networks with the help of approaches from constraint-based modeling and metabolic flux analysis. The focus is on providing a comprehensive summary of the state-of-the-art of modeling plant lipid metabolic pathways, which we then contrast with the existing modeling efforts in yeast and microalgae. We then point out the gaps in knowledge of lipid metabolism, and enumerate the recent advances of using genome-wide association and quantitative trait loci mapping studies to unravel the genetic regulations of lipid metabolism. Finally, we offer a perspective on how advances in the constraint-based modeling framework can propel further characterization of plant lipid metabolism and its rational manipulation.

Plant Chloroplast Stress Response: Insights from Thiol Redox Proteomics

Significance: Plant chloroplasts generate reactive oxygen species (ROS) during photosynthesis, especially under stresses. The sulfhydryl groups of protein cysteine residues are susceptible to redox modifications, which regulate protein structure and function, and thus different signaling and metabolic processes. The ROS-governed protein thiol redox switches play important roles in chloroplasts. Recent Advances: Various high-throughput thiol redox proteomic approaches have been developed, and they have enabled the improved understanding of redox regulatory mechanisms in chloroplasts. For example, the thioredoxin-modulated antioxidant enzymes help to maintain cellular ROS homeostasis. The light- and dark-dependent redox regulation of photosynthetic electron transport, the Calvin/Benson cycle, and starch biosynthesis ensures metabolic coordination and efficient energy utilization. In addition, redox cascades link the light with the dynamic changes of metabolites in nitrate and sulfur assimilation, shikimate pathway, and biosynthesis of fatty acid hormone as well as purine, pyrimidine, and thiamine. Importantly, redox regulation of tetrapyrrole and chlorophyll biosynthesis is critical to balance the photodynamic tetrapyrrole intermediates and prevent oxidative damage. Moreover, redox regulation of diverse elongation factors, chaperones, and kinases plays an important role in the modulation of gene expression, protein conformation, and posttranslational modification that contribute to photosystem II (PSII) repair, state transition, and signaling in chloroplasts. Critical Issues: This review focuses on recent advances in plant thiol redox proteomics and redox protein networks toward understanding plant chloroplast signaling, metabolism, and stress responses. Future Directions: Using redox proteomics integrated with biochemical and molecular genetic approaches, detailed studies of cysteine residues, their redox states, cross talk with other modifications, and the functional implications will yield a holistic understanding of chloroplast stress responses.

Non-Catalytic Subunits Facilitate Quaternary Organization of Plastidic Acetyl-CoA Carboxylase

Arabidopsis (Arabidopsis thaliana), like most dicotyledonous plants, expresses a multicomponent, heteromeric acetyl-CoA carboxylase (htACCase), which catalyzes the generation of the malonyl-CoA precursor of de novo fatty acid biosynthesis. This enzyme consists of four catalytic subunits: biotin carboxylase (BC), carboxyltransferase (CT)-α, CT-β, and biotin carboxyl carrier protein (BCCP1 or BCCP2). By coexpressing combinations of components in a bacterial expression system, we demonstrate noncatalytic BADCs facilitate the assembly and activation of BCCP-BADC-BC subcomplexes catalyzing the bicarbonate-dependent hydrolysis of ATP, which is the first half-reaction catalyzed by the htACCase enzyme. Although BADC proteins do not directly impact formation of the CT-αβ subcomplex, the BADC-facilitated BCCP-BADC-BC subcomplex can more readily interact with the CT-αβ subcomplex to facilitate the generation of malonyl-CoA. The Arabidopsis genome encodes three BADC isoforms (BADC1, BADC2, and BADC3), and BADC2 and BADC3 (rather than BADC1), in combination with BCCP1, best support this quaternary-structural organization and catalytic activation of the htACCase enzyme. Physiological genetic studies validate these attributes as Arabidopsis double mutants singularly expressing BADC2, BADC3, or BADC1 present increasingly greater deleterious impacts on morphological and biochemical phenotypes. Specifically, plants expressing only BADC2 develop normally, plants only expressing BADC3 suffer a stunted root-growth phenotype, and plants expressing only BADC1 are embryo-lethal. The latter phenotype may also be associated with the distinct suborganelle localization of BADC1 in plastids as compared to the localization of the other two BADC homologs. These finding can inspire novel strategies to improve the biological sources of fats and oils for dietary and industrial applications.

PUB11-Dependent Ubiquitination of the Phospholipid Flippase ALA10 Modifies ALA10 Localization and Affects the Pool of Linolenic Phosphatidylcholine

Biogenesis of photosynthetic membranes depends on galactolipid synthesis, which relies on several cell compartments, notably the endoplasmic reticulum (ER) and the chloroplast envelope. Galactolipid synthesis involves lipid trafficking between both membrane compartments. In Arabidopsis, ALA10, a phospholipid flippase of the P₄ type-ATPase family, counteracts the limitation of monogalactosyldiacylglycerol (MGDG) production and has a positive effect on leaf development. ALA10 locates in distinct domains of the ER depending on the ALIS (ALA interacting subunit) subunit it interacts with: close to the plasma membrane with ALIS1, or next to chloroplasts with ALIS5. It interacts with FAD2 (Fatty acid desaturase 2) and prevents accumulation of linolenic (18:3) containing phosphatidylcholine (PC) stimulating an increase of MGDG synthesis. Here we report that ALA10 interacts with PUB11 (plant U-box type 11), an E3 protein ubiquitin ligase, in vitro and in vivo. ALA10 is however ubiquitinated and degraded by the 26S proteasome in a PUB11-independent process. In pub11 null mutant, the proteasome-dependent degradation of ALA10 is retained and ALA10 is still subject to ubiquitination although its ubiquitination profile appears different. In the absence of PUB11, ALA10 is constrained to the ER close to chloroplasts, which is the usual location when ALA10 is overexpressed. Additionally, in this condition, the decrease of 18:3 containing PC is no longer observed. Taken together these results suggest, that ALA10 contributes in chloroplast-distal ER interacting domains, to reduce the 18:3 desaturation of PC and that PUB11 is involved in reconditioning of ALA10 from chloroplast-proximal to chloroplast-distal ER interacting domains.

