Proteomic identification and characterization of a novel peroxisomal adenine nucleotide transporter supplying ATP for fatty acid beta-oxidation in soybean and Arabidopsis

PLANT UNCOUPLING MITOCHONDRIAL PROTEIN 2 localizes to the Golgi

In contrast to its close homolog PLANT UNCOUPLING MITOCHONDRIAL PROTEIN 1 (UCP1), which is an abundant carrier protein in the mitochondria, UCP2 localizes to the Golgi.

Transcriptome profiling reveals multiple regulatory pathways of Tamarix chinensis in response to salt stress

KEY MESSAGE

Multiple regulatory pathways of T. chinensis to salt stress were identified through transcriptome data analysis. Tamarix chinensis (Tamarix chinensis Lour.) is a typical halophyte capable of completing its life cycle in soils with medium to high salinity. However, the mechanisms underlying its resistance to high salt stress are still largely unclear. In this study, transcriptome profiling analyses in different organs of T. chinensis plants in response to salt stress were carried out. A total number of 2280, 689, and 489 differentially expressed genes (DEGs) were, respectively, identified in roots, stems, and leaves, with more DEGs detected in roots than in stems and leaves. Gene Ontology (GO) term analysis revealed that they were significantly enriched in "biological processes" and "molecular functions". Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis revealed that "Beta-alanine metabolism" was the most differentially enriched pathway in roots, stems, and leaves. In pair-to-pair comparison of the most differentially enriched pathways, a total of 14 pathways, including 5 pathways in roots and leaves, 6 pathways in roots and stems, and 3 pathways in leaves and stems, were identified. Furthermore, genes encoding transcription factor, such as bHLH, bZIP, HD-Zip, MYB, NAC, WRKY, and genes associated with oxidative stress, starch and sucrose metabolism, and ion homeostasis, were differentially expressed with distinct organ specificity in roots, stems, and leaves. Our findings in this research provide a novel approach for exploring the salt tolerance mechanism of halophytes and identifying new gene targets for the genetic breeding of new plant cultivars with improved resistance to salt stress.

Mapping the castor bean endosperm proteome revealed a metabolic interaction between plastid, mitochondria, and peroxisomes to optimize seedling growth

In this work, we studied castor-oil plant Ricinus communis as a classical system for endosperm reserve breakdown. The seeds of castor beans consist of a centrally located embryo with the two thin cotyledons surrounded by the endosperm. The endosperm functions as major storage tissue and is packed with nutritional reserves, such as oil, proteins, and starch. Upon germination, mobilization of the storage reserves requires inter-organellar interplay of plastids, mitochondria, and peroxisomes to optimize growth for the developing seedling. To understand their metabolic interactions, we performed a large-scale organellar proteomic study on castor bean endosperm. Organelles from endosperm of etiolated seedlings were isolated and subjected to liquid chromatography-tandem mass spectrometry (LC-MS/MS). Computer-assisted deconvolution algorithms were applied to reliably assign the identified proteins to their correct subcellular localization and to determine the abundance of the different organelles in the heterogeneous protein samples. The data obtained were used to build a comprehensive metabolic model for plastids, mitochondria, and peroxisomes during storage reserve mobilization in castor bean endosperm.

Subcellular Proteomics to Elucidate Soybean Response to Abiotic Stress

Climate change jeopardizes soybean production by declining seed yield and quality. In this review, the morphophysiological alterations of soybean in response to abiotic stress are summarized, followed by illustrations of cellular metabolisms and regulatory mechanisms to organellar stress based on subcellular proteomics. This highlights the communications associated with reactive oxygen species scavenging, molecular chaperones, and phytohormone signals among subcellular compartments. Given the complexity of climate change and the limitations of plants in coping with multiple abiotic stresses, a generic response to environmental constraints is proposed between calcium and abscisic acid signals in subcellular organelles. This review summarizes the findings of subcellular proteomics in stressed soybean and discusses the future prospects of subcellular proteomics for promoting the improvement of climate-tolerant crops.

Integrative transcriptomic and TMT-based proteomic analysis reveals the mechanism by which AtENO2 affects seed germination under salt stress

Seed germination is critical for plant survival and agricultural production and is affected by many cues, including internal factors and external environmental conditions. As a key enzyme in glycolysis, enolase 2 (ENO2) also plays a vital role in plant growth and abiotic stress responses. In our research, we found that the seed germination rate was lower in the AtENO2 mutation (eno2^- ) than in the wild type (WT) under salt stress in Arabidopsis thaliana, while there was no significant difference under normal conditions. However, the mechanisms by which AtENO2 regulates seed germination under salt stress remain limited. In the current study, transcriptome and proteome analyses were used to compare eno2^- and the WT under normal and salt stress conditions at the germination stage. There were 417 and 4442 differentially expressed genes (DEGs) identified by transcriptome, and 302 and 1929 differentially expressed proteins (DEPs) qualified by proteome under normal and salt stress conditions, respectively. The combined analysis found abundant DEGs and DEPs related to stresses and hydrogen peroxide removal were highly down-regulated in eno2^- . In addition, several DEGs and DEPs encoding phytohormone transduction pathways were identified, and the DEGs and DEPs related to ABA signaling were relatively greatly up-regulated in eno2^- . Moreover, we constructed an interactive network and further identified GAPA1 and GAPB that could interact with AtENO2, which may explain the function of AtENO2 under salt stress during seed germination. Together, our results reveal that under salt stress, AtENO2 mainly affects the expression of genes and proteins related to the phytohormone signal transduction pathways, stress response factors, and reactive oxygen species (ROS), and then affects seed germination. Our study lays the foundation for further exploration of the molecular function of AtENO2 under salt stress at the seed germination stage in Arabidopsis thaliana.

Tissue-targeted inorganic pyrophosphate hydrolysis in a fugu5 mutant reveals that excess inorganic pyrophosphate triggers developmental defects in a cell-autonomous manner

Excess PPi triggers developmental defects in a cell-autonomous manner. The level of inorganic pyrophosphate (PPi) must be tightly regulated in all kingdoms for the proper execution of cellular functions. In plants, the vacuolar proton pyrophosphatase (H+-PPase) has a pivotal role in PPi homeostasis. We previously demonstrated that the excess cytosolic PPi in the H+-PPase loss-of-function fugu5 mutant inhibits gluconeogenesis from seed storage lipids, arrests cell division in cotyledonary palisade tissue, and triggers a compensated cell enlargement (CCE). Moreover, PPi alters pavement cell (PC) shape, stomatal patterning, and functioning, supporting specific yet broad inhibitory effects of PPi on leaf morphogenesis. Whereas these developmental defects were totally rescued by the expression of the yeast soluble pyrophosphatase IPP1, sucrose supply alone canceled CCE in the palisade tissue but not the epidermal developmental defects. Hence, we postulated that the latter are likely triggered by excess PPi rather than a sucrose deficit. To formally test this hypothesis, we adopted a spatiotemporal approach by constructing and analyzing fugu5-1 PDF1 pro ::IPP1, fugu5-1 CLV1 pro ::IPP1, and fugu5-1 ICL pro ::IPP1, whereby PPi was removed specifically from the epidermis, palisade tissue cells, or during the 4 days following seed imbibition, respectively. It is important to note that whereas PC defects in fugu5-1 PDF1 pro ::IPP1 were completely recovered, those in fugu5-1 CLV1 pro ::IPP1 were not. In addition, phenotypic analyses of fugu5-1 ICL pro ::IPP1 lines demonstrated that the immediate removal of PPi after seed imbibition markedly improved overall plant growth, abolished CCE, but only partially restored the epidermal developmental defects. Next, the impact of spatial and temporal removal of PPi was investigated by capillary electrophoresis time-of-flight mass spectrometry (CE-TOF MS). Our analysis revealed that the metabolic profiles are differentially affected among all the above transgenic lines, and consistent with an axial role of central metabolism of gluconeogenesis in CCE. Taken together, this study provides a conceptual framework to unveil metabolic fluctuations within leaf tissues with high spatio-temporal resolution. Finally, our findings suggest that excess PPi exerts its inhibitory effect in planta in the early stages of seedling establishment in a tissue- and cell-autonomous manner.

