A Central Role for Triacylglycerol in Membrane Lipid Breakdown, Fatty Acid β-Oxidation, and Plant Survival under Extended Darkness

Autophagy mutants show delayed chloroplast development during de-etiolation in carbon limiting conditions

Autophagy is a conserved catabolic process that plays an essential role under nutrient starvation conditions and influences different developmental processes. We observed that seedlings of autophagy mutants (atg2, atg5, atg7, and atg9) germinated in the dark showed delayed chloroplast development following illumination. The delayed chloroplast development was characterized by a decrease in photosynthetic and chlorophyll biosynthetic proteins, lower chlorophyll content, reduced chloroplast size, and increased levels of proteins involved in lipid biosynthesis. Confirming the biological impact of these differences, photosynthetic performance was impaired in autophagy mutants 12 h post-illumination. We observed that while gene expression for photosynthetic machinery during de-etiolation was largely unaffected in atg mutants, several genes involved in photosystem assembly were transcriptionally downregulated. We also investigated if the delayed chloroplast development could be explained by lower lipid import to the chloroplast or lower triglyceride (TAG) turnover. We observed that the limitations in the chloroplast lipid import imposed by trigalactosyldiacylglycerol1 are unlikely to explain the delay in chloroplast development. However, we found that lower TAG mobility in the triacylglycerol lipase mutant sugardependent1 significantly affected de-etiolation. Moreover, we showed that lower levels of carbon resources exacerbated the slow greening phenotype whereas higher levels of carbon resources had an opposite effect. This work suggests a lack of autophagy machinery limits chloroplast development during de-etiolation, and this is exacerbated by limited lipid turnover (lipophagy) that physically or energetically restrains chloroplast development.

Pathogens and Elicitors Induce Local and Systemic Changes in Triacylglycerol Metabolism in Roots and in Leaves of Arabidopsis thaliana

Simple Summary

Abiotic and biotic stress conditions result in profound changes in plant lipid metabolism. Vegetable oil consists of triacylglycerols, which are important energy and carbon storage compounds in seeds of various plant species. These compounds are also present in vegetative tissue, and levels have been reported to increase with different abiotic stresses in leaves. This work shows that triacylglycerols accumulate in roots and in distal, non-treated leaves upon treatment with a fungal pathogen or lipopolysaccharide (a common bacterial-derived elicitor in animals and plants). Treatment of leaves with a bacterial pathogen or a bacterial effector molecule results in triacylglycerol accumulation in leaves, but not systemically in roots. These results suggest that elicitor molecules are sufficient to induce an increase in triacylglycerol levels, and that unidirectional long-distance signaling from roots to leaves is involved in pathogen and elicitor-induced triacylglycerol accumulation.

Abstract

Interaction of plants with the environment affects lipid metabolism. Changes in the pattern of phospholipids have been reported in response to abiotic stress, particularly accumulation of triacylglycerols, but less is known about the alteration of lipid metabolism in response to biotic stress and leaves have been more intensively studied than roots. This work investigates the levels of lipids in roots as well as leaves of Arabidopsis thaliana in response to pathogens and elicitor molecules by UPLC-TOF-MS. Triacylglycerol levels increased in roots and systemically in leaves upon treatment of roots with the fungus Verticillium longisporum. Upon spray infection of leaves with the bacterial pathogen Pseudomonas syringae, triacylglycerols accumulated locally in leaves but not in roots. Treatment of roots with a bacterial lipopolysaccharide elicitor induced a strong triacylglycerol accumulation in roots and leaves. Induction of the expression of the bacterial effector AVRRPM1 resulted in a dramatic increase of triacylglycerol levels in leaves, indicating that elicitor molecules are sufficient to induce accumulation of triacylglycerols. These results give insight into local and systemic changes to lipid metabolism in roots and leaves in response to biotic stresses.

Blue Light Improves Photosynthetic Performance during Healing and Acclimatization of Grafted Watermelon Seedlings

To investigate the importance of light on healing and acclimatization, in the present study, grafted watermelon seedlings were exposed to darkness (D) or light, provided by blue (B), red (R), a mixture of R (68%) and B (RB), or white (W; 35% B, 49% intermediate spectra, 16% R) LEDs for 12 days. Survival ratio, root and shoot growth, soluble carbohydrate content, photosynthetic pigments content, and photosynthetic performance were evaluated. Seedling survival was not only strongly limited in D but the survived seedlings had an inferior shoot and root development, reduced chlorophyll content, and attenuated photosynthetic efficiency. RB-exposed seedlings had a less-developed root system. R-exposed seedlings showed leaf epinasty, and had the smallest leaf area, reduced chlorophyll content, and suppressed photosynthetic apparatus performance. The R-exposed seedlings contained the highest amount of soluble carbohydrate and together with D-exposed seedlings the lowest amount of chlorophyll in their scions. B-exposed seedlings showed the highest chlorophyll content and improved overall PSII photosynthetic functioning. W-exposed seedling had the largest leaf area, and closely resembled the photosynthetic properties of RB-exposed seedlings. We assume that, during healing of grafted seedlings monochromatic R light should be avoided. Instead, W and monochromatic B light may be willingly adopted due to their promoting effect on shoot, pigments content, and photosynthetic efficiency.

Mechanisms and functions of membrane lipid remodeling in plants

Lipid remodeling, defined herein as post-synthetic structural modifications of membrane lipids, play crucial roles in regulating the physicochemical properties of cellular membranes and hence their many functions. Processes affected by lipid remodeling include lipid metabolism, membrane repair, cellular homeostasis, fatty acid trafficking, cellular signaling and stress tolerance. Glycerolipids are the major structural components of cellular membranes and their composition can be adjusted by modifying their head groups, their acyl chain lengths and the number and position of double bonds. This review summarizes recent advances in our understanding of mechanisms of membrane lipid remodeling with emphasis on the lipases and acyltransferases involved in the modification of phosphatidylcholine and monogalactosyldiacylglycerol, the major membrane lipids of extraplastidic and photosynthetic membranes, respectively. We also discuss the role of triacylglycerol metabolism in membrane acyl chain remodeling. Finally, we discuss emerging data concerning the functional roles of glycerolipid remodeling in plant stress responses. Illustrating the molecular basis of lipid remodeling may lead to novel strategies for crop improvement and other biotechnological applications such as bioenergy production.

A transcriptomic, metabolomic and cellular approach to the physiological adaptation of tomato fruit to high temperature

High temperatures can negatively influence plant growth and development. Besides yield, the effects of heat stress on fruit quality traits remain poorly characterised. In tomato, insights into how fruits regulate cellular metabolism in response to heat stress could contribute to the development of heat-tolerant varieties, without detrimental effects on quality. In the present study, the changes occurring in wild type tomato fruits after exposure to transient heat stress have been elucidated at the transcriptome, cellular and metabolite level. An impact on fruit quality was evident as nutritional attributes changed in response to heat stress. Fruit carotenogenesis was affected, predominantly at the stage of phytoene formation, although altered desaturation/isomerisation arose during the transient exposure to high temperatures. Plastidial isoprenoid compounds showed subtle alterations in their distribution within chromoplast sub-compartments. Metabolite profiling suggests limited effects on primary/intermediary metabolism but lipid remodelling was evident. The heat-induced molecular signatures included the accumulation of sucrose and triacylglycerols, and a decrease in the degree of membrane lipid unsaturation, which influenced the volatile profile. Collectively, these data provide valuable insights into the underlying biochemical and molecular adaptation of fruit to heat stress and will impact on our ability to develop future climate resilient tomato varieties.