Plant Unsaturated Fatty Acids: Biosynthesis and Regulation

In most plants, major unsaturated fatty acids (UFAs) are three C18 species, namely, oleic (18:1), linoleic (18:2), and α-linolenic (18:3) acids. These simple compounds play multiple crucial roles in planta and are also important economic traits of oil crops. The enzymatic steps of C18 UFA biosynthesis have been well established. However, the associated FA/lipid trafficking between the plastid and the endoplasmic reticulum remains largely unclear, as does the regulation of the expression and activities of the involved enzymes. In this review, we will revisit the biosynthesis of C18 UFAs with an emphasis on the trafficking, and present an overview of the key enzymes and their regulation. Of particular interest is the emerging regulatory network composed of transcriptional factors and upstream signaling pathways. The review thereby provides the promise of using physical, biochemical and/or genetic means to manipulate FA composition and increase oil yield in crop improvement.

Insights into the function of NADPH thioredoxin reductase C (NTRC) based on identification of NTRC-interacting proteins in vivo

Redox regulation in heterotrophic organisms relies on NADPH, thioredoxins (TRXs), and an NADPH-dependent TRX reductase (NTR). In contrast, chloroplasts harbor two redox systems, one that uses photoreduced ferredoxin (Fd), an Fd-dependent TRX reductase (FTR), and TRXs, which links redox regulation to light, and NTRC, which allows the use of NADPH for redox regulation. It has been shown that NTRC-dependent regulation of 2-Cys peroxiredoxin (PRX) is critical for optimal function of the photosynthetic apparatus. Thus, the objective of the present study was the analysis of the interaction of NTRC and 2-Cys PRX in vivo and the identification of proteins interacting with them with the aim of identifying chloroplast processes regulated by this redox system. To assess this objective, we generated Arabidopsis thaliana plants expressing either an NTRC-tandem affinity purification (TAP)-Tag or a green fluorescent protein (GFP)-TAP-Tag, which served as a negative control. The presence of 2-Cys PRX and NTRC in complexes isolated from NTRC-TAP-Tag-expressing plants confirmed the interaction of these proteins in vivo. The identification of proteins co-purified in these complexes by MS revealed the relevance of the NTRC-2-Cys PRX system in the redox regulation of multiple chloroplast processes. The interaction of NTRC with selected targets was confirmed in vivo by bimolecular fluorescence complementation (BiFC) assays.

Identification and Validation of Candidate Genes Involved in Fatty Acid Content in Oil Palm by Genome-Wide Association Analysis

Oil palm (Elaeis guineensis) is the highest yielding oil crop per unit area worldwide, but its oil is considered unhealthy for human consumption due to its high palmitic acid content (C16:0). In order to facilitate breeding for fatty acid content in oil palm, genome-wide association analysis (GWAS) was used to identify and validate single-nucleotide polymorphism (SNP) markers and underlying candidate genes associated with fatty acid content in a diversity panel of 200 oil palm individuals. A total of 1,261,501 SNP markers previously developed using SLAF-seq (specific locus amplified fragment sequencing) were used for GWAS. Based on this analysis, 62 SNP markers were significantly associated with fatty acid composition, and 223 candidate genes were identified in the flanking regions of these SNPs. We found one gene (acyl-ACP thioesterase B genes) that was involved in fatty acid biosynthesis and that was associated with high palmitic acid content in the mesocarp. Over-expression of this gene caused a significant increase in palmitic acid content. Our study provides key loci that can be used for breeding oil palm cultivars with low palmitic acid content.

Cellular Organization and Regulation of Plant Glycerolipid Metabolism

Great strides have been made in understanding how membranes and lipid droplets are formed and maintained in land plants, yet much more is to be learned given the complexity of plant lipid metabolism. A complicating factor is the multi-organellar presence of biosynthetic enzymes and unique compositional requirements of different membrane systems. This necessitates a rich network of transporters and transport mechanisms that supply fatty acids, membrane lipids and storage lipids to their final cellular destination. Though we know a large number of the biosynthetic enzymes involved in lipid biosynthesis and a few transport proteins, the regulatory mechanisms, in particular, coordinating expression and/or activity of the majority remain yet to be described. Plants undergoing stress alter their membranes' compositions, and lipids such as phosphatidic acid have been implicated in stress signaling. Additionally, lipid metabolism in chloroplasts supplies precursors for jasmonic acid (JA) biosynthesis, and perturbations in lipid homeostasis has consequences on JA signaling. In this review, several aspects of plant lipid metabolism are discussed that are currently under investigation: cellular transport of lipids, regulation of lipid biosynthesis, roles of lipids in stress signaling, and lastly the structural and oligomeric states of lipid enzymes.

Highly Resolved Systems Biology to Dissect the Etioplast-to-Chloroplast Transition in Tobacco Leaves

Upon exposure to light, plant cells quickly acquire photosynthetic competence by converting pale etioplasts into green chloroplasts. This developmental transition involves the de novo biogenesis of the thylakoid system and requires reprogramming of metabolism and gene expression. Etioplast-to-chloroplast differentiation involves massive changes in plastid ultrastructure, but how these changes are connected to specific changes in physiology, metabolism, and expression of the plastid and nuclear genomes is poorly understood. Here, we describe a new experimental system in the dicotyledonous model plant tobacco (Nicotiana tabacum) that allows us to study the leaf deetiolation process at the systems level. We have determined the accumulation kinetics of photosynthetic complexes, pigments, lipids, and soluble metabolites and recorded the dynamic changes in plastid ultrastructure and in the nuclear and plastid transcriptomes. Our data describe the greening process at high temporal resolution, resolve distinct genetic and metabolic phases during deetiolation, and reveal numerous candidate genes that may be involved in light-induced chloroplast development and thylakoid biogenesis.

