Redefining the metabolic continuity of chloroplasts and ER

ApCPS2 contributes to medicinal diterpenoid biosynthesis and defense against insect herbivore in Andrographis paniculata

Kalmegh (Andrographis paniculata) spatiotemporally produces medicinally-important ent-labdane-related diterpenoids (ent-LRDs); andrographolide (AD), 14-deoxy-11,12-didehydroandrographolide (DDAD), neoandrographolide (NAD). ApCPS1 and ApCPS2, the ent-copalyl pyrophosphate (ent-CPP)-producing class II diterpene synthases (diTPSs) were identified, but their contributions to ent-CPP precursor supply for ent-LRD biosynthesis were not well understood. Here, we characterized ApCPS4, an additional ent-CPP-forming diTPS. Further, we elucidated in planta function of the ent-CPP-producing diTPSs (ApCPS1,2,4) by integrating transcript-metabolite co-profiles, biochemical analysis and gene functional characterization. ApCPS1,2,4 localized to the plastids, where diterpenoid biosynthesis occurs in plants, but ApCPS1,2,4 transcript expression patterns and ent-LRD contents revealed a strong correlation of ApCPS2 expression and ent-LRD accumulation in kalmegh. ApCPS1,2,4 upstream sequences differentially activated β-glucuronidase (GUS) in Arabidopsis and transiently-transformed kalmegh. Similar to higher expression of ApCPS1 in kalmegh stem, ApCPS1 upstream sequence activated GUS in stem/hypocotyl of Arabidopsis and kalmegh. However, ApCPS2,4 upstream sequences weakly activated GUS expression in Arabidopsis, which was not well correlated with ApCPS2,4 transcript expression in kalmegh tissues. Whereas, ApCPS2,4 upstream sequences could activate GUS expression at a considerable level in kalmegh leaf and roots/calyx, respectively, suggesting the involvement of transcriptional regulator(s) of ApCPS2,4 that might participate in kalmegh-specific diterpenoid pathway. Interestingly, ApCPS2-silenced kalmegh showed a drastic reduction in AD, DDAD and NAD contents and compromised defense against insect herbivore Spodoptera litura. However, ent-LRD contents and herbivore defense in ApCPS1 or ApCPS4-silenced plants remained largely unaltered. Overall, these results suggested an important role of ApCPS2 in producing ent-CPP for medicinal ent-LRD biosynthesis and defense against insect herbivore.

Characterization and heterologous reconstitution of Taxus biosynthetic enzymes leading to baccatin III

Paclitaxel is a well known anticancer compound. Its biosynthesis involves the formation of a highly functionalized diterpenoid core skeleton (baccatin III) and the subsequent assembly of a phenylisoserinoyl side chain. Despite intensive investigation for half a century, the complete biosynthetic pathway of baccatin III remains unknown. In this work, we identified a bifunctional cytochrome P450 enzyme [taxane oxetanase 1 (TOT1)] in Taxus mairei that catalyzes an oxidative rearrangement in paclitaxel oxetane formation, which represents a previously unknown enzyme mechanism for oxetane ring formation. We created a screening strategy based on the taxusin biosynthesis pathway and uncovered the enzyme responsible for the taxane oxidation of the C9 position (T9αH1). Finally, we artificially reconstituted a biosynthetic pathway for the production of baccatin III in tobacco.

The genomes of medicinal skullcaps reveal the polyphyletic origins of clerodane diterpene biosynthesis in the family Lamiaceae

The presence of anticancer clerodane diterpenoids is a chemotaxonomic marker for the traditional Chinese medicinal plant Scutellaria barbata, although the molecular mechanisms behind clerodane biosynthesis are unknown. Here, we report a high-quality assembly of the 414.98 Mb genome of S. barbata into 13 pseudochromosomes. Using phylogenomic and biochemical data, we mapped the plastidial metabolism of kaurene (gibberellins), abietane, and clerodane diterpenes in three species of the family Lamiaceae (Scutellaria barbata, Scutellaria baicalensis, and Salvia splendens), facilitating the identification of genes involved in the biosynthesis of the clerodanes, kolavenol, and isokolavenol. We show that clerodane biosynthesis evolved through recruitment and neofunctionalization of genes from gibberellin and abietane metabolism. Despite the assumed monophyletic origin of clerodane biosynthesis, which is widespread in species of the Lamiaceae, our data show distinct evolutionary lineages and suggest polyphyletic origins of clerodane biosynthesis in the family Lamiaceae. Our study not only provides significant insights into the evolution of clerodane biosynthetic pathways in the mint family, Lamiaceae, but also will facilitate the production of anticancer clerodanes through future metabolic engineering efforts.

Terpenoid Transport in Plants: How Far from the Final Picture?

Contrary to the biosynthetic pathways of many terpenoids, which are well characterized and elucidated, their transport inside subcellular compartments and the secretion of reaction intermediates and final products at the short- (cell-to-cell), medium- (tissue-to-tissue), and long-distance (organ-to-organ) levels are still poorly understood, with some limited exceptions. In this review, we aim to describe the state of the art of the transport of several terpene classes that have important physiological and ecological roles or that represent high-value bioactive molecules. Among the tens of thousands of terpenoids identified in the plant kingdom, only less than 20 have been characterized from the point of view of their transport and localization. Most terpenoids are secreted in the apoplast or stored in the vacuoles by the action of ATP-binding cassette (ABC) transporters. However, little information is available regarding the movement of terpenoid biosynthetic intermediates from plastids and the endoplasmic reticulum to the cytosol. Through a description of the transport mechanisms of cytosol- or plastid-synthesized terpenes, we attempt to provide some hypotheses, suggestions, and general schemes about the trafficking of different substrates, intermediates, and final products, which might help develop novel strategies and approaches to allow for the future identification of terpenoid transporters that are still uncharacterized.

A polarized supercell produces specialized metabolites in cannabis trichomes

For centuries, humans have cultivated cannabis for the pharmacological properties that result from consuming its specialized metabolites, primarily cannabinoids and terpenoids. Today, cannabis is a multi-billion-dollar industry whose existence rests on the biological activity of tiny cell clusters, called glandular trichomes, found mainly on flowers. Cannabinoids are toxic to cannabis cells,¹ and how the trichome cells can produce and secrete massive quantities of lipophilic metabolites is not known.¹ To address this gap in knowledge, we investigated cannabis glandular trichomes using ultra-rapid cryofixation, quantitative electron microscopy, and immuno-gold labeling of cannabinoid pathway enzymes. We demonstrate that the metabolically active cells in cannabis form a "supercell," with extensive cytoplasmic bridges across the cell walls and a polar distribution of organelles adjacent to the apical surface where metabolites are secreted. The predicted metabolic role of the non-photosynthetic plastids is supported by unusual membrane arrays in the plastids and the localization of the start of the cannabinoid/terpene pathway in the stroma of the plastids. Abundant membrane contact sites connected plastid paracrystalline cores with the plastid envelope, plastid with endoplasmic reticulum (ER), and ER with plasma membrane. The final step of cannabinoid biosynthesis, catalyzed by tetrahydrocannabinolic acid synthase (THCAS), was localized in the cell-surface wall facing the extracellular storage cavity. We propose a new model of how the cannabis cells can support abundant metabolite production, with emphasis on the key role of membrane contact sites and extracellular THCA biosynthesis. This new model can inform synthetic biology approaches for cannabinoid production in yeast or cell cultures.

