Physical interaction between peroxisomes and chloroplasts elucidated by in situ laser analysis

Evidence for dual targeting control of Arabidopsis 6-phosphogluconate dehydrogenase isoforms by N-terminal phosphorylation

The oxidative pentose-phosphate pathway (OPPP) retrieves NADPH from glucose-6-phosphate, which is important in chloroplasts at night and in plastids of heterotrophic tissues. We previously studied how OPPP enzymes may transiently locate to peroxisomes, but how this is achieved for the third enzyme remained unclear. By extending our genetic approach, we demonstrated that Arabidopsis isoform 6-phosphogluconate dehydrogenase 2 (PGD2) is indispensable in peroxisomes during fertilization, and investigated why all PGD-reporter fusions show a mostly cytosolic pattern. A previously published interaction of a plant PGD with thioredoxin m was confirmed using Trxm2 for yeast two-hybrid (Y2H) and bimolecular fluorescent complementation (BiFC) assays, and medial reporter fusions (with both ends accessible) proved to be beneficial for studying peroxisomal targeting of PGD2. Of special importance were phosphomimetic changes at Thr6, resulting in a clear targeting switch to peroxisomes, while a similar change at position Ser7 in PGD1 conferred plastid import. Apparently, efficient subcellular localization can be achieved by activating an unknown kinase, either early after or during translation. N-terminal phosphorylation of PGD2 interfered with dimerization in the cytosol, thus allowing accessibility of the C-terminal peroxisomal targeting signal (PTS1). Notably, we identified amino acid positions that are conserved among plant PGD homologues, with PTS1 motifs first appearing in ferns, suggesting a functional link to fertilization during the evolution of seed plants.

A century journey of organelles research in the plant endomembrane system

We are entering an exciting century in the study of the plant organelles in the endomembrane system. Over the past century, especially within the past 50 years, tremendous advancements have been made in the complex plant cell to generate a much clearer and informative picture of plant organelles, including the molecular/morphological features, dynamic/spatial behavior, and physiological functions. Importantly, all these discoveries and achievements in the identification and characterization of organelles in the endomembrane system would not have been possible without: (1) the innovations and timely applications of various state-of-art cell biology tools and technologies for organelle biology research; (2) the continuous efforts in developing and characterizing new organelle markers by the plant biology community; and (3) the landmark studies on the identification and characterization of the elusive organelles. While molecular aspects and results for individual organelles have been extensively reviewed, the development of the techniques for organelle research in plant cell biology is less appreciated. As one of the ASPB Centennial Reviews on "organelle biology," here we aim to take a journey across a century of organelle biology research in plants by highlighting the important tools (or landmark technologies) and key scientists that contributed to visualize organelles. We then highlight the landmark studies leading to the identification and characterization of individual organelles in the plant endomembrane systems.

A molecular atlas of plastid and mitochondrial proteins reveals organellar remodeling during plant evolutionary transitions from algae to angiosperms

Algae and plants carry 2 organelles of endosymbiotic origin that have been co-evolving in their host cells for more than a billion years. The biology of plastids and mitochondria can differ significantly across major lineages and organelle changes likely accompanied the adaptation to new ecological niches such as the terrestrial habitat. Based on organelle proteome data and the genomes of 168 phototrophic (Archaeplastida) versus a broad range of 518 non-phototrophic eukaryotes, we screened for changes in plastid and mitochondrial biology across 1 billion years of evolution. Taking into account 331,571 protein families (or orthogroups), we identify 31,625 protein families that are unique to primary plastid-bearing eukaryotes. The 1,906 and 825 protein families are predicted to operate in plastids and mitochondria, respectively. Tracing the evolutionary history of these protein families through evolutionary time uncovers the significant remodeling the organelles experienced from algae to land plants. The analyses of gained orthogroups identifies molecular changes of organelle biology that connect to the diversification of major lineages and facilitated major transitions from chlorophytes en route to the global greening and origin of angiosperms.

Collective mitochondrial dynamics resolve conflicting cellular tensions: From plants to general principles

Mitochondria play diverse and essential roles in eukaryotic cells, and plants are no exception. Plant mitochondria have several differences from their metazoan and fungal cousins: they often exist in a fragmented state, move rapidly on actin rather than microtubules, have many plant-specific metabolic features and roles, and usually contain only a subset of the complete mtDNA genome, which itself undergoes frequent recombination. This arrangement means that exchange and complementation is essential for plant mitochondria, and recent work has begun to reveal how their collective dynamics and resultant "social networks" of encounters support this exchange, connecting plant mitochondria in time rather than in space. This review will argue that this social network perspective can be extended to a "societal network", where mitochondrial dynamics are an essential part of the interacting cellular society of organelles and biomolecules. Evidence is emerging that mitochondrial dynamics allow optimal resolutions to competing cellular priorities; we will survey this evidence and review potential future research directions, highlighting that plant mitochondria can help reveal and test principles that apply across other kingdoms of life. In parallel with this fundamental cell biology, we also highlight the translational "One Health" importance of plant mitochondrial behaviour - which is exploited in the production of a vast amount of crops consumed worldwide - and the potential for multi-objective optimisation to understand and rationally re-engineer the evolved resolutions to these tensions.

Peroxisome-based metabolic engineering for biomanufacturing and agriculture

Subcellular compartmentalization of metabolic pathways plays a crucial role in metabolic engineering. The peroxisome has emerged as a highly valuable and promising compartment for organelle engineering, particularly in the fields of biological manufacturing and agriculture. In this review, we summarize the remarkable achievements in peroxisome engineering in yeast, the industrially popular biomanufacturing chassis host, to produce various biocompounds. We also review progress in plant peroxisome engineering, a field that has already exhibited high potential in both biomanufacturing and agriculture. Moreover, we outline various experimentally validated strategies to improve the efficiency of engineered pathways in peroxisomes, as well as prospects of peroxisome engineering.

Plastid Transient and Stable Interactions with Other Cell Compartments

Plastids are organelles delineated by two envelopes playing important roles in different cellular processes such as energy production or lipid biosynthesis. To regulate their biogenesis and their function, plastids have to communicate with other cellular compartments. This communication can be mediated by metabolites, signaling molecules, and by the establishment of direct contacts between the plastid envelope and other organelles such as the endoplasmic reticulum, mitochondria, peroxisomes, plasma membrane, and the nucleus. These interactions are highly dynamic and respond to different biotic and abiotic stresses. However, the mechanisms involved in the formation of plastid-organelle contact sites and their functions are still far from being understood. In this chapter, we summarize our current knowledge about plastid contact sites and their role in the regulation of plastid biogenesis and function.