Arabidopsis FAR-RED ELONGATED HYPOCOTYL3 negatively regulates carbon starvation responses

Light is one of the most important environmental factors that affects various cellular processes in plant growth and development; it is also crucial for the metabolism of carbohydrates as it provides the energy source for photosynthesis. Under extended darkness conditions, carbon starvation responses are triggered by depletion of stored energy. Although light rapidly inhibits starvation responses, the molecular mechanisms by which light signalling affects this process remain largely unknown. In this study, we showed that the Arabidopsis thaliana light signalling protein FAR-RED ELONGATED HYPOCOTYL3 (FHY3) and its homolog FAR-RED IMPAIRED RESPONSE1 (FAR1) are essential for plant survival after extended darkness treatment at both seedling and adult stages. Transmission electron microscopy analyses revealed that disruption of both FHY3 and FAR1 resulted in destruction of chloroplast envelopes and thylakoid membranes under extended darkness conditions. Furthermore, treatment with sucrose, but not glucose, completely rescued carbon starvation-induced cell death in the rosette leaves and arrested early seedling establishment in the fhy3 far1 plants. We thus concluded that the light signalling proteins FHY3 and FAR1 negatively regulate carbon starvation responses in Arabidopsis.

Autophagy is required for lipid homeostasis during dark-induced senescence

Autophagy is an evolutionarily conserved mechanism that mediates the degradation of cytoplasmic components in eukaryotic cells. In plants, autophagy has been extensively associated with the recycling of proteins during carbon-starvation conditions. Even though lipids constitute a significant energy reserve, our understanding of the function of autophagy in the management of cell lipid reserves and components remains fragmented. To further investigate the significance of autophagy in lipid metabolism, we performed an extensive lipidomic characterization of Arabidopsis (Arabidopsis thaliana) autophagy mutants (atg) subjected to dark-induced senescence conditions. Our results revealed an altered lipid profile in atg mutants, suggesting that autophagy affects the homeostasis of multiple lipid components under dark-induced senescence. The acute degradation of chloroplast lipids coupled with the differential accumulation of triacylglycerols (TAGs) and plastoglobuli indicates an alternative metabolic reprogramming toward lipid storage in atg mutants. The imbalance of lipid metabolism compromises the production of cytosolic lipid droplets and the regulation of peroxisomal lipid oxidation pathways in atg mutants.

Chloroplast lipid biosynthesis is fine-tuned to thylakoid membrane remodeling during light acclimation

Reprogramming metabolism, in addition to modifying the structure and function of the photosynthetic machinery, is crucial for plant acclimation to changing light conditions. One of the key acclimatory responses involves reorganization of the photosynthetic membrane system including changes in thylakoid stacking. Glycerolipids are the main structural component of thylakoids and their synthesis involves two main pathways localized in the plastid and the endoplasmic reticulum (ER); however, the role of lipid metabolism in light acclimation remains poorly understood. We found that fatty acid synthesis, membrane lipid content, the plastid lipid biosynthetic pathway activity, and the degree of thylakoid stacking were significantly higher in plants grown under low light compared with plants grown under normal light. Plants grown under high light, on the other hand, showed a lower rate of fatty acid synthesis, a higher fatty acid flux through the ER pathway, higher triacylglycerol content, and thylakoid membrane unstacking. We additionally demonstrated that changes in rates of fatty acid synthesis under different growth light conditions are due to post-translational regulation of the plastidic acetyl-CoA carboxylase activity. Furthermore, Arabidopsis mutants defective in one of the two glycerolipid biosynthetic pathways displayed altered growth patterns and a severely reduced ability to remodel thylakoid architecture, particularly under high light. Overall, this study reveals how plants fine-tune fatty acid and glycerolipid biosynthesis to cellular metabolic needs in response to long-term changes in light conditions, highlighting the importance of lipid metabolism in light acclimation.

Ascorbate inactivates the oxygen-evolving complex in prolonged darkness

Ascorbate (Asc, vitamin C) is an essential metabolite participating in multiple physiological processes of plants, including environmental stress management and development. In this study, we acquired knowledge on the role of Asc in dark-induced leaf senescence using Arabidopsis thaliana as a model organism. One of the earliest effects of prolonged darkness is the inactivation of oxygen-evolving complexes (OEC) as demonstrated here by fast chlorophyll a fluorescence and thermoluminescence measurements. We found that inactivation of OEC due to prolonged darkness was attenuated in the Asc-deficient vtc2-4 mutant. On the other hand, the severe photosynthetic phenotype of a psbo1 knockout mutant, lacking the major extrinsic OEC subunit PSBO1, was further aggravated upon a 24-h dark treatment. The psbr mutant, devoid of the PSBR subunit of OEC, performed only slightly disturbed photosynthetic activity under normal growth conditions, whereas it showed a strongly diminished B thermoluminescence band upon dark treatment. We have also generated a double psbo1 vtc2 mutant, and it showed a slightly milder photosynthetic phenotype than the single psbo1 mutant. Our results, therefore, suggest that Asc leads to the inactivation of OEC in prolonged darkness by over-reducing the Mn-complex that is probably enabled by a dark-induced dissociation of the extrinsic OEC subunits. Our study is an example that Asc may negatively affect certain cellular processes and thus its concentration and localization need to be highly controlled.

Identification, Classification, and Expression Analysis of the Triacylglycerol Lipase (TGL) Gene Family Related to Abiotic Stresses in Tomato

Triacylglycerol Lipases (TGLs) are the major enzymes involved in triacylglycerol catabolism. TGLs hydrolyze long-chain fatty acid triglycerides, which are involved in plant development and abiotic stress responses. Whereas most studies of TGLs have focused on seed oil metabolism and biofuel in plants, limited information is available regarding the genome-wide identification and characterization of the TGL gene family in tomato (Solanum lycopersicum L.). Based on the latest published tomato genome annotation ITAG4.0, 129 SlTGL genes were identified and classified into 5 categories according to their structural characteristics. Most SlTGL genes were distributed on 3 of 12 chromosomes. Segment duplication appeared to be the driving force underlying expansion of the TGL gene family in tomato. The promoter analysis revealed that the promoters of SlTGLs contained many stress responsiveness cis-elements, such as ARE, LTR, MBS, WRE3, and WUN-motifs. Expression of the majority of SlTGL genes was suppressed following exposure to chilling and heat, while it was induced under drought stress, such as SlTGLa9, SlTGLa6, SlTGLa25, SlTGLa26, and SlTGLa13. These results provide valuable insights into the roles of the SlTGL genes family and lay a foundation for further functional studies on the linkage between triacylglycerol catabolism and abiotic stress responses in tomato.