Fatty acid desaturases (FADs) modulate multiple lipid metabolism pathways to improve plant resistance

BACKGROUND

Biological and abiotic stresses such as salt, extreme temperatures, and pests and diseases place major constraints on plant growth and crop yields. Fatty acids (FAs) and FA- derivatives are unique biologically active substance that show a wide range of functions in biological systems. They are not only participated in the regulation of energy storage substances and cell membrane plasm composition, but also extensively participate in the regulation of plant basic immunity, effector induced resistance and systemic resistance and other defense pathways, thereby improving plant resistance to adversity stress. Fatty acid desaturases (FADs) is involved in the desaturation of fatty acids, where desaturated fatty acids can be used as substrates for FA-derivatives.

OBJECTIVE

In this paper, the role of omega-FADs (ω-3 FADs and ω-6 FADs) in the prokaryotic and eukaryotic pathways of fatty acid biosynthesis in plant defense against stress (biological and abiotic stress) and the latest research progress were summarized. Moreover' the existing problems in related research and future research directions were also discussed.

RESULTS

Fatty acid desaturases are involved in various responses of plants during biotic and abiotic stress. For example, it is involved in regulating the stability and fluidity of cell membranes, reactive oxygen species signaling pathways, etc. In this review, we have collected several experimental studies to represent the differential effects of fatty acid desaturases on biotic and abiotic species.

CONCLUSION

Fatty acid desaturases play an important role in regulating biotic and abiotic stresses.

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.

How retrograde signaling is intertwined with the evolution of photosynthetic eukaryotes

Chloroplasts and mitochondria evolved from free-living prokaryotic organisms that entered the eukaryotic cell through endosymbiosis. The gradual conversion from endosymbiont to organelle during the course of evolution was accompanied by the development of a communication system between the host and the endosymbiont, referred to as retrograde signaling or organelle-to-nucleus signaling. In higher plants, plastid-to-nucleus signaling involves multiple signaling pathways necessary to coordinate plastid function and cellular responses to developmental and environmental stimuli. Phylogenetic reconstructions using sequence information from evolutionarily diverse photosynthetic eukaryotes have begun to provide information about how retrograde signaling pathways were adopted and modified in different lineages over time. A tight communication system was likely a major facilitator of plants conquest of the land because it would have enabled the algal ancestors of land plants to better allocate their cellular resources in response to high light and desiccation, the major stressor for streptophyte algae in a terrestrial habitat. In this review, we aim to give an evolutionary perspective on plastid-to-nucleus signaling.

Plant cytoskeletons and the endoplasmic reticulum network organization

Plant endoplasmic reticulum (ER) remodelling is likely to be important for its function in targeted protein secretion, organelle interaction and signal exchange. It has been known for decades that the structure and movement of the ER network is mainly regulated by the actin cytoskeleton through actin motor proteins and membrane-cytoskeleton adaptors. Recent discoveries also revealed alternative pathways that influence ER movement, through a microtubule-based machinery. Therefore, plants utilize both cytoskeletal components to drive ER dynamics, a process that is likely to be dependent on the cell type and the developmental stages. On the other hand, the ER membrane also has a direct effect towards the organization of the cytoskeletal network and disrupting the tethering factors at the ER-PM interface also rearranges the cytoskeletal structure. However, the influence of the ER network on the cytoskeleton organization has not been studied. In this review, we will provide an overview of the ER-cytoskeleton network in plants, and discuss the most recent discoveries in the field.

TGD5 is required for normal morphogenesis of non-mesophyll plastids, but not mesophyll chloroplasts, in Arabidopsis

Stromules are dynamic membrane-bound tubular structures that emanate from plastids. Stromule formation is triggered in response to various stresses and during plant development, suggesting that stromules may have physiological and developmental roles in these processes. Despite the possible biological importance of stromules and their prevalence in green plants, their exact roles and formation mechanisms remain unclear. To explore these issues, we obtained Arabidopsis thaliana mutants with excess stromule formation in the leaf epidermis by microscopy-based screening. Here, we characterized one of these mutants, stromule biogenesis altered 1 (suba1). suba1 forms plastids with severely altered morphology in a variety of non-mesophyll tissues, such as leaf epidermis, hypocotyl epidermis, floral tissues, and pollen grains, but apparently normal leaf mesophyll chloroplasts. The suba1 mutation causes impaired chloroplast pigmentation and altered chloroplast ultrastructure in stomatal guard cells, as well as the aberrant accumulation of lipid droplets and their autophagic engulfment by the vacuole. The causal defective gene in suba1 is TRIGALACTOSYLDIACYLGLYCEROL5 (TGD5), which encodes a protein putatively involved in the endoplasmic reticulum (ER)-to-plastid lipid trafficking required for the ER pathway of thylakoid lipid assembly. These findings suggest that a non-mesophyll-specific mechanism maintains plastid morphology. The distinct mechanisms maintaining plastid morphology in mesophyll versus non-mesophyll plastids might be attributable, at least in part, to the differential contributions of the plastidial and ER pathways of lipid metabolism between mesophyll and non-mesophyll plastids.

Fatty Acid Desaturases in the Chloroplast and Endoplasmic Reticulum Promote Susceptibility to the Green Peach Aphid Myzus persicae in Arabidopsis thaliana

Fatty acid desaturases (FADs) in plants influence levels of susceptibility to multiple stresses, including insect infestations. In this study, populations of the green peach aphid (Myzus persicae) on Arabidopsis thaliana were reduced by mutations in three desaturases: AtFAB2/SSI2, which encodes a chloroplastic stearoyl-[acyl-carrier-protein] 9-desaturase, and AtFAD7 or AtFAD3, which encode ω-3 FADs in the chloroplast and endoplasmic reticulum (ER), respectively. These data indicate that certain FADs promote susceptibility to aphids and that aphids are impacted by desaturases in both the chloroplast and ER. Aphid resistance in ssi2, fad3, and fad7, singly or in combination, might involve altered signaling between these subcellular compartments. C18:1 levels are depleted in ssi2, whereas C18:2 accumulation is enhanced in fad3 and fad7. In contrast, fad8 has higher than normal C18:2 levels but also high C18:1 and low C18:0 and does not impact aphid numbers. Potentially, aphids may be influenced by the balance of multiple fatty acids (FAs) rather than by a single species, with C18:2 promoting aphid resistance and C18:1 promoting susceptibility. Although the fad7 mutant also accumulates higher-than-normal levels of C16:2, this FA does not contribute to aphid resistance because a triple mutant line that lacks detectable levels of C16:2 (fad2fad6fad7) retains comparable levels of aphid resistance as fad7. In addition, aphid numbers are unaffected by the fad5 mutation that inhibits C16:1 synthesis. Together, these results demonstrate that certain FADs are important susceptibility factors in plant-aphid interactions and that aphid resistance is more strongly associated with differences in C18 abundance than C16 abundance.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

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.

