Arabidopsis Small Rubber Particle Protein Homolog SRPs Play Dual Roles as Positive Factors for Tissue Growth and Development and in Drought Stress Responses

Identification and Characterization of Lipid Droplet-Associated Protein (LDAP) Isoforms from Tung Tree (Vernicia fordii)

Lipid droplets (LDs) are cytoplasmic organelles responsible primarily for the storage of neutral lipids, such as triacyclglycerols (TAGs). Derived from the endoplasmic reticulum bilayer, LDs are composed of a hydrophobic lipid core encased by a phospholipid monolayer and surface-associated proteins. To date, only a relatively few LD 'coat' proteins in plants have been identified and characterized, most of which come from studies of the model plant Arabidopsis thaliana. To expand our knowledge of the plant LD proteome, the LD-associated protein (LDAP) family from the tung tree (Vernicia fordii), whose seeds are rich in a commercially valuable TAG containing the conjugated fatty acid α-eleostearic acid (C18:3Δ9cis,11trans,13trans [α-ESA]), was identified and characterized. Based on the tung tree transcriptome, three LDAP isoforms (VfLDAP1-3) were elucidated and the encoded proteins distinctly clustered into three clades along with their respective isoforms from other angiosperm species. Ectopic expression of the VfLDAPs in Nicotiana benthamiana leaves revealed that they localized specifically to LDs and influenced LD numbers and sizes, as well as increasing TAG content and altering TAG fatty acid composition. Interestingly, in a partially reconstructed TAG-ESA biosynthetic pathway, the co-expression of VfLDAP3 and, to a lesser degree, VfLDAP2, significantly increased the content of α-ESA stored within the LDs. These results suggest that the VfLDAPs can influence the steady-state content and composition of TAG in plant cells and that certain LDAP isoforms may have evolved to more efficiently package TAGs into LDs containing unusual fatty acids, such as α-ESA.

Plasticity of the Arabidopsis leaf lipidome and proteome in response to pathogen infection and heat stress

Plants must cope with a variety of stressors during their life cycle, and the adaptive responses to these environmental cues involve all cellular organelles. Among them, comparatively little is known about the contribution of cytosolic lipid droplets (LDs) and their core set of neutral lipids and associated surface proteins to the rewiring of cellular processes in response to stress. Here, we analyzed the changes that occur in the lipidome and proteome of Arabidopsis (Arabidopsis thaliana) leaves after pathogen infection with Botrytis cinerea or Pseudomonas syringae, or after heat stress. Analyses were carried out in wild-type plants and the oil-rich double mutant trigalactosyldiacylglycerol1-1 sugar dependent 1-4 (tgd1-1 sdp1-4) that allowed for an allied study of the LD proteome in stressed leaves. Using liquid chromatography-tandem mass spectrometry-based methods, we showed that a hyperaccumulation of the primary LD core lipid TAG is a general response to stress and that acyl chain and sterol composition are remodeled during cellular adaptation. Likewise, comparative analysis of the LD protein composition in stress-treated leaves highlighted the plasticity of the LD proteome as part of the general stress response. We further identified at least two additional LD-associated proteins, whose localization to LDs in leaves was confirmed by confocal microscopy of fluorescent protein fusions. Taken together, these results highlight LDs as dynamic contributors to the cellular adaptation processes that underlie how plants respond to environmental stress.

Packaging "vegetable oils": Insights into plant lipid droplet proteins

Plant neutral lipids, also known as "vegetable oils", are synthesized within the endoplasmic reticulum (ER) membrane and packaged into subcellular compartments called lipid droplets (LDs) for stable storage in the cytoplasm. The biogenesis, modulation, and degradation of cytoplasmic LDs in plant cells are orchestrated by a variety of proteins localized to the ER, LDs, and peroxisomes. Recent studies of these LD-related proteins have greatly advanced our understanding of LDs not only as steady oil depots in seeds but also as dynamic cell organelles involved in numerous physiological processes in different tissues and developmental stages of plants. In the past 2 decades, technology advances in proteomics, transcriptomics, genome sequencing, cellular imaging and protein structural modeling have markedly expanded the inventory of LD-related proteins, provided unprecedented structural and functional insights into the protein machinery modulating LDs in plant cells, and shed new light on the functions of LDs in nonseed plant tissues as well as in unicellular algae. Here, we review critical advances in revealing new LD proteins in various plant tissues, point out structural and mechanistic insights into key proteins in LD biogenesis and dynamic modulation, and discuss future perspectives on bridging our knowledge gaps in plant LD biology.

Diurnal Regulation of SOS Pathway and Sodium Excretion Underlying Salinity Tolerance of Vigna marina

Vigna marina (Barm.) Merr. is adapted to tropical marine beaches and has an outstanding tolerance to salt stress. Given there are growing demands for cultivating crops in saline soil or with saline water, it is important to understand how halophytic species are adapted to the saline environments. Here we revealed by positron-emitting tracer imaging system (PETIS) that V. marina actively excretes sodium from the root during the light period but not the dark period. The following whole genome sequencing accompanied with forward genetic study identified a QTL region harbouring SOS1, encoding plasma membrane Na+/H+ antiporter, which was associated with not only salt tolerance but also the ability of sodium excretion. We also found the QTL region contained a large structural rearrangement that suppressed recombination across ~14 Mbp, fixing multiple gene loci potentially involved in salt tolerance. RNA-seq and promoter analyses revealed SOS1 in V. marina was highly expressed even without salt stress and its promoter shared common cis-regulatory motifs with those exhibiting similar expression profiles. Interestingly, the cis-regulatory motifs seemed installed by a transposable element (TE) insertion. Though not identified by genetic analysis, the transcriptome data also revealed SOS2 transcription was under diurnal regulation, explaining the pattern of sodium excretion together with upregulated expression of SOS1. Altogether, the study elucidated one aspect of the strategy adopted by V. marina to adapt to marine beach, which is highly saline and transpiring.

The interaction networks of small rubber particle proteins in the latex of Taraxacum koksaghyz reveal diverse functions in stress responses and secondary metabolism

The Russian dandelion (Taraxacum koksaghyz) is a promising source of natural rubber (NR). The synthesis of NR takes place on the surface of organelles known as rubber particles, which are found in latex - the cytoplasm of specialized cells known as laticifers. As well as the enzymes directly responsible for NR synthesis, the rubber particles also contain small rubber particle proteins (SRPPs), the most abundant of which are SRPP3, 4 and 5. These three proteins support NR synthesis by maintaining rubber particle stability. We used homology-based searches to identify the whole TkSRPP gene family and qPCR to create their spatial expression profiles. Affinity enrichment-mass spectrometry was applied to identify TkSRPP3/4/5 protein interaction partners in T. koksaghyz latex and selected interaction partners were analyzed using qPCR, confocal laser scanning microscopy and heterologous expression in yeast. We identified 17 SRPP-like sequences in the T. koksaghyz genome, including three apparent pseudogenes, 10 paralogs arranged as an inverted repeat in a cluster with TkSRPP3/4/5, and one separate gene (TkSRPP6). Their sequence diversity and different expression profiles indicated distinct functions and the latex interactomes obtained for TkSRPP3/4/5 suggested that TkSRPP4 is a promiscuous hub protein that binds many partners from different compartments, whereas TkSRPP3 and 5 have more focused interactomes. Two interactors shared by TkSRPP3/4/5 (TkSRPP6 and TkUGT80B1) were chosen for independent validation and detailed characterization. TkUGT80B1 triterpenoid glycosylating activity provided first evidence for triterpenoid saponin synthesis in T. koksaghyz latex. Based on its identified interaction partners, TkSRPP4 appears to play a special role in the endoplasmic reticulum, interacting with lipidmodifying enzymes that may facilitate rubber particle formation. TkSRPP5 appears to be involved in GTPase-dependent signaling and TkSRPP3 may act as part of a kinase signaling cascade, with roles in stress tolerance. TkSRPP interaction with TkUGT80B1 draws a new connection between TkSRPPs and triterpenoid saponin synthesis in T. koksaghyz latex. Our data contribute to the functional differentiation between TkSRPP paralogs and demonstrate unexpected interactions that will help to further elucidate the network of proteins linking TkSRPPs, stress responses and NR biosynthesis within the cellular complexity of latex.

