Functional Characterization of Four Olive Squalene Synthases with Respect to the Squalene Content of the Virgin Olive Oil

The release of new olive cultivars with an increased squalene content in their virgin olive oil is considered an important target in olive breeding programs. In this work, the variability of the squalene content in a core collection of 36 olive cultivars was first studied, revealing two olive cultivars, 'Dokkar' and 'Klon-14', with extremely low and high squalene contents in their oils, respectively. Next, four cDNA sequences encoding squalene synthases (SQS) were cloned from olive. Sequence analysis and functional expression in bacteria confirmed that they encode squalene synthases. Transcriptional analysis in distinct olive tissues and cultivars indicated that expression levels of these four SQS genes are spatially and temporally regulated in a cultivar-dependent manner and pointed to OeSQS2 as the gene mainly involved in squalene biosynthesis in olive mesocarp and, therefore, in the olive oil. In addition, the biosynthesis of squalene appears to be transcriptionally regulated in water-stressed olive mesocarp.

Modification of 13-hydroperoxide lyase expression in olive affects plant growth and results in altered volatile profile

The C6 aldehydes, alcohols, and the corresponding esters are the most important compounds of virgin olive oil aroma. These C6 volatile compounds are synthesized via the 13-hydroperoxide lyase (13-HPL) branch of the lipoxygenase pathway. In this investigation, a functional analysis of the olive (Olea europaea L.) 13-HPL gene by its overexpression and silencing in olive transgenic lines was carried out. With this aim, sense and RNAi constructs of the olive 13-HPL gene were generated and used for the transformation of embryogenic olive cultures. Leaves from overexpressing lines showed a slight increase in 13-HPL gene expression, whereas RNAi lines exhibited a strong decrease in their transcript levels. Quantification of 13-HPL activity in two overexpressing and two RNAi lines showed a positive correlation with levels of transcripts. Interestingly, RNAi lines showed a high decrease in the content of C6 volatiles linked to a strong increase of C5 volatile compounds, altering the volatile profile in the leaves. In addition, the silencing of the 13-HPL gene severely affected plant growth and development. This investigation demonstrates the role of the 13-HPL gene in the biogenesis of olive volatile compounds and constitutes a functional genomics study in olive related to virgin olive oil quality.

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.

The Oleic/Linoleic Acid Ratio in Olive (Olea europaea L.) Fruit Mesocarp Is Mainly Controlled by OeFAD2-2 and OeFAD2-5 Genes Together With the Different Specificity of Extraplastidial Acyltransferase Enzymes

Fatty acid composition of olive oil has an important effect on the oil quality to such an extent that oils with a high oleic and low linoleic acid contents are preferable from a nutritional and technological point of view. In the present work, we have first studied the diversity of the fatty acid composition in a set of eighty-nine olive cultivars from the Worldwide Olive Germplasm Bank of IFAPA Cordoba (WOGBC-IFAPA), and in a core collection (Core-36), which includes 28 olive cultivars from the previously mentioned set. Our results indicate that oleic and linoleic acid contents displayed the highest degree of variability of the different fatty acids present in the olive oil of the 89 cultivars under study. In addition, the independent study of the Core-36 revealed two olive cultivars, Klon-14 and Abou Kanani, with extremely low and high linoleic acid contents, respectively. Subsequently, these two cultivars were used to investigate the specific contribution of different fatty acid desaturases to the linoleic acid content of mesocarp tissue during olive fruit development and ripening. Fatty acid desaturase gene expression levels, together with lipid analysis, suggest that not only OeFAD2-2 and OeFAD2-5 but also the different specificities of extraplastidial acyltransferase enzymes are responsible for the variability of the oleic/linoleic acid ratio in olive cultivars. All this information allows for an advancement in the knowledge of the linoleic acid biosynthesis in different olive cultivars, which can impact olive breeding programs to improve olive oil quality.

Lipid Composition and Associated Gene Expression Patterns during Pollen Germination and Pollen Tube Growth in Olive (Olea europaea L.)