A central role of the endoplasmic reticulum in the cell emerges from its functional contact sites with multiple organelles

Early eukaryotic cells emerged from the compartmentalization of metabolic processes into specific organelles through the development of an endomembrane system (ES), a precursor of the endoplasmic reticulum (ER), which was essential for their survival. Recently, substantial evidence emerged on how organelles communicate among themselves and with the plasma membrane (PM) through contact sites (CSs). From these studies, the ER-the largest single structure in eukaryotic cells-emerges as a central player communicating with all organelles to coordinate cell functions and respond to external stimuli to maintain cellular homeostasis. Herein we review the functional insights into the ER-CSs with other organelles in a physiological perspective. We hypothesize that, in addition to the primitive role by the ES in the appearance of proto-eukaryotes, its successor-the ER-emerges as the key coordinator of inter-organelle/PM communication. The ER thus appears to be the 'maestro' driving eukaryotic cell evolution by incorporating new functions/organelles, while remaining the real coordinator overarching cellular functions and orchestrating them with the external milieu.

The Chloroplast Reactive Oxygen Species-Redox System in Plant Immunity and Disease

Pathogen infections limit plant growth and productivity, thus contributing to crop losses. As the site of photosynthesis, the chloroplast is vital for plant productivity. This organelle, communicating with other cellular compartments challenged by infection (e.g., apoplast, mitochondria, and peroxisomes), is also a key battlefield in the plant-pathogen interaction. Here, we focus on the relation between reactive oxygen species (ROS)-redox signaling, photosynthesis which is governed by redox control, and biotic stress response. We also discuss the pathogen strategies to weaken the chloroplast-mediated defense responses and to promote pathogenesis. As in the next decades crop yield increase may depend on the improvement of photosynthetic efficiency, a comprehensive understanding of the integration between photosynthesis and plant immunity is required to meet the future food demand.

Light microscopy of the endoplasmic reticulum-membrane contact sites in plants

The existence of membrane contact sites (MCS) has been reported in different systems in the past decade, and their importance has been recognised by the cell biology community. Amongst all endomembrane structures, the endoplasmic reticulum (ER) plays vital roles in organising the organelle interaction network with the plasma membrane (PM), Golgi bodies, mitochondria, plastids, endosomes and autophagosomes. A number of methods have been used to study the establishment and functions of these interactions, among them, light microscopy appears to be one of the most effective approaches. Here, we present an overview of the discovery of ER-PM contact sites, and highlight the latest developments in light microscopical-based techniques that can be used for their study.

FAX2 Mediates Fatty Acid Export from Plastids in Developing Arabidopsis Seeds

Vegetable oils are mainly stored in the form of triacylglycerol (TAG) in oilseeds. Fatty acids (FAs), one of the building blocks for TAG assembly, are synthesized in plastids and then exported to the endoplasmic reticulum for storage oil synthesis. A recent study demonstrated that the export of FAs from plastids was mediated by a FAX (FA export) family protein. However, the significance of FAs export from plastid during seed oil accumulation has not been investigated. In this study, we found that FAX2 was highly expressed in developing Arabidopsis seeds and the expression level was consistent with FAs synthesis activity. FAX2 mutant seeds showed an approximately 18% reduction of lipid levels compared with wild-type seeds. By contrast, overexpression of FAX2 enhanced seed lipid accumulation by up to 30%. The FAs export activity of FAX2 was confirmed by yeast mutant cell complementation analysis. Our results showed that FAX2 could interact with other proteins to facilitate FAs transport. Taken together, these results indicate that FAX2-mediated FA export from plastids is important for seed oil accumulation, and that FAX2 can be used as a target gene for increasing lipid production in oilseeds.

Multiple genes recruited from hormone pathways partition maize diterpenoid defences

Duplication and divergence of primary pathway genes underlie the evolution of plant specialized metabolism; however, mechanisms partitioning parallel hormone and defence pathways are often speculative. For example, the primary pathway intermediate ent-kaurene is essential for gibberellin biosynthesis and is also a proposed precursor for maize antibiotics. By integrating transcriptional coregulation patterns, genome-wide association studies, combinatorial enzyme assays, proteomics and targeted mutant analyses, we show that maize kauralexin biosynthesis proceeds via the positional isomer ent-isokaurene formed by a diterpene synthase pair recruited from gibberellin metabolism. The oxygenation and subsequent desaturation of ent-isokaurene by three promiscuous cytochrome P450s and a new steroid 5α reductase indirectly yields predominant ent-kaurene-associated antibiotics required for Fusarium stalk rot resistance. The divergence and differential expression of pathway branches derived from multiple duplicated hormone-metabolic genes minimizes dysregulation of primary metabolism via the circuitous biosynthesis of ent-kaurene-related antibiotics without the production of growth hormone precursors during defence.

Dynamic metabolic solutions to the sessile life style of plants

Plants are sessile organisms. To compensate for not being able to escape when challenged by unfavorable growth conditions, pests or herbivores, plants have perfected their metabolic plasticity by having developed the capacity for on demand dynamic biosynthesis and storage of a plethora of phytochemicals.

Covering: up to 2018

Plants are sessile organisms. To compensate for not being able to escape when challenged by unfavorable growth conditions, pests or herbivores, plants have perfected their metabolic plasticity by having developed the capacity for on demand synthesis of a plethora of phytochemicals to specifically respond to the challenges arising during plant ontogeny. Key steps in the biosynthesis of phytochemicals are catalyzed by membrane-bound cytochrome P450 enzymes which in plants constitute a superfamily. In planta, the P450s may be organized in dynamic enzyme clusters (metabolons) and the genes encoding the P450s and other enzymes in a specific pathway may be clustered. Metabolon formation facilitates transfer of substrates between sequential enzymes and therefore enables the plant to channel the flux of general metabolites towards biosynthesis of specific phytochemicals. In the plant cell, compartmentalization of the operation of specific biosynthetic pathways in specialized plastids serves to avoid undesired metabolic cross-talk and offers distinct storage sites for molar concentrations of specific phytochemicals. Liquid–liquid phase separation may lead to formation of dense biomolecular condensates within the cytoplasm or vacuole allowing swift activation of the stored phytochemicals as required upon pest or herbivore attack. The molecular grid behind plant plasticity offers an endless reservoir of functional modules, which may be utilized as a synthetic biology tool-box for engineering of novel biological systems based on rational design principles. In this review, we highlight some of the concepts used by plants to coordinate biosynthesis and storage of phytochemicals.

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.