Translating the Arabidopsis thaliana Peroxisome Proteome Insights to Solanum lycopersicum: Consensus Versus Diversity

Peroxisomes are small, single-membrane specialized organelles present in all eukaryotic organisms. The peroxisome is one of the nodal centers of reactive oxygen species homeostasis in plants, which are generated in a high amount due to various stress conditions. Over the past decade, there has been extensive study on peroxisomal proteins and their signaling pathways in the model plant Arabidopsis thaliana, and a lot has been deciphered. However, not much impetus has been given to studying the peroxisome proteome of economically important crops. Owing to the significance of peroxisomes in the physiology of plants during normal and stress conditions, understating its proteome is of much importance. Hence, in this paper, we have made a snapshot of putative peroxisomal matrix proteins in the economically important vegetable crop tomato (Solanum lycopersicum, (L.) family Solanaceae). First, a reference peroxisomal matrix proteome map was generated for Arabidopsis thaliana using the available proteomic and localization studies, and proteins were categorized into various groups as per their annotations. This was used to create the putative peroxisomal matrix proteome map for S. lycopersicum. The putative peroxisome proteome in S. lycopersicum retains the basic framework: the bulk of proteins had peroxisomal targeting signal (PTS) type 1, a minor group had PTS2, and the catalase family retained its characteristic internal PTS. Apart from these, a considerable number of S. lycopersicum orthologs did not contain any "obvious" PTS. The number of PTS2 isoforms was found to be reduced in S. lycopersicum. We further investigated the PTS1s in the case of both the plant species and generated a pattern for canonical and non-canonical PTS1s. The number of canonical PTS1 proteins was comparatively lesser in S. lycopersicum. The non-canonical PTS1s were found to be comparable in both the plant species; however, S. lycopersicum showed greater diversity in the composition of the signal tripeptide. Finally, we have tried to address the lacunas and probable strategies to fill those gaps.

Overexpression of Peroxisome-Localized GmABCA7 Promotes Seed Germination in Arabidopsis thaliana

Peroxisome is one of the important organelles for intracellular lipid metabolism in plant cells and β-oxidation of fatty acids in peroxisomes provides the energy for oil-containing seed germination. In this study, we identified an ATP-binding cassette (ABC) transporter gene, GmABCA7 from soybean, which is highly expressed in the different developmental stages of seeds. Transient expression of GmABCA7 in tobacco epidermal cells showed that GmABCA7 was specifically localized at the peroxisomes. Overexpression of GmABCA7 in Arabidopsis does not change seed phenotypes, or the overall levels of lipid, protein and sugar stored in the seeds; however, the transgenic seeds produced more gluconeogenic pathway precursors such as succinate and malate and germinated earlier compared to the wild type seeds. These results suggest that GmABCA7 may affect the β-oxidation of fatty acids and play an important role in seed germination.

Subcellular Proteomics as a Unified Approach of Experimental Localizations and Computed Prediction Data for Arabidopsis and Crop Plants

In eukaryotic organisms, subcellular protein location is critical in defining protein function and understanding sub-functionalization of gene families. Some proteins have defined locations, whereas others have low specificity targeting and complex accumulation patterns. There is no single approach that can be considered entirely adequate for defining the in vivo location of all proteins. By combining evidence from different approaches, the strengths and weaknesses of different technologies can be estimated, and a location consensus can be built. The Subcellular Location of Proteins in Arabidopsis database ( http://suba.live/ ) combines experimental data sets that have been reported in the literature and is analyzing these data to provide useful tools for biologists to interpret their own data. Foremost among these tools is a consensus classifier (SUBAcon) that computes a proposed location for all proteins based on balancing the experimental evidence and predictions. Further tools analyze sets of proteins to define the abundance of cellular structures. Extending these types of resources to plant crop species has been complex due to polyploidy, gene family expansion and contraction, and the movement of pathways and processes within cells across the plant kingdom. The Crop Proteins of Annotated Location database ( http://crop-pal.org/ ) has developed a range of subcellular location resources including a species-specific voting consensus for 12 plant crop species that offers collated evidence and filters for current crop proteomes akin to SUBA. Comprehensive cross-species comparison of these data shows that the sub-cellular proteomes (subcellulomes) depend only to some degree on phylogenetic relationship and are more conserved in major biosynthesis than in metabolic pathways. Together SUBA and cropPAL created reference subcellulomes for plants as well as species-specific subcellulomes for cross-species data mining. These data collections are increasingly used by the research community to provide a subcellular protein location layer, inform models of compartmented cell function and protein-protein interaction network, guide future molecular crop breeding strategies, or simply answer a specific question-where is my protein of interest inside the cell?

Transcriptome Profiling of the Salt Stress Response in the Leaves and Roots of Halophytic Eutrema salsugineum

Eutrema salsugineum can grow in natural harsh environments; however, the underlying mechanisms for salt tolerance of Eutrema need to be further understood. Herein, the transcriptome profiling of Eutrema leaves and roots exposed to 300 mM NaCl is investigated, and the result emphasized the role of genes involved in lignin biosynthesis, autophagy, peroxisome, and sugar metabolism upon salt stress. Furthermore, the expression of the lignin biosynthesis and autophagy-related genes, as well as 16 random selected genes, was validated by qRT-PCR. Notably, the transcript abundance of a large number of lignin biosynthesis genes such as CCoAOMT, C4H, CCR, CAD, POD, and C3′H in leaves was markedly elevated by salt shock. And the examined lignin content in leaves and roots demonstrated salt stress led to lignin accumulation, which indicated the enhanced lignin level could be an important mechanism for Eutrema responding to salt stress. Additionally, the differentially expressed genes (DEGs) assigned in the autophagy pathway including Vac8, Atg8, and Atg4, as well as DEGs enriched in the peroxisome pathway such as EsPEX7, EsCAT, and EsSOD2, were markedly induced in leaves and/or roots. In sugar metabolism pathways, the transcript levels of most DEGs associated with the synthesis of sucrose, trehalose, raffinose, and xylose were significantly enhanced. Furthermore, the expression of various stress-related transcription factor genes including WRKY, AP2/ERF-ERF, NAC, bZIP, MYB, C2H2, and HSF was strikingly improved. Collectively, the increased expression of biosynthesis genes of lignin and soluble sugars, as well as the genes in the autophagy and peroxisome pathways, suggested that Eutrema encountering salt shock possibly possess a higher capacity to adjust osmotically and facilitate water transport and scavenge reactive oxidative species and oxidative proteins to cope with the salt environment. Thus, this study provides a new insight for exploring the salt tolerance mechanism of halophytic Eutrema and discovering new gene targets for the genetic improvement of crops.

An Oil Hyper-Accumulator Mutant Highlights Peroxisomal ATP Import as a Regulatory Step for Fatty Acid Metabolism in Aurantiochytrium limacinum

Thraustochytrids are marine protists that naturally accumulate triacylglycerol with long chains of polyunsaturated fatty acids, such as ω3-docosahexaenoic acid (DHA). They represent a sustainable response to the increasing demand for these "essential" fatty acids (FAs). Following an attempt to transform a strain of Aurantiochytrium limacinum, we serendipitously isolated a clone that did not incorporate any recombinant DNA but contained two to three times more DHA than the original strain. Metabolic analyses indicated a deficit in FA catabolism. However, whole transcriptome analysis did not show down-regulation of genes involved in FA catabolism. Genome sequencing revealed extensive DNA deletion in one allele encoding a putative peroxisomal adenylate transporter. Phylogenetic analyses and yeast complementation experiments confirmed the gene as a peroxisomal adenylate nucleotide transporter (AlANT1), homologous to yeast ScANT1 and plant peroxisomal adenylate nucleotide carrier AtPNC genes. In yeast and plants, a deletion of the peroxisomal adenylate transporter inhibits FA breakdown and induces FA accumulation, a phenotype similar to that described here. In response to this metabolic event, several compensatory mechanisms were observed. In particular, genes involved in FA biosynthesis were upregulated, also contributing to the high FA accumulation. These results support AlANT1 as a promising target for enhancing DHA production in Thraustochytrids.