Pollen lipids are essential for sexual reproduction, but our current knowledge regarding lipid dynamics in growing pollen tubes is still very scarce. Here, we report unique lipid composition and associated gene expression patterns during olive pollen germination. Up to 376 genes involved in the biosynthesis of all lipid classes, except suberin, cutin and lipopolysaccharides, are expressed in olive pollen. The fatty acid profile of olive pollen is markedly different compared with other plant organs. Triacylglycerol (TAG), containing mostly C12-C16 saturated fatty acids, constitutes the bulk of olive pollen lipids. These compounds are partially mobilized, and the released fatty acids enter the β-oxidation pathway to yield acetyl-CoA, which is converted into sugars through the glyoxylate cycle during the course of pollen germination. Our data suggest that fatty acids are synthesized de novo and incorporated into glycerolipids by the 'eukaryotic pathway' in elongating pollen tubes. Phosphatidic acid is synthesized de novo in the endomembrane system during pollen germination and seems to have a central role in pollen tube lipid metabolism. The coordinated action of fatty acid desaturases FAD2-3 and FAD3B might explain the increase in linoleic and alpha-linolenic acids observed in germinating pollen. Continuous synthesis of TAG by the action of diacylglycerol acyltransferase 1 (DGAT1) enzyme, but not phosphoplipid:diacylglycerol acyltransferase (PDAT), also seems plausible. All these data allow for a better understanding of lipid metabolism during the olive reproductive process, which can impact, in the future, on the increase in olive fruit yield and, therefore, olive oil production.

Specialized Functions of Olive FAD2 Gene Family Members Related to Fruit Development and the Abiotic Stress Response

Three different cDNA sequences, designated OepFAD2-3, OepFAD2-4 and OepFAD2-5, encoding three microsomal oleate desaturases (FAD2) have been isolated from olive (Olea europaea cv. Picual). Sequence analysis and functional expression in yeast of the corresponding cDNAs confirm that they encode microsomal oleate desaturases. Gene expression and lipid analysis indicate that these three genes are not involved in the linoleic acid present in seed lipids, while OeFAD2-5, together with OeFAD2-2, contributes mostly to the linoleic acid present in the mesocarp and, therefore, in the olive oil. Our results have also shown that olive FAD2-3, FAD2-4 and FAD2-5 gene expression is not only spatially and temporally regulated in olive fruit, but also is cultivar-dependent, as well as regulated by water regime, temperature, light and wounding. All these data suggest specialized physiological roles for the olive FAD2 gene family members with respect to both aspects of the biosynthesis of the linoleic acid, either present in storage lipids that constitute the olive oil or being part of membrane lipids, which are involved in the response to abiotic stresses, and highlight the differences on FAD2 gene regulation between oilseeds and oil fruits.

Effect of saline irrigation on physiological traits, fatty acid composition and desaturase genes expression in olive fruit mesocarp

The effect of salinity on physiological traits, fatty acid composition and desaturase genes expression in fruit mesocarp of olive cultivar Leccino was investigated. Significant reduction of shoot elongation (-12%) during salt treatments (80 mM NaCl) was associated with the translocation of Na in the aerial part. After 75 days of treatment, fruits from each plant were subdivided into four maturation groups (MG₀, MG₁, MG₂, MG₃) according to ripening degrees. Na accumulation increased in each MG under salinity, reaching the highest values in MG₁ fruits (2654 mg kg^-1 DW). Salinity caused an acceleration of the ripening process, increased fruit number and decreased total fatty acids content in MG₃. An increase in oleic acid at MG₁ (53%) was detected, with consequent increase in the oleic/linoleic (41%) and decrease in the polyunsaturated/monounsaturated ratios (30%). Those variations could be explained by the synergic up-regulation of OeSAD1, together with the down-regulation of OeFAD6 transcript levels.

Transcriptional Regulation of Stearoyl-Acyl Carrier Protein Desaturase Genes in Response to Abiotic Stresses Leads to Changes in the Unsaturated Fatty Acids Composition of Olive Mesocarp

In higher plants, the stearoyl-acyl carrier protein desaturase (SAD) catalyzes the first desaturation step leading to oleic acid, which can be further desaturated to linoleic and α-linolenic acids. Therefore, SAD plays an essential role in determining the overall content of unsaturated fatty acids (UFA). We have investigated how SAD genes expression and UFA composition are regulated in olive (Olea europaea) mesocarp tissue from Picual and Arbequina cultivars in response to different abiotic stresses. The results showed that olive SAD genes are transcriptionally regulated by temperature, darkness and wounding. The increase in SAD genes expression levels observed in Picual mesocarp exposed to low temperature brought about a modification in the UFA content of microsomal membrane lipids. In addition, darkness caused the down-regulation of SAD genes transcripts, together with a decrease in the UFA content of chloroplast lipids. The differential role of olive SAD genes in the wounding response was also demonstrated. These data point out that different environmental stresses can modify the UFA composition of olive mesocarp through the transcriptional regulation of SAD genes, affecting olive oil quality.