Communications Between the Endoplasmic Reticulum and Other Organelles During Abiotic Stress Response in Plants

To adapt to constantly changing environmental conditions, plants have evolved sophisticated tolerance mechanisms to integrate various stress signals and to coordinate plant growth and development. It is well known that inter-organellar communications play important roles in maintaining cellular homeostasis in response to environmental stresses. The endoplasmic reticulum (ER), extending throughout the cytoplasm of eukaryotic cells, is a central organelle involved in lipid metabolism, Ca²⁺ homeostasis, and synthesis and folding of secretory and transmembrane proteins crucial to perceive and transduce environmental signals. The ER communicates with the nucleus via the highly conserved unfolded protein response pathway to mitigate ER stress. Importantly, recent studies have revealed that the dynamic ER network physically interacts with other intracellular organelles and endomembrane compartments, such as the Golgi complex, mitochondria, chloroplast, peroxisome, vacuole, and the plasma membrane, through multiple membrane contact sites between closely apposed organelles. In this review, we will discuss the signaling and metabolite exchanges between the ER and other organelles during abiotic stress responses in plants as well as the ER-organelle membrane contact sites and their associated tethering complexes.

The essential chloroplast ribosomal protein uL15c interacts with the chloroplast RNA helicase ISE2 and affects intercellular trafficking through plasmodesmata

Chloroplasts retain part of their ancestral genomes and the machinery for expression of those genomes. The nucleus-encoded chloroplast RNA helicase INCREASED SIZE EXCLUSION LIMIT2 (ISE2) is required for chloroplast ribosomal RNA processing and chloro-ribosome assembly. To further elucidate ISE2's role in chloroplast translation, two independent approaches were used to identify its potential protein partners. Both a yeast two-hybrid screen and a pull-down assay identified plastid ribosomal protein L15, uL15c (formerly RPL15), as interacting with ISE2. The interaction was confirmed in vivo by co-immunoprecipitation. Interestingly, we found that rpl15 null mutants do not complete embryogenesis, indicating that RPL15 is an essential gene for autotrophic growth of Arabidopsis thaliana. Arabidopsis and Nicotiana benthamiana plants with reduced expression of RPL15 developed chlorotic leaves, had reduced photosynthetic capacity and exhibited defective chloroplast development. Processing of chloroplast ribosomal RNAs and assembly of ribosomal subunits were disrupted by reduced expression of RPL15. Chloroplast translation was also decreased, reducing accumulation of chloroplast-encoded proteins, in such plants compared to wild-type plants. Notably, knockdown of RPL15 expression increased intercellular trafficking, a phenotype also observed in plants with reduced ISE2 expression. This finding provides further evidence for chloroplast function in modulating intercellular trafficking via plasmodesmata.

Interplay between the unfolded protein response and reactive oxygen species: a dynamic duo

Secretory proteins undergo modifications such as glycosylation and disulphide bond formation before proper folding, and move to their final destination via the endomembrane system. Accumulation of unfolded proteins in the endoplasmic reticulum (ER) due to suboptimal environmental conditions triggers a response called the unfolded protein response (UPR), which induces a set of genes that elevate protein folding capacity in the ER. This review aims to establish a connection among ER stress, UPR, and reactive oxygen species (ROS), which remains an unexplored topic in plants. For this, we focused on mechanisms of ROS production originating from ER stress, the interaction between ER stress and overall ROS signalling process in the cell, and the interaction of ER stress with other organellar ROS signalling pathways such as of the mitochondria and chloroplasts. The roles of the UPR during plant hormone signalling and abiotic and biotic stress responses are also discussed in connection with redox and ROS signalling.

Spatial distribution of organelles in leaf cells and soybean root nodules revealed by focused ion beam-scanning electron microscopy

Analysis of cellular ultrastructure has been dominated by transmission electron microscopy (TEM), so images collected by this technique have shaped our current understanding of cellular structure. More recently, three-dimensional (3D) analysis of organelle structures has typically been conducted using TEM tomography. However, TEM tomography application is limited by sample thickness. Focused ion beam-scanning electron microscopy (FIB-SEM) uses a dual beam system to perform serial sectioning and imaging of a sample. Thus FIB-SEM is an excellent alternative to TEM tomography and serial section TEM tomography. Animal tissue samples have been more intensively investigated by this technique than plant tissues. Here, we show that FIB-SEM can be used to study the 3D ultrastructure of plant tissues in samples previously prepared for TEM via commonly used fixation and embedding protocols. Reconstruction of FIB-SEM sections revealed ultra-structural details of the plant tissues examined. We observed that organelles packed tightly together in Nicotiana benthamiana Domin leaf cells may form membrane contacts. 3D models of soybean nodule cells suggest that the bacteroids in infected cells are contained within one large membrane-bound structure and not the many individual symbiosomes that TEM thin-sections suggest. We consider the implications of these organelle arrangements for intercellular signalling.

Unravelling Protein-Protein Interaction Networks Linked to Aliphatic and Indole Glucosinolate Biosynthetic Pathways in Arabidopsis

Within the cell, biosynthetic pathways are embedded in protein-protein interaction networks. In Arabidopsis, the biosynthetic pathways of aliphatic and indole glucosinolate defense compounds are well-characterized. However, little is known about the spatial orchestration of these enzymes and their interplay with the cellular environment. To address these aspects, we applied two complementary, untargeted approaches-split-ubiquitin yeast 2-hybrid and co-immunoprecipitation screens-to identify proteins interacting with CYP83A1 and CYP83B1, two homologous enzymes specific for aliphatic and indole glucosinolate biosynthesis, respectively. Our analyses reveal distinct functional networks with substantial interconnection among the identified interactors for both pathway-specific markers, and add to our knowledge about how biochemical pathways are connected to cellular processes. Specifically, a group of protein interactors involved in cell death and the hypersensitive response provides a potential link between the glucosinolate defense compounds and defense against biotrophic pathogens, mediated by protein-protein interactions.

Assembly of Dynamic P450-Mediated Metabolons-Order Versus Chaos

Purpose of Review

We provide an overview of the current knowledge on cytochrome P450-mediated metabolism organized as metabolons and factors that facilitate their stabilization. Essential parameters will be discussed including those that are commonly disregarded using the dhurrin metabolon from Sorghum bicolor as a case study.

Recent Findings

Sessile plants control their metabolism to prioritize their resources between growth and development, or defense. This requires fine-tuned complex dynamic regulation of the metabolic networks involved. Within the recent years, numerous studies point to the formation of dynamic metabolons playing a major role in controlling the metabolic fluxes within such networks.

Summary

We propose that P450s and their partners interact and associate dynamically with POR, which acts as a charging station possibly in concert with Cytb5. Solvent environment, lipid composition, and non-catalytic proteins guide metabolon formation and thereby activity, which have important implications for synthetic biology approaches aiming to produce high-value specialized metabolites in heterologous hosts.