An auxin signaling network translates low-sugar-state input into compensated cell enlargement in the fugu5 cotyledon

In plants, the effective mobilization of seed nutrient reserves is crucial during germination and for seedling establishment. The Arabidopsis H+-PPase-loss-of-function fugu5 mutants exhibit a reduced number of cells in the cotyledons. This leads to enhanced post-mitotic cell expansion, also known as compensated cell enlargement (CCE). While decreased cell numbers have been ascribed to reduced gluconeogenesis from triacylglycerol, the molecular mechanisms underlying CCE remain ill-known. Given the role of indole 3-butyric acid (IBA) in cotyledon development, and because CCE in fugu5 is specifically and completely cancelled by ech2, which shows defective IBA-to-indoleacetic acid (IAA) conversion, IBA has emerged as a potential regulator of CCE. Here, to further illuminate the regulatory role of IBA in CCE, we used a series of high-order mutants that harbored a specific defect in IBA-to-IAA conversion, IBA efflux, IAA signaling, or vacuolar type H+-ATPase (V-ATPase) activity and analyzed the genetic interaction with fugu5-1. We found that while CCE in fugu5 was promoted by IBA, defects in IBA-to-IAA conversion, IAA response, or the V-ATPase activity alone cancelled CCE. Consistently, endogenous IAA in fugu5 reached a level 2.2-fold higher than the WT in 1-week-old seedlings. Finally, the above findings were validated in icl-2, mls-2, pck1-2 and ibr10 mutants, in which CCE was triggered by low sugar contents. This provides a scenario in which following seed germination, the low-sugar-state triggers IAA synthesis, leading to CCE through the activation of the V-ATPase. These findings illustrate how fine-tuning cell and organ size regulation depend on interplays between metabolism and IAA levels in plants.

Genomic variation of an endosymbiotic dinoflagellate (Symbiodinium 'fitti') among closely related coral hosts

Mutualisms where hosts are coupled metabolically to their symbionts often exhibit high partner fidelity. Most reef-building coral species form obligate symbioses with a specific species of photosymbionts, dinoflagellates in the family Symbiodiniaceae, despite needing to acquire symbionts early in their development from environmental sources. Three Caribbean acroporids (Acropora palmata, A. cervicornis and their F₁ hybrid) are sympatric across much of their range, but often occupy different depth and light habitats. Throughout this range, both species and their hybrid associate with the endosymbiotic dinoflagellate Symbiodinium 'fitti'. Because light (and therefore depth) influences the physiology of dinoflagellates, we investigated whether S. 'fitti' populations from each host taxon were differentiated genetically. Single nucleotide polymorphisms (SNPs) among S. 'fitti' strains were identified by aligning shallow metagenomic sequences of acroporid colonies sampled from across the Caribbean to a ~600-Mb draft assembly of the S. 'fitti' genome (from the CFL14120 A. cervicornis metagenome). Phylogenomic and multivariate analyses revealed that genomic variation among S. 'fitti' strains partitioned to each host taxon rather than by biogeographical origin. This is particularly noteworthy because the hybrid has a sparse fossil record and may be of relatively recent origin. A subset (37.6%) of the SNPs putatively under selection were nonsynonymous mutations predicted to alter protein efficiency. Differences in genomic variation of S. 'fitti' strains from each host taxon may reflect the unique selection pressures created by the microenvironments associated with each host. The nonrandom sorting among S. 'fitti' strains to different hosts could be the basis for lineage diversification via disruptive selection, leading to ecological specialization and ultimately speciation.

The Saccharomyces cerevisiae ABC subfamily D transporter Pxa1/Pxa2p co-imports CoASH into the peroxisome

ATP-binding cassette (ABC) subfamily D transporters are important for the uptake of fatty acids and other beta-oxidation substrates into peroxisomes. Genetic and biochemical evidence indicates that the transporters accept fatty acyl-coenzyme A that is cleaved during the transport cycle and then re-esterified in the peroxisomal lumen. However, it is not known whether free coenzyme A (CoA) is released inside or outside the peroxisome. Here we have used Saccharomyces cerevisiae and isolated peroxisomes to demonstrate that free CoA is released in the peroxisomal lumen. Thus, ABC subfamily D transporter provide an import pathway for free CoA that controls peroxisomal CoA homeostasis and tunes metabolism according to the cell's demands.

CropPAL for discovering divergence in protein subcellular location in crops to support strategies for molecular crop breeding

Agriculture faces increasing demand for yield, higher plant-derived protein content and diversity while facing pressure to achieve sustainability. Although the genomes of many of the important crops have been sequenced, the subcellular locations of most of the encoded proteins remain unknown or are only predicted. Protein subcellular location is crucial in determining protein function and accumulation patterns in plants, and is critical for targeted improvements in yield and resilience. Integrating location data from over 800 studies for 12 major crop species into the cropPAL2020 data collection showed that while >80% of proteins in most species are not localised by experimental data, combining species data or integrating predictions can help bridge gaps at similar accuracy. The collation and integration of over 61 505 experimental localisations and more than 6 million predictions showed that the relative sizes of the protein catalogues located in different subcellular compartments are comparable between crops and Arabidopsis. A comprehensive cross-species comparison showed that between 50% and 80% of the subcellulomes are conserved across species and that conservation only depends to some degree on the phylogenetic relationship of the species. Protein subcellular locations in major biosynthesis pathways are more often conserved than in metabolic pathways. Underlying this conservation is a clear potential for subcellular diversity in protein location between species by means of gene duplication and alternative splicing. Our cropPAL data set and search platform (https://crop-pal.org) provide a comprehensive subcellular proteomics resource to drive compartmentation-based approaches for improving yield, protein composition and resilience in future crop varieties.

Characterization of In Vivo Function(s) of Members of the Plant Mitochondrial Carrier Family

Although structurally related, mitochondrial carrier family (MCF) proteins catalyze the specific transport of a range of diverse substrates including nucleotides, amino acids, dicarboxylates, tricarboxylates, cofactors, vitamins, phosphate and H⁺. Despite their name, they do not, however, always localize to the mitochondria, with plasma membrane, peroxisomal, chloroplast and thylakoid and endoplasmic reticulum localizations also being reported. The existence of plastid-specific MCF proteins is suggestive that the evolution of these proteins occurred after the separation of the green lineage. That said, plant-specific MCF proteins are not all plastid-localized, with members also situated at the endoplasmic reticulum and plasma membrane. While by no means yet comprehensive, the in vivo function of a wide range of these transporters is carried out here, and we discuss the employment of genetic variants of the MCF as a means to provide insight into their in vivo function complementary to that obtained from studies following their reconstitution into liposomes.

Peroxisomal Cofactor Transport

Peroxisomes are eukaryotic organelles that are essential for growth and development. They are highly metabolically active and house many biochemical reactions, including lipid metabolism and synthesis of signaling molecules. Most of these metabolic pathways are shared with other compartments, such as Endoplasmic reticulum (ER), mitochondria, and plastids. Peroxisomes, in common with all other cellular organelles are dependent on a wide range of cofactors, such as adenosine 5′-triphosphate (ATP), Coenzyme A (CoA), and nicotinamide adenine dinucleotide (NAD). The availability of the peroxisomal cofactor pool controls peroxisome function. The levels of these cofactors available for peroxisomal metabolism is determined by the balance between synthesis, import, export, binding, and degradation. Since the final steps of cofactor synthesis are thought to be located in the cytosol, cofactors must be imported into peroxisomes. This review gives an overview about our current knowledge of the permeability of the peroxisomal membrane with the focus on ATP, CoA, and NAD. Several members of the mitochondrial carrier family are located in peroxisomes, catalyzing the transfer of these organic cofactors across the peroxisomal membrane. Most of the functions of these peroxisomal cofactor transporters are known from studies in yeast, humans, and plants. Parallels and differences between the transporters in the different organisms are discussed here.