An Oleuropein β-Glucosidase from Olive Fruit Is Involved in Determining the Phenolic Composition of Virgin Olive Oil

Phenolic composition of virgin olive oil is determined by the enzymatic and/or chemical reactions that take place during olive fruit processing. Of these enzymes, β-glucosidase activity plays a relevant role in the transformation of the phenolic glycosides present in the olive fruit, generating different secoiridoid derivatives. The main goal of the present study was to characterize olive fruit β-glucosidase genes and enzymes responsible for the phenolic composition of virgin olive oil. To achieve that, we have isolated an olive β-glucosidase gene from cultivar Picual (OepGLU), expressed in Nicotiana benthamiana leaves and purified its corresponding recombinant enzyme. Western blot analysis showed that recombinant OepGLU protein is detected by an antibody raised against the purified native olive mesocarp β-glucosidase enzyme, and exhibits a deduced molecular mass of 65.0 kDa. The recombinant OepGLU enzyme showed activity on the major olive phenolic glycosides, with the highest levels with respect to oleuropein, followed by ligstroside and demethyloleuropein. In addition, expression analysis showed that olive GLU transcript level in olive fruit is spatially and temporally regulated in a cultivar-dependent manner. Furthermore, temperature, light and water regime regulate olive GLU gene expression in olive fruit mesocarp. All these data are consistent with the involvement of OepGLU enzyme in the formation of the major phenolic compounds present in virgin olive oil.

Volatile Compound Profiling by HS-SPME/GC-MS-FID of a Core Olive Cultivar Collection as a Tool for Aroma Improvement of Virgin Olive Oil

Virgin olive oil (VOO) is the only food product requiring official sensory analysis to be classified in commercial categories, in which the evaluation of the aroma plays a very important role. The selection of parents, with the aim of obtaining new cultivars with improved oil aroma, is of paramount importance in olive breeding programs. We have assessed the volatile fraction by headspace-solid-phase microextraction/gas chromatography-mass spectrometry-flame ionization detection (HS-SPME/GC-MS-FID) and the deduced aroma properties of VOO from a core set of olive cultivars (Core-36) which possesses most of the genetic diversity found in the World Olive Germplasm Collection (IFAPA Alameda del Obispo) located in Cordoba, Spain. The VOO volatile fractions of Core-36 cultivars display a high level of variability. It is mostly made of compounds produced from polyunsaturated fatty acids through the lipoxygenase pathway, which confirms to be a general characteristic of the olive species (Olea europaea L.). The main group of volatile compounds in the oils was six straight-chain carbon compounds derived from linolenic acid, some of them being the main contributors to the aroma of the olive oils according to their odor activity values (OAV). The high level of variability found for the volatile fraction of the oils from Core-36 and, therefore, for the aroma odor notes, suggest that this core set may be a very useful tool for the choice of optimal parents in olive breeding programs in order to raise new cultivars with improved VOO aroma.

Transcriptional Analysis of Stearoyl-Acyl Carrier Protein Desaturase Genes from Olive (Olea europaea) in Relation to the Oleic Acid Content of the Virgin Olive Oil

The specific contribution of different stearoyl-ACP desaturase (SAD) genes to the oleic acid content in olive (Olea europaea) fruit has been studied. Toward that end, we isolated three distinct cDNA clones encoding three SAD isoforms from olive (cv. Picual), as revealed by sequence analysis. The expression levels of olive SAD genes were determined in different tissues from Picual and Arbequina cultivars, including developing mesocarp and seed, together with the unsaturated fatty acid content. Lipid and gene expression analyses indicate that OeSAD2 seems to be the main gene contributing to the oleic acid content of the olive fruit and, therefore, of the virgin olive oil. This conclusion was confirmed when the study was extended to Hojiblanca, Picudo, and Manzanilla cultivars. Furthermore, our data indicate that the olive microsomal oleate desaturase gene OeFAD2-2, but not OeSAD2, is responsible for the linoleic acid content in the virgin olive oil.