Visualizing the dynamics of plant energy organelles

Plant organelles predominantly rely on the actin cytoskeleton and the myosin motors for long-distance trafficking, while using microtubules and the kinesin motors mostly for short-range movement. The distribution and motility of organelles in the plant cell are fundamentally important to robust plant growth and defense. Chloroplasts, mitochondria, and peroxisomes are essential organelles in plants that function independently and coordinately during energy metabolism and other key metabolic processes. In response to developmental and environmental stimuli, these energy organelles modulate their metabolism, morphology, abundance, distribution and motility in the cell to meet the need of the plant. Consistent with their metabolic links in processes like photorespiration and fatty acid mobilization is the frequently observed inter-organellar physical interaction, sometimes through organelle membranous protrusions. The development of various organelle-specific fluorescent protein tags has allowed the simultaneous visualization of organelle movement in living plant cells by confocal microscopy. These energy organelles display an array of morphology and movement patterns and redistribute within the cell in response to changes such as varying light conditions, temperature fluctuations, ROS-inducible treatments, and during pollen tube development and immune response, independently or in association with one another. Although there are more reports on the mechanism of chloroplast movement than that of peroxisomes and mitochondria, our knowledge of how and why these three energy organelles move and distribute in the plant cell is still scarce at the functional and mechanistic level. It is critical to identify factors that control organelle motility coupled with plant growth, development, and stress response.

Peroxisomes undergo morphological changes in a light-dependent manner with proximity to the nucleus

The size and shape of organelles can influence the rate of biochemical reactions in cells. Previous studies have suggested that organelle morphology changes due to intra- and extracellular environmental responses, affecting the metabolic efficiency of and signal transduction emanating from neighboring organelles. In this study, we tested the possibility that intracellularly distributed organelles exhibit a heterogeneous response to intra- and extracellular environments. We detected a high correlation between peroxisome morphology and distance to the nucleus in light-exposed cells. Moreover, the proximity area between chloroplasts and peroxisomes varied with distance to the nucleus. These results indicate that peroxisome morphology varies with proximity to the nucleus, suggesting the presence of a nucleus-peroxisome signal transduction cascade mediated by chloroplasts.

Applying Optical Tweezers with TIRF Microscopy to Quantify Physical Interactions Between Organelles in the Plant Endomembrane System

Plant organelles are associated with each other through tethering proteins at membrane contact sites (MCS). Methods such as total internal reflection fluorescence (TIRF) optical tweezers allow us to probe organelle interactions in live plant cells. Optical tweezers (focused infrared laser beams) can trap organelles that have a different refractive index to their surrounding medium (cytosol), whilst TIRF allows us to simultaneously image behaviors of organelles in the thin region of cortical cytoplasm. However, few MCS tethering proteins have so far been identified and tested in a quantitative manner. Automated routines (such as setting trapping laser power and controlling the stage speed and distance) mean we can quantify organelle interactions in a repeatable and reproducible manner. Here we outline a series of protocols which describe laser calibrations required to collect robust data sets, generation of fluorescent plant material (Nicotiana tabacum, tobacco), how to set up an automated organelle trapping routine, and how to quantify organelle interactions (particularly organelle interactions with the endoplasmic reticulum). TIRF-optical tweezers enable quantitative testing of putative tethering proteins to reveal their role in plant organelle associations at MCS. © 2023 Wiley Periodicals LLC. Basic Protocol 1: Microscope system set-up and stability Basic Protocol 2: Generation of transiently expressed fluorescent tobacco tissue by Agrobacterium-mediated infiltration Basic Protocol 3: Setting up an automated organelle trapping routine Basic Protocol 4: Quantifying organelle interactions.

Keep in contact: multiple roles of endoplasmic reticulum-membrane contact sites and the organelle interaction network in plants

Functional regulation and structural maintenance of the different organelles in plants contribute directly to plant development, reproduction and stress responses. To ensure these activities take place effectively, cells have evolved an interconnected network amongst various subcellular compartments, regulating rapid signal transduction and the exchange of biomaterial. Many proteins that regulate membrane connections have recently been identified in plants, and this is the first step in elucidating both the mechanism and function of these connections. Amongst all organelles, the endoplasmic reticulum is the key structure, which likely links most of the different subcellular compartments through membrane contact sites (MCS) and the ER-PM contact sites (EPCS) have been the most intensely studied in plants. However, the molecular composition and function of plant MCS are being found to be different from other eukaryotic systems. In this article, we will summarise the most recent advances in this field and discuss the mechanism and biological relevance of these essential links in plants.

Patterned proliferation orients tissue-wide stress to control root vascular symmetry in Arabidopsis

Symmetric tissue alignment is pivotal to the functions of plant vascular tissue, such as long-distance molecular transport and lateral organ formation. During the vascular development of the Arabidopsis roots, cytokinins initially determine cell-type boundaries among vascular stem cells and subsequently promote cell proliferation to establish vascular tissue symmetry. Although it is unknown whether and how the symmetry of initially defined boundaries is progressively refined under tissue growth in plants, such boundary shapes in animal tissues are regulated by cell fluidity, e.g., cell migration and intercalation, lacking in plant tissues. Here, we uncover that cell proliferation during vascular development produces anisotropic compressive stress, smoothing, and symmetrizing cell arrangement of the vascular-cell-type boundary. Mechanistically, the GATA transcription factor HANABA-TARANU cooperates with the type-B Arabidopsis response regulators to form an incoherent feedforward loop in cytokinin signaling. The incoherent feedforward loop fine-tunes the position and frequency of vascular cell proliferation, which in turn restricts the source of mechanical stress to the position distal and symmetric to the boundary. By combinatorial analyses of mechanical simulations and laser cell ablation, we show that the spatially constrained environment of vascular tissue efficiently entrains the stress orientation among the cells to produce a tissue-wide stress field. Together, our data indicate that the localized proliferation regulated by the cytokinin signaling circuit is decoded into a globally oriented mechanical stress to shape the vascular tissue symmetry, representing a reasonable mechanism controlling the boundary alignment and symmetry in tissue lacking cell fluidity.