Slc25a17 Gene Trapped Mice: PMP34 Plays a Role in the Peroxisomal Degradation of Phytanic and Pristanic Acid

Mice lacking PMP34, a peroxisomal membrane transporter encoded by Slc25a17, did not manifest any obvious phenotype on a Swiss Webster genetic background, even with various treatments designed to unmask impaired peroxisomal functioning. Peroxisomal α- and β-oxidation rates in PMP34 deficient fibroblasts or liver slices were not or only modestly affected and in bile, no abnormal bile acid intermediates were detected. Peroxisomal content of cofactors like CoA, ATP, NAD+, thiamine-pyrophosphate and pyridoxal-phosphate, based on direct or indirect data, appeared normal as were tissue plasmalogen and very long chain fatty acid levels. However, upon dietary phytol administration, the knockout mice displayed hepatomegaly, liver inflammation, and an induction of peroxisomal enzymes. This phenotype was partially mediated by PPARα. Hepatic triacylglycerols and cholesterylesters were elevated and both phytanic acid and pristanic acid accumulated in the liver lipids, in females to higher extent than in males. In addition, pristanic acid degradation products were detected, as wells as the CoA-esters of all these branched fatty acids. Hence, PMP34 is important for the degradation of phytanic/pristanic acid and/or export of their metabolites. Whether this is caused by a shortage of peroxisomal CoA affecting the intraperoxisomal formation of pristanoyl-CoA (and perhaps of phytanoyl-CoA), or the SCPx-catalyzed thiolytic cleavage during pristanic acid β-oxidation, could not be proven in this model, but the phytol-derived acyl-CoA profile is compatible with the latter possibility. On the other hand, the normal functioning of other peroxisomal pathways, and especially bile acid formation, seems to exclude severe transport problems or a shortage of CoA, and other cofactors like FAD, NAD(P)+, TPP. Based on our findings, PMP34 deficiency in humans is unlikely to be a life threatening condition but could cause elevated phytanic/pristanic acid levels in older adults.

Peroxisomes: versatile organelles with diverse roles in plants

Peroxisomes are small, ubiquitous organelles that are delimited by a single membrane and lack genetic material. However, these simple-structured organelles are highly versatile in morphology, abundance and protein content in response to various developmental and environmental cues. In plants, peroxisomes are essential for growth and development and perform diverse metabolic functions, many of which are carried out coordinately by peroxisomes and other organelles physically interacting with peroxisomes. Recent studies have added greatly to our knowledge of peroxisomes, addressing areas such as the diverse proteome, regulation of division and protein import, pexophagy, matrix protein degradation, solute transport, signaling, redox homeostasis and various metabolic and physiological functions. This review summarizes our current understanding of plant peroxisomes, focusing on recent discoveries. Current problems and future efforts required to better understand these organelles are also discussed. An improved understanding of peroxisomes will be important not only to the understanding of eukaryotic cell biology and metabolism, but also to agricultural efforts aimed at improving crop performance and defense.

Plant peroxisomal solute transporter proteins

Plant peroxisomes are unique subcellular organelles which play an indispensable role in several key metabolic pathways, including fatty acid β-oxidation, photorespiration, and degradation of reactive oxygen species. The compartmentalization of metabolic pathways into peroxisomes is a strategy for organizing the metabolic network and improving pathway efficiency. An important prerequisite, however, is the exchange of metabolites between peroxisomes and other cell compartments. Since the first studies in the 1970s scientists contributed to understanding how solutes enter or leave this organelle. This review gives an overview about our current knowledge of the solute permeability of peroxisomal membranes described in plants, yeast, mammals and other eukaryotes. In general, peroxisomes contain in their bilayer membrane specific transporters for hydrophobic fatty acids (ABC transporter) and large cofactor molecules (carrier for ATP, NAD and CoA). Smaller solutes with molecular masses below 300-400 Da, like the organic acids malate, oxaloacetate, and 2-oxoglutarate, are shuttled via non-selective channels across the peroxisomal membrane. In comparison to yeast, human, mammals and other eukaryotes, the function of these known peroxisomal transporters and channels in plants are discussed in this review.

Proteomic analysis of glycosomes from Trypanosoma cruzi epimastigotes

In Trypanosoma cruzi, the causal agent of Chagas disease, the first seven steps of glycolysis are compartmentalized in glycosomes, which are authentic but specialized peroxisomes. Besides glycolysis, activity of enzymes of other metabolic processes have been reported to be present in glycosomes, such as β-oxidation of fatty acids, purine salvage, pentose-phosphate pathway, gluconeogenesis and biosynthesis of ether-lipids, isoprenoids, sterols and pyrimidines. In this study, we have purified glycosomes from T. cruzi epimastigotes, collected the soluble and membrane fractions of these organelles, and separated peripheral and integral membrane proteins by Na₂CO₃ treatment and osmotic shock. Proteomic analysis was performed on each of these fractions, allowing us to confirm the presence of enzymes involved in various metabolic pathways as well as identify new components of this parasite's glycosomes.

Comprehensive evaluation of fuel properties and complex regulation of intracellular transporters for high oil production in developing seeds of Prunus sibirica for woody biodiesel

BACKGROUND

Based on our previous studies of 17 Prunus sibirica germplasms, one plus tree with high quality and quantity of seed oils has emerged as novel potential source of biodiesel. To better develop P. sibirica seed oils as woody biodiesel, a concurrent exploration of oil content, FA composition, biodiesel yield and fuel properties as well as prediction model construction for fuel properties was conducted on developing seeds to determine the optimal seed harvest time for producing high-quality biodiesel. Oil synthesis required supply of carbon source, energy and FA, but their transport mechanisms still remains enigmatic. Our recent 454 sequencing of P. sibirica could provide long-read sequences to identify membrane transporters for a better understanding of regulatory mechanism for high oil production in developing seeds.

RESULTS

To better develop the seed oils of P. sibirica as woody biodiesel, we firstly focused on a temporal and comparative evaluation of growth tendency, oil content, FA composition, biodiesel yield and fuel properties as well as model construction for biodiesel property prediction in different developing seeds from P. sibirica plus tree (accession AS-80), revealing that the oils from developing seeds harvested after 60 days after flowering (DAF) could be as novel potential feedstock for producing biodiesel with ideal fuel property. To gain new insight into membrane transport mechanism for high oil yield in developing seeds of P. sibirica, we presented a global analysis of transporter based on our recent 454 sequencing data of P. sibirica. We annotated a total of 116 genes for membrane-localized transporters at different organelles (plastid, endoplasmatic reticulum, tonoplast, mitochondria and peroxisome), of which some specific transporters were identified to be involved in carbon allocation, metabolite transport and energy supply for oil synthesis by both RT-PCR and qRT-PCR. Importantly, the transporter-mediated model was well established for high oil synthesis in developing P. sibirica seeds. Our findings could help to reveal molecular mechanism of increased oil production and may also present strategies for engineering oil accumulation in oilseed plants.

CONCLUSIONS

This study presents a temporal and comparative evaluation of developing P. sibirica seed oils as a potential feedstock for producing high-quality biodiesel and a global identification for membrane transporters was to gain better insights into regulatory mechanism of high oil production in developing seeds of P. sibirica. Our findings may present strategies for developing woody biodiesel resources and engineering oil accumulation.

Changes in gene expression during germination reveal pea genotypes with either "quiescence" or "escape" mechanisms of waterlogging tolerance

Waterlogging causes germination failure in pea (Pisum sativum L.). Three genotypes (BARI Motorshuti-3, Natore local-2 [NL-2], and Kaspa) contrasting in ability to germinate in waterlogged soil were exposed to different durations of waterlogging. Whole genome RNAseq was employed to capture differentially expressing genes. The ability to germinate in waterlogged soil was associated with testa colour and testa membrane integrity as confirmed by electrical conductivity measurements. Genotypes Kaspa and NL-2 displayed different mechanisms of tolerance. In Kaspa, an energy conserving strategy was indicated by a strong upregulation of tyrosine protein kinsase and down regulation of linoleate 9S-lipoxygenase 5, a fat metabolism gene. In contrast, a faster energy utilization strategy was suggested in NL-2 by the marked upregulation of a subtilase family protein and peroxisomal adenine nucleotide carrier 2, a fat metabolizing gene. Waterlogging susceptibility in germinating seeds of genotype BARI Motorshuti-3 was linked to upregulation of a kunitz-type trypsin/protease inhibitor that blocks protein metabolism and may lead to excessive lipid metabolism and the membrane leakage associated with waterlogging damage. Pathway analyses based on gene ontologies showed seed storage protein metabolism as upregulated in tolerant genotypes and downregulated in the sensitive genotype. Understanding the tolerance mechanism provides a platform to breed for adaptation to waterlogging stress at germination in pea.

Data-Mining Bioinformatics: Connecting Adenylate Transport and Metabolic Responses to Stress

Adenine nucleotides are essential in countless processes within the cellular metabolism. In plants, ATP is mainly produced in chloroplasts and mitochondria through photophosphorylation and oxidative phosphorylation, respectively. Thus, efficient adenylate transport systems are required for intracellular energy partitioning between the cell organelles. Adenylate carriers present in different subcellular compartments have been previously identified and biochemically characterized in plants. Here, by using data-mining bioinformatics tools, we propose how, and to what extent, these carriers integrate energy metabolism within a plant cell under different environmental conditions. We demonstrate that the expression pattern of the corresponding genes is variable under different environmental conditions, suggesting that specific adenylate carriers have distinct and nonredundant functions in plants.