Transcriptome analysis of the common moss Bryum pseudotriquetrum grown under Antarctic field condition

Mosses are distributed all over the world including Antarctica. Although Antarctic mosses show active growth in a short summer season under harsh environments such as low temperature, drought and high levels of UV radiation, survival mechanisms for such multiple environmental stresses of Antarctic mosses have not yet been clarified. In the present study, transcriptome analyses were performed using one of the common mosses Bryum pseudotriquetrum grown under an Antarctic field and artificial cultivation conditions. Totally 88 205 contigs were generated by de novo assembly, among which 1377 and 435 genes were significantly up and downregulated, respectively, under Antarctic field conditions compared with artificial cultivation conditions at 15°C. Among the upregulated genes, a number of lipid metabolism-related and oil body formation-related genes were identified. Expression levels of these genes were increased by artificial environmental stress treatments such as low temperature, salt and osmic stress treatments. Consistent with these results, B. pseudotriquetrum grown under Antarctic field conditions contained large amounts of fatty acids, especially α-linolenic acid, linolenic acid and arachidonic acid. In addition, proportion of unsaturated fatty acids, which enhance membrane fluidity, to the total fatty acids was also higher in B. pseudotriquetrum grown under Antarctic field conditions. Since lipid accumulation and unsaturation of fatty acids are generally important factors for the acquisition of various environmental stress tolerance in plants, these intracellular physiological and metabolic changes may be responsible for the survival of B. pseudotriquetrum under Antarctic harsh environments.

Hypoxia increases triacylglycerol levels and unsaturation in tomato roots

BACKGROUND

Plants are designed to endure stress, but increasingly extreme weather events are testing the limits. Events like flooding result in submergence of plant organs, triggering an energy crisis due to hypoxia and threaten plant growth and productivity. Lipids are relevant as building blocks and energy vault and are substantially intertwined with primary metabolism, making them an ideal readout for plant stress.

RESULTS

By high resolution mass spectrometry, a distinct, hypoxia-related lipid composition of Solanum lycopersicum root tissue was observed. Out of 491 lipid species, 11 were exclusively detected in this condition. Among the lipid classes observed, glycerolipids and glycerophospholipids dominated by far (78%). Differences between the lipidomic profiles of both analyzed conditions were significantly driven by changes in the abundance of triacylglycerols (TGs) whereas sitosterol esters, digalactosyldiacylglycerols, and phosphatidylcholine play a significantly negligible role in separation. Alongside, an increased level of polyunsaturation was observed in the fatty acid chains, with 18:2 and 18:3 residues showing a significant increase. Of note, hexadecatetraenoic acid (16:4) was identified in hypoxia condition samples. Changes in gene expression of enzymes related to lipid metabolism corroborate the above findings.

CONCLUSION

To our knowledge, this is the first report on a hypoxia-induced increase in TG content in tomato root tissue, closing a knowledge gap in TG abiotic stress response. The results suggest that the increase in TGs and TG polyunsaturation degree are common features of hypoxic response in plant roots.

Maize lipid droplet-associated protein 2 is recruited by a virus to enhance viral multiplication and infection through regulating cellular fatty acid metabolism

Pathogen infection induces massive reprogramming of host primary metabolism. Lipid and fatty acid (FA) metabolism is generally disrupted by pathogens and co-opted for their proliferation. Lipid droplets (LDs) that play important roles in regulating cellular lipid metabolism are utilized by a variety of pathogens in mammalian cells. However, the function of LDs during pathogenic infection in plants remains unknown. We show here that infection by rice black streaked dwarf virus (RBSDV) affects the lipid metabolism of maize, which causes elevated accumulation of C18 polyunsaturated fatty acids (PUFAs) leading to viral proliferation and symptom development. The overexpression of one of the two novel LD-associated proteins (LDAPs) of maize (ZmLDAP1 and ZmLDAP2) induces LD clustering. The core capsid protein P8 of RBSDV interacts with ZmLDAP2 and prevents its degradation through the ubiquitin-proteasome system mediated by a UBX domain-containing protein, PUX10. In addition, silencing of ZmLDAP2 downregulates the expression of FA desaturase genes in maize, leading to a decrease in C18 PUFAs levels and suppression of RBSDV accumulation. Our findings reveal that plant virus may recruit LDAP to regulate cellular FA metabolism to promote viral multiplication and infection. These results expand the knowledge of LD functions and viral infection mechanisms in plants.

Adaptive divergence, historical population dynamics, and simulation of suitable distributions for Picea Meyeri and P. Mongolica at the whole-genome level

The taxonomic classification of Picea meyeri and P. mongolica has long been controversial. To investigate the genetic relatedness, evolutionary history, and population history dynamics of these species, genotyping-by-sequencing (GBS) technology was utilized to acquire whole-genome single nucleotide polymorphism (SNP) markers, which were subsequently used to assess population structure, population dynamics, and adaptive differentiation. Phylogenetic and population structural analyses at the genomic level indicated that although the ancestor of P. mongolica was a hybrid of P. meyeri and P. koraiensis, P. mongolica is an independent Picea species. Additionally, P. mongolica is more closely related to P. meyeri than to P. koraiensis, which is consistent with its geographic distribution. There were up to eight instances of interspecific and intraspecific gene flow between P. meyeri and P. mongolica. The P. meyeri and P. mongolica effective population sizes generally decreased, and Maxent modeling revealed that from the Last Glacial Maximum (LGM) to the present, their habitat areas decreased initially and then increased. However, under future climate scenarios, the habitat areas of both species were projected to decrease, especially under high-emission scenarios, which would place P. mongolica at risk of extinction and in urgent need of protection. Local adaptation has promoted differentiation between P. meyeri and P. mongolica. Genotype‒environment association analysis revealed 96,543 SNPs associated with environmental factors, mainly related to plant adaptations to moisture and temperature. Selective sweeps revealed that the selected genes among P. meyeri, P. mongolica and P. koraiensis are primarily associated in vascular plants with flowering, fruit development, and stress resistance. This research enhances our understanding of Picea species classification and provides a basis for future genetic improvement and species conservation efforts.