Using the organelle glue technique to engineer the plant cell metabolome

By using the organelle glue technique, we artificially manipulated organelle interactions and controlled the plant metabolome at the pathway level. Plant cell metabolic activity changes with fluctuating environmental conditions, in part via adjustments in the arrangement and interaction of organelles. This hints at the potential for designing plants with desirable metabolic activities for food and pharmaceutical industries by artificially controlling the interaction of organelles through genetic modification. We previously developed a method called the organelle glue technique, in which chloroplast-chloroplast adhesion is induced in plant cells using the multimerization properties of split fluorescent proteins. Here, we generated transgenic Arabidopsis (Arabidopsis thaliana) plants in which chloroplasts adhere to each other and performed metabolome analysis to examine the metabolic changes in these lines. In plant cells expressing a construct encoding the red fluorescent protein mCherry targeted to the chloroplast outer envelope by fusion with a signal sequence (cTP-mCherry), chloroplasts adhered to each other and formed chloroplast aggregations. Mitochondria and peroxisomes were embedded in the aggregates, suggesting that normal interactions between chloroplasts and these organelles were also affected. Metabolome analysis of the cTP-mCherry-expressing Arabidopsis shoots revealed significantly higher levels of glycine, serine, and glycerate compared to control plants. Notably, these are photorespiratory metabolites that are normally transported between chloroplasts, mitochondria, and peroxisomes. Together, our data indicate that chloroplast-chloroplast adhesion alters organellar interactions with mitochondria and peroxisomes and disrupts photorespiratory metabolite transport. These results highlight the possibility of controlling plant metabolism at the pathway level by manipulating organelle interactions.

Tissue-Specific Isolation of Tagged Arabidopsis Plastids

Plastids are found in all plant cell types. However, most extraction methods to study these organelles are performed at the organ level (e.g., leaf, root, fruit) and do not allow for tissue-specific resolution, which hinders our understanding of their physiology. Therefore, IPTACT (Isolation of Plastids TAgged in specific Cell Types) was developed to isolate plastids in a tissue-specific manner in Arabidopsis thaliana (Arabidopsis). Plastids are biotinylated using one-shot transgenic lines, and tissue specificity is achieved with a suitable promoter as long as such a promoter exists. Cell-specific biotinylated plastids are then isolated with 2.8-µm streptavidin beads. Plastids extracted by IPTACT are suitable for RNA or protein isolation and subsequent tissue-specific OMICs analyses. This method provides the user with a powerful tool to investigate plastidial functions at cell-type resolution. Furthermore, it can easily be combined with studies using diverse genetic backgrounds and/or different developmental or stress conditions. © 2022 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Promoter cloning and plant selection Basic Protocol 2: Isolation of biotinylated plastids Basic Protocol 3: Quality control of isolated plastids.

Co-regulation of mitochondrial and chloroplast function: Molecular components and mechanisms

The metabolic interdependence, interactions, and coordination of functions between chloroplasts and mitochondria are established and intensively studied. However, less is known about the regulatory components that control these interactions and their responses to external stimuli. Here, we outline how chloroplastic and mitochondrial activities are coordinated via common components involved in signal transduction pathways, gene regulatory events, and post-transcriptional processes. The endoplasmic reticulum emerges as a point of convergence for both transcriptional and post-transcriptional pathways that coordinate chloroplast and mitochondrial functions. Although the identification of molecular components and mechanisms of chloroplast and mitochondrial signaling increasingly suggests common players, this raises the question of how these allow for distinct organelle-specific downstream pathways. Outstanding questions with respect to the regulation of post-transcriptional pathways and the cell and/or tissue specificity of organelle signaling are crucial for understanding how these pathways are integrated at a whole-plant level to optimize plant growth and its response to changing environmental conditions.

Pexophagy suppresses ROS-induced damage in leaf cells under high-intensity light

Although light is essential for photosynthesis, it has the potential to elevate intracellular levels of reactive oxygen species (ROS). Since high ROS levels are cytotoxic, plants must alleviate such damage. However, the cellular mechanism underlying ROS-induced leaf damage alleviation in peroxisomes was not fully explored. Here, we show that autophagy plays a pivotal role in the selective removal of ROS-generating peroxisomes, which protects plants from oxidative damage during photosynthesis. We present evidence that autophagy-deficient mutants show light intensity-dependent leaf damage and excess aggregation of ROS-accumulating peroxisomes. The peroxisome aggregates are specifically engulfed by pre-autophagosomal structures and vacuolar membranes in both leaf cells and isolated vacuoles, but they are not degraded in mutants. ATG18a-GFP and GFP-2×FYVE, which bind to phosphatidylinositol 3-phosphate, preferentially target the peroxisomal membranes and pre-autophagosomal structures near peroxisomes in ROS-accumulating cells under high-intensity light. Our findings provide deeper insights into the plant stress response caused by light irradiation.

Organellar Glue: A Molecular Tool to Artificially Control Chloroplast-Chloroplast Interactions

Organelles can physically interact to facilitate various cellular processes such as metabolite exchange. Artificially regulating these interactions represents a promising approach for synthetic biology. Here, we artificially controlled chloroplast-chloroplast interactions in living plant cells with our organelle glue (ORGL) technique, which is based on reconstitution of a split fluorescent protein. We simultaneously targeted N-terminal and C-terminal fragments of a fluorescent protein to the chloroplast outer envelope membrane or cytosol, respectively, which induced chloroplast-chloroplast interactions. The cytosolic C-terminal fragment likely functions as a bridge between two N-terminal fragments, thereby bringing the chloroplasts in close proximity to interact. We modulated the frequency of chloroplast-chloroplast interactions by altering the ratio of N- and C-terminal fragments. We conclude that the ORGL technique can successfully control chloroplast-chloroplast interactions in plants, providing a proof of concept for the artificial regulation of organelle interactions in living cells.

A high-throughput quantitative method to evaluate peroxisome-chloroplast interactions in Arabidopsis thaliana

Organelles contribute to plant growth via their movements and interactions, which ensure efficient metabolic flow and help plants adapt to environmental stress. Live-cell imaging of the interactions of organelles has been performed in yeast, plant, and animal cells. However, high-throughput quantitative methods are needed to simultaneously analyze the interactions of many organelles in living plant cells. Here, we developed a semi-automatic high-throughput method to quantitatively evaluate the interactions between peroxisomes and chloroplasts using a distance transformation algorithm and high-resolution 3D fluorescent images taken by confocal laser scanning microscopy. Using this method, we measured the 3D distance between the center of peroxisome and chloroplast surface in Arabidopsis thaliana. We then compared the distances between these organelles in leaf mesophyll cells under light and dark conditions. This distance was shorter in the light than in the dark, which is in agreement with the findings of previous studies. We used our method to evaluate peroxisome-chloroplast (plastid) interactions in different cell types in the light and dark, including guard, stem, and root cells. Like in mesophyll cells, the distance between the peroxisome and chloroplast was shorter in the light in guard and stem cells, but not in root cells, suggesting that photosynthetic plastids (chloroplasts) play important roles in these interactions. When leaf mesophyll cells were incubated under high-intensity light, the frequency of shorter distances between peroxisomes and chloroplasts significantly increased. Our high-throughput, semi-automatic method represents a powerful tool for evaluating peroxisome-chloroplast interactions in different types of plant cells under various environmental conditions.