Uncoupling proteins 1 and 2 (UCP1 and UCP2) from Arabidopsis thaliana are mitochondrial transporters of aspartate, glutamate, and dicarboxylates

The Arabidopsis thaliana genome contains 58 members of the solute carrier family SLC25, also called the mitochondrial carrier family, many of which have been shown to transport specific metabolites, nucleotides, and cofactors across the mitochondrial membrane. Here, two Arabidopsis members of this family, AtUCP1 and AtUCP2, which were previously thought to be uncoupling proteins and hence named UCP1/PUMP1 and UCP2/PUMP2, respectively, are assigned with a novel function. They were expressed in bacteria, purified, and reconstituted in phospholipid vesicles. Their transport properties demonstrate that they transport amino acids (aspartate, glutamate, cysteine sulfinate, and cysteate), dicarboxylates (malate, oxaloacetate, and 2-oxoglutarate), phosphate, sulfate, and thiosulfate. Transport was saturable and inhibited by mercurials and other mitochondrial carrier inhibitors to various degrees. AtUCP1 and AtUCP2 catalyzed a fast counterexchange transport as well as a low uniport of substrates, with transport rates of AtUCP1 being much higher than those of AtUCP2 in both cases. The aspartate/glutamate heteroexchange mediated by AtUCP1 and AtUCP2 is electroneutral, in contrast to that mediated by the mammalian mitochondrial aspartate glutamate carrier. Furthermore, both carriers were found to be targeted to mitochondria. Metabolite profiling of single and double knockouts shows changes in organic acid and amino acid levels. Notably, AtUCP1 and AtUCP2 are the first reported mitochondrial carriers in Arabidopsis to transport aspartate and glutamate. It is proposed that the primary function of AtUCP1 and AtUCP2 is to catalyze an aspartate_out/glutamate_in exchange across the mitochondrial membrane and thereby contribute to the export of reducing equivalents from the mitochondria in photorespiration.

Proteome of Plant Peroxisomes

Plant peroxisomes are required for a number of fundamental physiological processes, such as primary and secondary metabolism, development and stress response. Indexing the dynamic peroxisome proteome is prerequisite to fully understanding the importance of these organelles. Mass Spectrometry (MS)-based proteome analysis has allowed the identification of novel peroxisomal proteins and pathways in a relatively high-throughput fashion and significantly expanded the list of proteins and biochemical reactions in plant peroxisomes. In this chapter, we summarize the experimental proteomic studies performed in plants, compile a list of ~200 confirmed Arabidopsis peroxisomal proteins, and discuss the diverse plant peroxisome functions with an emphasis on the role of Arabidopsis MS-based proteomics in discovering new peroxisome functions. Many plant peroxisome proteins and biochemical pathways are specific to plants, substantiating the complexity, plasticity and uniqueness of plant peroxisomes. Mapping the full plant peroxisome proteome will provide a knowledge base for the improvement of crop production, quality and stress tolerance.

How to move an amphipathic molecule across a lipid bilayer: different mechanisms for different ABC transporters?

Import of β-oxidation substrates into peroxisomes is mediated by ATP binding cassette (ABC) transporters belonging to subfamily D. In order to enter the β-oxidation pathway, fatty acids are activated by conversion to fatty acyl-CoA esters, a reaction which is catalysed by acyl-CoA synthetases (ACSs). Here, we present evidence for an unusual transport mechanism, in which fatty acyl-CoA substrates are accepted by ABC subclass D protein (ABCD) transporters, cleaved by the transporters during transit across the lipid bilayer to release CoA, and ultimately re-esterified in the peroxisome lumen by ACSs which interact with the transporter. We propose that this solves the biophysical problem of moving an amphipathic molecule across the peroxisomal membrane, since the intrinsic thioesterase activity of the transporter permits separate membrane translocation pathways for the hydrophobic fatty acid moiety and the polar CoA moiety. The cleavage/re-esterification mechanism also has the potential to control entry of disparate substrates into the β-oxidation pathway when coupled with distinct peroxisomal ACSs. A different solution to the movement of amphipathic molecules across a lipid bilayer is deployed by the bacterial lipid-linked oligosaccharide (LLO) flippase, PglK, in which the hydrophilic head group and the hydrophobic polyprenyl tail of the substrate are proposed to have distinct translocation pathways but are not chemically separated during transport. We discuss a speculative alternating access model for ABCD proteins based on the mammalian ABC transporter associated with antigen processing (TAP) and compare it to the novel mechanism suggested by the recent PglK crystal structures and biochemical data.

Plant peroxisomes: A nitro-oxidative cocktail

Although peroxisomes are very simple organelles, research on different species has provided us with an understanding of their importance in terms of cell viability. In addition to the significant role played by plant peroxisomes in the metabolism of reactive oxygen species (ROS), data gathered over the last two decades show that these organelles are an endogenous source of nitric oxide (NO) and related molecules called reactive nitrogen species (RNS). Molecules such as NO and H2O2 act as retrograde signals among the different cellular compartments, thus facilitating integral cellular adaptation to physiological and environmental changes. However, under nitro-oxidative conditions, part of this network can be overloaded, possibly leading to cellular damage and even cell death. This review aims to update our knowledge of the ROS/RNS metabolism, whose important role in plant peroxisomes is still underestimated. However, this pioneering approach, in which key elements such as β-oxidation, superoxide dismutase (SOD) and NO have been mainly described in relation to plant peroxisomes, could also be used to explore peroxisomes from other organisms.

Compensated Cell Enlargement in fugu5 is Specifically Triggered by Lowered Sucrose Production from Seed Storage Lipids

To reveal the logic of size regulation in multicellular organisms, we have used Arabidopsis thaliana as a model organism and its leaves as a model organ. We discovered the existence of a compensatory system, whereby a decrease in leaf cell number often triggers unusual cell enlargement. However, despite the large number of compensation-exhibiting mutants analyzed to date, we have only a limited understanding of the detailed molecular mechanisms triggering the decrease in cell number and subsequent compensated cell enlargement (CCE). CCE in fugu5, the vacuolar type H+-pyrophosphatase loss-of-function mutant, is specific to cotyledons and completely suppressed when sucrose (Suc) is supplied or cytosolic pyrophosphate (PPi) is specifically removed. In addition, several lines of evidence suggest that excess cytosolic PPi in fugu5 impairs gluconeogenesis from triacylglycerol (TAG) to Suc. Here, detailed cellular phenotyping revealed that the loss-of-function mutants icl-2, mls-2 and pck1-2 triggered CCE in cotyledons. All double mutant combinations between fugu5-1 and the above three mutants exhibited compensation, but did not display a further increase in cell size. Importantly, similar phenotypes were observed in icl-2 mls-2, icl-2 pck1-2 and mls-2 pck1-2. Quantification of TAG breakdown and Suc contents further supported our findings. Taken together, we demonstrate that de novo Suc synthesis from TAG is fundamentally important for proper resumption of post-germinative cotyledon development. Moreover, provided that icl-2, mls-2 and pck1-2 are only compromised in Suc biosynthesis de novo from TAG, our findings clearly indicate that lowered Suc production in fugu5, rather than excess cytosolic PPi, is the direct trigger of CCE.

Plant peroxisomes: recent discoveries in functional complexity, organelle homeostasis, and morphological dynamics

Peroxisomes are essential for life in plants. These organelles house a variety of metabolic processes that generate and inactivate reactive oxygen species. Our knowledge of pathways and mechanisms that depend on peroxisomes and their constituent enzymes continues to grow, and in this review we highlight recent advances in understanding the identity and biological functions of peroxisomal enzymes and metabolic processes. We also review how peroxisomal matrix and membrane proteins enter the organelle from their sites of synthesis. Peroxisome homeostasis is regulated by specific degradation mechanisms, and we discuss the contributions of specialized autophagy and a peroxisomal protease to the degradation of entire peroxisomes and peroxisomal enzymes that are damaged or superfluous. Finally, we review how peroxisomes can flexibly change their morphology to facilitate inter-organellar contacts.