Lipid droplets in Arabidopsis thaliana leaves contain myosin-binding proteins and enzymes associated with furan-containing fatty acid biosynthesis

Lipid droplets (LDs) are lipid storage organelles in plant leaves and seeds. Seed LD proteins are well known, and their functions in lipid metabolism have been characterized; however, many leaf LD proteins remain to be identified. We therefore isolated LDs from leaves of the leaf LD-overaccumulating mutant high sterol ester 1 (hise1) of Arabidopsis thaliana by centrifugation or co-immunoprecipitation. We then performed LD proteomics by mass spectrometry and identified 3,206 candidate leaf LD proteins. In this study, we selected 31 candidate proteins for transient expression assays using a construct encoding the candidate protein fused with green fluorescent protein (GFP). Fluorescence microscopy showed that MYOSIN BINDING PROTEIN14 (MYOB14) and two uncharacterized proteins localized to LDs labeled with the LD marker. Subcellular localization analysis of MYOB family members revealed that MYOB1, MYOB2, MYOB3, and MYOB5 localized to LDs. LDs moved along actin filaments together with the endoplasmic reticulum. Co-immunoprecipitation of myosin XIK with MYOB2-GFP or MYOB14-GFP suggested that LD-localized MYOBs are involved in association with the myosin XIK-LDs. The two uncharacterized proteins were highly similar to enzymes for furan fatty acid biosynthesis in the photosynthetic bacterium Cereibacter sphaeroides, suggesting a relationship between LDs and furan fatty acid biosynthesis. Our findings thus reveal potential molecular functions of LDs and provide a valuable resource for further studies of the leaf LD proteome.

Golgi-localized MORN1 promotes lipid droplet abundance and enhances tolerance to multiple stresses in Arabidopsis

Lipid droplet (LD) in vegetative tissues has recently been implicated in environmental responses in plants, but its regulation and its function in stress tolerance are not well understood. Here, we identified a Membrane Occupation and Recognition Nexus 1 (MORN1) gene as a contributor to natural variations of stress tolerance through genome-wide association study in Arabidopsis thaliana. Characterization of its loss-of-function mutant and natural variants revealed that the MORN1 gene is a positive regulator of plant growth, disease resistance, cold tolerance, and heat tolerance. The MORN1 protein is associated with the Golgi and is also partly associated with LD. Protein truncations that disrupt these associations abolished the biological function of the MORN1 protein. Furthermore, the MORN1 gene is a positive regulator of LD abundance, and its role in LD number regulation and stress tolerance is highly linked. Therefore, this study identifies MORN1 as a positive regulator of LD abundance and a contributor to natural variations of stress tolerance. It implicates a potential involvement of Golgi in LD biogenesis and strongly suggests a contribution of LD to diverse processes of plant growth and stress responses.

Cosuppression of AtGELP22 and AtGELP23, two ubiquitinated target proteins of RING E3 ligase AtAIRP5, increases tolerance to drought stress in Arabidopsis

AtAIRP5 RING E3 ubiquitin ligase was recently identified as a positive regulator of the abscisic acid (ABA)-mediated drought stress response by stimulating the degradation of serine carboxypeptidase-like 1. Here, we identified GDSL-type esterase/lipase 22 (AtGELP22) and AtGELP23 as additional interacting partners of AtAIRP5. Yeast two-hybrid, pull-down, co-immunoprecipitation, and ubiquitination analyses verified that AtGELP22 and AtGELP23 are ubiquitinated target proteins of AtAIRP5. AtGELP22 and AtGELP23 were colocalized with AtAIRP5 to punctate-like structures in the cytosolic fraction, in which PYK10 and NAI2, two ER body marker proteins, are localized. T-DNA insertion atgelp22 and atgelp23 single knockout mutant plants showed phenotypes indistinguishable from those of wild-type plants under ABA treatment. In contrast, RNAi-mediated cosuppression of AtGELP22 and AtGELP23 resulted in hypersensitive ABA-mediated stomatal movements and higher tolerance to drought stress than that of the single mutant and wild-type plants. Taken together, our results suggest that the putative GDSL-type esterases/lipases AtGELP22 and AtGELP23 act as redundant negative regulators of the ABA-mediated drought stress response in Arabidopsis.

Exploring the Potential Role of Ribosomal Proteins to Enhance Potato Resilience in the Face of Changing Climatic Conditions

Potatoes have emerged as a key non-grain crop for food security worldwide. However, the looming threat of climate change poses significant risks to this vital food source, particularly through the projected reduction in crop yields under warmer temperatures. To mitigate potential crises, the development of potato varieties through genome editing holds great promise. In this study, we performed a comprehensive transcriptomic analysis to investigate microtuber development and identified several differentially expressed genes, with a particular focus on ribosomal proteins-RPL11, RPL29, RPL40 and RPL17. Our results reveal, by protein-protein interaction (PPI) network analyses, performed with the highest confidence in the STRING database platform (v11.5), the critical involvement of these ribosomal proteins in microtuber development, and highlighted their interaction with PEBP family members as potential microtuber activators. The elucidation of the molecular biological mechanisms governing ribosomal proteins will help improve the resilience of potato crops in the face of today's changing climatic conditions.

Lipid droplets are versatile organelles involved in plant development and plant response to environmental changes

Since decades plant lipid droplets (LDs) are described as storage organelles accumulated in seeds to provide energy for seedling growth after germination. Indeed, LDs are the site of accumulation for neutral lipids, predominantly triacylglycerols (TAGs), one of the most energy-dense molecules, and sterol esters. Such organelles are present in the whole plant kingdom, from microalgae to perennial trees, and can probably be found in all plant tissues. Several studies over the past decade have revealed that LDs are not merely simple energy storage compartments, but also dynamic structures involved in diverse cellular processes like membrane remodeling, regulation of energy homeostasis and stress responses. In this review, we aim to highlight the functions of LDs in plant development and response to environmental changes. In particular, we tackle the fate and roles of LDs during the plant post-stress recovery phase.

Identification and characterization of the LDAP family revealed GhLDAP2_Dt enhances drought tolerance in cotton

Lipid droplet-associated proteins (LDAPs) play essential roles in tissue growth and development and in drought stress responses in plants. Cotton is an important fiber and cash crop; however, the LDAP family has not been characterized in cotton. In this study, a total of 14, six, seven, and seven genes were confirmed as LDAP family members in Gossypium hirsutum, Gossypium raimondii, Gossypium arboreum, and Gossypium stocksii, respectively. Additionally, expansion in the LDAP family occurred with the formation of Gossypium, which is mirrored in the number of LDAPs found in five Malvaceae species (Gossypioides kirkii, Bombax ceiba, Durio zibethinus, Theobroma cacao, and Corchorus capsularis), Arabidopsis thaliana, and Carica papaya. The phylogenetic tree showed that the LDAP genes in cotton can be divided into three groups (I, II, and III). The analysis of gene structure and conserved domains showed that LDAPs derived from group I (LDAP1/2/3) are highly conserved during evolution, while members from groups II and III had large variations in both domains and gene structures. The gene expression pattern analysis of LDAP genes showed that they are expressed not only in the reproductive organs (ovule) but also in vegetative organs (root, stem, and leaves). The expression level of two genes in group III, GhLDAP6_At/Dt, were significantly higher in fiber development than in other tissues, indicating that it may be an important regulator of cotton fiber development. In group III, GhLDAP2_At/Dt, especially GhLDAP2_Dt was strongly induced by various abiotic stresses. Decreasing the expression of GhLDAP2_Dt in cotton via virus-induced gene silencing increased the drought sensitivity, and the over-expression of GhLDAP2_Dt led to increased tolerance to mannitol-simulated osmotic stress at the germination stage. Thus, we conclude that GhLDAP2_Dt plays a positive role in drought tolerance.