Distinct Physiological Roles of Three Phospholipid:Diacylglycerol Acyltransferase Genes in Olive Fruit with Respect to Oil Accumulation and the Response to Abiotic Stress

Three different cDNA sequences, designated OepPDAT1-1, OepPDAT1-2, and OepPDAT2, encoding three phospholipid:diacylglycerol acyltransferases (PDAT) have been isolated from olive (Olea europaea cv. Picual). Sequence analysis showed the distinctive features typical of the PDAT family and together with phylogenetic analysis indicated that they encode PDAT. Gene expression analysis in different olive tissues showed that transcript levels of these three PDAT genes are spatially and temporally regulated and suggested that, in addition to acyl-CoA:diacylglycerol acyltransferase, OePDAT1-1 may contribute to the biosynthesis of triacylglycerols in the seed, whereas OePDAT1-2 could be involved in the triacylglycerols content in the mesocarp and, therefore, in the olive oil. The relative contribution of PDAT and acyl-CoA:diacylglycerol acyltransferase enzymes to the triacylglycerols content in olive appears to be tissue-dependent. Furthermore, water regime, temperature, light, and wounding regulate PDAT genes at transcriptional level in the olive fruit mesocarp, indicating that PDAT could be involved in the response to abiotic stresses. Altogether, this study represents an advance in our knowledge on the regulation of oil accumulation in oil fruit.

Glycerolipid profile differences between perennial and annual stem zones in the perennial model plant Arabis alpina

The perennial life style is a successful ecological strategy, and Arabis alpina is a recently developed model Brassicaceae species for studying it. One aspect, poorly investigated until today, concerns the differing patterns of allocation, storage, and metabolism of nutrients between perennials and annuals and the yet unknown signals that regulate this process. A. alpina has a complex lateral stem architecture with a proximal vegetative perennial (PZ) and a distal annual flowering zone (AZ) inside the same stems. Lipid bodies (LBs) with triacylglycerols (TAGs) accumulate in the PZ. To identify potential processes of lipid metabolism linked with the perennial lifestyle, we analyzed lipid species in the PZ versus AZ. Glycerolipid fractions, including neutral lipids with mainly TAGs, phospholipids, and glycolipids, were present at higher levels in the PZ as compared to AZ or roots. Concomitantly, contents of specific long-chain and very long-chain fatty acids increased during formation of the PZ. Corresponding gene expression data, gene ontology term enrichment, and correlation analysis with lipid species pinpoint glycerolipid-related genes to be active during the development of the PZ. Possibilities that lipid metabolism genes may be targets of regulatory mechanisms specifying PZ differentiation in A. alpina are discussed.

Bacillus subtilis CP4, isolated from native soil in combination with arbuscular mycorrhizal fungi promotes biofortification, yield and metabolite production in wheat under field conditions

AIMS

The aim of this study was to identify the best combination of plant growth promoting bacteria (PGPB) and arbuscular mycorrhizal fungi (AMF) for biofortification and enhancing yield in wheat as well as improve soil health under field conditions. Another aim was to get insights into metabolite dynamics in plants treated with PGPB and AMF.

METHODS AND RESULTS

Different combinations of PGPB and AMF that gave good results in greenhouse study were used in a field study. The combined application of Bacillus subtilis CP4 (native PGPB) and AMF gave the best results with a significant increase in biomass, macronutrient and micronutrient content in wheat grains and improvement in yield-related parameters relative to the untreated control. PGPB and AMF treatment increased antioxidant enzymes and compounds and decreased the level of an oxidation marker. Metabolite profiling performed using Gas Chromatography-Mass Spectrometry (GC-MS) showed significant upregulation of specific organic acids, amino acids, sugars and sugar alcohols in plants treated with CP4 and AMF. The altered pathways due to CP4 and AMF inoculation mainly belong to carbohydrate and amino acid metabolism. A positive correlation was observed between some organic acids, sugars and amino acids with wheat growth and yield parameters. The activities of soil enzymes increased significantly with the best results shown by native PGPB and AMF combination.

CONCLUSIONS

A native bacterial isolate Bacillus subtilis CP4 in combination with AMF showed exceptional ability for biofortification and yield enhancement under field conditions. The upregulation of a number of metabolites showed correlation plant growth promotion and nutrients.

SIGNIFICANCE AND IMPACT OF THE STUDY

The combined application of native B. subtilis CP4 and AMF could offer a more sustainable approach for the development of a biofertilizer to enhance wheat nutrient content and production and soil health thereby advancing agriculture.

Multifaceted Roles of Plant Autophagy in Lipid and Energy Metabolism

Together with sugars and proteins, lipids constitute the main carbon reserves in plants. Lipids are selectively recycled and catabolized for energy production during development and in response to environmental stresses. Autophagy is a major catabolic pathway, operating in the recycling of cellular components in eukaryotes. Although the autophagic degradation of lipids has been mainly characterized in mammals and yeast, growing evidence has highlighted the role of autophagy in several aspects of lipid metabolism in plants. Here, we summarize recent findings focusing on autophagy functions in lipid droplet (LD) metabolism. We further provide novel insights regarding the relevance of autophagy in the maintenance and clearance of mitochondria and peroxisomes and its consequences for proper lipid usage and energy homeostasis in plants.

Adjustment of Photosynthetic and Antioxidant Activities to Water Deficit Is Crucial in the Drought Tolerance of Lolium multiflorum/Festuca arundinacea Introgression Forms

Lolium multiflorum/Festuca arundinacea introgression forms have been proved several times to be good models to identify key components of grass metabolism involved in the mechanisms of tolerance to water deficit. Here, for the first time, a relationship between photosynthetic and antioxidant capacities with respect to drought tolerance of these forms was analyzed in detail. Two closely related L. multiflorum/F. arundinacea introgression forms distinct in their ability to re-grow after cessation of prolonged water deficit in the field were selected and subjected to short-term drought in pots to dissect precisely mechanisms of drought tolerance in this group of plants. The studies revealed that the form with higher drought tolerance was characterized by earlier and higher accumulation of abscisic acid, more stable cellular membranes, and more balanced reactive oxygen species metabolism associated with a higher capacity of the antioxidant system under drought conditions. On the other hand, both introgression forms revealed the same levels of stomatal conductance, CO₂ assimilation, and consequently, intrinsic water use efficiency under drought and recovery conditions. However, simultaneous higher adjustment of the Calvin cycle to water deficit and reduced CO₂ availability, with respect to the accumulation and activity of plastid fructose-1,6-bisphosphate aldolase, were clearly visible in the form with higher drought tolerance.

Reserve lipids and plant autophagy

Autophagy is a universal mechanism that facilitates the degradation of unwanted cytoplasmic components in eukaryotic cells. In this review, we highlight recent developments in the investigation of the role of autophagy in lipid homeostasis in plants by comparison with algae, yeast, and animals. We consider the storage compartments that form the sources of lipids in plants, and the roles that autophagy plays in the synthesis of triacylglycerols and in the formation and maintenance of lipid droplets. We also consider the relationship between lipids and the biogenesis of autophagosomes, and the role of autophagy in the degradation of lipids in plants.