Characterization, prediction and evolution of plant peroxisomal targeting signals type 1 (PTS1s)

Our knowledge of the proteome of plant peroxisomes and their functional plasticity is far from being complete, primarily due to major technical challenges in experimental proteome research of the fragile cell organelle. Several unexpected novel plant peroxisome functions, for instance in biotin and phylloquinone biosynthesis, have been uncovered recently. Nevertheless, very few regulatory and membrane proteins of plant peroxisomes have been identified and functionally described up to now. To define the matrix proteome of plant peroxisomes, computational methods have emerged as important powerful tools. Novel prediction approaches of high sensitivity and specificity have been developed for peroxisome targeting signals type 1 (PTS1) and have been validated by in vivo subcellular targeting analyses and thermodynamic binding studies with the cytosolic receptor, PEX5. Accordingly, the algorithms allow the correct prediction of many novel peroxisome-targeted proteins from plant genome sequences and the discovery of additional organelle functions. In this review, we provide an overview of methodologies, capabilities and accuracies of available prediction algorithms for PTS1 carrying proteins. We also summarize and discuss recent quantitative, structural and mechanistic information of the interaction of PEX5 with PTS1 carrying proteins in relation to in vivo import efficiency. With this knowledge, we develop a model of how proteins likely evolved peroxisomal targeting signals in the past and still nowadays, in which order the two import pathways might have evolved in the ancient eukaryotic cell, and how the secondary loss of the PTS2 pathway probably happened in specific organismal groups.

Discoveries, metabolic roles and diseases of mitochondrial carriers: A review

Mitochondrial carriers (MCs) are a superfamily of nuclear-encoded proteins that are mostly localized in the inner mitochondrial membrane and transport numerous metabolites, nucleotides, cofactors and inorganic anions. Their unique sequence features, i.e., a tripartite structure, six transmembrane α-helices and a three-fold repeated signature motif, allow MCs to be easily recognized. This review describes how the functions of MCs from Saccharomyces cerevisiae, Homo sapiens and Arabidopsis thaliana (listed in the first table) were discovered after the genome sequence of S. cerevisiae was determined in 1996. In the genomic era, more than 50 previously unknown MCs from these organisms have been identified and characterized biochemically using a method consisting of gene expression, purification of the recombinant proteins, their reconstitution into liposomes and transport assays (EPRA). Information derived from studies with intact mitochondria, genetic and metabolic evidence, sequence similarity, phylogenetic analysis and complementation of knockout phenotypes have guided the choice of substrates that were tested in the transport assays. In addition, the diseases associated to defects of human MCs have been briefly reviewed. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.

Towards understanding peroxisomal phosphoregulation in Arabidopsis thaliana

This work identifies new protein phosphatases and phosphatase-related proteins targeting peroxisomes, and raises the question of a novel protein import pathway from ER to peroxisomes involving peroxisomal targeting signal type 1 (PTS1) Plant peroxisomes are essential for several processes, for example lipid metabolism, free radical detoxification, development, and stress-related functions. Although research on peroxisomes has been intensified, reversible phosphorylation as a control mechanism in peroxisomes is barely studied. Therefore, it is crucial to identify all peroxisomal proteins involved in phosphoregulation. We here started with protein phosphatases, and searched the Arabidopsis thaliana genome for phosphatase-related proteins with putative peroxisomal targeting signals (PTS). Five potential peroxisomal candidates were detected, from which four were confirmed to target peroxisomes or have a functional PTS. The highly conserved Ser-Ser-Met> was validated for two protein phosphatase 2C (PP2C) family members (POL like phosphatases, PLL2 and PLL3) as a functional peroxisomal targeting signal type 1 (PTS1). Full-length PLL2 and PLL3 fused with a reporter protein targeted peroxisomes in two plant expression systems. A putative protein phosphatase, purple acid phosphatase 7 (PAP7), was found to be dually targeted to ER and peroxisomes and experiments indicated a possible trafficking to peroxisomes via the ER depending on peroxisomal PTS1. In addition, a protein phosphatase 2A regulator (TIP41) was validated to harbor a functional PTS1 (Ser-Lys-Val>), but the full-length protein targeted cytosol and nucleus. Reverse genetics indicated a role for TIP41 in senescence signaling. Mass spectrometry of whole seedlings and isolated peroxisomes was employed, and identified new putative phosphorylated peroxisomal proteins. Previously, only one protein phosphatase, belonging to the phospho-protein phosphatase (PPP) family, was identified as a peroxisomal protein. The present work implies that members of two other main protein phosphatase families, i.e. PP2C and PAP, are also targeting peroxisomes.

Suppressor Screen and Phenotype Analyses Revealed an Emerging Role of the Monofunctional Peroxisomal Enoyl-CoA Hydratase 2 in Compensated Cell Enlargement

Efficient use of seed nutrient reserves is crucial for germination and establishment of plant seedlings. Mobilizing seed oil reserves in Arabidopsis involves β-oxidation, the glyoxylate cycle, and gluconeogenesis, which provide essential energy and the carbon skeletons needed to sustain seedling growth until photoautotrophy is acquired. We demonstrated that H(+)-PPase activity is required for gluconeogenesis. Lack of H(+)-PPase in fugu5 mutants increases cytosolic pyrophosphate (PPi) levels, which partially reduces sucrose synthesis de novo and inhibits cell division. In contrast, post-mitotic cell expansion in cotyledons was unusually enhanced, a phenotype called compensation. Therefore, it appears that PPi inhibits several cellular functions, including cell cycling, to trigger compensated cell enlargement (CCE). Here, we mutagenized fugu5-1 seeds with (12)C(6+) heavy-ion irradiation and screened mutations that restrain CCE to gain insight into the genetic pathway(s) involved in CCE. We isolated A#3-1, in which cell size was severely reduced, but cell number remained similar to that of original fugu5-1. Moreover, cell number decreased in A#3-1 single mutant (A#3-1sm), similar to that of fugu5-1, but cell size was almost equal to that of the wild type. Surprisingly, A#3-1 mutation did not affect CCE in other compensation exhibiting mutant backgrounds, such as an3-4 and fugu2-1/fas1-6. Subsequent map-based cloning combined with genome sequencing and HRM curve analysis identified enoyl-CoA hydratase 2 (ECH2) as the causal gene of A#3-1. The above phenotypes were consistently observed in the ech2-1 allele and supplying sucrose restored the morphological and cellular phenotypes in fugu5-1, ech2-1, A#3-1sm, fugu5-1 ech2-1, and A#3-1; fugu5-1. Taken together, these results suggest that defects in either H(+)-PPase or ECH2 compromise cell proliferation due to defects in mobilizing seed storage lipids. In contrast, ECH2 alone likely promotes CCE during the post-mitotic cell expansion stage of cotyledon development, probably by converting indolebutyric acid to indole acetic acid.

Dynamics of the Light-Dependent Transition of Plant Peroxisomes

Peroxisomes are present in almost all plant cells. These organelles are involved in various metabolic processes, such as lipid catabolism and photorespiration. A notable feature of plant peroxisomes is their flexible adaptive responses to environmental conditions such as light. When plants shift from heterotrophic to autotrophic growth during the post-germinative stage, peroxisomes undergo a dynamic response, i.e. enzymes involved in lipid catabolism are replaced with photorespiratory enzymes. Although the detailed molecular mechanisms underlying the functional transition of peroxisomes have previously been unclear, recent analyses at the cellular level have enabled this detailed machinery to be characterized. During the functional transition, obsolete enzymes are degraded inside peroxisomes by Lon protease, while newly synthesized enzymes are transported into peroxisomes. In parallel, mature and oxidized peroxisomes are eliminated via autophagy; this functional transition occurs in an efficient manner. Moreover, it has become clear that quality control mechanisms are important for the peroxisomal response to environmental stimuli. In this review, we highlight recent advances in elucidating the molecular mechanisms required for the regulation of peroxisomal roles in response to changes in environmental conditions.