Functional Characterization of TkSRPP Promoter in Response to Hormones and Wounding Stress in Transgenic Tobacco

Taraxacum kok-saghyz is a model species for studying natural rubber biosynthesis because its root can produce high-quality rubber. Small rubber particle protein (SRPP), a stress-related gene to multiple stress responses, involves in natural rubber biosynthesis. To investigate the transcriptional regulation of the TkSRPP promoter, the full-length promoter PR0 (2188 bp) and its four deletion derivatives, PR1 (1592 bp), PR2 (1274 bp), PR3 (934 bp), and PR4 (450 bp), were fused to β-glucuronidase (GUS) reporter gene and transformed into tobacco. The GUS tissue staining showed that the five promoters distinctly regulated GUS expression utilizing transient transformation of tobacco. The GUS activity driven by a PR0 promoter was detected in transgenic tobacco leaves, stem and roots, suggesting that the TkSRPP promoter was not tissue-specific. Deletion analyses in transgenic tobacco have demonstrated that the PR3 from -934 bp to -450 bp core region responded strongly to the hormones, methyl jasmonate (MeJA), abscisic acid (ABA), and salicylic acid (SA), and also to injury induction. The TkSRPP gene was highly expressed under hormones and wound-induced conditions. This study reveals the regulation pattern of the SRPP promoter, and provides valuable information for studying natural rubber biosynthesis under hormones and wounding stress.

The plant ESCRT component FREE1 regulates peroxisome-mediated turnover of lipid droplets in germinating Arabidopsis seedlings

Lipid droplets (LDs) stored during seed development are mobilized and provide essential energy and lipids to support seedling growth upon germination. Triacylglycerols (TAGs) are the main neutral lipids stored in LDs. The lipase SUGAR DEPENDENT 1 (SDP1), which hydrolyzes TAGs in Arabidopsis thaliana, is localized on peroxisomes and traffics to the LD surface through peroxisomal extension, but the underlying mechanism remains elusive. Here, we report a previously unknown function of a plant-unique endosomal sorting complex required for transport (ESCRT) component FYVE DOMAIN PROTEIN REQUIRED FOR ENDOSOMAL SORTING 1 (FREE1) in regulating peroxisome/SDP1-mediated LD turnover in Arabidopsis. We showed that LD degradation was impaired in germinating free1 mutant; moreover, the tubulation of SDP1- or PEROXIN 11e (PEX11e)-marked peroxisomes and the migration of SDP1-positive peroxisomes to the LD surface were altered in the free1 mutant. Electron tomography analysis showed that peroxisomes failed to form tubules to engulf LDs in free1, unlike in the wild-type. FREE1 interacted directly with both PEX11e and SDP1, suggesting that these interactions may regulate peroxisomal extension and trafficking of the lipase SDP1 to LDs. Taken together, our results demonstrate a pivotal role for FREE1 in LD degradation in germinating seedlings via regulating peroxisomal tubulation and SDP1 targeting.

Transcriptional regulation of triacylglycerol accumulation in plants under environmental stress conditions

Triacylglycerol (TAG), a major energy reserve in lipid form, accumulates mainly in seeds. Although TAG concentrations are usually low in vegetative tissues because of the repression of seed maturation programs, these programs are derepressed upon the exposure of vegetative tissues to environmental stresses. Metabolic reprogramming of TAG accumulation is driven primarily by transcriptional regulation. A substantial proportion of transcription factors regulating seed TAG biosynthesis also participates in stress-induced TAG accumulation in vegetative tissues. TAG accumulation leads to the formation of lipid droplets and plastoglobules, which play important roles in plant tolerance to environmental stresses. Toxic lipid intermediates generated from environmental-stress-induced lipid membrane degradation are captured by TAG-containing lipid droplets and plastoglobules. This review summarizes recent advances in the transcriptional control of metabolic reprogramming underlying stress-induced TAG accumulation, and provides biological insight into the plant adaptive strategy, linking TAG biosynthesis with plant survival.

Biogenesis and Lipase-Mediated Mobilization of Lipid Droplets in Plants

Cytosolic lipid droplets (LDs) derived from the endoplasmic reticulum (ER) mainly contain neutral lipids, such as triacylglycerols (TAGs) and sterol esters, which are considered energy reserves. The metabolic pathways associated with LDs in eukaryotic species are involved in diverse cellular functions. TAG synthesis in plants is mediated by the sequential involvement of two subcellular organelles, i.e., plastids - plant-specific organelles, which serve as the site of lipid synthesis, and the ER. TAGs and sterol esters synthesized in the ER are sequestered to form LDs through the cooperative action of several proteins, such as SEIPINs, LD-associated proteins, LDAP-interacting proteins, and plant-specific proteins such as oleosins. The integrity and stability of LDs are highly dependent on oleosins, especially in the seeds, and oleosin degradation is critical for efficient mobilization of the TAGs of plant LDs. As the TAGs mobilize in LDs during germination and post-germinative growth, a plant-specific lipase-sugar-dependent 1 (SDP1)-plays a major role, through the inter-organellar communication between the ER and peroxisomes. In this review, we briefly recapitulate the different processes involved in the biogenesis and degradation of plant LDs, followed by a discussion of future perspectives in this field.

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.

OsATL38 mediates mono-ubiquitination of the 14-3-3 protein OsGF14d and negatively regulates the cold stress response in rice

One of the major regulatory pathways that permits plants to convert an external stimulus into an internal cellular response within a short period of time is the ubiquitination pathway. In this study, OsATL38 was identified as a low temperature-induced gene that encodes a rice homolog of Arabidopsis Tóxicos en Levadura RING-type E3 ubiquitin (Ub) ligase, which was predominantly localized to the plasma membrane. OsATL38-overexpressing transgenic rice plants exhibited decreased tolerance to cold stress as compared with wild-type rice plants. In contrast, RNAi-mediated OsATL38 knockdown transgenic progeny exhibited markedly increased tolerance to cold stress relative to that of wild-type plants, which indicated a negative role of OsATL38 in response to cold stress. Yeast two-hybrid, in vitro pull-down, and co-immunoprecipitation assays revealed that OsATL38 physically interacted with OsGF14d, a rice 14-3-3 protein. An in vivo target ubiquitination assay indicated that OsGF14d was mono-ubiquitinated by OsATL38. osgf14d knockout mutant plants were more sensitive to cold stress than wild-type rice plants, indicating that OsGF14d is a positive factor in the response to cold stress. These results provide evidence that the RING E3 Ub ligase OsATL38 negatively regulates the cold stress response in rice via mono-ubiquitination of OsGF14d 14-3-3 protein.

Characterization of Differentially Expressed Genes under Salt Stress in Olive

Climate change, currently taking place worldwide and also in the Mediterranean area, is leading to a reduction in water availability and to groundwater salinization. Olive represents one of the most efficient tree crops to face these scenarios, thanks to its natural ability to tolerate moderate salinity and drought. In the present work, four olive cultivars (Koroneiki, Picual, Royal de Cazorla and Fadak86) were exposed to high salt stress conditions (200 mM of NaCl) in greenhouse, in order to evaluate their tolerance level and to identify key genes involved in salt stress response. Molecular and physiological parameters, as well as plant growth and leaves’ ions Na⁺ and K⁺ content were measured. Results of the physiological measurements showed Royal de Cazorla as the most tolerant cultivar, and Fadak86 and Picual as the most susceptible ones. Ten candidate genes were analyzed and their complete genomic, CDS and protein sequences were identified. The expression analysis of their transcripts through reverse transcriptase quantitative PCR (RT-qPCR) demonstrated that only OeNHX7, OeP5CS, OeRD19A and OePetD were upregulated in tolerant cultivars, thus suggesting their key role in the activation of a salt tolerance mechanism.