Interaction and Regulation Between Lipid Mediator Phosphatidic Acid and Circadian Clock Regulators

Circadian clocks play important roles in regulating cellular metabolism, but the reciprocal effect that metabolism has on the clock is largely unknown in plants. Here, we show that the central glycerolipid metabolite and lipid mediator phosphatidic acid (PA) interacts with and modulates the function of the core clock regulators LATE ELONGATED HYPOCOTYL (LHY) and CIRCADIAN CLOCK ASSOCIATED1 (CCA1) in Arabidopsis (Arabidopsis thaliana). PA reduced the ability of LHY and CCA1 to bind the promoter of their target gene TIMING OF CAB EXPRESSION1 Increased PA accumulation and inhibition of PA-producing enzymes had opposite effects on circadian clock outputs. Diurnal change in levels of several membrane phospholipid species, including PA, observed in wild type was lost in the LHY and CCA1 double knockout mutant. Storage lipid accumulation was also affected in the clock mutants. These results indicate that the interaction of PA with the clock regulator may function as a cellular conduit to integrate the circadian clock with lipid metabolism.

Salidroside and Curcumin Formula Prevents Liver Injury in Nonalcoholic Fatty Liver Disease in Rats

INTRODUCTION AND AIM

Salidroside and curcumin (SC) formula could alleviate lipid deposition in high fat diet-induced nonalcoholic fatty liver disease (NAFLD). However, the mechanisms are still unknown, and the magnitude of potential therapeutic benefit remains understudied.

MATERIAL AND METHODS

The rats were treated with high fat diet for 14 weeks to induce NAFLD. The experiment was divided into control, model (NAFLD), SC formula and rosiglitazone groups (n = 7 in each group). Hematoxylin-eosin (H&E) staining was applied to detect liver morphological changes. Biochemical, metabolic indices and inflammation factors in liver tissue and serum were detected. Additionally, the activities of related enzymes were detected by enzyme-linked immunosorbent assay.

RESULTS

In the established rat model, typical lipid deposition and liver steatosis were observed. Liver triglyceride, free fatty acids, sera alanine aminotransferase, aspartate aminotransferase, gamma-glutamyl transferase, fasting insulin, fasting blood glucose and homeostasis model assessment of insulin resistance were elevated in model group. Liver malondialdehyde was significantly elevated, while superoxide dismutase was significantly decreased in model group, compared with control. Moreover, tumor necrosis factor-α and Interleukin-1 were significantly produced in model group, compared with control. As a mechanism, high fat diet decreased tissue AMP-activated protein kinase (AMPK), phosphorylated AMPK, carnitine palmitoyltransferase 1 and increased inacetyl-CoA carboxylase (ACCase), phosphorylated ACCase. Importantly, these abnormal changes caused by high fat diet were reduced by SC formula administration.

CONCLUSION

SC formula could ameliorate the injury caused by high fat diet. The effect was likely mediated via its influence on insulin resistance, lipid peroxidation injury and AMPK signaling pathway.

The effect of light conditions on interpreting oil composition engineering in Arabidopsis seeds

Arabidopsis thaliana is the most developed and utilized model plant. In particular, it is an excellent model for proof-of-concept seed oil engineering studies because it accumulates approximately 37% seed oil by weight, and it is closely related to important Brassicaceae oilseed crops. Arabidopsis can be grown under a wide variety of conditions including continuous light; however, the amount of light is strongly correlated with total seed oil accumulation. In addition, many attempts to engineer novel seed oil fatty acid compositions in Arabidopsis have reported significant reductions in oil accumulation; however, the relative reduction from the nontransgenic controls varies greatly within the literature. A set of experiments were conducted to systematically analyze the effect of light conditions (including day/night cycle vs. continuous light, and different light intensities) on the relative accumulation of seed oil between three different transgenic lines producing novel hydroxy fatty acids and their nontransgenic background. Oil content was measured per seed and as a percentage of seed weight. Our results indicate the relative amount of seed oil between transgenic lines and nontransgenic controls is dependent on both the light conditions and the type of oil content measurement utilized. In addition, the light conditions effect the relative accumulation of the novel fatty acids between various transgenic lines. Therefore, the success of novel fatty acid proof-of-concept engineering strategies on both oil accumulation and fatty acid composition in Arabidopsis seeds should be considered in light of the select growth and measurement conditions prior to moving engineering strategies into crop plants.

M-type thioredoxins are involved in the xanthophyll cycle and proton motive force to alter NPQ under low-light conditions in Arabidopsis

M-type thioredoxins are required to regulate zeaxanthin epoxidase activity and to maintain the steady-state level of the proton motive force, thereby influencing NPQ properties under low-light conditions in Arabidopsis. Non-photochemical quenching (NPQ) helps protect photosynthetic organisms from photooxidative damage via the non-radiative dissipation of energy as heat. Energy-dependent quenching (qE) is a major constituent of NPQ. However, the mechanism underlying the regulation of qE is not well understood. In this study, we demonstrate that the m-type thioredoxins TRX-m1, TRX-m2, and TRX-m4 (TRX-ms) interact with the xanthophyll cycle enzyme zeaxanthin epoxidase (ZE) and are required for maintaining the redox-dependent stabilization of ZE by regulating its intermolecular disulfide bridges. Reduced ZE activity and accumulated zeaxanthin levels were observed under TRX-ms deficiency. Furthermore, concurrent deficiency of TRX-ms resulted in a significant increase in proton motive force (pmf) and acidification of the thylakoid lumen under low irradiance, perhaps due to the significantly reduced ATP synthase activity under TRX-ms deficiency. The increased pmf, combined with acidification of the thylakoid lumen and the accumulation of zeaxanthin, ultimately contribute to the elevated stable qE in VIGS-TRX-m2m4/m1 plants under low-light conditions. Taken together, these results indicate that TRX-ms are involved in regulating NPQ-dependent photoprotection in Arabidopsis.