Defense-Related Transcriptional Reprogramming in Vitamin E-Deficient Arabidopsis Mutants Exposed to Contrasting Phosphate Availability

Vitamin E inhibits the propagation of lipid peroxidation and helps protecting photosystem II from photoinhibition, but little is known about its possible role in plant response to Pi availability. Here, we aimed at examining the effect of vitamin E deficiency in Arabidopsis thaliana vte mutants on phytohormone contents and the expression of transcription factors in plants exposed to contrasting Pi availability. Plants were subjected to two doses of Pi, either unprimed (controls) or previously exposed to low Pi (primed). In the wild type, α-tocopherol contents increased significantly in response to repeated periods of low Pi, which was paralleled by increased growth, indicative of a priming effect. This growth-stimulating effect was, however, abolished in vte mutants. Hormonal profiling revealed significant effects of Pi availability, priming and genotype on the contents of jasmonates and salicylates; remarkably, vte mutants showed enhanced accumulation of both hormones under low Pi. Furthermore, expression profiling of 1,880 transcription factors by qRT-PCR revealed a pronounced effect of priming on the transcript levels of 45 transcription factors mainly associated with growth and stress in wild-type plants in response to low Pi availability; while distinct differences in the transcriptional response were detected in vte mutants. We conclude that α-tocopherol plays a major role in the response of plants to Pi availability not only by protecting plants from photo-oxidative stress, but also by exerting a control over growth- and defense-related transcriptional reprogramming and hormonal modulation.

Proteomic Analysis of Lipid Droplets from Arabidopsis Aging Leaves Brings New Insight into Their Biogenesis and Functions

Lipid droplets (LDs) are cell compartments specialized for oil storage. Although their role and biogenesis are relatively well documented in seeds, little is known about their composition, structure and function in senescing leaves where they also accumulate. Here, we used a label free quantitative mass spectrometry approach to define the LD proteome of aging Arabidopsis leaves. We found that its composition is highly different from that of seed/cotyledon and identified 28 proteins including 9 enzymes of the secondary metabolism pathways involved in plant defense response. With the exception of the TRIGALACTOSYLDIACYLGLYCEROL2 protein, we did not identify enzymes implicated in lipid metabolism, suggesting that growth of leaf LDs does not occur by local lipid synthesis but rather through contact sites with the endoplasmic reticulum (ER) or other membranes. The two most abundant proteins of the leaf LDs are the CALEOSIN3 and the SMALL RUBBER PARTICLE1 (AtSRP1); both proteins have structural functions and participate in plant response to stress. CALEOSIN3 and AtSRP1 are part of larger protein families, yet no other members were enriched in the LD proteome suggesting a specific role of both proteins in aging leaves. We thus examined the function of AtSRP1 at this developmental stage and found that AtSRP1 modulates the expression of CALEOSIN3 in aging leaves. Furthermore, AtSRP1 overexpression induces the accumulation of triacylglycerol with an unusual composition compared to wild-type. We demonstrate that, although AtSRP1 expression is naturally increased in wild type senescing leaves, its overexpression in senescent transgenic lines induces an over-accumulation of LDs organized in clusters at restricted sites of the ER. Conversely, atsrp1 knock-down mutants displayed fewer but larger LDs. Together our results reveal that the abundancy of AtSRP1 regulates the neo-formation of LDs during senescence. Using electron tomography, we further provide evidence that LDs in leaves share tenuous physical continuity as well as numerous contact sites with the ER membrane. Thus, our data suggest that leaf LDs are functionally distinct from seed LDs and that their biogenesis is strictly controlled by AtSRP1 at restricted sites of the ER.

Biochemical properties and subcellular localization of tyrosine aminotransferases in Arabidopsis thaliana

Plants produce various L-tyrosine (Tyr)-derived compounds that are of pharmaceutical or nutritional importance to humans. Tyr aminotransferase (TAT) catalyzes the reversible transamination between Tyr and 4-hydroxyphenylpyruvate (HPP), the initial step in the biosynthesis of many Tyr-derived plant natural products. Herein reported is the biochemical characterization and subcellular localization of TAT enzymes from the model plant Arabidopsis thaliana. Phylogenetic analysis showed that Arabidopsis has at least two homologous TAT genes, At5g53970 (AtTAT1) and At5g36160 (AtTAT2). Their recombinant enzymes showed distinct biochemical properties: AtTAT1 had the highest activity towards Tyr, while AtTAT2 exhibited a broad substrate specificity for both amino and keto acid substrates. Also, AtTAT1 favored the direction of Tyr deamination to HPP, whereas AtTAT2 preferred transamination of HPP to Tyr. Subcellular localization analysis using GFP-fusion proteins and confocal microscopy showed that AtTAT1, AtTAT2, and HPP dioxygenase (HPPD), which catalyzes the subsequent step of TAT, are localized in the cytosol, unlike plastid-localized Tyr and tocopherol biosynthetic enzymes. Furthermore, subcellular fractionation indicated that, while HPPD activity is restricted to the cytosol, TAT activity is detected in both cytosolic and plastidic fractions of Arabidopsis leaf tissue, suggesting that an unknown aminotransferase(s) having TAT activity is also present in the plastids. Biochemical and cellular analyses of Arabidopsis TATs provide a fundamental basis for future in vivo studies and metabolic engineering for enhanced production of Tyr-derived phytochemicals in plants.

Synthesis and Function of Apocarotenoid Signals in Plants

In plants, carotenoids are essential for photosynthesis and photoprotection. However, carotenoids are not the end products of the pathway; apocarotenoids are produced by carotenoid cleavage dioxygenases (CCDs) or non-enzymatic processes. Apocarotenoids are more soluble or volatile than carotenoids but they are not simply breakdown products, as there can be modifications post-cleavage and their functions include hormones, volatiles, and signals. Evidence is emerging for a class of apocarotenoids, here referred to as apocarotenoid signals (ACSs), that have regulatory roles throughout plant development beyond those ascribed to abscisic acid (ABA) and strigolactone (SL). In this context we review studies of carotenoid feedback regulation, chloroplast biogenesis, stress signaling, and leaf and root development providing evidence that apocarotenoids may fine-tune plant development and responses to environmental stimuli.

Extending the biosynthetic repertoires of cyanobacteria and chloroplasts

Chloroplasts in plants and algae and photosynthetic microorganisms such as cyanobacteria are emerging hosts for sustainable production of valuable biochemicals, using only inorganic nutrients, water, CO2 and light as inputs. In the past decade, many bioengineering efforts have focused on metabolic engineering and synthetic biology in the chloroplast or in cyanobacteria for the production of fuels, chemicals and complex, high-value bioactive molecules. Biosynthesis of all these compounds can be performed in photosynthetic organelles/organisms by heterologous expression of the appropriate pathways, but this requires optimization of carbon flux and reducing power, and a thorough understanding of regulatory pathways. Secretion or storage of the compounds produced can be exploited for the isolation or confinement of the desired compounds. In this review, we explore the use of chloroplasts and cyanobacteria as biosynthetic compartments and hosts, and we estimate the levels of production to be expected from photosynthetic hosts in light of the fraction of electrons and carbon that can potentially be diverted from photosynthesis. The supply of reducing power, in the form of electrons derived from the photosynthetic light reactions, appears to be non-limiting, but redirection of the fixed carbon via precursor molecules presents a challenge. We also discuss the available synthetic biology tools and the need to expand the molecular toolbox to facilitate cellular reprogramming for increased production yields in both cyanobacteria and chloroplasts.