Differential Contribution of Endoplasmic Reticulum and Chloroplast ω-3 Fatty Acid Desaturase Genes to the Linolenic Acid Content of Olive (Olea europaea) Fruit

Linolenic acid is a polyunsaturated fatty acid present in plant lipids, which plays key roles in plant metabolism as a structural component of storage and membrane lipids, and as a precursor of signaling molecules. The synthesis of linolenic acid is catalyzed by two different ω-3 fatty acid desaturases, which correspond to microsomal- (FAD3) and chloroplast- (FAD7 and FAD8) localized enzymes. We have investigated the specific contribution of each enzyme to the linolenic acid content in olive fruit. With that aim, we isolated two different cDNA clones encoding two ω-3 fatty acid desaturases from olive (Olea europaea cv. Picual). Sequence analysis indicates that they code for microsomal (OepFAD3B) and chloroplast (OepFAD7-2) ω-3 fatty acid desaturase enzymes, different from the previously characterized OekFAD3A and OekFAD7-1 genes. Functional expression in yeast of the corresponding OepFAD3A and OepFAD3B cDNAs confirmed that they encode microsomal ω-3 fatty acid desaturases. The linolenic acid content and transcript levels of olive FAD3 and FAD7 genes were measured in different tissues of Picual and Arbequina cultivars, including mesocarp and seed during development and ripening of olive fruit. Gene expression and lipid analysis indicate that FAD3A is the gene mainly responsible for the linolenic acid present in the seed, while FAD7-1 and FAD7-2 contribute mostly to the linolenic acid present in the mesocarp and, therefore, in the olive oil. These results also indicate the relevance of lipid trafficking between the endoplasmic reticulum and chloroplast in determining the linolenic acid content of membrane and storage lipids in oil-accumulating photosynthetic tissues.

Virus-induced alterations in primary metabolism modulate susceptibility to Tobacco rattle virus in Arabidopsis

During compatible virus infections, plants respond by reprogramming gene expression and metabolite content. While gene expression studies are profuse, our knowledge of the metabolic changes that occur in the presence of the virus is limited. Here, we combine gene expression and metabolite profiling in Arabidopsis (Arabidopsis thaliana) infected with Tobacco rattle virus (TRV) in order to investigate the influence of primary metabolism on virus infection. Our results revealed that primary metabolism is reconfigured in many ways during TRV infection, as reflected by significant changes in the levels of sugars and amino acids. Multivariate data analysis revealed that these alterations were particularly conspicuous at the time points of maximal accumulation of TRV, although infection time was the dominant source of variance during the process. Furthermore, TRV caused changes in lipid and fatty acid composition in infected leaves. We found that several Arabidopsis mutants deficient in branched-chain amino acid catabolism or fatty acid metabolism possessed altered susceptibility to TRV. Finally, we showed that increments in the putrescine content in TRV-infected plants correlated with enhanced tolerance to freezing stress in TRV-infected plants and that impairment of putrescine biosynthesis promoted virus multiplication. Our results thus provide an interesting overview for a better understanding of the relationship between primary metabolism and virus infection.

Stress-dependent regulation of 13-lipoxygenases and 13-hydroperoxide lyase in olive fruit mesocarp

The effect of different environmental stresses on the expression and enzyme activity levels of 13-lipoxygenases (13-LOX) and 13-hydroperoxide lyase (13-HPL) and on the volatile compounds synthesized by their sequential action has been studied in the mesocarp tissue of olive fruit from the Picual and Arbequina cultivars. The results showed that temperature, light, wounding and water regime regulate olive 13-LOXs and 13-HPL genes at transcriptional level. Low temperature and wounding brought about an increase in LOX and HPL enzyme activities. A very slight increase in the total content of six straight-chain carbons (C6) volatile compounds was also observed in the case of low temperature and wounding treatments. The physiological roles of 13-LOXs and 13-HPL in the olive fruit stress response are discussed.