Thioredoxin and NADPH-Dependent Thioredoxin Reductase C Regulation of Tetrapyrrole Biosynthesis

In chloroplasts, thioredoxin (TRX) isoforms and NADPH-dependent thioredoxin reductase C (NTRC) act as redox regulatory factors involved in multiple plastid biogenesis and metabolic processes. To date, less is known about the functional coordination between TRXs and NTRC in chlorophyll biosynthesis. In this study, we aimed to explore the potential functions of TRX m and NTRC in the regulation of the tetrapyrrole biosynthesis (TBS) pathway. Silencing of three genes, TRX m1, TRX m2, and TRX m4 (TRX ms), led to pale-green leaves, a significantly reduced 5-aminolevulinic acid (ALA)-synthesizing capacity, and reduced accumulation of chlorophyll and its metabolic intermediates in Arabidopsis (Arabidopsis thaliana). The contents of ALA dehydratase, protoporphyrinogen IX oxidase, the I subunit of Mg-chelatase, Mg-protoporphyrin IX methyltransferase (CHLM), and NADPH-protochlorophyllide oxidoreductase were decreased in triple TRX m-silenced seedlings compared with the wild type, although the transcript levels of the corresponding genes were not altered significantly. Protein-protein interaction analyses revealed a physical interaction between the TRX m isoforms and CHLM. 4-Acetoamido-4-maleimidylstilbene-2,2-disulfonate labeling showed the regulatory impact of TRX ms on the CHLM redox status. Since CHLM also is regulated by NTRC (Richter et al., 2013), we assessed the concurrent functions of TRX m and NTRC in the control of CHLM. Combined deficiencies of three TRX m isoforms and NTRC led to a cumulative decrease in leaf pigmentation, TBS intermediate contents, ALA synthesis rate, and CHLM activity. We discuss the coordinated roles of TRX m and NTRC in the redox control of CHLM stability with its corollary activity in the TBS pathway.

The Deep Thioredoxome in Chlamydomonas reinhardtii: New Insights into Redox Regulation

Thiol-based redox post-translational modifications have emerged as important mechanisms of signaling and regulation in all organisms, and thioredoxin plays a key role by controlling the thiol-disulfide status of target proteins. Recent redox proteomic studies revealed hundreds of proteins regulated by glutathionylation and nitrosylation in the unicellular green alga Chlamydomonas reinhardtii, while much less is known about the thioredoxin interactome in this organism. By combining qualitative and quantitative proteomic analyses, we have comprehensively investigated the Chlamydomonas thioredoxome and 1188 targets have been identified. They participate in a wide range of metabolic pathways and cellular processes. This study broadens not only the redox regulation to new enzymes involved in well-known thioredoxin-regulated metabolic pathways but also sheds light on cellular processes for which data supporting redox regulation are scarce (aromatic amino acid biosynthesis, nuclear transport, etc). Moreover, we characterized 1052 thioredoxin-dependent regulatory sites and showed that these data constitute a valuable resource for future functional studies in Chlamydomonas. By comparing this thioredoxome with proteomic data for glutathionylation and nitrosylation at the protein and cysteine levels, this work confirms the existence of a complex redox regulation network in Chlamydomonas and provides evidence of a tremendous selectivity of redox post-translational modifications for specific cysteine residues.

Variation in Leaf Respiration Rates at Night Correlates with Carbohydrate and Amino Acid Supply

Plant respiration can theoretically be fueled by and dependent upon an array of central metabolism components; however, which ones are responsible for the quantitative variation found in respiratory rates is unknown. Here, large-scale screens revealed 2-fold variation in nighttime leaf respiration rate (R_N) among mature leaves from an Arabidopsis (Arabidopsis thaliana) natural accession collection grown under common favorable conditions. R_N variation was mostly maintained in the absence of genetic variation, which emphasized the low heritability of R_N and its plasticity toward relatively small environmental differences within the sampling regime. To pursue metabolic explanations for leaf R_N variation, parallel metabolite level profiling and assays of total protein and starch were performed. Within an accession, R_N correlated strongly with stored carbon substrates, including starch and dicarboxylic acids, as well as sucrose, major amino acids, shikimate, and salicylic acid. Among different accessions, metabolite-R_N correlations were maintained with protein, sucrose, and major amino acids but not stored carbon substrates. A complementary screen of the effect of exogenous metabolites and effectors on leaf R_N revealed that (1) R_N is stimulated by the uncoupler FCCP and high levels of substrates, demonstrating that both adenylate turnover and substrate supply can limit leaf R_N, and (2) inorganic nitrogen did not stimulate R_N, consistent with limited nighttime nitrogen assimilation. Simultaneous measurements of R_N and protein synthesis revealed that these processes were largely uncorrelated in mature leaves. These results indicate that differences in preceding daytime metabolic activities are the major source of variation in mature leaf R_N under favorable controlled conditions.

Maternal/fetal metabolomes appear to mediate the impact of arsenic exposure on birth weight: A pilot study

Arsenic exposure has been associated with low birth weight. However, the underlying mechanisms are not well understood. Alterations to metabolites may act as causal mediators of the effect of arsenic exposure on low birth weight. This pilot study aimed to explore the role of metabolites in mediating the association of arsenic exposure on infant birth weight. Study samples were selected from a well-established prospectively enrolled cohort in Bangladesh comprising 35 newborns and a subset of 20 matched mothers. Metabolomics profiling was performed on 35 cord blood samples and 20 maternal peripheral blood samples collected during the second trimester of pregnancy. Inorganic arsenic (iAs) exposure was evaluated via cord blood samples and maternal toenail samples collected during the first trimester. Multiple linear regression and mediation analyses were used to explore the relationship between iAs exposure, metabolite alterations, and low birth weight. Cord blood arsenic level was correlated with elevated levels of 17-methylstearate, laurate (12:0) and 4-vinylphenol sulfate along with lower birth weight. Prenatal maternal toenail iAs level was associated with two peripheral blood metabolites (butyrylqlycine and tartarate), which likely contributed to higher cord blood iAs levels both independently and interactively. Findings of this pilot study indicate that both intrauterine and maternal peripheral blood metabolites appear to influence the toxic effect of inorganic arsenic exposure on low birth weight.