Hitchhiking Across Kingdoms: Cotransport of Cargos in Fungal, Animal, and Plant Cells

Eukaryotic cells across the tree of life organize their subcellular components via intracellular transport mechanisms. In canonical transport, myosin, kinesin, and dynein motor proteins interact with cargos via adaptor proteins and move along filamentous actin or microtubule tracks. In contrast to this canonical mode, hitchhiking is a newly discovered mode of intracellular transport in which a cargo attaches itself to an already-motile cargo rather than directly associating with a motor protein itself. Many cargos including messenger RNAs, protein complexes, and organelles hitchhike on membrane-bound cargos. Hitchhiking-like behaviors have been shown to impact cellular processes including local protein translation, long-distance signaling, and organelle network reorganization. Here, we review instances of cargo hitchhiking in fungal, animal, and plant cells and discuss the potential cellular and evolutionary importance of hitchhiking in these different contexts.

Critical roles of mitochondrial fatty acid synthesis in tomato development and environmental response

Plant mitochondrial fatty acid synthesis (mtFAS) appears to be important in photorespiration based on the reverse genetics research from Arabidopsis (Arabidopsis thaliana) in recent years, but its roles in plant development have not been completely explored. Here, we identified a tomato (Solanum lycopersicum) mutant, fern-like, which displays pleiotropic phenotypes including dwarfism, yellowing, curly leaves, and increased axillary buds. Positional cloning and genetic and heterozygous complementation tests revealed that the underlying gene FERN encodes a 3-hydroxyl-ACP dehydratase enzyme involved in mtFAS. FERN was causally involved in tomato morphogenesis by affecting photorespiration, energy supply, and the homeostasis of reactive oxygen species. Based on lipidome data, FERN and the mtFAS pathway may modulate tomato development by influencing mitochondrial membrane lipid composition and other lipid metabolic pathways. These findings provide important insights into the roles and importance of mtFAS in tomato development.

Links between autophagy and lipid droplet dynamics

Autophagy is a catabolic process in which cytoplasmic components are delivered to vacuoles or lysosomes for degradation and nutrient recycling. Autophagy-mediated degradation of membrane lipids provides a source of fatty acids for the synthesis of energy-rich, storage lipid esters such as triacylglycerol (TAG). In eukaryotes, storage lipids are packaged into dynamic subcellular organelles, lipid droplets. In times of energy scarcity, lipid droplets can be degraded via autophagy in a process termed lipophagy to release fatty acids for energy production via fatty acid β-oxidation. On the other hand, emerging evidence suggests that lipid droplets are required for the efficient execution of autophagic processes. Here, we review recent advances in our understanding of metabolic interactions between autophagy and TAG storage, and discuss mechanisms of lipophagy. Free fatty acids are cytotoxic due to their detergent-like properties and their incorporation into lipid intermediates that are toxic at high levels. Thus, we also discuss how cells manage lipotoxic stresses during autophagy-mediated mobilization of fatty acids from lipid droplets and organellar membranes for energy generation.

Image-Based Analysis Revealing the Molecular Mechanism of Peroxisome Dynamics in Plants

Peroxisomes are present in eukaryotic cells and have essential roles in various biological processes. Plant peroxisomes proliferate by de novo biosynthesis or division of pre-existing peroxisomes, degrade, or replace metabolic enzymes, in response to developmental stages, environmental changes, or external stimuli. Defects of peroxisome functions and biogenesis alter a variety of biological processes and cause aberrant plant growth. Traditionally, peroxisomal function-based screening has been employed to isolate Arabidopsis thaliana mutants that are defective in peroxisomal metabolism, such as lipid degradation and photorespiration. These analyses have revealed that the number, subcellular localization, and activity of peroxisomes are closely related to their efficient function, and the molecular mechanisms underlying peroxisome dynamics including organelle biogenesis, protein transport, and organelle interactions must be understood. Various approaches have been adopted to identify factors involved in peroxisome dynamics. With the development of imaging techniques and fluorescent proteins, peroxisome research has been accelerated. Image-based analyses provide intriguing results concerning the movement, morphology, and number of peroxisomes that were hard to obtain by other approaches. This review addresses image-based analysis of peroxisome dynamics in plants, especially A. thaliana and Marchantia polymorpha.

The ER Is a Common Mediator for the Behavior and Interactions of Other Organelles

Optimal functioning of a plant cell depends upon the efficient exchange of genetic information, ions, proteins and metabolites between the different organelles. Intuitively, increased proximity between organelles would be expected to play an important role in facilitating exchanges between them. However, it remains to be seen whether under normal, relatively non-stressed conditions organelles maintain close proximity at all. Moreover, does interactivity involve direct and frequent physical contact between the different organelles? Further, many organelles transition between spherical and tubular forms or sporadically produce thin tubular extensions, but it remains unclear whether changes in organelle morphology play a role in increasing their interactivity. Here, using targeted multicolored fluorescent fusion proteins, we report observations on the spatiotemporal relationship between plastids, mitochondria, peroxisomes and the endoplasmic reticulum in living plant cells. Under normal conditions of growth, we observe that the smaller organelles do not establish direct, physical contacts with each other but, irrespective of their individual form they all maintain intimate connectivity with the ER. Proximity between organelles does increase in response to stress through concomitant alterations in ER dynamics. Significantly, even under increased proximity the ER still remains sandwiched between the different organelles. Our observations provide strong live-imaging-based evidence for the ER acting as a common mediator in interactions between other organelles.

Identification of Arabidopsis Protein Kinases That Harbor Functional Type 1 Peroxisomal Targeting Signals

Peroxisomes are eukaryotic specific organelles that perform diverse metabolic functions including fatty acid β-oxidation, reactive species metabolism, photorespiration, and responses to stress. However, the potential regulation of these functions by post-translational modifications, including protein phosphorylation, has had limited study. Recently, we identified and catalogued a large number of peroxisomal phosphorylated proteins, implicating the presence of protein kinases in this organelle. Here, we employed available prediction models coupled with sequence conservation analysis to identify 31 protein kinases from the Arabidopsis kinome (all protein kinases) that contain a putative, non-canonical peroxisomal targeting signal type 1 (PTS1). From this, twelve C-terminal domain-PTS1s were demonstrated to be functional in vivo, targeting enhanced yellow fluorescent protein to peroxisomes, increasing the list of presumptive peroxisomal protein kinases to nineteen. Of the twelve protein kinases with functional PTS1s, we obtained full length clones for eight and demonstrated that seven target to peroxisomes in vivo. Screening homozygous mutants of the presumptive nineteen protein kinases revealed one candidate (GPK1) that harbors a sugar-dependence phenotype, suggesting it is involved in regulating peroxisomal fatty acid β-oxidation. These results present new opportunities for investigating the regulation of peroxisome functions.