A new insight into the mechanism for cytosolic lipid droplet degradation in senescent leaves

Leaf senescence involves lipid droplet (LD) degradation that produces toxic fatty acids, but little is known about how the toxic metabolites are isolated from the rest of the cellular components. Our ultramicroscopic characterization of cytosolic LD degradation in central vacuole-absent cells and central vacuole-containing cells of senescent watermelon leaves demonstrated two degradation pathways: the small vacuole-associated pathway and the central vacuole-associated pathway. This provided an insight into the subcellular mechanisms for the isolation of the fatty acids derived from LDs. The central vacuole-containing cells, including mesophyll cells and vascular parenchyma cells, adopted the central vacuole-associated pathway, indicated by the presence of LDs in the central vacuole, which is believed to play a crucial role in scavenging toxic metabolites. The central vacuole-absent intermediary cells, where senescence caused the occurrence of numerous small vacuoles, adopted the small vacuole-associated pathway, evidenced by the occurrence of LDs in the small vacuoles. The assembly of organelles, including LDs, small vacuoles, mitochondria and peroxisome-like organelles, occurred in the central vacuole-absent intermediary cell in response to leaf senescence.

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.

Deconstructing the genetic architecture of iron deficiency chlorosis in soybean using genome-wide approaches

BACKGROUND

Iron (Fe) is an essential micronutrient for plant growth and development. Iron deficiency chlorosis (IDC), caused by calcareous soils or high soil pH, can limit iron availability, negatively affecting soybean (Glycine max) yield. This study leverages genome-wide association study (GWAS) and a genome-wide epistatic study (GWES) with previous gene expression studies to identify regions of the soybean genome important in iron deficiency tolerance.

RESULTS

A GWAS and a GWES were performed using 460 diverse soybean PI lines from 27 countries, in field and hydroponic iron stress conditions, using more than 36,000 single nucleotide polymorphism (SNP) markers. Combining this approach with available RNA-sequencing data identified significant markers, genomic regions, and novel genes associated with or responding to iron deficiency. Sixty-nine genomic regions associated with IDC tolerance were identified across 19 chromosomes via the GWAS, including the major-effect quantitative trait locus (QTL) on chromosome Gm03. Cluster analysis of significant SNPs in this region deconstructed this historically prominent QTL into four distinct linkage blocks, enabling the identification of multiple candidate genes for iron chlorosis tolerance. The complementary GWES identified SNPs in this region interacting with nine other genomic regions, providing the first evidence of epistatic interactions impacting iron deficiency tolerance.

CONCLUSIONS

This study demonstrates that integrating cutting edge genome wide association (GWA), genome wide epistasis (GWE), and gene expression studies is a powerful strategy to identify novel iron tolerance QTL and candidate loci from diverse germplasm. Crops, unlike model species, have undergone selection for thousands of years, constraining and/or enhancing stress responses. Leveraging genomics-enabled approaches to study these adaptations is essential for future crop improvement.

A Biostimulant Obtained from the Seaweed Ascophyllum nodosum Protects Arabidopsis thaliana from Severe Oxidative Stress

Abiotic stresses cause oxidative damage in plants. Here, we demonstrate that foliar application of an extract from the seaweed Ascophyllum nodosum, SuperFifty (SF), largely prevents paraquat (PQ)-induced oxidative stress in Arabidopsis thaliana. While PQ-stressed plants develop necrotic lesions, plants pre-treated with SF (i.e., primed plants) were unaffected by PQ. Transcriptome analysis revealed induction of reactive oxygen species (ROS) marker genes, genes involved in ROS-induced programmed cell death, and autophagy-related genes after PQ treatment. These changes did not occur in PQ-stressed plants primed with SF. In contrast, upregulation of several carbohydrate metabolism genes, growth, and hormone signaling as well as antioxidant-related genes were specific to SF-primed plants. Metabolomic analyses revealed accumulation of the stress-protective metabolite maltose and the tricarboxylic acid cycle intermediates fumarate and malate in SF-primed plants. Lipidome analysis indicated that those lipids associated with oxidative stress-induced cell death and chloroplast degradation, such as triacylglycerols (TAGs), declined upon SF priming. Our study demonstrated that SF confers tolerance to PQ-induced oxidative stress in A. thaliana, an effect achieved by modulating a range of processes at the transcriptomic, metabolic, and lipid levels.

Plant Unsaturated Fatty Acids: Multiple Roles in Stress Response

Land plants are exposed to not only biotic stresses such as pathogen infection and herbivore wounding, but abiotic stresses such as cold, heat, drought, and salt. Elaborate strategies have been developed to avoid or abide the adverse effects, with unsaturated fatty acids (UFAs) emerging as general defenders. In higher plants, the most common UFAs are three 18-carbon species, namely, oleic (18:1), linoleic (18:2), and α-linolenic (18:3) acids. These simple compounds act as ingredients and modulators of cellular membranes in glycerolipids, reserve of carbon and energy in triacylglycerol, stocks of extracellular barrier constituents (e.g., cutin and suberin), precursors of various bioactive molecules (e.g., jasmonates and nitroalkenes), and regulators of stress signaling. Nevertheless, they are also potential inducers of oxidative stress. In this review, we will present an overview of these roles and then shed light on genetic engineering of FA synthetic genes for improving plant/crop stress tolerance.

Autophagy: An Intracellular Degradation Pathway Regulating Plant Survival and Stress Response

Autophagy is an intracellular process that facilitates the bulk degradation of cytoplasmic materials by the vacuole or lysosome in eukaryotes. This conserved process is achieved through the coordination of different autophagy-related genes (ATGs). Autophagy is essential for recycling cytoplasmic material and eliminating damaged or dysfunctional cell constituents, such as proteins, aggregates or even entire organelles. Plant autophagy is necessary for maintaining cellular homeostasis under normal conditions and is upregulated during abiotic and biotic stress to prolong cell life. In this review, we present recent advances on our understanding of the molecular mechanisms of autophagy in plants and how autophagy contributes to plant development and plants' adaptation to the environment.