Engineering oxidative stress defense pathways to build a robust lipid production platform in Yarrowia lipolytica

Microbially derived lipids have recently attracted renewed interests due to their broad applications in production of green diesels, cosmetic additives, and oleochemicals. Metabolic engineering efforts have targeted a large portfolio of biosynthetic pathways to efficiently convert sugar to lipids in oleaginous yeast. In the engineered overproducing strains, endogenous cell metabolism typically generates harmful electrophilic molecules that compromise cell fitness and productivity. Lipids, particularly unsaturated fatty acids, are highly susceptible to oxygen radical attack and the resulting oxidative species are detrimental to cell metabolism and limit lipid productivity. In this study, we investigated cellular oxidative stress defense pathways in Yarrowia lipolytica to further improve the lipid titer, yield, and productivity. Specifically, we determined that coupling glutathione disulfide reductase and glucose-6-phosphate dehydrogenase along with aldehyde dehydrogenase are efficient solutions to combat reactive oxygen and aldehyde stress in Y. lipolytica. With the reported engineering strategies, we were able to synchronize cell growth and lipid production, improve cell fitness and morphology, and achieved industrially-relevant level of lipid titer (72.7 g/L), oil content (81.4%) and productivity (0.97 g/L/h) in controlled bench-top bioreactors. The strategies reported here represent viable steps in the development of sustainable biorefinery platforms that potentially upgrade low value carbons to high value oleochemicals and biofuels. Biotechnol. Bioeng. 2017;114: 1521-1530. © 2017 Wiley Periodicals, Inc.

The Unprecedented Versatility of the Plant‎ Thioredoxin System

Thioredoxins are ubiquitous enzymes catalyzing reversible disulfide-bond formation to regulate structure and function of many proteins in diverse organisms. In recent years, reverse genetics and biochemical approaches were used to resolve the functions, specificities, and interactions of the different thioredoxin isoforms and reduction systems in planta and revealed the most versatile thioredoxin system of all organisms. Here we review the emerging roles of the thioredoxin system, namely the integration of thylakoid energy transduction, metabolism, gene expression, growth, and development under fluctuating environmental conditions. We argue that these new developments help us to understand why plants organize such a divergent composition of thiol redox networks and provide insights into the regulatory hierarchy that operates between them.

A Family of Negative Regulators Targets the Committed Step of de Novo Fatty Acid Biosynthesis

Acetyl-CoA carboxylase (ACCase) catalyzes the committed step of de novo fatty acid biosynthesis. In prokaryotes, green algae, and most plants, this enzyme is a heteromeric complex requiring four different subunits for activity. The plant complex is recalcitrant to conventional purification schemes and hence the structure and composition of the full assembly have been unclear. In vivo coimmunoprecipitation using subunit-specific antibodies identified a novel family of proteins in Arabidopsis thaliana annotated as biotin/lipoyl attachment domain containing (BADC) proteins. Results from yeast two-hybrid and coexpression in Escherichia coli confirmed that all three BADC isoforms interact with the two biotin carboxyl carrier protein (BCCP) isoforms of Arabidopsis ACCase. These proteins resemble BCCP subunits but are not biotinylated due to a mutated biotinylation motif. We demonstrate that BADC proteins significantly inhibit ACCase activity in both E. coli and Arabidopsis. Targeted gene silencing of BADC isoform 1 in Arabidopsis significantly increased seed oil content when normalized to either mass or individual seed. We conclude the BADC proteins are ancestral BCCPs that gained a new function as negative regulators of ACCase after initial loss of the biotinylation motif. A functional model is proposed.

Role of membrane glycerolipids in photosynthesis, thylakoid biogenesis and chloroplast development

The lipid bilayer of the thylakoid membrane in plant chloroplasts and cyanobacterial cells is predominantly composed of four unique lipid classes; monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG). MGDG and DGDG are uncharged galactolipids that constitute the bulk of thylakoid membrane lipids and provide a lipid bilayer matrix for photosynthetic complexes as the main constituents. The glycolipid SQDG and phospholipid PG are anionic lipids with a negative charge on their head groups. SQDG and PG substitute for each other to maintain the amount of total anionic lipids in the thylakoid membrane, with PG having indispensable functions in photosynthesis. In addition to biochemical studies, extensive analyses of mutants deficient in thylakoid lipids have revealed important roles of these lipids in photosynthesis and thylakoid membrane biogenesis. Moreover, recent studies of Arabidopsis thaliana suggest that thylakoid lipid biosynthesis triggers the expression of photosynthesis-associated genes in both the nucleus and plastids and activates the formation of photosynthetic machineries and chloroplast development. Meanwhile, galactolipid biosynthesis is regulated in response to chloroplast functionality and lipid metabolism at transcriptional and post-translational levels. This review summarizes the roles of thylakoid lipids with their biosynthetic pathways in plants and discusses the coordinated regulation of thylakoid lipid biosynthesis with the development of photosynthetic machinery during chloroplast biogenesis.