Stitching Organelles: Organization and Function of Specialized Membrane Contact Sites in Plants

The coordination of multiple metabolic activities in plants relies on an interorganelle communication network established through membrane contact sites (MCS). The MCS are maintained in transient or durable configurations by tethering structures which keep the two membranes in close proximity, and create chemical microdomains that allow localized and targeted exchange of small molecules and possibly proteins. The past few years have witnessed a dramatic increase in our understanding of the structural and molecular organization of plant interorganelle MCS, and their crucial roles in plant specialized functions including stress responses, cell to cell communication, and lipid transport. In this review we summarize recent advances in understanding the molecular components, structural organization, and functions of different plant-specific MCS architectures.

Plant VAP27 proteins: domain characterization, intracellular localization and role in plant development

The endoplasmic reticulum (ER) is connected to the plasma membrane (PM) through the plant-specific NETWORKED protein, NET3C, and phylogenetically conserved vesicle-associated membrane protein-associated proteins (VAPs). Ten VAP homologues (VAP27-1 to 27-10) can be identified in the Arabidopsis genome and can be divided into three clades. Representative members from each clade were tagged with fluorescent protein and expressed in Nicotiana benthamiana. Proteins from clades I and III localized to the ER as well as to ER/PM contact sites (EPCSs), whereas proteins from clade II were found only at the PM. Some of the VAP27-labelled EPCSs localized to plasmodesmata, and we show that the mobility of VAP27 at EPCSs is influenced by the cell wall. EPCSs closely associate with the cytoskeleton, but their structure is unaffected when the cytoskeleton is removed. VAP27-labelled EPCSs are found in most cell types in Arabidopsis, with the exception of cells in early trichome development. Arabidopsis plants expressing VAP27-GFP fusions exhibit pleiotropic phenotypes, including defects in root hair morphogenesis. A similar effect is also observed in plants expressing VAP27 RNAi. Taken together, these data indicate that VAP27 proteins used at EPCSs are essential for normal ER-cytoskeleton interaction and for plant development.

Two ω-3 FADs Are Associated with Peach Fruit Volatile Formation

Aroma-related volatiles, together with sugars and acids, play an important role in determining fruit flavor quality. Characteristic volatiles of peach fruit are mainly derived from fatty acids such as linoleic acid (18:2) and linolenic acid (18:3). In the present study, six genes encoding fatty acid desaturases (FAD) were cloned, including two ω-6 FAD genes (PpFAD2, PpFAD6) and four ω-3 FAD genes (PpFAD3-1, PpFAD3-2, PpFAD7 and PpFAD8). Heterologous expression of peach FADs in tobacco plants showed that PpFAD3-1, and PpFAD3-2 significantly reduced contents of 18:2, and accumulated significant higher levels of 18:3. In the case of volatiles, transgenic plants produced lower concentrations of hexanal and higher levels of (E)-2-hexenal. Consequently, the ratio of the (E)-2-hexenal and hexanal was about 5- and 3-fold higher than that of wild type (WT) in PpFAD3-1 and PpFAD3-2 transformants, respectively. No significant changes in volatile profiles were observed in transgenic plants overexpressing the four other peach FAD genes. Real-time quantitative polymerase chain reaction (qPCR) analysis showed that ripe fruit had high PpFAD3-1 and low PpFAD3-2 transcript levels. In contrast, high PpFAD3-2 and low PpFAD3-1 transcript levels were observed in young fruit. These results indicate a temporal regulation of these two ω-3 FADs during development and ripening, influencing peach fruit volatile formation.

Proteomics of terpenoid biosynthesis and secretion in trichomes of higher plant species

Among the specialized (secondary) plant metabolites, terpenoids represent the most diverse family and are often involved in the defense against pathogens and herbivores. Terpenoids can be produced both constitutively and in response to the environment. At the front line of this defense strategy are the glandular trichomes, which are organs dedicated primarily to the production of specialized metabolites. Mass spectrometry-based proteomics is a powerful tool, which is very useful to investigate enzymes involved in metabolic pathways, such as the synthesis and secretion of terpenoids in glandular trichomes. Here we review the strategies used to investigate the specific roles of these particular organs from non-model plant species, mainly belonging to the Lamiaceae, Solanaceae, and Cannabaceae families. We discuss how proteomics helps to accurately pinpoint candidate proteins to be functionally characterized, and how technological progresses create opportunities for studying low-abundance proteins, such as the ones related to the synthesis and transport of specialized metabolites. This article is part of a Special Issue entitled: Plant Proteomics--a bridge between fundamental processes and crop production, edited by Dr. Hans-Peter Mock.

Fatty Acid and Lipid Transport in Plant Cells

Fatty acids (FAs) and lipids are essential - not only as membrane constituents but also for growth and development. In plants and algae, FAs are synthesized in plastids and to a large extent transported to the endoplasmic reticulum for modification and lipid assembly. Subsequently, lipophilic compounds are distributed within the cell, and thus are transported across most membrane systems. Membrane-intrinsic transporters and proteins for cellular FA/lipid transfer therefore represent key components for delivery and dissemination. In addition to highlighting their role in lipid homeostasis and plant performance, different transport mechanisms for land plants and green algae - in the model systems Arabidopsis thaliana, Chlamydomonas reinhardtii - are compared, thereby providing a current perspective on protein-mediated FA and lipid trafficking in photosynthetic cells.

Mitochondrial Dynamics and the ER: The Plant Perspective

Whereas contact sites between mitochondria and the ER have been in the focus of animal and fungal research for several years, the importance of this organellar interface and the molecular effectors are largely unknown for plants. This work gives an introduction into known evolutionary differences of molecular effectors of mitochondrial dynamics and interactions between animals, fungi, and plants. Using the model plant Physcomitrella patens, we provide microscopic evidence for the existence of mitochondria-ER interactions in plants and their correlation with mitochondrial constriction and fission. We further investigate a previously identified protein of unknown function (MELL1), and show that it modulates the amount of mitochondrial association to the ER, as well as mitochondrial shape and number.

A Conserved Cytochrome P450 Evolved in Seed Plants Regulates Flower Maturation

Global inspection of plant genomes identifies genes maintained in low copies across taxa and under strong purifying selection, which are likely to have essential functions. Based on this rationale, we investigated the function of the low-duplicated CYP715 cytochrome P450 gene family that appeared early in seed plants and evolved under strong negative selection. Arabidopsis CYP715A1 showed a restricted tissue-specific expression in the tapetum of flower buds and in the anther filaments upon anthesis. cyp715a1 insertion lines showed a strong defect in petal development, and transient alteration of pollen intine deposition. Comparative expression analysis revealed the downregulated expression of genes involved in pollen development, cell wall biogenesis, hormone homeostasis, and floral sesquiterpene biosynthesis, especially TPS21 and several key genes regulating floral development such as MYB21, MYB24, and MYC2. Accordingly, floral sesquiterpene emission was suppressed in the cyp715a1 mutants. Flower hormone profiling, in addition, indicated a modification of gibberellin homeostasis and a strong disturbance of the turnover of jasmonic acid derivatives. Petal growth was partially restored by the active gibberellin GA3 or the functional analog of jasmonoyl-isoleucine, coronatine. CYP715 appears to function as a key regulator of flower maturation, synchronizing petal expansion and volatile emission. It is thus expected to be an important determinant of flower-insect interaction.