Multilocation proteins in organelle communication: Based on protein-protein interactions

Protein-protein interaction (PPI) plays a crucial role in most biological processes, including signal transduction and cell apoptosis. Importantly, the knowledge of PPIs can be useful for identification of multimeric protein complexes and elucidation of uncharacterized protein functions. Arabidopsis thaliana, the best-characterized dicotyledonous plant, the steadily increasing amount of information on the levels of its proteome and signaling pathways is progressively enabling more researchers to construct models for cellular processes for the plant, which in turn encourages more experimental data to be generated. In this study, we performed an overview analysis of the 10 major organelles and their associated proteins of the dicotyledonous model plant Arabidopsis thaliana via PPI network, and found that PPI may play an important role in organelle communication. Further, multilocation proteins, especially phosphorylation-related multilocation proteins, can function as a "needle and thread" via PPIs and play an important role in organelle communication. Similar results were obtained in a monocotyledonous model crop, rice. Furthermore, we provide a research strategy for multilocation proteins by LOPIT technique, proteomics, and bioinformatics analysis and also describe their potential role in the field of plant science. The results provide a new view that the phosphorylation-related multilocation proteins play an important role in organelle communication and provide new insight into PPIs and novel directions for proteomic research. The research of phosphorylation-related multilocation proteins may promote the development of organelle communication and provide an important theoretical basis for plant responses to external stress.

A glossary of plant cell structures: Current insights and future questions

In this glossary of plant cell structures, we asked experts to summarize a present-day view of plant organelles and structures, including a discussion of outstanding questions. In the following short reviews, the authors discuss the complexities of the plant cell endomembrane system, exciting connections between organelles, novel insights into peroxisome structure and function, dynamics of mitochondria, and the mysteries that need to be unlocked from the plant cell wall. These discussions are focused through a lens of new microscopy techniques. Advanced imaging has uncovered unexpected shapes, dynamics, and intricate membrane formations. With a continued focus in the next decade, these imaging modalities coupled with functional studies are sure to begin to unravel mysteries of the plant cell.

Mitochondrial proteomic analysis reveals the regulation of energy metabolism and reactive oxygen species production in Clematis terniflora DC. leaves under high-level UV-B radiation followed by dark treatment

Clematis terniflora DC. is an important medicinal plant from the family Ranunculaceae. A previous study has shown that active ingredients in C. terniflora, such as flavonoids and coumarins, are increased under ultraviolet B radiation (UV-B) and dark treatment and that the numbers of genes related to the tricarboxylic acid cycle and mitochondrial electron transport chain (mETC) are changed. To uncover the mechanism of the response to UV-B radiation and dark treatment in C. terniflora, mitochondrial proteomics was performed. The results showed that proteins related to photorespiration, mitochondrial membrane permeability, the tricarboxylic acid cycle, and the mETC mainly showed differential expression profiles. Moreover, the increase in alternative oxidase indicated that another oxygen-consuming respiratory pathway in plant mitochondria was induced to minimize mitochondrial reactive oxygen species production. These results suggested that respiration and mitochondrial membrane permeability were deeply influenced to avoid energy consumption and maintain energy balance under UV-B radiation and dark treatment in C. terniflora leaf mitochondria. Furthermore, oxidative phosphorylation was able to regulate intracellular oxygen balance to resist oxidative stress. This study improves understanding of the function of mitochondria in response to UV-B radiation and dark treatment in C. terniflora. SIGNIFICANCE: C. terniflora was an important traditional Chinese medicine for anti-inflammatory. Previous study showed that the contents of coumarins which were the main active ingredient in C. terniflora were induced by UV-B radiation and dark treatment. In the present study, to uncover the regulatory mechanism of metabolic changes in C. terniflora, mitochondrial proteomics analysis of leaves was performed. The results showed that photorespiration and oxidative phosphorylation pathways were influenced under UV-B radiation and dark treatment. Mitochondria in C. terniflora leaf played a crucial role in energy mechanism and regulation of cellular oxidation-reduction to maintain cell homeostasis under UV-B radiation followed with dark treatment.

Filopodium-derived vesicles produced by MIM enhance the migration of recipient cells

Extracellular vesicles (EVs) are classified as large EVs (l-EVs, or microvesicles) and small EVs (s-EVs, or exosomes). S-EVs are thought to be generated from endosomes through a process that mainly depends on the ESCRT protein complex, including ALG-2 interacting protein X (ALIX). However, the mechanisms of l-EV generation from the plasma membrane have not been identified. Membrane curvatures are generated by the bin-amphiphysin-rvs (BAR) family proteins, among which the inverse BAR (I-BAR) proteins are involved in filopodial protrusions. Here, we show that the I-BAR proteins, including missing in metastasis (MIM), generate l-EVs by scission of filopodia. Interestingly, MIM-containing l-EV production was promoted by in vivo equivalent external forces and by the suppression of ALIX, suggesting an alternative mechanism of vesicle formation to s-EVs. The MIM-dependent l-EVs contained lysophospholipids and proteins, including IRS4 and Rac1, which stimulated the migration of recipient cells through lamellipodia formation. Thus, these filopodia-dependent l-EVs, which we named as filopodia-derived vesicles (FDVs), modify cellular behavior.

Ferroportin 3 is a dual-targeted mitochondrial/chloroplast iron exporter necessary for iron homeostasis in Arabidopsis

Mitochondria and chloroplasts are organelles with high iron demand that are particularly susceptible to iron-induced oxidative stress. Despite the necessity of strict iron regulation in these organelles, much remains unknown about mitochondrial and chloroplast iron transport in plants. Here, we propose that Arabidopsis ferroportin 3 (FPN3) is an iron exporter that is dual-targeted to mitochondria and chloroplasts. FPN3 is expressed in shoots, regardless of iron conditions, but its transcripts accumulate under iron deficiency in roots. fpn3 mutants cannot grow as well as the wild type under iron-deficient conditions and their shoot iron levels are lower compared with the wild type. Analyses of iron homeostasis gene expression in fpn3 mutants and inductively coupled plasma mass spectrometry (ICP-MS) measurements show that iron levels in the mitochondria and chloroplasts are increased relative to the wild type, consistent with the proposed role of FPN3 as a mitochondrial/plastid iron exporter. In iron-deficient fpn3 mutants, abnormal mitochondrial ultrastructure was observed, whereas chloroplast ultrastructure was not affected, implying that FPN3 plays a critical role in the mitochondria. Overall, our study suggests that FPN3 is essential for optimal iron homeostasis.