Comparing the growth-promoting potential of Paenibacillus lentimorbus and Bacillus amyloliquefaciens in Oryza sativa L. var. Sarju-52 under suboptimal nutrient conditions

An adequate supply of mineral nutrients is crucial to obtain optimum productivity in agriculture. The present investigation was carried to find the inoculation effect of plant growth-promoting rhizobacteria (PGPR), i.e., Paenibacillus lentimorbus B-30488 (B-30488), Bacillus amyloliquefaciens SN13 (SN13) and their consortium for the growth of rice var. Sarju-52, grown under suboptimal nutrient conditions. The study revealed that the individual PGPR treatments showed comparatively better performance than consortia in morphological, physiological, biochemical, and nutrient analysis. Towards understanding the complex mechanism(s), untargeted metabolite profiling was performed using GC-MS, showed alteration of metabolites in rice seedlings facing suboptimal nutrient conditions and inoculated with PGPR. Metabolites such as oleic acid, mannitol, and ethyl iso-allocol were accumulated significantly under starved conditions. Under suboptimal nutrient conditions, sugars such as ribose, glucose, fructose, trehalose, palmitic acid, and myristic acid were accumulated significantly in PGPR inoculated seedlings. The significantly altered pathways due to PGPR inoculation under suboptimal nutrient conditions mainly belongs to carbohydrate and fatty acid metabolism. Interestingly, it was observed that among all the treatments, inoculation with SN13 performed comparatively better than other treatments. Further, in SN13 inoculated samples the qRT-PCR analysis of transcription factors and metabolism-related genes were validated that indicates PGPR deploy metabolic re-programming in rice var. Sarju-52 to enhance its nutrient use efficiency, tolerance, and growth under suboptimum nutrient conditions.

An Advanced Lipid Metabolism System Revealed by Transcriptomic and Lipidomic Analyses Plays a Central Role in Peanut Cold Tolerance

Cold stress restricts peanut (Arachis hypogaea L.) growth, development, and yield. However, the specific mechanism of cold tolerance in peanut remains unknown. Here, the comparative physiological, transcriptomic, and lipidomic analyses of cold tolerant variety NH5 and cold sensitive variety FH18 at different time points of cold stress were conducted to fill this gap. Transcriptomic analysis revealed lipid metabolism including membrane lipid and fatty acid metabolism may be a significant contributor in peanut cold tolerance, and 59 cold-tolerant genes involved in lipid metabolism were identified. Lipidomic data corroborated the importance of membrane lipid remodeling and fatty acid unsaturation. It indicated that photosynthetic damage, resulted from the alteration in fluidity and integrity of photosynthetic membranes under cold stress, were mainly caused by markedly decreased monogalactosyldiacylglycerol (MGDG) levels and could be relieved by increased digalactosyldiacylglycerol (DGDG) and sulfoquinovosyldiacylglycerol (SQDG) levels. The upregulation of phosphatidate phosphatase (PAP1) and phosphatidate cytidylyltransferase (CDS1) inhibited the excessive accumulation of PA, thus may prevent the peroxidation of membrane lipids. In addition, fatty acid elongation and fatty acid β-oxidation were also worth further studied in peanut cold tolerance. Finally, we constructed a metabolic model for the regulatory mechanism of peanut cold tolerance, in which the advanced lipid metabolism system plays a central role. This study lays the foundation for deeply analyzing the molecular mechanism and realizing the genetic improvement of peanut cold tolerance.

Genetic Analyses of the Arabidopsis ATG1 Kinase Complex Reveal Both Kinase-Dependent and Independent Autophagic Routes during Fixed-Carbon Starvation

Under nutrient and energy-limiting conditions, plants up-regulate sophisticated catabolic pathways such as autophagy to remobilize nutrients and restore energy homeostasis. Autophagic flux is tightly regulated under these circumstances through the AuTophaGy-related1 (ATG1) kinase complex, which relays upstream nutrient and energy signals to the downstream components that drive autophagy. Here, we investigated the role(s) of the Arabidopsis (Arabidopsis thaliana) ATG1 kinase during autophagy through an analysis of a quadruple mutant deficient in all four ATG1 isoforms. These isoforms appear to act redundantly, including the plant-specific, truncated ATG1t variant, and like other well-characterized atg mutants, homozygous atg1abct quadruple mutants display early leaf senescence and hypersensitivity to nitrogen and fixed-carbon starvations. Although ATG1 kinase is essential for up-regulating autophagy under nitrogen deprivation and short-term carbon starvation, it did not stimulate autophagy under prolonged carbon starvation. Instead, an ATG1-independent response arose requiring phosphatidylinositol-3-phosphate kinase (PI3K) and SUCROSE NONFERMENTING1-RELATED PROTEIN KINASE1 (SnRK1), possibly through phosphorylation of the ATG6 subunit within the PI3K complex by the catalytic KIN10 subunit of SnRK1. Together, our data connect ATG1 kinase to autophagy and reveal that plants engage multiple pathways to activate autophagy during nutrient stress, which include the ATG1 route as well as an alternative route requiring SnRK1 and ATG6 signaling.plantcell;31/12/2973/FX1F1fx1.

How Does the Seed Pre-Germinative Metabolism Fight Against Imbibition Damage? Emerging Roles of Fatty Acid Cohort and Antioxidant Defence

During seed imbibition, lipids are engaged in membrane reorganization while facing free radical-mediated oxidative injury. In the present work, we explored changes in lipid components at different timepoints of imbibition (0.5, 2, 4, 6, and 8 h) in the legume Medicago truncatula, by combining biochemical approaches with targeted lipidomics and untargeted metabolomics. ROS and RNS (reactive oxygen and nitrogen species) accumulation was observed throughout the tested timepoints whereas lipid peroxidation increased at 4 h of imbibition. The seed response to oxidative damage was evidenced by a significant increase in tocopherols starting from 0.5 h of imbibition as well as by the reduction in total thiol content occurring at 2 h of imbibition. Since under physiological conditions, the proper functions of the cell membranes are strongly dependent on the qualitative and quantitative balance of fatty acid residues in phospholipids, the investigation was expanded to the fatty acid cohort of M. truncatula seeds. Total saturated fatty acids (SFAs), monounsaturated fatty acids (MUFAs), polyunsaturated fatty acids (PUFAs), omega(ω)-3 and omega(ω)-6 fatty acids showed fluctuations during seed imbibition. The most remarkable finding was the profile of the ω-3 PUFA docosopentaenoic acid (DPA, 7 cis, 10 cis, 13 cis, 16 cis, and 19 cis-22:5) that showed a peak (up to 1.0% of the total fatty acid content) at 0.5 and 8 h of imbibition, concomitant with the peaks observed in tocopherol levels. It is possible that the observed changes in DPA alter the physical properties of membranes, as reported in animal cells, triggering signaling pathways relevant for the cell defense against oxidative injury. Furthermore, the content and balance between tocopherols and PUFAs is regarded as a determinant of storage stability. No enhancement in trans-fatty acids occurred throughout imbibition, suggesting for a proper antioxidant response carried by the seed. Fatty acids profiles were integrated with data from untargeted metabolomics showing changes in lipid sub-pathways, among which fatty acid amide, lyso-phospholipids, and phospholipid metabolism. The emerging lipid profiles and dynamics are discussed in view of the overall imbibition damage generated during M. truncatula seed imbibition.