Triacylglycerol Metabolism, Function, and Accumulation in Plant Vegetative Tissues

Oils in the form of triacylglycerols are the most abundant energy-dense storage compounds in eukaryotes, and their metabolism plays a key role in cellular energy balance, lipid homeostasis, growth, and maintenance. Plants accumulate oils primarily in seeds and fruits. Plant oils are used for food and feed and, increasingly, as feedstocks for biodiesel and industrial chemicals. Although plant vegetative tissues do not accumulate significant levels of triacylglycerols, they possess a high capacity for their synthesis, storage, and metabolism. The development of plants that accumulate oil in vegetative tissues presents an opportunity for expanded production of triacylglycerols as a renewable and sustainable bioenergy source. Here, we review recent progress in the understanding of triacylglycerol synthesis, turnover, storage, and function in leaves and discuss emerging genetic engineering strategies targeted at enhancing triacylglycerol accumulation in biomass crops. Such plants could potentially be modified to produce oleochemical feedstocks or nutraceuticals.

Regulation and structure of the heteromeric acetyl-CoA carboxylase

The enzyme acetyl-CoA carboxylase (ACCase) catalyzes the committed step of the de novo fatty acid biosynthesis (FAS) pathway by converting acetyl-CoA to malonyl-CoA. Two forms of ACCase exist in nature, a homomeric and heteromic form. The heteromeric form of this enzyme requires four different subunits for activity: biotin carboxylase; biotin carboxyl carrier protein; and α- and β-carboxyltransferases. Heteromeric ACCases (htACCase) can be found in prokaryotes and the plastids of most plants. The plant htACCase is regulated by diverse mechanisms reflected by the biochemical and genetic complexity of this multienzyme complex and the plastid stroma where it resides. In this review we summarize the regulation of the plant htACCase and also describe the structural characteristics of this complex from both prokaryotes and plants. This article is part of a Special Issue entitled: Plant Lipid Biology edited by Kent D. Chapman and Ivo Feussner.

The influences of cultivation setting on inflorescence lipid distributions, concentrations, and carbon isotope ratios of Cannabis sp

While much is known about how the growth environment influences many aspects of floral morphology and physiology, little is known about how the growth setting influences floral lipid composition. We explored variations in paraffin wax composition in Cannabis sp., a cash crop grown both indoors and outdoors across the United States today. Given an increased focus on regulation of this crop, there are additional incentives to certify the setting of Cannabis cultivation. To understand the impacts of the growth environment, we studied distributions, concentrations, and carbon isotope ratios of n-alkanes isolated from Cannabis sp. inflorescences to assess if variations within these lipid parameters were related to known growth settings of specimens seized by federal agents. We found that Cannabis plants cultivated under open-field settings had increased inflorescence paraffin wax abundances and greater production of lower molecular weight n-alkanes relative to plants grown in enclosed environments. Further, the carbon isotope ratios of n-C29 from Cannabis plants grown in enclosed environments had relatively lower carbon isotope (δ(13)C) values compared to plants from open-field environments. While this set of observations on seized plant specimens cannot address the particular driver behind these observations, we posit that (a) variations in irradiance and/or photoperiod may influence the distribution and concentration of inflorescence lipids, and (b) the δ(13)C value of source CO2 and lipid concentration regulates the δ(13)C values of inflorescence n-C29 and bulk Cannabis plant materials. Nonetheless, by using a cultivation model based on δ(13)C values of n-C29, the model correctly identified the growth environment 90% of time. We suggest that these lipid markers may be used to trace cultivation methods of Cannabis sp. now and become a more powerful marker in the future, once the mechanism(s) behind these patterns is uncovered.

Time-Series Analyses of Transcriptomes and Proteomes Reveal Molecular Networks Underlying Oil Accumulation in Canola

Understanding the regulation of lipid metabolism is vital for genetic engineering of canola (Brassica napus L.) to increase oil yield or modify oil composition. We conducted time-series analyses of transcriptomes and proteomes to uncover the molecular networks associated with oil accumulation and dynamic changes in these networks in canola. The expression levels of genes and proteins were measured at 2, 4, 6, and 8 weeks after pollination (WAP). Our results show that the biosynthesis of fatty acids is a dominant cellular process from 2 to 6 WAP, while the degradation mainly happens after 6 WAP. We found that genes in almost every node of fatty acid synthesis pathway were significantly up-regulated during oil accumulation. Moreover, significant expression changes of two genes, acetyl-CoA carboxylase and acyl-ACP desaturase, were detected on both transcriptomic and proteomic levels. We confirmed the temporal expression patterns revealed by the transcriptomic analyses using quantitative real-time PCR experiments. The gene set association analysis show that the biosynthesis of fatty acids and unsaturated fatty acids are the most significant biological processes from 2-4 WAP and 4-6 WAP, respectively, which is consistent with the results of time-series analyses. These results not only provide insight into the mechanisms underlying lipid metabolism, but also reveal novel candidate genes that are worth further investigation for their values in the genetic engineering of canola.

Acyl-CoA-Binding Proteins (ACBPs) in Plant Development

Acyl-CoA-binding proteins (ACBPs) play a pivotal role in fatty acid metabolism because they can transport medium- and long-chain acyl-CoA esters. In eukaryotic cells, ACBPs are involved in intracellular trafficking of acyl-CoA esters and formation of a cytosolic acyl-CoA pool. In addition to these ubiquitous functions, more specific non-redundant roles of plant ACBP subclasses are implicated by the existence of multigene families with variable molecular masses, ligand specificities, functional domains (e.g. protein-protein interaction domains), subcellular locations and gene expression patterns. In this chapter, recent progress in the characterization of ACBPs from the model dicot plant, Arabidopsis thaliana, and the model monocot, Oryza sativa, and their emerging roles in plant growth and development are discussed. The functional significance of respective members of the plant ACBP families in various developmental and physiological processes such as seed development and germination, stem cuticle formation, pollen development, leaf senescence, peroxisomal fatty acid β-oxidation and phloem-mediated lipid transport is highlighted.