Peroxisomes as redox-signaling nodes in intracellular communication and stress responses

Peroxisomes are redox nodes playing a diverse range of roles in cell functionality and in the perception of and responses to changes in their environment.

Organelle extensions in plant cells

The life strategy of plants includes their ability to respond quickly at the cellular level to changes in their environment. The use of targeted fluorescent protein probes and imaging of living cells has revealed several rapidly induced organelle responses that create the efficient sub-cellular machinery for maintaining homeostasis in the plant cell. Several organelles, including plastids, mitochondria, and peroxisomes, extend and retract thin tubules that have been named stromules, matrixules, and peroxules, respectively. Here, I combine all these thin tubular forms under the common head of organelle extensions. All extensions change shape continuously and in their elongated form considerably increase organelle outreach into the surrounding cytoplasm. Their pleomorphy reflects their interactions with the dynamic endoplasmic reticulum and cytoskeletal elements. Here, using foundational images and time-lapse movies, and providing salient information on some molecular and biochemically characterized mutants with increased organelle extensions, I draw attention to their common role in maintaining homeostasis in plant cells.

FMT, a protein that affects mitochondrial distribution, interacts with translation-related proteins in Arabidopsis thaliana

Two translation-related proteins are identified as FMT-interacting proteins. However, FMT, unlike mutants of other CLU genes in fly and human, has no clear impact on the accumulation of mitochondrial proteins. Organelle distribution is critical for effective metabolism and stress response and is controlled by various environmental factors. Clustered mitochondria (CLU) superfamily genes affect mitochondrial distribution and their disruptions cause mitochondria to cluster within a cell in various species including yeast, fly, mammals and Arabidopsis. In Arabidopsis thaliana, Friendly mitochondria (FMT) is a CLU gene that is required for normal mitochondrial distribution, but its molecular function is unclear. Here, we demonstrate that FMT interacts with some translation-related proteins (translation initiation factor eIFiso4G1 and glutamyl-tRNA synthetase OVA9), as well as itself. We also show FMT forms dynamic particles in the cytosol that sometimes move with mitochondria, and their movements are mainly controlled by actin filaments but also by microtubules. Similar results have been reported for animal CLU orthologs. However, an fmt mutant, unlike animal clu mutants, did not show any clear decrease of nuclear-encoded mitochondrial protein levels. This difference may reflect a functional divergence of FMT from other CLU superfamily genes.

Hydrodynamic particle focusing enhanced by femtosecond laser deep grooving at low Reynolds numbers

Microfluidic focusing of particles (both synthetic and biological), which enables precise control over the positions of particles in a tightly focused stream, is a prerequisite step for the downstream processing, such as detection, trapping and separation. In this study, we propose a novel hydrodynamic focusing method by taking advantage of open v-shaped microstructures on a glass substrate engraved by femtosecond pulse (fs) laser. The fs laser engraved microstructures were capable of focusing polystyrene particles and live cells in rectangular microchannels at relatively low Reynolds numbers (Re). Numerical simulations were performed to explain the mechanisms of particle focusing and experiments were carried out to investigate the effects of groove depth, groove number and flow rate on the performance of the groove-embedded microchannel for particle focusing. We found out that 10-µm polystyrene particles are directed toward the channel center under the effects of the groove-induced secondary flows in low-Re flows, e.g. Re < 1. Moreover, we achieved continuous focusing of live cells with different sizes ranging from 10 to 15 µm, i.e. human T-cell lymphoma Jurkat cells, rat adrenal pheochromocytoma PC12 cells and dog kidney MDCK cells. The glass grooves fabricated by fs laser are expected to be integrated with on-chip detection components, such as contact imaging and fluorescence lifetime-resolved imaging, for various biological and biomedical applications, where particle focusing at a relatively low flow rate is desirable.

Modulating the activities of chloroplasts and mitochondria promotes adenosine triphosphate production and plant growth

Efficient photosynthesis requires a balance of ATP and NADPH production/consumption in chloroplasts, and the exportation of reducing equivalents from chloroplasts is important for balancing stromal ATP/NADPH ratio. Here, we showed that the overexpression of purple acid phosphatase 2 on the outer membranes of chloroplasts and mitochondria can streamline the production and consumption of reducing equivalents in these two organelles, respectively. A higher capacity of consumption of reducing equivalents in mitochondria can indirectly help chloroplasts to balance the ATP/NADPH ratio in stroma and recycle NADP⁺, the electron acceptors of the linear electron flow (LEF). A higher rate of ATP and NADPH production from the LEF, a higher capacity of carbon fixation by the Calvin-Benson-Bassham (CBB) cycle and a greater consumption of NADH in mitochondria enhance photosynthesis in the chloroplasts, ATP production in the mitochondria and sucrose synthesis in the cytosol and eventually boost plant growth and seed yields in the overexpression lines.

Phototropin- and photosynthesis-dependent mitochondrial positioning in Arabidopsis thaliana mesophyll cells

Mitochondria are frequently observed in the vicinity of chloroplasts in photosynthesizing cells, and this association is considered necessary for their metabolic interactions. We previously reported that, in leaf palisade cells of Arabidopsis thaliana, mitochondria exhibit blue-light-dependent redistribution together with chloroplasts, which conduct accumulation and avoidance responses under the control of blue-light receptor phototropins. In this study, precise motility analyses by fluorescent microscopy revealed that the individual mitochondria in palisade cells, labeled with green fluorescent protein, exhibit typical stop-and-go movement. When exposed to blue light, the velocity of moving mitochondria increased in 30 min, whereas after 4 h, the frequency of stoppage of mitochondrial movement markedly increased. Using different mutant plants, we concluded that the presence of both phototropin1 and phototropin2 is necessary for the early acceleration of mitochondrial movement. On the contrary, the late enhancement of stoppage of mitochondrial movement occurs only in the presence of phototropin2 and only when intact photosynthesis takes place. A plasma-membrane ghost assay suggested that the stopped mitochondria are firmly adhered to chloroplasts. These results indicate that the physical interaction between mitochondria and chloroplasts is cooperatively mediated by phototropin2- and photosynthesis-dependent signals. The present study might add novel regulatory mechanism for light-dependent plant organelle interactions.