Aspergillus niger upregulated glycerolipid metabolism and ethanol utilization pathway under ethanol stress

The knowledge of how Aspergillus niger responds to ethanol can lead to the design of strains with enhanced ethanol tolerance to be utilized in numerous industrial bioprocesses. However, the current understanding about the response mechanisms of A. niger toward ethanol stress remains quite limited. Here, we first applied a cell growth assay to test the ethanol tolerance of A. niger strain ES4, which was isolated from the wall near a chimney of an ethanol tank of a petroleum company, and found that it was capable of growing in 5% (v/v) ethanol to 30% of the ethanol‐free control level. Subsequently, the metabolic responses of this strain toward ethanol were investigated using untargeted metabolomics, which revealed the elevated levels of triacylglycerol (TAG) in the extracellular components, and of diacylglycerol, TAG, and hydroxy‐TAG in the intracellular components. Lastly, stable isotope labeling mass spectrometry with ethanol‐d₆ showed altered isotopic patterns of molecular ions of lipids in the ethanol‐d₆ samples, compared with the nonlabeled ethanol controls, suggesting the ability of A. niger ES4 to utilize ethanol as a carbon source. Together, the studies revealed the upregulation of glycerolipid metabolism and ethanol utilization pathway as novel response mechanisms of A. niger ES4 toward ethanol stress, thereby underlining the utility of untargeted metabolomics and the overall approaches as tools for elucidating new biological insights.

Biomass and lipid induction strategies in microalgae for biofuel production and other applications

The use of fossil fuels has been strongly related to critical problems currently affecting society, such as: global warming, global greenhouse effects and pollution. These problems have affected the homeostasis of living organisms worldwide at an alarming rate. Due to this, it is imperative to look for alternatives to the use of fossil fuels and one of the relevant substitutes are biofuels. There are different types of biofuels (categories and generations) that have been previously explored, but recently, the use of microalgae has been strongly considered for the production of biofuels since they present a series of advantages over other biofuel production sources: (a) they don’t need arable land to grow and therefore do not compete with food crops (like biofuels produced from corn, sugar cane and other plants) and; (b) they exhibit rapid biomass production containing high oil contents, at least 15 to 20 times higher than land based oleaginous crops. Hence, these unicellular photosynthetic microorganisms have received great attention from researches to use them in the large-scale production of biofuels. However, one disadvantage of using microalgae is the high economic cost due to the low-yields of lipid content in the microalgae biomass. Thus, development of different methods to enhance microalgae biomass, as well as lipid content in the microalgae cells, would lead to the development of a sustainable low-cost process to produce biofuels. Within the last 10 years, many studies have reported different methods and strategies to induce lipid production to obtain higher lipid accumulation in the biomass of microalgae cells; however, there is not a comprehensive review in the literature that highlights, compares and discusses these strategies. Here, we review these strategies which include modulating light intensity in cultures, controlling and varying CO₂ levels and temperature, inducing nutrient starvation in the culture, the implementation of stress by incorporating heavy metal or inducing a high salinity condition, and the use of metabolic and genetic engineering techniques coupled with nanotechnology.

Dual Role for Autophagy in Lipid Metabolism in Arabidopsis

Autophagy is a major catabolic pathway whereby cytoplasmic constituents including lipid droplets (LDs), storage compartments for neutral lipids, are delivered to the lysosome or vacuole for degradation. The autophagic degradation of cytosolic LDs, a process termed lipophagy, has been extensively studied in yeast and mammals, but little is known about the role for autophagy in lipid metabolism in plants. Organisms maintain a basal level of autophagy under favorable conditions and upregulate the autophagic activity under stress including starvation. Here, we demonstrate that Arabidopsis (Arabidopsis thaliana) basal autophagy contributes to triacylglycerol (TAG) synthesis, whereas inducible autophagy contributes to LD degradation. We found that disruption of basal autophagy impedes organellar membrane lipid turnover and hence fatty acid mobilization from membrane lipids to TAG. We show that lipophagy is induced under starvation as indicated by colocalization of LDs with the autophagic marker and the presence of LDs in vacuoles. We additionally show that lipophagy occurs in a process morphologically resembling microlipophagy and requires the core components of the macroautophagic machinery. Together, this study provides mechanistic insight into lipophagy and reveals a dual role for autophagy in regulating lipid synthesis and turnover in plants.

Functional diversity of glycerolipid acylhydrolases in plant metabolism and physiology

Most current knowledge about plant lipid metabolism has focused on the biosynthesis of lipids and their transport between different organelles. However, lipid composition changes during development and in response to environmental cues often go beyond adjustments of lipid biosynthesis. When lipids have to be removed to adjust the extent of membranes during down regulation of photosynthesis, or lipid composition has to be adjusted to alter the biophysical properties of membranes, or lipid derived chemical signals have to be produced, lipid-degrading enzymes come into play. This review focuses on glycerolipid acylhydrolases that remove acyl groups from glycerolipids and will highlight their roles in lipid remodeling and lipid-derived signal generation. One emerging theme is that these enzymes are involved in the dynamic movement of acyl groups through different lipid pools, for example from polar membrane lipids to neutral lipids sequestered in lipid droplets during de novo triacylglycerol synthesis. Another example of acyl group sequestration in the form of triacylglycerols in lipid droplets is membrane lipid remodeling in response to abiotic stresses. Fatty acids released for membrane lipids can also give rise to potent signaling molecules and acylhydrolases are therefore often the first step in initiating the formation of these lipid signals.

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.

Peroxisomal fatty acid β-oxidation negatively impacts plant survival under salt stress

Peroxisomal β-oxidation is the sole pathway for metabolic breakdown of fatty acids to generate energy and carbon skeletons in plants, is essential for oilseed germination and plays an important role in growth, development and cellular homeostasis. Yet, this process also produces cytotoxic reactive oxygen species (ROS) as byproducts. We recently showed that disruption of fatty acid β-oxidation enhance plant survival under carbon starvation conditions. Here, we extend these findings by demonstrating that blocking fatty acid import into peroxisomes reduces ROS accumulation and increases plant tolerance to salt stress, whereas increasing fatty acid flux into the β-oxidation pathway has opposite effects. Together, these results support the view that peroxisomal β-oxidation of fatty acids enhances stress-induced ROS production, thereby negatively impacting plant survival under adverse environmental conditions.

Up-regulation of lipid biosynthesis increases the oil content in leaves of Sorghum bicolor

Synthesis and accumulation of the storage lipid triacylglycerol in vegetative plant tissues has emerged as a promising strategy to meet the world's future need for vegetable oil. Sorghum (Sorghum bicolor) is a particularly attractive target crop given its high biomass, drought resistance and C₄ photosynthesis. While oilseed-like triacylglycerol levels have been engineered in the C₃ model plant tobacco, progress in C₄ monocot crops has been lagging behind. In this study, we report the accumulation of triacylglycerol in sorghum leaf tissues to levels between 3 and 8.4% on a dry weight basis depending on leaf and plant developmental stage. This was achieved by the combined overexpression of genes encoding the Zea mays WRI1 transcription factor, Umbelopsis ramanniana UrDGAT2a acyltransferase and Sesamum indicum Oleosin-L oil body protein. Increased oil content was visible as lipid droplets, primarily in the leaf mesophyll cells. A comparison between a constitutive and mesophyll-specific promoter driving WRI1 expression revealed distinct changes in the overall leaf lipidome as well as transitory starch and soluble sugar levels. Metabolome profiling uncovered changes in the abundance of various amino acids and dicarboxylic acids. The results presented here are a first step forward towards the development of sorghum as a dedicated biomass oil crop and provide a basis for further combinatorial metabolic engineering.