The binding versatility of plant acyl-CoA-binding proteins and their significance in lipid metabolism

Acyl-CoA esters are the activated form of fatty acids and play important roles in lipid metabolism and the regulation of cell functions. They are bound and transported by nonenzymic proteins such as the acyl-CoA-binding proteins (ACBPs). Although plant ACBPs were so named by virtue of amino acid homology to existing yeast and mammalian counterparts, recent studies revealed that ligand specificities of plant ACBPs are not restricted to acyl-CoA esters. Arabidopsis and rice ACBPs also interact with phospholipids, and their affinities to different acyl-CoA species and phospholipid classes vary amongst isoforms. Their ligands also include heavy metals. Interactors of plant ACBPs are further diversified due to the evolution of protein-protein interacting domains. This review summarizes our current understanding of plant ACBPs with a focus on their binding versatility. Their broad ligand range is of paramount significance in serving a multitude of functions during development and stress responses as discussed herein. This article is part of a Special Issue entitled: Plant Lipid Biology edited by Kent D. Chapman and Ivo Feussner.

Plant Cytosolic Acyl-CoA-Binding Proteins

A gene family encoding six members of acyl-CoA-binding proteins (ACBP) exists in Arabidopsis and they are designated as AtACBP1-AtACBP6. They have been observed to play pivotal roles in plant lipid metabolism, consistent to the abilities of recombinant AtACBP in binding different medium- and long-chain acyl-CoA esters in vitro. While AtACBP1 and AtACBP2 are membrane-associated proteins with ankyrin repeats and AtACBP3 contains a signaling peptide for targeting to the apoplast, AtACBP4, AtACBP5 and AtACBP6 represent the cytosolic forms in the AtACBP family. They were verified to be subcellularly localized in the cytosol using diverse experimental methods, including cell fractionation followed by western blot analysis, immunoelectron microscopy and confocal laser-scanning microscopy using autofluorescence-tagged fusions. AtACBP4 (73.2 kDa) and AtACBP5 (70.1 kDa) are the largest, while AtACBP6 (10.4 kDa) is the smallest. Their binding affinities to oleoyl-CoA esters suggested that they can potentially transfer oleoyl-CoA esters from the plastids to the endoplasmic reticulum, facilitating the subsequent biosynthesis of non-plastidial membrane lipids in Arabidopsis. Recent studies on ACBP, extended from a dicot (Arabidopsis) to a monocot, revealed that six ACBP are also encoded in rice (Oryza sativa). Interestingly, three small rice ACBP (OsACBP1, OsACBP2 and OsACBP3) are present in the cytosol in comparison to one (AtACBP6) in Arabidopsis. In this review, the combinatory and distinct roles of the cytosolic AtACBP are discussed, including their functions in pollen and seed development, light-dependent regulation and substrate affinities to acyl-CoA esters.

Genomic Foundation of Starch-to-Lipid Switch in Oleaginous Chlorella spp

The ability to rapidly switch the intracellular energy storage form from starch to lipids is an advantageous trait for microalgae feedstock. To probe this mechanism, we sequenced the 56.8-Mbp genome of Chlorella pyrenoidosa FACHB-9, an industrial production strain for protein, starch, and lipids. The genome exhibits positive selection and gene family expansion in lipid and carbohydrate metabolism and genes related to cell cycle and stress response. Moreover, 10 lipid metabolism genes might be originated from bacteria via horizontal gene transfer. Transcriptomic dynamics tracked via messenger RNA sequencing over six time points during metabolic switch from starch-rich heterotrophy to lipid-rich photoautotrophy revealed that under heterotrophy, genes most strongly expressed were from the tricarboxylic acid cycle, respiratory chain, oxidative phosphorylation, gluconeogenesis, glyoxylate cycle, and amino acid metabolisms, whereas those most down-regulated were from fatty acid and oxidative pentose phosphate metabolism. The shift from heterotrophy into photoautotrophy highlights up-regulation of genes from carbon fixation, photosynthesis, fatty acid biosynthesis, the oxidative pentose phosphate pathway, and starch catabolism, which resulted in a marked redirection of metabolism, where the primary carbon source of glycine is no longer supplied to cell building blocks by the tricarboxylic acid cycle and gluconeogenesis, whereas carbon skeletons from photosynthesis and starch degradation may be directly channeled into fatty acid and protein biosynthesis. By establishing the first genetic transformation in industrial oleaginous C. pyrenoidosa, we further showed that overexpression of an NAD(H) kinase from Arabidopsis (Arabidopsis thaliana) increased cellular lipid content by 110.4%, yet without reducing growth rate. These findings provide a foundation for exploiting the metabolic switch in microalgae for improved photosynthetic production of food and fuels.

Thioredoxin f1 and NADPH-Dependent Thioredoxin Reductase C Have Overlapping Functions in Regulating Photosynthetic Metabolism and Plant Growth in Response to Varying Light Conditions

Two different thiol redox systems exist in plant chloroplasts, the ferredoxin-thioredoxin (Trx) system, which depends on ferredoxin reduced by the photosynthetic electron transport chain and, thus, on light, and the NADPH-dependent Trx reductase C (NTRC) system, which relies on NADPH and thus may be linked to sugar metabolism in the dark. Previous studies suggested, therefore, that the two different systems may have different functions in plants. We now report that there is a previously unrecognized functional redundancy of Trx f1 and NTRC in regulating photosynthetic metabolism and growth. In Arabidopsis (Arabidopsis thaliana) mutants, combined, but not single, deficiencies of Trx f1 and NTRC led to severe growth inhibition and perturbed light acclimation, accompanied by strong impairments of Calvin-Benson cycle activity and starch accumulation. Light activation of key enzymes of these pathways, fructose-1,6-bisphosphatase and ADP-glucose pyrophosphorylase, was almost completely abolished. The subsequent increase in NADPH-NADP(+) and ATP-ADP ratios led to increased nitrogen assimilation, NADP-malate dehydrogenase activation, and light vulnerability of photosystem I core proteins. In an additional approach, reporter studies show that Trx f1 and NTRC proteins are both colocalized in the same chloroplast substructure. Results provide genetic evidence that light- and NADPH-dependent thiol redox systems interact at the level of Trx f1 and NTRC to coordinately participate in the regulation of the Calvin-Benson cycle, starch metabolism, and growth in response to varying light conditions.