Rethinking how volatiles are released from plant cells

For plant volatile organic compounds (VOCs) to be emitted, they must cross membrane(s), the aqueous cell wall, and sometimes the cuticle, before moving into the gas phase. It is presumed that VOC movement through each barrier occurs via passive diffusion. However, VOCs, which are primarily nonpolar compounds, will preferentially partition into membranes, making diffusion into aqueous compartments slow. Using Fick's first law, we calculated that to achieve observed VOC emission rates by diffusion alone would necessitate toxic VOC levels in membranes. Here, we propose that biological mechanisms, such as those involved in trafficking other hydrophobic compounds, must contribute to VOC emission. Such parallel biological pathways would lower barrier resistances and, thus, steady-state emission rates could be maintained with significantly reduced intramembrane VOC concentrations.

Oil biosynthesis in a basal angiosperm: transcriptome analysis of Persea Americana mesocarp

BACKGROUND

The mechanism by which plants synthesize and store high amounts of triacylglycerols (TAG) in tissues other than seeds is not well understood. The comprehension of controls for carbon partitioning and oil accumulation in nonseed tissues is essential to generate oil-rich biomass in perennial bioenergy crops. Persea americana (avocado), a basal angiosperm with unique features that are ancestral to most flowering plants, stores ~ 70 % TAG per dry weight in its mesocarp, a nonseed tissue. Transcriptome analyses of select pathways, from generation of pyruvate and leading up to TAG accumulation, in mesocarp tissues of avocado was conducted and compared with that of oil-rich monocot (oil palm) and dicot (rapeseed and castor) tissues to identify tissue- and species-specific regulation and biosynthesis of TAG in plants.

RESULTS

RNA-Seq analyses of select lipid metabolic pathways of avocado mesocarp revealed patterns similar to that of other oil-rich species. However, only some predominant orthologs of the fatty acid biosynthetic pathway genes in this basal angiosperm were similar to those of monocots and dicots. The accumulation of TAG, rich in oleic acid, was associated with higher transcript levels for a putative stearoyl-ACP desaturase and endoplasmic reticulum (ER)-associated acyl-CoA synthetases, during fruit development. Gene expression levels for enzymes involved in terminal steps to TAG biosynthesis in the ER further indicated that both acyl-CoA-dependent and -independent mechanisms might play a role in TAG assembly, depending on the developmental stage of the fruit. Furthermore, in addition to the expression of an ortholog of WRINKLED1 (WRI1), a regulator of fatty acid biosynthesis, high transcript levels for WRI2-like and WRI3-like suggest a role for additional transcription factors in nonseed oil accumulation. Plastid pyruvate necessary for fatty acid synthesis is likely driven by the upregulation of genes involved in glycolysis and transport of its intermediates. Together, a comparative transcriptome analyses for storage oil biosynthesis in diverse plants and tissues suggested that several distinct and conserved features in this basal angiosperm species might contribute towards its rich TAG content.

CONCLUSIONS

Our work represents a comprehensive transcriptome resource for a basal angiosperm species and provides insight into their lipid metabolism in mesocarp tissues. Furthermore, comparison of the transcriptome of oil-rich mesocarp of avocado, with oil-rich seed and nonseed tissues of monocot and dicot species, revealed lipid gene orthologs that are highly conserved during evolution. The orthologs that are distinctively expressed in oil-rich mesocarp tissues of this basal angiosperm, such as WRI2, ER-associated acyl-CoA synthetases, and lipid-droplet associated proteins were also identified. This study provides a foundation for future investigations to increase oil-content and has implications for metabolic engineering to enhance storage oil content in nonseed tissues of diverse species.

Oil biosynthesis in a basal angiosperm: transcriptome analysis of Persea Americana mesocarp

BACKGROUND

The mechanism by which plants synthesize and store high amounts of triacylglycerols (TAG) in tissues other than seeds is not well understood. The comprehension of controls for carbon partitioning and oil accumulation in nonseed tissues is essential to generate oil-rich biomass in perennial bioenergy crops. Persea americana (avocado), a basal angiosperm with unique features that are ancestral to most flowering plants, stores ~ 70 % TAG per dry weight in its mesocarp, a nonseed tissue. Transcriptome analyses of select pathways, from generation of pyruvate and leading up to TAG accumulation, in mesocarp tissues of avocado was conducted and compared with that of oil-rich monocot (oil palm) and dicot (rapeseed and castor) tissues to identify tissue- and species-specific regulation and biosynthesis of TAG in plants.

RESULTS

RNA-Seq analyses of select lipid metabolic pathways of avocado mesocarp revealed patterns similar to that of other oil-rich species. However, only some predominant orthologs of the fatty acid biosynthetic pathway genes in this basal angiosperm were similar to those of monocots and dicots. The accumulation of TAG, rich in oleic acid, was associated with higher transcript levels for a putative stearoyl-ACP desaturase and endoplasmic reticulum (ER)-associated acyl-CoA synthetases, during fruit development. Gene expression levels for enzymes involved in terminal steps to TAG biosynthesis in the ER further indicated that both acyl-CoA-dependent and -independent mechanisms might play a role in TAG assembly, depending on the developmental stage of the fruit. Furthermore, in addition to the expression of an ortholog of WRINKLED1 (WRI1), a regulator of fatty acid biosynthesis, high transcript levels for WRI2-like and WRI3-like suggest a role for additional transcription factors in nonseed oil accumulation. Plastid pyruvate necessary for fatty acid synthesis is likely driven by the upregulation of genes involved in glycolysis and transport of its intermediates. Together, a comparative transcriptome analyses for storage oil biosynthesis in diverse plants and tissues suggested that several distinct and conserved features in this basal angiosperm species might contribute towards its rich TAG content.

CONCLUSIONS

Our work represents a comprehensive transcriptome resource for a basal angiosperm species and provides insight into their lipid metabolism in mesocarp tissues. Furthermore, comparison of the transcriptome of oil-rich mesocarp of avocado, with oil-rich seed and nonseed tissues of monocot and dicot species, revealed lipid gene orthologs that are highly conserved during evolution. The orthologs that are distinctively expressed in oil-rich mesocarp tissues of this basal angiosperm, such as WRI2, ER-associated acyl-CoA synthetases, and lipid-droplet associated proteins were also identified. This study provides a foundation for future investigations to increase oil-content and has implications for metabolic engineering to enhance storage oil content in nonseed tissues of diverse species.