Mitochondrial transporters for ornithine and related amino acids: a review

Among the members of the mitochondrial carrier family, there are transporters that catalyze the translocation of ornithine and related substrates, such as arginine, homoarginine, lysine, histidine, and citrulline, across the inner mitochondrial membrane. The mitochondrial carriers ORC1, ORC2, and SLC25A29 from Homo sapiens, BAC1 and BAC2 from Arabidopsis thaliana, and Ort1p from Saccharomyces cerevisiae have been biochemically characterized by transport assays in liposomes. All of them transport ornithine and amino acids with side chains terminating at least with one amine. There are, however, marked differences in their substrate specificities including their affinity for ornithine (KM values in the mM to μM range). These differences are most likely reflected by minor differences in the substrate binding sites of these carriers. The physiological role of the above-mentioned mitochondrial carriers is to link several metabolic pathways that take place partly in the cytosol and partly in the mitochondrial matrix and to provide basic amino acids for mitochondrial translation. In the liver, human ORC1 catalyzes the citrulline/ornithine exchange across the mitochondrial inner membrane, which is required for the urea cycle. Human ORC1, ORC2, and SLC25A29 are likely to be involved in the biosynthesis and transport of arginine, which can be used as a precursor for the synthesis of NO, agmatine, polyamines, creatine, glutamine, glutamate, and proline, as well as in the degradation of basic amino acids. BAC1 and BAC2 are implicated in some processes similar to those of their human counterparts and in nitrogen and amino acid metabolism linked to stress conditions and the development of plants. Ort1p is involved in the biosynthesis of arginine and polyamines in yeast.

Systematic phenotypic screen of Arabidopsis peroxisomal mutants identifies proteins involved in β-oxidation

Peroxisomes are highly dynamic and multifunctional organelles essential to development. Plant peroxisomes accommodate a multitude of metabolic reactions, many of which are related to the β-oxidation of fatty acids or fatty acid-related metabolites. Recently, several dozens of novel peroxisomal proteins have been identified from Arabidopsis (Arabidopsis thaliana) through in silico and experimental proteomic analyses followed by in vivo protein targeting validations. To determine the functions of these proteins, we interrogated their transfer DNA insertion mutants with a series of physiological, cytological, and biochemical assays to reveal peroxisomal deficiencies. Sugar dependence and 2,4-dichlorophenoxybutyric acid and 12-oxo-phytodienoic acid response assays uncovered statistically significant phenotypes in β-oxidation-related processes in mutants for 20 of 27 genes tested. Additional investigations uncovered a subset of these mutants with abnormal seed germination, accumulation of oil bodies, and delayed degradation of long-chain fatty acids during early seedling development. Mutants for seven genes exhibited deficiencies in multiple assays, strongly suggesting the involvement of their gene products in peroxisomal β-oxidation and initial seedling growth. Proteins identified included isoforms of enzymes related to β-oxidation, such as acyl-CoA thioesterase2, acyl-activating enzyme isoform1, and acyl-activating enzyme isoform5, and proteins with functions previously unknown to be associated with β-oxidation, such as Indigoidine synthase A, Senescence-associated protein/B12D-related protein1, Betaine aldehyde dehydrogenase, and Unknown protein5. This multipronged phenotypic screen allowed us to reveal β-oxidation proteins that have not been discovered by single assay-based mutant screens and enabled the functional dissection of different isoforms of multigene families involved in β-oxidation.

Metabolic control of redox and redox control of metabolism in plants

SIGNIFICANCE

Reduction-oxidation (Redox) status operates as a major integrator of subcellular and extracellular metabolism and is simultaneously itself regulated by metabolic processes. Redox status not only dominates cellular metabolism due to the prominence of NAD(H) and NADP(H) couples in myriad metabolic reactions but also acts as an effective signal that informs the cell of the prevailing environmental conditions. After relay of this information, the cell is able to appropriately respond via a range of mechanisms, including directly affecting cellular functioning and reprogramming nuclear gene expression.

RECENT ADVANCES

The facile accession of Arabidopsis knockout mutants alongside the adoption of broad-scale post-genomic approaches, which are able to provide transcriptomic-, proteomic-, and metabolomic-level information alongside traditional biochemical and emerging cell biological techniques, has dramatically advanced our understanding of redox status control. This review summarizes redox status control of metabolism and the metabolic control of redox status at both cellular and subcellular levels.

CRITICAL ISSUES

It is becoming apparent that plastid, mitochondria, and peroxisome functions influence a wide range of processes outside of the organelles themselves. While knowledge of the network of metabolic pathways and their intraorganellar redox status regulation has increased in the last years, little is known about the interorganellar redox signals coordinating these networks. A current challenge is, therefore, synthesizing our knowledge and planning experiments that tackle redox status regulation at both inter- and intracellular levels.

FUTURE DIRECTIONS

Emerging tools are enabling ever-increasing spatiotemporal resolution of metabolism and imaging of redox status components. Broader application of these tools will likely greatly enhance our understanding of the interplay of redox status and metabolism as well as elucidating and characterizing signaling features thereof. We propose that such information will enable us to dissect the regulatory hierarchies that mediate the strict coupling of metabolism and redox status which, ultimately, determine plant growth and development.

RuBPCase activase (RCA) mediates growth-defense trade-offs: silencing RCA redirects jasmonic acid (JA) flux from JA-isoleucine to methyl jasmonate (MeJA) to attenuate induced defense responses in Nicotiana attenuata

• RuBPCase activase (RCA), an abundant photosynthetic protein, is strongly down-regulated in response to Manduca sexta's oral secretion (OS) in Nicotiana attenuata. RCA-silenced plants are impaired not only in photosynthetic capacity and growth, but also in jasmonic acid-isoleucine (JA-Ile) signaling, and herbivore resistance mediated by JA-Ile-dependent defense traits. These responses are consistent with a resource-based growth-defense trade-off. • As JA + Ile supplementation of OS restored wild-type (WT) levels of JA-Ile, defenses and resistance to M. sexta, but OS supplemented individually with JA or Ile did not, the JA-Ile deficiency of RCA-silenced plants could not be attributed to lower JA or Ile pools or JAR4/6 conjugating activity. Similar levels of JA-Ile derivatives after OS elicitation indicated unaltered JA-Ile turnover, and lower levels of other JA conjugates ruled out competition from other conjugation reactions. • RCA-silenced plants accumulated more methyl jasmonate (MeJA) after OS elicitation, which corresponded to increased jasmonate methyltransferase (JMT) activity. RCA silencing phenocopies JMT overexpression, wherein elevated JMT activity redirects OS-elicited JA flux towards inactive MeJA, creating a JA sink which depletes JA-Ile and its associated defense responses. • Hence, RCA plays an additional non-photosynthetic role in attenuating JA-mediated defenses and their associated costs, potentially allowing plants to anticipate resource-based constraints on growth before they actually occur.

Highly oxidized peroxisomes are selectively degraded via autophagy in Arabidopsis

The positioning of peroxisomes in a cell is a regulated process that is closely associated with their functions. Using this feature of the peroxisomal positioning as a criterion, we identified three Arabidopsis thaliana mutants (peroxisome unusual positioning1 [peup1], peup2, and peup4) that contain aggregated peroxisomes. We found that the PEUP1, PEUP2, and PEUP4 were identical to Autophagy-related2 (ATG2), ATG18a, and ATG7, respectively, which are involved in the autophagic system. The number of peroxisomes was increased and the peroxisomal proteins were highly accumulated in the peup1 mutant, suggesting that peroxisome degradation by autophagy (pexophagy) is deficient in the peup1 mutant. These aggregated peroxisomes contained high levels of inactive catalase and were more oxidative than those of the wild type, indicating that peroxisome aggregates comprise damaged peroxisomes. In addition, peroxisome aggregation was induced in wild-type plants by exogenous application of hydrogen peroxide. The cat2 mutant also contained peroxisome aggregates. These findings demonstrate that hydrogen peroxide as a result of catalase inactivation is the inducer of peroxisome aggregation. Furthermore, an autophagosome marker, ATG8, frequently colocalized with peroxisome aggregates, indicating that peroxisomes damaged by hydrogen peroxide are selectively degraded by autophagy in the wild type. Our data provide evidence that autophagy is crucial for quality control mechanisms for peroxisomes in Arabidopsis.