RNASeq analysis of drought-stressed guayule reveals the role of gene transcription for modulating rubber, resin, and carbohydrate synthesis

The drought-adapted shrub guayule (Parthenium argentatum) produces rubber, a natural product of major commercial importance, and two co-products with potential industrial use: terpene resin and the carbohydrate fructan. The rubber content of guayule plants subjected to water stress is higher compared to that of well-irrigated plants, a fact consistently reported in guayule field evaluations. To better understand how drought influences rubber biosynthesis at the molecular level, a comprehensive transcriptome database was built from drought-stressed guayule stem tissues using de novo RNA-seq and genome-guided assembly, followed by annotation and expression analysis. Despite having higher rubber content, most rubber biosynthesis related genes were down-regulated in drought-stressed guayule, compared to well-irrigated plants, suggesting post-transcriptional effects may regulate drought-induced rubber accumulation. On the other hand, terpene resin biosynthesis genes were unevenly affected by water stress, implying unique environmental influences over transcriptional control of different terpene compounds or classes. Finally, drought induced expression of fructan catabolism genes in guayule and significantly suppressed these fructan biosynthesis genes. It appears then, that in guayule cultivation, irrigation levels might be calibrated in such a regime to enable tunable accumulation of rubber, resin and fructan.

LDIP cooperates with SEIPIN and LDAP to facilitate lipid droplet biogenesis in Arabidopsis

Cytoplasmic lipid droplets (LDs) are evolutionarily conserved organelles that store neutral lipids and play critical roles in plant growth, development, and stress responses. However, the molecular mechanisms underlying their biogenesis at the endoplasmic reticulum (ER) remain obscure. Here we show that a recently identified protein termed LD-associated protein [LDAP]-interacting protein (LDIP) works together with both endoplasmic reticulum-localized SEIPIN and the LD-coat protein LDAP to facilitate LD formation in Arabidopsis thaliana. Heterologous expression in insect cells demonstrated that LDAP is required for the targeting of LDIP to the LD surface, and both proteins are required for the production of normal numbers and sizes of LDs in plant cells. LDIP also interacts with SEIPIN via a conserved hydrophobic helix in SEIPIN and LDIP functions together with SEIPIN to modulate LD numbers and sizes in plants. Further, the co-expression of both proteins is required to restore normal LD production in SEIPIN-deficient yeast cells. These data, combined with the analogous function of LDIP to a mammalian protein called LD Assembly Factor 1, are discussed in the context of a new model for LD biogenesis in plant cells with evolutionary connections to LD biogenesis in other eukaryotes.

Isolation of Lipid Droplets for Protein and Lipid Analysis

Cytosolic lipid droplets (LDs) are organelles which emulsify a variety of hydrophobic molecules in the aqueous cytoplasm of essentially all plant cells. Most familiar are the LDs from oilseeds or oleaginous fruits that primarily store triacylglycerols and serve a storage function. However, similar hydrophobic particles are found in cells of plant tissues that package terpenoids, sterol esters, wax esters, or other types of nonpolar lipids. The various hydrophobic lipids inside LDs are coated with a phospholipid monolayer, mostly derived from membrane phospholipids during their ontogeny. Various proteins have been identified to be associated with LDs, and these may be cell-type, tissue-type, or even species specific. While major LD proteins like oleosins have been known for decades, more recently a growing list of LD proteins has been identified, primarily by proteomics analyses of isolated LDs and confirmation of their localization by confocal microscopy. LDs, unlike other organelles, have a density less than that of water, and consequently can be isolated and enriched in cellular fractions by flotation centrifugation for composition studies. However, due to its deep coverage, modern proteomics approaches are also prone to identify contaminants, making control experiments necessary. Here, procedures for the isolation of LDs, and analysis of LD components are provided as well as methods to validate the LD localization of proteins.

Lipid Body Dynamics in Shoot Meristems: Production, Enlargement, and Putative Organellar Interactions and Plasmodesmal Targeting

Post-embryonic cells contain minute lipid bodies (LBs) that are transient, mobile, engage in organellar interactions, and target plasmodesmata (PD). While LBs can deliver γ-clade 1,3-β-glucanases to PD, the nature of other cargo is elusive. To gain insight into the poorly understood role of LBs in meristems, we investigated their dynamics by microscopy, gene expression analyzes, and proteomics. In developing buds, meristems accumulated LBs, upregulated several LB-specific OLEOSIN genes and produced OLEOSINs. During bud maturation, the major gene OLE6 was strongly downregulated, OLEOSINs disappeared from bud extracts, whereas lipid biosynthesis genes were upregulated, and LBs were enlarged. Proteomic analyses of the LB fraction of dormant buds confirmed that OLEOSINs were no longer present. Instead, we identified the LB-associated proteins CALEOSIN (CLO1), Oil Body Lipase 1 (OBL1), Lipid Droplet Interacting Protein (LDIP), Lipid Droplet Associated Protein1a/b (LDAP1a/b) and LDAP3a/b, and crucial components of the OLEOSIN-deubiquitinating and degradation machinery, such as PUX10 and CDC48A. All mRFP-tagged LDAPs localized to LBs when transiently expressed in Nicotiana benthamiana. Together with gene expression analyzes, this suggests that during bud maturation, OLEOSINs were replaced by LDIP/LDAPs at enlarging LBs. The LB fraction contained the meristem-related actin7 (ACT7), "myosin XI tail-binding" RAB GTPase C2A, an LB/PD-associated γ-clade 1,3-β-glucanase, and various organelle- and/or PD-localized proteins. The results are congruent with a model in which LBs, motorized by myosin XI-k/1/2, traffic on F-actin, transiently interact with other organelles, and deliver a diverse cargo to PD.

Arabidopsis thaliana EARLY RESPONSIVE TO DEHYDRATION 7 Localizes to Lipid Droplets via Its Senescence Domain

Lipid droplets (LDs) are neutral-lipid-containing organelles found in all kingdoms of life and are coated with proteins that carry out a vast array of functions. Compared to mammals and yeast, relatively few LD proteins have been identified in plants, particularly those associated with LDs in vegetative (non-seed) cell types. Thus, to better understand the cellular roles of LDs in plants, a more comprehensive inventory and characterization of LD proteins is required. Here, we performed a proteomics analysis of LDs isolated from drought-stressed Arabidopsis leaves and identified EARLY RESPONSIVE TO DEHYDRATION 7 (ERD7) as a putative LD protein. mCherry-tagged ERD7 localized to both LDs and the cytosol when ectopically expressed in plant cells, and the protein's C-terminal senescence domain (SD) was both necessary and sufficient for LD targeting. Phylogenetic analysis revealed that ERD7 belongs to a six-member family in Arabidopsis that, along with homologs in other plant species, is separated into two distinct subfamilies. Notably, the SDs of proteins from each subfamily conferred targeting to either LDs or mitochondria. Further, the SD from the ERD7 homolog in humans, spartin, localized to LDs in plant cells, similar to its localization in mammals; although, in mammalian cells, spartin also conditionally localizes to other subcellular compartments, including mitochondria. Disruption of ERD7 gene expression in Arabidopsis revealed no obvious changes in LD numbers or morphology under normal growth conditions, although this does not preclude a role for ERD7 in stress-induced LD dynamics. Consistent with this possibility, a yeast two-hybrid screen using ERD7 as bait identified numerous proteins involved in stress responses, including some that have been identified in other LD proteomes. Collectively, these observations provide new insight to ERD7 and the SD-containing family of proteins in plants and suggest that ERD7 may be involved in functional aspects of plant stress response that also include localization to the LD surface.