Light induces gene expression to enhance the synthesis of storage reserves in Brassica napus L. embryos

In this manuscript, we disclosed the influence of light on the accumulation of storage reserves in B. napus embryos.1.Light induced the gene expression in the developing embryos of B. napus.2.Light promoted the starch synthesis in chloroplasts of B. napus embryos.3.Light enhanced the metabolic activity of storage reserve synthesis in B. napus embryos. Light influences the accumulation of storage reserves in embryos, but the molecular mechanism was not fully understood. Here, we monitored the effects of light on reserve biosynthesis in Brassica napus by comparing embryos from siliques grown in normal light conditions to those that were shaded or masked (i.e., darkened completely). Masked embryos developed more slowly, weighed less, and contained fewer proteins and lipids than control embryos. They also had fewer and smaller oil bodies than control embryos and lacked chloroplasts, where starch grains are usually synthesized. The levels of most amino acids, carbohydrates, and fatty acids were higher in masked embryos than in control or shaded embryos, whereas the levels of these metabolites in the masked endosperms were lower than those in control and shaded endosperm. Transcriptome analysis indicated that genes involved in photosynthesis (42 genes), amino acid biosynthesis (51 genes), lipid metabolism (61 genes), and sugar transport (13 genes) were significantly repressed in masked embryos. Our results suggest that light contributes to reserve accumulation in embryos by inducing the expression of metabolic genes, thereby enhancing the biosynthesis of storage reserves.

In planta study of photosynthesis and photorespiration using NADPH and NADH/NAD+ fluorescent protein sensors

The challenge of monitoring in planta dynamic changes of NADP(H) and NAD(H) redox states at the subcellular level is considered a major obstacle in plant bioenergetics studies. Here, we introduced two circularly permuted yellow fluorescent protein sensors, iNAP and SoNar, into Arabidopsis thaliana to monitor the dynamic changes in NADPH and the NADH/NAD+ ratio. In the light, photosynthesis and photorespiration are linked to the redox states of NAD(P)H and NAD(P) pools in several subcellular compartments connected by the malate-OAA shuttles. We show that the photosynthetic increases in stromal NADPH and NADH/NAD+ ratio, but not ATP, disappear when glycine decarboxylation is inhibited. These observations highlight the complex interplay between chloroplasts and mitochondria during photosynthesis and support the suggestions that, under normal conditions, photorespiration supplies a large amount of NADH to mitochondria, exceeding its NADH-dissipating capacity, and the surplus NADH is exported from the mitochondria to the cytosol through the malate-OAA shuttle.

Hechtian Strands Transmit Cell Wall Integrity Signals in Plant Cells

Hechtian strands are thread-like structures in plasmolyzed plant cells that connect the cell wall to the plasma membrane. Although these strands were first observed more than 100 years ago, their physiological roles are largely unknown. Here, we used intracellular laser microdissection to examine the effects of disrupting Hechtian strands on plasmolyzed tobacco BY-2 cells. When we focused femtosecond laser pulses on Hechtian strands, targeted disruptions were induced, but no visible changes in cell morphology were detected. However, the calcofluor white signals from β-glucans was detected in plasmolyzed cells with disrupted Hechtian strands, whereas no signals were detected in untreated plasmolyzed cells. These results suggest that Hechtian strands play roles in sensing cell wall integrity.

It Started With a Kiss: Monitoring Organelle Interactions and Identifying Membrane Contact Site Components in Plants

Organelle movement and interaction are dynamic processes. Interpreting the functional role and mechanistic detail of interactions at membrane contact sites requires careful quantification of parameters such as duration, frequency, proximity, and surface area of contact, and identification of molecular components. We provide an overview of current methods used to quantify organelle interactions in plants and other organisms and propose novel applications of existing technologies to tackle this emerging topic in plant cell biology.

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.

Sucrose Starvation Induces Microautophagy in Plant Root Cells

Autophagy is an essential system for degrading and recycling cellular components for survival during starvation conditions. Under sucrose starvation, application of a papain protease inhibitor E-64d to the Arabidopsis root and tobacco BY-2 cells induced the accumulation of vesicles, labeled with a fluorescent membrane marker FM4-64. The E-64d-induced vesicle accumulation was reduced in the mutant defective in autophagy-related genes ATG2, ATG5, and ATG7, suggesting autophagy is involved in the formation of these vesicles. To clarify the formation of these vesicles in detail, we monitored time-dependent changes of tonoplast, and vesicle accumulation in sucrose-starved cells. We found that these vesicles were derived from the tonoplast and produced by microautophagic process. The tonoplast proteins were excluded from the vesicles, suggesting that the vesicles are generated from specific membrane domains. Concanamycin A treatment in GFP-ATG8a transgenic plants showed that not all FM4-64-labeled vesicles, which were derived from the tonoplast, contained the ATG8a-containing structure. These results suggest that ATG8a may not always be necessary for microautophagy.

Plant peroxisomes at the crossroad of NO and H2 O2 metabolism

Plant peroxisomes are subcellular compartments involved in many biochemical pathways during the life cycle of a plant but also in the mechanism of response against adverse environmental conditions. These organelles have an active nitro-oxidative metabolism under physiological conditions but this could be exacerbated under stress situations. Furthermore, peroxisomes have the capacity to proliferate and also undergo biochemical adaptations depending on the surrounding cellular status. An important characteristic of peroxisomes is that they have a dynamic metabolism of reactive nitrogen and oxygen species (RNS and ROS) which generates two key molecules, nitric oxide (NO) and hydrogen peroxide (H₂ O₂ ). These molecules can exert signaling functions by means of post-translational modifications that affect the functionality of target molecules like proteins, peptides or fatty acids. This review provides an overview of the endogenous metabolism of ROS and RNS in peroxisomes with special emphasis on polyamine and uric acid metabolism as well as the possibility that these organelles could be a source of signal molecules involved in the functional interconnection with other subcellular compartments.

Re-evaluation of physical interaction between plant peroxisomes and other organelles using live-cell imaging techniques

The dynamic behavior of organelles is essential for plant survival under various environmental conditions. Plant organelles, with various functions, migrate along actin filaments and contact other types of organelles, leading to physical interactions at a specific site called the membrane contact site. Recent studies have revealed the importance of physical interactions in maintaining efficient metabolite flow between organelles. In this review, we first summarize peroxisome function under different environmental conditions and growth stages to understand organelle interactions. We then discuss current knowledge regarding the interactions between peroxisome and other organelles, i.e., the oil bodies, chloroplast, and mitochondria from the perspective of metabolic and physiological regulation, with reference to various organelle interactions and techniques for estimating organelle interactions occurring in plant cells.