Proteome analysis of peroxisomes from etiolated Arabidopsis seedlings identifies a peroxisomal protease involved in β-oxidation and development

Plant peroxisomes are highly dynamic organelles that mediate a suite of metabolic processes crucial to development. Peroxisomes in seeds/dark-grown seedlings and in photosynthetic tissues constitute two major subtypes of plant peroxisomes, which had been postulated to contain distinct primary biochemical properties. Multiple in-depth proteomic analyses had been performed on leaf peroxisomes, yet the major makeup of peroxisomes in seeds or dark-grown seedlings remained unclear. To compare the metabolic pathways of the two dominant plant peroxisomal subtypes and discover new peroxisomal proteins that function specifically during seed germination, we performed proteomic analysis of peroxisomes from etiolated Arabidopsis (Arabidopsis thaliana) seedlings. The detection of 77 peroxisomal proteins allowed us to perform comparative analysis with the peroxisomal proteome of green leaves, which revealed a large overlap between these two primary peroxisomal variants. Subcellular targeting analysis by fluorescence microscopy validated around 10 new peroxisomal proteins in Arabidopsis. Mutant analysis suggested the role of the cysteine protease RESPONSE TO DROUGHT21A-LIKE1 in β-oxidation, seed germination, and growth. This work provides a much-needed road map of a major type of plant peroxisome and has established a basis for future investigations of peroxisomal proteolytic processes to understand their roles in development and in plant interaction with the environment.

An inhibitor of oil body mobilization in Arabidopsis

Fatty acid β-oxidation is an essential process in many aspects of plant development, and storage oil in the form of triacylglycerol (TAG) is an important food source for humans and animals, for biofuel and for industrial feedstocks. In this study we characterize the effects of a small molecule, diphenyl methylphosphonate, on oil mobilization in Arabidopsis thaliana. Confocal laser scanning microscopy, transmission electron microscopy and quantitative lipid profiling were used to examine the effects of diphenyl methylphosphonate treatment on seedlings. Diphenyl methylphosphonate causes peroxisome clustering around oil bodies but does not affect morphology of other cellular organelles. We show that this molecule blocks the breakdown of pre-existing oil bodies resulting in retention of TAG and accumulation of acyl CoAs. The biochemical and phenotypic effects are consistent with a block in the early part of the β-oxidation pathway. Diphenyl methylphosphonate appears to be a fairly specific inhibitor of TAG mobilization in plants and whilst further work is required to identify the molecular target of the compound it should prove a useful tool to interrogate and manipulate these pathways in a controlled and reproducible manner.

An engineered plant peroxisome and its application in biotechnology

Plant metabolic engineering is a promising tool for biotechnological applications. Major goals include enhancing plant fitness for an increased product yield and improving or introducing novel pathways to synthesize industrially relevant products. Plant peroxisomes are favorable targets for metabolic engineering, because they are involved in diverse functions, including primary and secondary metabolism, development, abiotic stress response, and pathogen defense. This review discusses targets for manipulating endogenous peroxisomal pathways, such as fatty acid β-oxidation, or introducing novel pathways, such as the synthesis of biodegradable polymers. Furthermore, strategies to bypass peroxisomal pathways for improved energy efficiency and detoxification of environmental pollutants are discussed. In sum, we highlight the biotechnological potential of plant peroxisomes and indicate future perspectives to exploit peroxisomes as biofactories.

Identification of a dual-targeted protein belonging to the mitochondrial carrier family that is required for early leaf development in rice

A dual-targeted protein belonging to the mitochondrial carrier family was characterized in rice (Oryza sativa) and designated 3'-Phosphoadenosine 5'-Phosphosulfate Transporter1 (PAPST1). The papst1 mutant plants showed a defect in thylakoid development, resulting in leaf chlorosis at an early leaf developmental stage, while normal leaf development was restored 4 to 6 d after leaf emergence. OsPAPST1 is highly expressed in young leaves and roots, while the expression is reduced in mature leaves, in line with the recovery of chloroplast development seen in the older leaves of papst1 mutant plants. OsPAPST1 is located on the outer mitochondrial membrane and chloroplast envelope. Whole-genome transcriptomic analysis reveals reduced expression of genes encoding photosynthetic components (light reactions) in papst1 mutant plants. In addition, sulfur metabolism is also perturbed in papst1 plants, and it was seen that PAPST1 can act as a nucleotide transporter when expressed in Escherichia coli that can be inhibited significantly by 3'-phosphoadenosine 5'-phosphosulfate. Given these findings, together with the altered phenotype seen only when leaves are first exposed to light, it is proposed that PAPST1 may act as a 3'-phosphoadenosine 5'-phosphosulfate carrier that has been shown to act as a retrograde signal between chloroplasts and the nucleus.

Metabolite transporters of the plant peroxisomal membrane: known and unknown

Tremendous progress in plant peroxisome research has revealed unexpected metabolic functions for plant peroxisomes. Besides photorespiration and lipid metabolism, plant peroxisomes play a key role in many metabolic and signaling pathways, such as biosynthesis of phytohormones, pathogen defense, senescence-associated processes, biosynthesis of biotin and isoprenoids, and metabolism of urate, polyamines, sulfite, phylloquinone, volatile benzenoids, and branched chain amino acids. These peroxisomal pathways require an interplay with other cellular compartments, including plastids, mitochondria, and the cytosol. Consequently, a considerable number of substrates, intermediates, end products, and cofactors have to shuttle across peroxisome membranes. However, our knowledge of their membrane passage is still quite limited. This review describes the solute transport processes required to connect peroxisomes with other cell compartments. Furthermore, we discuss the known and yet-to-be-defined transport proteins that mediate these metabolic exchanges across the peroxisomal bilayer.

Seed storage oil catabolism: a story of give and take

The transition from seed to seedling is an important step in the life cycle of plants, which is fuelled primarily by the breakdown of triacylglycerol (TAG) in 'oilseed' species. TAG is stored within cytosolic oil bodies, while the pathway for fatty acid β-oxidation resides in the peroxisome. Although the enzymology of fatty acid β-oxidation has been relatively well characterised, the processes by which fatty acids are liberated from oil bodies and enter the peroxisome are less well understood and, together with metabolite, cofactor and co-substrate transporters, represent key targets for future research in order to understand co-ordination of peroxisomal metabolism with that of other subcellular compartments.

Plant peroxisomes: biogenesis and function

Peroxisomes are eukaryotic organelles that are highly dynamic both in morphology and metabolism. Plant peroxisomes are involved in numerous processes, including primary and secondary metabolism, development, and responses to abiotic and biotic stresses. Considerable progress has been made in the identification of factors involved in peroxisomal biogenesis, revealing mechanisms that are both shared with and diverged from non-plant systems. Furthermore, recent advances have begun to reveal an unexpectedly large plant peroxisomal proteome and have increased our understanding of metabolic pathways in peroxisomes. Coordination of the biosynthesis, import, biochemical activity, and degradation of peroxisomal proteins allows for highly dynamic responses of peroxisomal metabolism to meet the needs of a plant. Knowledge gained from plant peroxisomal research will be instrumental to fully understanding the organelle's dynamic behavior and defining peroxisomal metabolic networks, thus allowing the development of molecular strategies for rational engineering of plant metabolism, biomass production, stress tolerance, and pathogen defense.

Glyoxylate reductase isoform 1 is localized in the cytosol and not peroxisomes in plant cells

Glyoxylate reductase (GLYR) is a key enzyme in plant metabolism which catalyzes the detoxification of both photorespiratory glyoxylate and succinic semialdehdye, an intermediate of the γ-aminobutyrate (GABA) pathway. Two isoforms of GLYR exist in plants, GLYR1 and GLYR2, and while GLYR2 is known to be localized in plastids, GLYR1 has been reported to be localized in either peroxisomes or the cytosol. Here, we reappraised the intracellular localization of GLYR1 in Arabidopsis thaliana L. Heynh (ecotype Lansberg erecta) using both transiently-transformed suspension cells and stably-transformed plants, in combination with fluorescence microscopy. The results indicate that GLYR1 is localized exclusively to the cytosol regardless of the species, tissue and/or cell type, or exposure of plants to environmental stresses that would increase flux through the GABA pathway. Moreover, the C-terminal tripeptide sequence of GLYR1, -SRE, despite its resemblance to a type 1 peroxisomal targeting signal, is not sufficient for targeting to peroxisomes. Collectively, these results define the cytosol as the intracellular location of GLYR1 and provide not only important insight to the metabolic roles of GLYR1 and the compartmentation of the GABA and photorespiratory pathways in plant cells, but also serve as a useful reference for future studies of proteins proposed to be localized to peroxisomes and/or the cytosol.