Connecting moss lipid droplets to patchoulol biosynthesis

Plant-derived terpenoids are extensively used in perfume, food, cosmetic and pharmaceutical industries, and several attempts are being made to produce terpenes in heterologous hosts. Native hosts have evolved to accumulate large quantities of terpenes in specialized cells. However, heterologous cells lack the capacity needed to produce and store high amounts of non-native terpenes, leading to reduced growth and loss of volatile terpenes by evaporation. Here, we describe how to direct the sesquiterpene patchoulol production into cytoplasmic lipid droplets (LDs) in Physcomitrium patens (syn. Physcomitrella patens), by attaching patchoulol synthase (PTS) to proteins linked to plant LD biogenesis. Three different LD-proteins: Oleosin (PpOLE1), Lipid Droplet Associated Protein (AtLDAP1) and Seipin (PpSeipin325) were tested as anchors. Ectopic expression of PTS increased the number and size of LDs, implying an unknown mechanism between heterologous terpene production and LD biogenesis. The expression of PTS physically linked to Seipin increased the LD size and the retention of patchoulol in the cell. Overall, the expression of PTS was lower in the anchored mutants than in the control, but when normalized to the expression the production of patchoulol was higher in the seipin-linked mutants.

Ties between Stress and Lipid Droplets Pre-date Seeds

Seeds were a key evolutionary innovation. These durable structures provide a concerted solution to two challenges on land: dispersal and stress. Lipid droplets (LDs) that act as nutrient storage reservoirs are one of the main cell-biological reasons for seed endurance. Although LDs are key structures in spermatophytes and are especially abundant in seeds, they are found across plants and algae, and increase during stress. Further, the proteins that underpin their form and function often have deep homologs. We propose an evolutionary scenario in which (i) the generation of LDs arose as a mechanism to mediate general drought and desiccation resilience, and (ii) the required protein framework was co-opted by spermatophytes for a seed-specific program.

AtKPNB1, an Arabidopsis importin-β protein, is downstream of the RING E3 ubiquitin ligase AtAIRP1 in the ABA-mediated drought stress response

AtKPNB1, an Arabidopsis importin-β protein, was regulated by AtAIRP1 E3 ubiquitin ligase, which intensified the ABA-mediated drought stress response. As an early step in the abscisic acid (ABA)-mediated drought response, the ABA signal is transduced into the nucleus, and thus the nuclear transport system is crucially involved in the drought stress response. AtKPNB1, an importin-β protein, which is a core component of nuclear transport, was previously reported to be a negative factor in the ABA-mediated drought stress response (Luo et al. Luo et al., Plant J 75:377-389, 2013). Here, we report that AtAIPR1, an Arabidopsis RING-type E3 ubiquitin (Ub) ligase, interacted with and ubiquitinated AtKPNB1. A null mutation of AtKPNB1 suppressed the ABA-insensitive germination phenotype of atairp1 mutant seedlings as compared to that of the wild-type plants. Furthermore, the ABA-insensitive stomatal closure and drought-susceptible phenotypes of atairp1 were rescued in atairp1atkpnb1 double mutant progeny, indicating that AtKPNB1 functions downstream of AtAIRP1. These data suggest that AtAIRP1 regulates the ABA-mediated drought response in Arabidopsis via ubiquitination of AtKPNB1.

Towards model-driven characterization and manipulation of plant lipid metabolism

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

The Arabidopsis transcription factor AINTEGUMENTA orchestrates patterning genes and auxin signaling in the establishment of floral growth and form

Understanding how flowers form is an important problem in plant biology, as human food supply depends on flower and seed production. Flower development also provides an excellent model for understanding how cell division, expansion and differentiation are coordinated during organogenesis. In the model plant Arabidopsis thaliana, floral organogenesis requires AINTEGUMENTA (ANT) and AINTEGUMENTA-LIKE 6 (AIL6)/PLETHORA 3 (PLT3), two members of the Arabidopsis AINTEGUMENTA-LIKE/PLETHORA (AIL/PLT) transcription factor family. Together, ANT and AIL6/PLT3 regulate aspects of floral organogenesis, including floral organ initiation, growth, identity specification and patterning. Previously, we used RNA-Seq to identify thousands of genes with disrupted expression in ant ail6 mutant flowers, indicating that ANT and AIL6/PLT3 influence a vast transcriptional network. The immediate downstream targets of ANT and AIL6/PLT3 in flowers are unknown, however. To identify direct targets of ANT regulation, we performed an RNA-Seq time-course experiment in which we induced ANT activity in transgenic plants bearing an ANT-glucocorticoid receptor fusion construct. In addition, we performed a ChIP-Seq experiment that identified ANT binding sites in developing flowers. These experiments identified 200 potential ANT target genes based on their proximity to ANT binding sites and differential expression in response to ANT. These 200 candidate target genes were involved in functions such as polarity specification, floral organ development, meristem development and auxin signaling. In addition, we identified several genes associated with lateral organ growth that may mediate the role of ANT in organ size control. These results reveal new features of the ANT transcriptional network by linking ANT to previously unknown regulatory targets.

Lipid droplets throughout the evolutionary tree

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

The Role of Triacylglycerol in Plant Stress Response

Vegetable oil is mainly composed of triacylglycerol (TAG), a storage lipid that serves as a major commodity for food and industrial purposes, as well as an alternative biofuel source. While TAG is typically not produced at significant levels in vegetative tissues, emerging evidence suggests that its accumulation in such tissues may provide one mechanism by which plants cope with abiotic stress. Different types of abiotic stress induce lipid remodeling through the action of specific lipases, which results in various alterations in membrane lipid composition. This response induces the formation of toxic lipid intermediates that cause membrane damage or cell death. However, increased levels of TAG under stress conditions are believed to function, at least in part, as a means of sequestering these toxic lipid intermediates. Moreover, the lipid droplets (LDs) in which TAG is enclosed also function as a subcellular factory to provide binding sites and substrates for the biosynthesis of bioactive compounds that protect against insects and fungi. Though our knowledge concerning the role of TAG in stress tolerance is expanding, many gaps in our understanding of the mechanisms driving these processes are still evident. In this review, we highlight progress that has been made to decipher the role of TAG in plant stress response, and we discuss possible ways in which this information could be utilized to improve crops in the future.

Lipid droplets in plants and algae: Distribution, formation, turnover and function

Plant oils represent an energy-rich and carbon-dense group of hydrophobic compounds. These oils are not only of economic interest, but also play important, fundamental roles in plant and algal growth and development. The subcellular storage compartments of plant lipids, referred to as lipid droplets (LDs), have long been considered relatively inert oil vessels. However, research in the last decade has revealed that LDs play far more dynamic roles in plant biology than previously appreciated, including transient neutral lipid storage, membrane remodeling, lipid signaling, and stress responses. Here we discuss recent developments in the understanding of LD formation, turnover and function in land plants and algae.