Spatiotemporal observations of light propagation in multiple polarization states

Real-time imaging techniques involving light propagation are commonly applied in the fields of physics, chemistry, and biomedicine. However, conventional techniques provide only the intensity change associated with light propagation. Here, we propose an imaging technique to visualize the ultrafast behavior of the polarization state of a propagating light pulse with four different linear polarization components. This approach provides ultrahigh temporal resolution to observe the light in motion. We recorded a motion picture of a three-dimensional image of a light pulse propagating through a diffuser and a calcite crystal at 56.8 and 75.4 ps, respectively. This technique can contribute to revealing the polarization state of propagating light pulses in a medium and ultrafast phenomenon.

A Framework to Investigate Peroxisomal Protein Phosphorylation in Arabidopsis

Peroxisomes perform essential roles in a range of cellular processes, highlighted by lipid metabolism, reactive species detoxification, and response to a variety of stimuli. The ability of peroxisomes to grow, divide, respond to changing cellular needs, interact with other organelles, and adjust their proteome as required, suggest that, like other organelles, their specialized roles are highly regulated. Similar to most other cellular processes, there is an emerging role for protein phosphorylation to regulate these events. In this review, we establish a knowledge framework of key players that control protein phosphorylation events in the plant peroxisome (i.e., the protein kinases and phosphatases), and highlight a vastly expanded set of (phospho)substrates.

Analyzing the Function of Catalase and the Ascorbate-Glutathione Pathway in H2O2 Processing: Insights from an Experimentally Constrained Kinetic Model

SIGNIFICANCE

Plant stress involves redox signaling linked to reactive oxygen species such as hydrogen peroxide (H₂O₂), which can be generated at high rates in photosynthetic cells. The systems that process H₂O₂ include catalase (CAT) and the ascorbate-glutathione pathway, but interactions between them remain unclear. Modeling can aid interpretation and pinpoint areas for investigation. Recent Advances: Based on emerging data and concepts, we introduce a new experimentally constrained kinetic model to analyze interactions between H₂O₂, CAT, ascorbate, glutathione, and NADPH. The sensitivity points required for accurate simulation of experimental observations are analyzed, and the implications for H₂O₂-linked redox signaling are discussed.

CRITICAL ISSUES

We discuss several implications of the modeled results, in particular the following. (i) CAT and ascorbate peroxidase can share the load in H₂O₂ processing even in optimal conditions. (ii) Intracellular H₂O₂ concentrations more than the low μM range may rarely occur. (iii) Ascorbate redox turnover is largely independent of glutathione until ascorbate peroxidation exceeds a certain value. (iv) NADPH availability may determine glutathione redox status through its influence on monodehydroascorbate reduction. (v) The sensitivity of glutathione status to oxidative stress emphasizes its potential suitability as a sensor of increased H₂O₂.

FUTURE DIRECTIONS

Important future questions include the roles of other antioxidative systems in interacting with CAT and the ascorbate-glutathione pathway as well as the nature and significance of processes that achieve redox exchange between different subcellular compartments. Progress in these areas is likely to be favored by integrating kinetic modeling analyses into experimentally based programs, allowing each approach to inform the other.

Coming together to define membrane contact sites

Close proximities between organelles have been described for decades. However, only recently a specific field dealing with organelle communication at membrane contact sites has gained wide acceptance, attracting scientists from multiple areas of cell biology. The diversity of approaches warrants a unified vocabulary for the field. Such definitions would facilitate laying the foundations of this field, streamlining communication and resolving semantic controversies. This opinion, written by a panel of experts in the field, aims to provide this burgeoning area with guidelines for the experimental definition and analysis of contact sites. It also includes suggestions on how to operationally and tractably measure and analyze them with the hope of ultimately facilitating knowledge production and dissemination within and outside the field of contact-site research.

Dynamics of Peroxisome Homeostasis and Its Role in Stress Response and Signaling in Plants

Peroxisomes play vital roles in plant growth, development, and environmental stress response. During plant development and in response to environmental stresses, the number and morphology of peroxisomes are dynamically regulated to maintain peroxisome homeostasis in cells. To execute their various functions in the cell, peroxisomes associate and communicate with other organelles. Under stress conditions, reactive oxygen species (ROS) produced in peroxisomes and other organelles activate signal transduction pathways, in a process known as retrograde signaling, to synergistically regulate defense systems. In this review, we focus on the recent advances in the plant peroxisome field to provide an overview of peroxisome biogenesis, degradation, crosstalk with other organelles, and their role in response to environmental stresses.

Lessons from optical tweezers: quantifying organelle interactions, dynamics and modelling subcellular events

Optical tweezers enable users to physically trap organelles and move them laterally within the plant cell. Recent advances have highlighted physical interactions between functionally related organelle pairs, such as ER-Golgi and peroxisome-chloroplast, and have shown how organelle positioning affects plant growth. Quantification of these processes has provided insight into the force components which ultimately drive organelle movement and positioning in plant cells. Application of optical tweezers has therefore revolutionised our understanding of plant organelle dynamics.

Plant organelle dynamics: cytoskeletal control and membrane contact sites

Contents Summary 381 I. Introduction 381 II. Basic movement characteristics 382 III. Actin and associated motors, myosins, play a primary role in plant organelle movement and positioning 382 IV. Mechanisms of myosin recruitment: a tightly regulated system? 384 V. Microtubules, associated motors and interplay with actin 386 VI. Role of organelle interactions: tales of tethers 387 VII. Summary model to describe organelle movement in higher plants 390 VIII. Why is organelle movement important? 390 IX. Conclusions and future perspectives 391 Acknowledgements 391 References 391 SUMMARY: Organelle movement and positioning are correlated with plant growth and development. Movement characteristics are seemingly erratic yet respond to external stimuli including pathogens and light. Given these clear correlations, we still do not understand the specific roles that movement plays in these processes. There are few exceptions including organelle inheritance during cell division and photorelocation of chloroplasts to prevent photodamage. The molecular and biophysical components that drive movement can be broken down into cytoskeletal components, motor proteins and tethers, which allow organelles to physically interact with one another. Our understanding of these components and concepts has exploded over the past decade, with recent technological advances allowing an even more in-depth profiling. Here, we provide an overview of the cytoskeletal and tethering components and discuss the mechanisms behind organelle movement in higher plants.