Transcriptome reprogramming during severe dehydration contributes to physiological and metabolic changes in the resurrection plant Haberlea rhodopensis

BACKGROUND

Water shortage is a major factor that harms agriculture and ecosystems worldwide. Plants display various levels of tolerance to water deficit, but only resurrection plants can survive full desiccation of their vegetative tissues. Haberlea rhodopensis, an endemic plant of the Balkans, is one of the few resurrection plants found in Europe. We performed transcriptomic analyses of this species under slight, severe and full dehydration and recovery to investigate the dynamics of gene expression and associate them with existing physiological and metabolomics data.

RESULTS

De novo assembly yielded a total of 142,479 unigenes with an average sequence length of 1034 nt. Among them, 18,110 unigenes were differentially expressed. Hierarchical clustering of all differentially expressed genes resulted in seven clusters of dynamic expression patterns. The most significant expression changes, involving more than 15,000 genes, started at severe dehydration (~ 20% relative water content) and were partially maintained at full desiccation (< 10% relative water content). More than a hundred pathways were enriched and functionally organized in a GO/pathway network at the severe dehydration stage. Transcriptomic changes in key pathways were analyzed and discussed in relation to metabolic processes, signal transduction, quality control of protein and DNA repair in this plant during dehydration and rehydration.

CONCLUSION

Reprograming of the transcriptome occurs during severe dehydration, resulting in a profound alteration of metabolism toward alternative energy supply, hormone signal transduction, and prevention of DNA/protein damage under very low cellular water content, underlying the observed physiological and metabolic responses and the resurrection behavior of H. rhodopensis.

Single cell-type analysis of cellular lipid remodelling in response to salinity in the epidermal bladder cells of the model halophyte Mesembryanthemum crystallinum

Salt stress causes dramatic changes in the organization and dynamic properties of membranes, however, little is known about the underlying mechanisms involved. Modified trichomes, known as epidermal bladder cells (EBC), on the leaves and stems of the halophyte Mesembryanthemum crystallinum can be successfully exploited as a single-cell-type system to investigate salt-induced changes to cellular lipid composition. In this study, alterations in key molecular species from different lipid classes highlighted an increase in phospholipid species, particularly those from phosphatidylcholine and phosphatidic acid (PA), where the latter is central to the synthesis of membrane lipids. Triacylglycerol (TG) species decreased during salinity, while there was little change in plastidic galactolipids. EBC transcriptomic and proteomic data mining revealed changes in genes and proteins involved in lipid metabolism and the upregulation of transcripts for PIPKIB, PI5PII, PIPKIII, and phospholipase D delta suggested the induction of signalling processes mediated by phosphoinositides and PA. TEM and flow cytometry showed the dynamic nature of lipid droplets in these cells under salt stress. Altogether, this work indicates that the metabolism of TG might play an important role in EBC response to salinity as either an energy reserve for sodium accumulation and/or driving membrane biosynthesis for EBC expansion.

Triacylglycerol accumulates exclusively outside the chloroplast in short-term nitrogen-deprived Chlamydomonas reinhardtii

In microalgae, triacylglycerol (TAG) biosynthesis occurs by parallel pathways involving both the chloroplast and endoplasmic reticulum. A better understanding of contribution of each pathway to TAG assembly facilitates enhanced TAG production via rational genetic engineering of microalgae. Here, using a UPLC-MS(/MS) coupled with TLC-GC-based lipidomic platform, the early response of the major glycerolipids to nitrogen stress was analyzed at both the cellular and chloroplastidic levels in the model green alga Chlamydomonas reinhardtii. Subcellular lipidomic analysis demonstrated that TAG was accumulated exclusively outside the chloroplast, and remained unaltered inside the chloroplast after 4 h of nitrogen starvation. This study ascertained the existence of the glycolipid, digalactosyldiacylglycerol (DGDG), outside the chloroplast and the betaine lipid, diacylglycerol-N,N,N-trimethylhomoserine (DGTS), inside the chloroplast. The newly synthesized DGDG and DGTS prominently increased at the extra-chloroplastidic compartments and served as the major precursors for TAG biosynthesis. In particular, DGDG contributed to the extra-chloroplastidic TAG assembly in form of diacylglycerol (DAG) and DGTS in form of acyl groups. The chloroplastidic membrane lipid, monogalactosyldiacylglycerol (MGDG), was proposed to primarily offer DAG for TAG formation outside the chloroplast. This study provides valuable insights into the subcellular glycerolipidomics and unveils the acyl flux into the extra-chloroplastidic TAG in microalgae.

Starch Deficiency Enhances Lipid Biosynthesis and Turnover in Leaves

Starch and lipids represent two major forms of carbon and energy storage in plants and play central roles in diverse cellular processes. However, whether and how starch and lipid metabolic pathways interact to regulate metabolism and growth are poorly understood. Here, we show that lipids can partially compensate for the lack of function of transient starch during normal growth and development in Arabidopsis (Arabidopsis thaliana). Disruption of starch synthesis resulted in a significant increase in fatty acid synthesis via posttranslational regulation of the plastidic acetyl-coenzyme A carboxylase and a concurrent increase in the synthesis and turnover of membrane lipids and triacylglycerol. Genetic analysis showed that blocking fatty acid peroxisomal β-oxidation, the sole pathway for metabolic breakdown of fatty acids in plants, significantly compromised or stunted the growth and development of mutants defective in starch synthesis under long days or short days, respectively. We also found that the combined disruption of starch synthesis and fatty acid turnover resulted in increased accumulation of membrane lipids, triacylglycerol, and soluble sugars and altered fatty acid flux between the two lipid biosynthetic pathways compartmentalized in either the chloroplast or the endoplasmic reticulum. Collectively, our findings provide insight into the role of fatty acid β-oxidation and the regulatory network controlling fatty acid synthesis, and they reveal the mechanistic basis by which starch and lipid metabolic pathways interact and undergo cross talk to modulate carbon allocation, energy homeostasis, and plant growth.

DIACYLGLYCEROL ACYLTRANSFERASE1 Contributes to Freezing Tolerance

Freezing limits plant growth and crop productivity, and plant species in temperate zones have the capacity to develop freezing tolerance through complex modulation of gene expression affecting various aspects of metabolism and physiology. While many components of freezing tolerance have been identified in model species under controlled laboratory conditions, little is known about the mechanisms that impart freezing tolerance in natural populations of wild species. Here, we performed a quantitative trait locus (QTL) study of acclimated freezing tolerance in seedlings of Boechera stricta, a highly adapted relative of Arabidopsis (Arabidopsis thaliana) native to the Rocky Mountains. A single QTL was identified that contained the gene encoding ACYL-COENZYME A:DIACYLGLYCEROL ACYLTRANSFERASE1 (BstDGAT1), whose expression is highly cold responsive. The primary metabolic enzyme DGAT1 catalyzes the final step in assembly of triacylglycerol (TAG) by acyl transfer from acyl-CoA to diacylglycerol. Freezing tolerant plants showed higher DGAT1 expression during cold acclimation than more sensitive plants, and this resulted in increased accumulation of TAG in response to subsequent freezing. Levels of oligogalactolipids that are produced by SFR2 (SENSITIVE TO FREEZING2), an indispensable element of freezing tolerance in Arabidopsis, were also higher in freezing-tolerant plants. Furthermore, overexpression of AtDGAT1 led to increased freezing tolerance. We propose that DGAT1 confers freezing tolerance in plants by supporting SFR2-mediated remodeling of chloroplast membranes.