Identification of Low-Abundance Lipid Droplet Proteins in Seeds and Seedlings

The developmental program of seed formation, germination, and early seedling growth requires not only tight regulation of cell division and metabolism, but also concerted control of the structure and function of organelles, which relies on specific changes in their protein composition. Of particular interest is the switch from heterotrophic to photoautotrophic seedling growth, for which cytoplasmic lipid droplets (LDs) play a critical role as depots for energy-rich storage lipids. Here, we present the results of a bottom-up proteomics study analyzing the total protein fractions and LD-enriched fractions in eight different developmental phases during silique (seed) development, seed germination, and seedling establishment in Arabidopsis (Arabidopsis thaliana). The quantitative analysis of the LD proteome using LD-enrichment factors led to the identification of six previously unidentified and comparably low-abundance LD proteins, each of which was confirmed by intracellular localization studies with fluorescent protein fusions. In addition to these advances in LD protein discovery and the potential insights provided to as yet unexplored aspects in plant LD functions, our data set allowed for a comparative analysis of the LD protein composition throughout the various developmental phases examined. Among the most notable of the alterations in the LD proteome were those during seedling establishment, indicating a switch in the physiological function(s) of LDs after greening of the cotyledons. This work highlights LDs as dynamic organelles with functions beyond lipid storage.

Arabidopsis LDIP protein locates at a confined area within the lipid droplet surface and favors lipid droplet formation

Lipid droplets (LDs) are cell organelles specialized in neutral lipid storage. Extendedly studied in seeds, LDs also accumulate in leaves during senescence or in response to abiotic stresses. However the mechanisms underlying their biogenesis remain relatively unknown. Here, we deciphered the distinct roles of two proteins during LD biogenesis: LD-associated protein 1 (AtLDAP1) and LDAP-interacting protein (AtLDIP). We demonstrated that AtLDIP overexpression favors the neo-formation of small LDs under growing conditions where LD accumulation is usually not observed. In addition, atldip knock-out mutant displayed fewer but larger LDs, confirming a role of AtLDIP in LD biogenesis. Interestingly, a synergistic effect of the overexpression of both AtLDIP and AtLDAP1 was observed, resulting in an increase of LD cluster occurrence and LD abundance within the clusters and the cells. AtLDIP overexpression has no significant impact on triacylglycerol and steryl ester accumulation but AtLDIP inactivation is associated with an increase of neutral lipid content, that is probably a consequence of the enlarged but less abundant LDs present in this line. Our localization study demonstrated that AtLDIP is localized at specific dotted sites within the LD in contrast to AtLDAP1 that covers the whole LD. In addition, AtLDIP sometimes localized away from the LD marker, but always associated with the ER network, suggesting a location at LD nascent sites within the ER. Taken together, our results suggested that AtLDIP promotes the formation of new LDs from ER localized TAG lenses.

In-depth proteome analysis reveals multiple pathways involved in tomato SlMPK1-mediated high-temperature responses

High temperature (HT) is one of the major environmental factors which limits plant growth and yield. The mitogen-activated protein kinase (MAPK) plays vital roles in environmental stress responses. However, the mechanisms triggered by MAPKs in plants in response to HT are still extremely limited. In this study, the proteomic data of differences between SlMPK1 RNA-interference mutant (SlMPK1i) and wild type and of tomato (Solanum lycopersicum) plants under HT stress using isobaric tags for relative and absolute quantitation (iTRAQ) was re-analyzed in depth. In total, 168 differently expressed proteins (DEPs) were identified in response to HT stress, including 38 DEPs only found in wild type, and 84 DEPs specifically observed in SlMPK1i after HT treatment. The majority of higher expression of 84 DEPs were annotated into photosynthesis, oxidation-reduction process, protein folding, translation, proteolysis, stress response, and amino acid biosynthetic process. More importantly, SlMPK1-mediated photosynthesis was confirmed by the physiological characterization of SlMPK1i with a higher level of photosynthetic capacity under HT stress. Overall, the results reveal a set of potential candidate proteins helping to further understand the intricate regulatory network regulated by SlMPK1 in response to HT.

The Arabidopsis MYB96 Transcription Factor Mediates ABA-Dependent Triacylglycerol Accumulation in Vegetative Tissues under Drought Stress Conditions

Triacylglycerols (TAGs), a major lipid form of energy storage, are involved in a variety of plant developmental processes. While carbon reserves mainly accumulate in seeds, significant amounts of TAG have also been observed in vegetative tissues. Notably, the accumulation of leaf TAGs is influenced by environmental stresses such as drought stress, although underlying molecular networks remain to be fully elucidated. In this study, we demonstrate that the R2R3-type MYB96 transcription factor promotes TAG biosynthesis in Arabidopsis thaliana seedlings. Core TAG biosynthetic genes were up-regulated in myb96-ox seedlings, but down-regulated in myb96-deficient seedlings. In particular, ABA stimulates TAG accumulation in the vegetative tissues, and MYB96 plays a fundamental role in this process. Considering that TAG accumulation contributes to plant tolerance to drought stress, MYB96-dependent TAG biosynthesis not only triggers plant adaptive responses but also optimizes energy metabolism to ensure plant fitness under unfavorable environmental conditions.

Mechanisms of lipid droplet biogenesis

Lipid droplets (LDs) are organelles that compartmentalize nonbilayer-forming lipids in the aqueous cytoplasm of cells. They are ubiquitous in most organisms, including in animals, protists, plants and microorganisms. In eukaryotes, LDs are believed to be derived by a budding and scission process from the surface of the endoplasmic reticulum, and this occurs concomitantly with the accumulation of neutral lipids, most often triacylglycerols and steryl esters. Overall, the mechanisms underlying LD biogenesis are difficult to generalize, in part because of the involvement of different sets of both evolutionarily conserved and organism-specific LD-packaging proteins. Here, we briefly compare and contrast these proteins and the allied processes responsible for LD biogenesis in cells of animals, yeasts and plants.

Enhanced ROS scavenging and sugar accumulation contribute to drought tolerance of naturally occurring autotetraploids in Poncirus trifoliata

Tetraploids have been reported to exhibit increased stress tolerance, but the underlying molecular and physiological mechanisms remain poorly understood. In this study, autotetraploid plants were identified by screening natural seedlings of trifoliate orange (Poncirus trifoliata). The tetraploids exhibited different morphology and displayed significantly enhanced drought and dehydration tolerance in comparison with the diploid progenitor. Transcriptome analysis indicated that a number of stress-responsive genes and pathways were differentially influenced and enriched in the tetraploids, in particular those coding for enzymes related to antioxidant process and sugar metabolism. Transcript levels and activities of antioxidant enzymes (peroxidase and superoxide dismutase) and sucrose-hydrolysing enzyme (vacuolar invertase) were increased in the tetraploids upon exposure to the drought, concomitant with greater levels of glucose but lower level of reactive oxygen species (ROS). These data indicate that the tetraploids might undergo extensive transcriptome reprogramming of genes involved in ROS scavenging and sugar metabolism, which contributes, synergistically or independently, to the enhanced stress tolerance of the tetraploid. Our results reveal that the tetraploids take priority over the diploid for stress tolerance by maintaining a more robust system of ROS detoxification and osmotic adjustment via elevating antioxidant capacity and sugar accumulation in comparison with the diploid counterpart.