Application of preparative disk gel electrophoresis for antigen purification from inclusion bodies

Specific antibodies are a reliable tool to examine protein expression patterns and to determine the protein localizations within cells. Generally, recombinant proteins are used as antigens for specific antibody production. However, recombinant proteins from mammals and plants are often overexpressed as insoluble inclusion bodies in Escherichia coli. Solubilization of these inclusion bodies is desirable because soluble antigens are more suitable for injection into animals to be immunized. Furthermore, highly purified proteins are also required for specific antibody production. Plastidic acetyl-CoA carboxylase (ACCase: EC 6.4.1.2) from Arabidopsis thaliana, which catalyzes the formation of malonyl-CoA from acetyl-CoA in chloroplasts, formed inclusion bodies when the recombinant protein was overexpressed in E. coli. To obtain the purified protein to use as an antigen, we applied preparative disk gel electrophoresis for protein purification from inclusion bodies. This method is suitable for antigen preparation from inclusion bodies because the purified protein is recovered as a soluble fraction in electrode running buffer containing 0.1% sodium dodecyl sulfate that can be directly injected into immune animals, and it can be used for large-scale antigen preparation (several tens of milligrams).

Enhanced acetyl-CoA production is associated with increased triglyceride accumulation in the green alga Chlorella desiccata

Triglycerides (TAGs) from microalgae can be utilized as food supplements and for biodiesel production, but little is known about the regulation of their biosynthesis. This work aimed to test the relationship between acetyl-CoA (Ac-CoA) levels and TAG biosynthesis in green algae under nitrogen deprivation. A novel, highly sensitive liquid chromatography mass spectrometry (LC-MS/MS) technique enabled us to determine the levels of Ac-CoA, malonyl-CoA, and unacetylated (free) CoA in green microalgae. A comparative study of three algal species that differ in TAG accumulation levels shows that during N starvation, Ac-CoA levels rapidly rise, preceding TAG accumulation in all tested species. The levels of Ac-CoA in the high TAG accumulator Chlorella desiccata exceed the levels in the moderate TAG accumulators Dunaliella tertiolecta and Chlamydomonas reinhardtii. Similarly, malonyl-CoA and free CoA levels also increase, but to lower extents. Calculated cellular concentrations of Ac-CoA are far lower than reported K mAc-CoA values of plastidic Ac-CoA carboxylase (ptACCase) in plants. Transcript level analysis of plastidic pyruvate dehydrogenase (ptPDH), the major chloroplastic Ac-CoA producer, revealed rapid induction in parallel with Ac-CoA accumulation in C. desiccata, but not in D. tertiolecta or C. reinhardtii. It is proposed that the capacity to accumulate high TAG levels in green algae critically depends on their ability to divert carbon flow towards Ac-CoA. This requires elevation of the chloroplastic CoA pool level and enhancement of Ac-CoA biosynthesis. These conclusions may have important implications for future genetic manipulation to enhance TAG biosynthesis in green algae.

New insights on the organization and regulation of the fatty acid biosynthetic network in the model higher plant Arabidopsis thaliana

In the plastids of plant cells, fatty acid (FA) production is a central biosynthetic process. It provides acyl chains for the formation of a variety of acyl lipids fulfilling different biological functions ranging from membrane synthesis to signaling or carbon and energy storage. The biochemical pathway leading to the synthesis of FA has been described for a long time. Over the last 15 years, and after the genome of the model higher plant Arabidopsis thaliana has been sequenced, the scientific community has deployed approaches of functional genomics to identify the actors comprising this pathway. One of the puzzling aspects of the emerging molecular biology of FA synthesis resided in the occurrence of multigene families encoding most enzymes of the pathway. Studies carried out to investigate these families led to the conclusion that most members have acquired non-redundant roles in planta. This is usually the consequence of divergent expression patterns of these isogenes and/or of different substrate specificities of the isoforms they encode. Nevertheless, much remains to be elucidated regarding the molecular bases underpinning these specificities. Protein biochemistry together with emerging quantitative proteomic technologies have then led to a better understanding of the structure of the network, which is composed of multiprotein complexes organized within the stromal compartment of plastids: whereas growing evidence suggests that the early steps of the pathway might be associated to the inner envelope membrane, several late enzymes might be localized next to the thylakoids. The question of the existence of a large integrated protein assembly channeling substrates through the whole pathway that would span the stroma remains uncertain. Finally, recent discoveries regarding the post-translational regulation of the pathway open new research horizons and may guide the development of relevant biotechnological strategies aimed at monitoring FA production in plant systems.

Photosynthetic light reactions increase total lipid accumulation in carbon-supplemented batch cultures of Chlorella vulgaris

Microalgae are an attractive biofuel feedstock because of their high lipid to biomass ratios, lipid compositions that are suitable for biodiesel production, and the ability to grow on varied carbon sources. While algae can grow autotrophically, supplying an exogenous carbon source can increase growth rates and allow heterotrophic growth in the absence of light. Time course analyses of dextrose-supplemented Chlorella vulgaris batch cultures demonstrate that light availability directly influences growth rate, chlorophyll production, and total lipid accumulation. Parallel photomixotrophic and heterotrophic cultures grown to stationary phase reached the same amount of biomass, but total lipid content was higher for algae grown in the presence of light (an average of 1.90 mg/mL vs. 0.77 mg/mL over 5 days of stationary phase growth).