Metabolism of acyl-lipids in Chlamydomonas reinhardtii

Microalgae are emerging platforms for production of a suite of compounds targeting several markets, including food, nutraceuticals, green chemicals, and biofuels. Many of these products, such as biodiesel or polyunsaturated fatty acids (PUFAs), derive from lipid metabolism. A general picture of lipid metabolism in microalgae has been deduced from well characterized pathways of fungi and land plants, but recent advances in molecular and genetic analyses of microalgae have uncovered unique features, pointing out the necessity to study lipid metabolism in microalgae themselves. In the past 10 years, in addition to its traditional role as a model for photosynthetic and flagellar motility processes, Chlamydomonas reinhardtii has emerged as a model organism to study lipid metabolism in green microalgae. Here, after summarizing data on total fatty acid composition, distribution of acyl-lipid classes, and major acyl-lipid molecular species found in C. reinhardtii, we review the current knowledge on the known or putative steps for fatty acid synthesis, glycerolipid desaturation and assembly, membrane lipid turnover, and oil remobilization. A list of characterized or putative enzymes for the major steps of acyl-lipid metabolism in C. reinhardtii is included, and subcellular localizations and phenotypes of associated mutants are discussed. Biogenesis and composition of Chlamydomonas lipid droplets and the potential importance of lipolytic processes in increasing cellular oil content are also highlighted.

Lipid trafficking at endoplasmic reticulum-chloroplast membrane contact sites

Glycerolipid synthesis in plant cells is characterized by an intense trafficking of lipids between the endoplasmic reticulum (ER) and chloroplasts. Initially, fatty acids are synthesized within chloroplasts and are exported to the ER where they are used to build up phospholipids and triacylglycerol. Ultimately, derivatives of these phospholipids return to chloroplasts to form galactolipids, monogalactosyldiacylglycerol and digalactosyldiacylglycerol, the main and essential lipids of photosynthetic membranes. Lipid trafficking was proposed to transit through membrane contact sites (MCSs) connecting both organelles. Here, we review recent insights into ER-chloroplast MCSs and lipid trafficking between chloroplasts and the ER.

Microalgal lipid droplets: composition, diversity, biogenesis and functions

Lipid droplet is the major site of neutral lipid storage in eukaryotic cells, and increasing evidence show its involvement in numerous cellular processes such as lipid homeostasis, signaling, trafficking and inter-organelle communications. Although the biogenesis, structure, and functions of lipid droplets have been well documented for seeds of vascular plants, mammalian adipose tissues, insects and yeasts, relative little is known about lipid droplets in microalgae. Over the past 5 years, the growing interest of microalgae as a platform for biofuel, green chemicals or value-added polyunsaturated fatty acid production has brought algal lipid droplets into spotlight. Studies conducted on the green microalga Chlamydomonas reinhardtii and other model microalgae such as Haematococcus and Nannochloropsis species have led to the identification of proteins associated with lipid droplets, which include putative structural proteins different from plant oleosins and animal perilipins, as well as candidate proteins for lipid biosynthesis, mobilization, trafficking and homeostasis. Biochemical and microscopy studies have also started to shed light on the role of chloroplasts in the biogenesis of lipid droplets in Chlamydomonas.

The role of plastoglobules in thylakoid lipid remodeling during plant development

Photosynthesis is the key bioenergetic process taking place in the chloroplast. The components of the photosynthetic machinery are embedded in a highly dynamic matrix, the thylakoid membrane. This membrane has the capacity to adapt during developmental transitions and under stress conditions. The galactolipids are the major polar lipid components of the thylakoid membrane conferring bilayer properties, while neutral thylakoid lipids such as the prenyllipids and carotenoids contribute to essential functions such as electron transport and photoprotection. Despite a large number of studies, the intriguing processes of thylakoid membrane biogenesis and dynamics remain unsolved. Plastoglobules, thylakoid-associated lipid droplets, appear to actively participate in thylakoid function from biogenesis to senescence. Recruitment of specific proteins enables the plastoglobules to act in metabolite synthesis, repair and disposal under changing environmental conditions and developmental stages. In this review, we describe plastoglobules as thylakoid membrane microdomains and discuss their involvement in lipid remodeling during stress and in the conversion from one plastid type to another. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

The endoplasmic reticulum: a dynamic and well-connected organelle

The endoplasmic reticulum forms the first compartment in a series of organelles which comprise the secretory pathway. It takes the form of an extremely dynamic and pleomorphic membrane-bounded network of tubules and cisternae which have numerous different cellular functions. In this review, we discuss the nature of endoplasmic reticulum structure and dynamics, its relationship with closely associated organelles, and its possible function as a highway for the distribution and delivery of a diverse range of structures from metabolic complexes to viral particles.

Chloroplast signaling within, between and beyond cells

The most conspicuous function of plastids is the oxygenic photosynthesis of chloroplasts, yet plastids are super-factories that produce a plethora of compounds that are indispensable for proper plant physiology and development. Given their origins as free-living prokaryotes, it is not surprising that plastids possess their own genomes whose expression is essential to plastid function. This semi-autonomous character of plastids requires the existence of sophisticated regulatory mechanisms that provide reliable communication between them and other cellular compartments. Such intracellular signaling is necessary for coordinating whole-cell responses to constantly varying environmental cues and cellular metabolic needs. This is achieved by plastids acting as receivers and transmitters of specific signals that coordinate expression of the nuclear and plastid genomes according to particular needs. In this review we will consider the so-called retrograde signaling occurring between plastids and nuclei, and between plastids and other organelles. Another important role of the plastid we will discuss is the involvement of plastid signaling in biotic and abiotic stress that, in addition to influencing retrograde signaling, has direct effects on several cellular compartments including the cell wall. We will also review recent evidence pointing to an intriguing function of chloroplasts in regulating intercellular symplasmic transport. Finally, we consider an intriguing yet less widely known aspect of plant biology, chloroplast signaling from the perspective of the entire plant. Thus, accumulating evidence highlights that chloroplasts, with their complex signaling pathways, provide a mechanism for exquisite regulation of plant development, metabolism and responses to the environment. As chloroplast processes are targeted for engineering for improved productivity the effect of such modifications on chloroplast signaling will have to be carefully considered in order to avoid unintended consequences on plant growth and development.

ER - the key to the highway

The endoplasmic reticulum (ER) is the key organelle at the start of the secretory pathway and the list of its functions is continually growing. The ER organization as a tubular/cisternal network at the cortex of plant cells has recently been shown to be governed by the membrane tubulation proteins of the reticulon family working alongside plant atlastin homologues, members of the RHD3 group of proteins. Such a network has intimate connections with other organelles such as peroxisomes via peroxules, chloroplasts, Golgi bodies and at the cell cortex to the plasma membrane with cytoskeleton at so called 'anchor/contact sites'. The ER network is by no means static displaying a range of different movements and acting as a subcellular highway supports the motility of organelles such as peroxisomes, mitochondria and Golgi bodies plus the transport of macromolecules such as viral movement proteins, nucleocapsid proteins and RNA. Here we highlight recent and exciting discoveries on the maintenance of the ER structure and its role on movement and biology of other organelles.