Proteomic Landscape of the Mature Roots in a Rubber-Producing Grass Taraxacum Kok-saghyz

The rubber grass Taraxacum kok-saghyz (TKS) contains large amounts of natural rubber (cis-1,4-polyisoprene) in its enlarged roots and it is an alternative crop source of natural rubber. Natural rubber biosynthesis (NRB) and storage in the mature roots of TKS is a cascade process involving many genes, proteins and their cofactors. The TKS genome has just been annotated and many NRB-related genes have been determined. However, there is limited knowledge about the protein regulation mechanism for NRB in TKS roots. We identified 371 protein species from the mature roots of TKS by combining two-dimensional gel electrophoresis (2-DE) and mass spectrometry (MS). Meanwhile, a large-scale shotgun analysis of proteins in TKS roots at the enlargement stage was performed, and 3545 individual proteins were determined. Subsequently, all identified proteins from 2-DE gel and shotgun MS in TKS roots were subject to gene ontology and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses and most proteins were involved in carbon metabolic process with catalytic activity in membrane-bounded organelles, followed by proteins with binding ability, transportation and phenylpropanoid biosynthesis activities. Fifty-eight NRB-related proteins, including eight small rubber particle protein (SRPP) and two rubber elongation factor(REF) members, were identified from the TKS roots, and these proteins were involved in both mevalonate acid (MVA) and methylerythritol phosphate (MEP) pathways. To our best knowledge, it is the first high-resolution draft proteome map of the mature TKS roots. Our proteomics of TKS roots revealed both MVA and MEP pathways are important for NRB, and SRPP might be more important than REF for NRB in TKS roots. These findings would not only deepen our understanding of the TKS root proteome, but also provide new evidence on the roles of these NRB-related proteins in the mature TKS roots.

Metabolic engineering for enhanced oil in biomass

The world is hungry for energy. Plant oils in the form of triacylglycerol (TAG) are one of the most reduced storage forms of carbon found in nature and hence represent an excellent source of energy. The myriad of applications for plant oils range across foods, feeds, biofuels, and chemical feedstocks as a unique substitute for petroleum derivatives. Traditionally, plant oils are sourced either from oilseeds or tissues surrounding the seed (mesocarp). Most vegetative tissues, such as leaves and stems, however, accumulate relatively low levels of TAG. Since non-seed tissues constitute the majority of the plant biomass, metabolic engineering to improve their low-intrinsic TAG-biosynthetic capacity has recently attracted significant attention as a novel, sustainable and potentially high-yielding oil production platform. While initial attempts predominantly targeted single genes, recent combinatorial metabolic engineering strategies have focused on the simultaneous optimization of oil synthesis, packaging and degradation pathways (i.e., 'push, pull, package and protect'). This holistic approach has resulted in dramatic, seed-like TAG levels in vegetative tissues. With the first proof of concept hurdle addressed, new challenges and opportunities emerge, including engineering fatty acid profile, translation into agronomic crops, extraction, and downstream processing to deliver accessible and sustainable bioenergy.

Possible Roles of Membrane Trafficking Components for Lipid Droplet Dynamics in Higher Plants and Green Algae

Lipid droplets are ubiquitous dynamic organelles that contain neutral lipids surrounded by a phospholipid monolayer. They can store and supply lipids for energy metabolism and membrane synthesis. In addition, protein transport and lipid exchange often occur between LDs and various organelles to control lipid homeostasis in response to multiple stress responses and cellular signaling. In recent years, multiple membrane trafficking proteins have been identified through LD proteomics and genetic analyses. These membrane trafficking machineries are emerging as critical regulators to function in different LD-organelle interactions, e.g., for LD dynamics, biogenesis and turnover. In this review, we will summarize recent advances in regard to LD-related membrane trafficking proteins and discuss future investigations in higher plants and green algae.

Terpenoid Esters Are the Major Constituents From Leaf Lipid Droplets of Camellia sinensis

Lipid droplets (LDs) have been widely found from diverse species and exhibit diverse functions. It remains unexplored what potential roles they played in tea. To address this question, we analyzed the chemical composition and the dynamic changes of cytosolic LDs during leaf growth and diurnal cycle. Using TopFluor cholesterol and Nile Red staining we demonstrated that cytosolic LDs were heterogeneous in tea tree (Camellia sinensis cv. Tieguanyin); the size and number of LDs increased with leaf growth. Compositional analysis showed that terpenoid esters and diacylglycerol are the major components of cytosolic LDs. The contents of total sterol esters (SEs) and β-amyrin esters increased with leaf expansion and growth; individual SE also showed diurnal changes. Our data suggest that cytosolic LDs from tea tree leave mainly serve as storage site for free sterols and triterpenoids in the form of esters. Cytosolic LDs were not the major contributors to the aroma quality of made tea.

Transcriptomics and co-expression networks reveal tissue-specific responses and regulatory hubs under mild and severe drought in papaya (Carica papaya L.)

Plants respond to drought stress through the ABA dependent and independent pathways, which in turn modulate transcriptional regulatory hubs. Here, we employed Illumina RNA-Seq to analyze a total of 18 cDNA libraries from leaves, sap, and roots of papaya plants under drought stress. Reference and de novo transcriptomic analyses identified 8,549 and 6,089 drought-responsive genes and unigenes, respectively. Core sets of 6 and 34 genes were simultaneously up- or down-regulated, respectively, in all stressed samples. Moreover, GO enrichment analysis revealed that under moderate drought stress, processes related to cell cycle and DNA repair were up-regulated in leaves and sap; while responses to abiotic stress, hormone signaling, sucrose metabolism, and suberin biosynthesis were up-regulated in roots. Under severe drought stress, biological processes related to abiotic stress, hormone signaling, and oxidation-reduction were up-regulated in all tissues. Moreover, similar biological processes were commonly down-regulated in all stressed samples. Furthermore, co-expression network analysis revealed three and eight transcriptionally regulated modules in leaves and roots, respectively. Seventeen stress-related TFs were identified, potentially serving as main regulatory hubs in leaves and roots. Our findings provide insight into the molecular responses of papaya plant to drought, which could contribute to the improvement of this important tropical crop.

PUX10 Is a Lipid Droplet-Localized Scaffold Protein That Interacts with CELL DIVISION CYCLE48 and Is Involved in the Degradation of Lipid Droplet Proteins

The number of known proteins associated with plant lipid droplets (LDs) is small compared with other organelles. Many aspects of LD biosynthesis and degradation are unknown, and identifying and characterizing candidate LD proteins could help elucidate these processes. Here, we analyzed the proteome of LD-enriched fractions isolated from tobacco (Nicotiana tabacum) pollen tubes. Proteins that were highly enriched in comparison with the total or cytosolic fraction were further tested for LD localization via transient expression in pollen tubes. One of these proteins, PLANT UBX DOMAIN-CONTAINING PROTEIN10 (PUX10), is a member of the plant UBX domain-containing (PUX) protein family. This protein localizes to LDs via a unique hydrophobic polypeptide sequence and can recruit the AAA-type ATPase CELL DIVISION CYCLE48 (CDC48) protein via its UBX domain. PUX10 is conserved in Arabidopsis thaliana and expressed in embryos, pollen tubes, and seedlings. In pux10 knockout mutants in Arabidopsis, LD size is significantly increased. Proteomic analysis of pux10 mutants revealed a delayed degradation of known LD proteins, some of which possessed ubiquitination sites. We propose that PUX10 is involved in a protein degradation pathway at LDs, mediating an interaction between polyubiquitinated proteins targeted for degradation and downstream effectors such as CDC48.