Molecular Mechanisms Preventing Senescence in Response to Prolonged Darkness in a Desiccation-Tolerant Plant

The desiccation-tolerant plant Haberlea rhodopensis can withstand months of darkness without any visible senescence. Here, we investigated the molecular mechanisms of this adaptation to prolonged (30 d) darkness and subsequent return to light. H. rhodopensis plants remained green and viable throughout the dark treatment. Transcriptomic analysis revealed that darkness regulated several transcription factor (TF) genes. Stress- and autophagy-related TFs such as ERF8, HSFA2b, RD26, TGA1, and WRKY33 were up-regulated, while chloroplast- and flowering-related TFs such as ATH1, COL2, COL4, RL1, and PTAC7 were repressed. PHYTOCHROME INTERACTING FACTOR4, a negative regulator of photomorphogenesis and promoter of senescence, also was down-regulated. In response to darkness, most of the photosynthesis- and photorespiratory-related genes were strongly down-regulated, while genes related to autophagy were up-regulated. This occurred concomitant with the induction of SUCROSE NON-FERMENTING1-RELATED PROTEIN KINASES (SnRK1) signaling pathway genes, which regulate responses to stress-induced starvation and autophagy. Most of the genes associated with chlorophyll catabolism, which are induced by darkness in dark-senescing species, were either unregulated (PHEOPHORBIDE A OXYGENASE, PAO; RED CHLOROPHYLL CATABOLITE REDUCTASE, RCCR) or repressed (STAY GREEN-LIKE, PHEOPHYTINASE, and NON-YELLOW COLORING1). Metabolite profiling revealed increases in the levels of many amino acids in darkness, suggesting increased protein degradation. In darkness, levels of the chloroplastic lipids digalactosyldiacylglycerol, monogalactosyldiacylglycerol, phosphatidylglycerol, and sulfoquinovosyldiacylglycerol decreased, while those of storage triacylglycerols increased, suggesting degradation of chloroplast membrane lipids and their conversion to triacylglycerols for use as energy and carbon sources. Collectively, these data show a coordinated response to darkness, including repression of photosynthetic, photorespiratory, flowering, and chlorophyll catabolic genes, induction of autophagy and SnRK1 pathways, and metabolic reconfigurations that enable survival under prolonged darkness.

Darkened Leaves Use Different Metabolic Strategies for Senescence and Survival

In plants, an individually darkened leaf initiates senescence much more rapidly than a leaf from a whole darkened plant. Combining transcriptomic and metabolomic approaches in Arabidopsis (Arabidopsis thaliana), we present an overview of the metabolic strategies that are employed in response to different darkening treatments. Under darkened plant conditions, the perception of carbon starvation drove a profound metabolic readjustment in which branched-chain amino acids and potentially monosaccharides released from cell wall loosening became important substrates for maintaining minimal ATP production. Concomitantly, the increased accumulation of amino acids with a high nitrogen-carbon ratio may provide a safety mechanism for the storage of metabolically derived cytotoxic ammonium and a pool of nitrogen for use upon returning to typical growth conditions. Conversely, in individually darkened leaf, the metabolic profiling that followed our 13C-enrichment assays revealed a temporal and differential exchange of metabolites, including sugars and amino acids, between the darkened leaf and the rest of the plant. This active transport could be the basis for a progressive metabolic shift in the substrates fueling mitochondrial activities, which are central to the catabolic reactions facilitating the retrieval of nutrients from the senescing leaf. We propose a model illustrating the specific metabolic strategies employed by leaves in response to these two darkening treatments, which support either rapid senescence or a strong capacity for survival.

Genome-wide (ChIP-seq) identification of target genes regulated by BdbZIP10 during paraquat-induced oxidative stress

BACKGROUND

bZIP transcription factors play a significant role in many aspects of plant growth and development and also play critical regulatory roles during plant responses to various stresses. Overexpression of the Brachypodium bZIP10 (Bradi1g30140) transcription factor conferred enhanced oxidative stress tolerance and increased viability when plants or cells were exposed to the herbicide paraquat. To gain a better understanding of genes involved in bZIP10 conferred oxidative stress tolerance, chromatin immunoprecipitation followed by high throughput sequencing (ChIP-Seq) was performed on BdbZIP10 overexpressing plants in the presence of oxidative stress.

RESULTS

We identified a transcription factor binding motif, TGDCGACA, different from most known bZIP TF motifs but with strong homology to the Arabidopsis zinc deficiency response element. Analysis of the immunoprecipitated sequences revealed an enrichment of gene ontology groups with metal ion transmembrane transporter, transferase, catalytic and binding activities. Functional categories including kinases and phosphotransferases, cation/ion transmembrane transporters, transferases (phosphorus-containing and glycosyl groups), and some nucleoside/nucleotide binding activities were also enriched.

CONCLUSIONS

Brachypodium bZIP10 is involved in zinc homeostasis, as it relates to oxidative stress.

Characterization of the enzymatic activity and physiological function of the lipid droplet-associated triacylglycerol lipase AtOBL1

Similar to seeds, pollen tubes contain lipid droplets that store triacylglycerol (TAG), but the fate of this TAG as well as the enzymes involved in its breakdown are unknown. Therefore, two potential TAG lipases from tobacco and Arabidopsis, NtOBL1 (Oil body lipase 1) and AtOBL1, were investigated, especially with respect to their importance for pollen tube growth. We expressed NtOBL1 and AtOBL1 as fluorescent fusion proteins to study their localization by confocal microscopy. Furthermore, we overexpressed AtOBL1 in Nicotiana benthamiana leaves to characterize it enzymatically. The obl1 mutant was studied in respect to its pollen tube growth in vivo and its seed germination. Both NtOBL1 and AtOBL1 localized to lipid droplets. AtOBL1 was abundant in pollen tubes and seedlings, and acted as a lipase on TAG, diacylglycerol and 1-monoacylglycerol at a pH optimum of 5.5. The obl1 mutant was hampered in pollen tube growth, whereas seedling establishment was not affected under optimal conditions, even though AtOBL1 accounted for a major lipase activity in seeds. TAG could be a direct precursor for the synthesis of membrane lipids in pollen tubes and proteins of the OBL family involved in the flux of acyl groups.

Peroxisome Function, Biogenesis, and Dynamics in Plants

Recent advances highlight understanding of the diversity of peroxisome contributions to plant biology and the mechanisms through which these essential organelles are generated.