Thermal inactivation kinetics of recombinant proteins of the lipoxygenase pathway related to the synthesis of virgin olive oil volatile compounds

The aim of this work was to characterize the thermal inactivation parameters of recombinant proteins related to the biosynthesis of virgin olive oil (VOO) volatile compounds through the lipoxygenase (LOX) pathway. Three purified LOX isoforms (Oep2LOX1, Oep1LOX2, and Oep2LOX2) and a hydroperoxide lyase (HPL) protein (OepHPL) were studied. According to their thermal inactivation parameters, recombinant Oep1LOX2 and Oep2LOX2 could be identified as the two LOX isoforms active in olive fruit crude preparations responsible for the synthesis of 13-hydroperoxides, the main substrates for the synthesis of VOO volatile compounds. Recombinant Oep2LOX1 displayed a low thermal stability, which suggests a weak actuation during the oil extraction process considering the current thermal conditions of this industrial process. In addition, recombinant OepHPL could be identified as the HPL activity in crude preparations. The thermal stability was the highest among the recombinant proteins studied, which suggests that HPL activity is not a limiting factor for the synthesis of VOO volatile compounds.

Molecular cloning, functional characterization and transcriptional regulation of a 9-lipoxygenase gene from olive

A lipoxygenase (LOX) cDNA clone (Oep2LOX1) has been isolated from olive fruit (Olea europaea cv. Picual). The deduced amino acid sequence displayed significant similarity to known plant LOX1 sequences. Genomic Southern blot analysis suggests that only one copy of Oep2LOX1 is present in the olive genome. Linolenic acid was the preferred substrate for the recombinant Oep2LOX1, which produced almost exclusively 9-hydroperoxide when linolenic acid was used as substrate, whereas a mixture of 9- and 13-hydroperoxides in a ratio 4:1 was formed from linoleic acid. Expression levels were measured in different tissues of Picual and Arbequina cultivars, including the mesocarp and seed during development and ripening of olive fruit. The results showed that Oep2LOX1 transcript level is spatially and temporally regulated. Besides, the transcriptional regulation of the Oep2LOX1 gene in response to different abiotic stresses was also investigated. Temperature, light and wounding regulate Oep2LOX1 gene expression in olive fruit mesocarp. The physiological role of the Oep2LOX1 gene during olive fruit ripening and in the stress response is discussed.

Effect of different environmental stresses on the expression of oleate desaturase genes and fatty acid composition in olive fruit

The regulation of microsomal and plastidial oleate desaturases by low and high temperature, darkness, and wounding was investigated. To this end, their gene expression levels and the fatty acid composition was determined in the mesocarp tissue of olive fruit from the Picual and Arbequina varieties subjected to the corresponding stress treatments. Firstly, a plastidial oleate desaturase from olive was cloned and its functional identity was confirmed by overexpression in Escherichia coli. The results showed that temperature and light regulate olive oleate desaturase genes at transcriptional level. However, no correlation between their expression levels and the linoleic acid content in microsomal and plastidial lipids was found. In addition, the involvement of microsomal but not plastidial oleate desaturases in the wounding response of olive fruit mesocarp is demonstrated. The fatty acid analysis revealed the appearance of palmitolinoleic acid only in microsomal lipids, reaching a maximum 3h after wounding.

Isolation, expression, and characterization of a 13-hydroperoxide lyase gene from olive fruit related to the biosynthesis of the main virgin olive oil aroma compounds

A full-length cDNA clone (OepHPL) coding for hydroperoxide lyase was isolated from olive fruit ( Olea europaea cv. Picual). The deduced amino acid sequence shows significant similarity to known plant hydroperoxide lyases and contains a N-terminal sequence that displays structural features of a chloroplast transit peptide. Genomic Southern blot analysis indicates that at least one copy of OepHPL is present in the olive genome. The recombinant hydroperoxide lyase was specific for 13-hydroperoxide derivatives of linolenic and linoleic acids but did not use 9-hydroperoxy isomers as substrates. Analyses of reaction products revealed that this enzyme produces primarily (Z)-hex-3-enal, which partially isomerizes to (E)-hex-2-enal, from 13-hydroperoxylinolenic acid and hexanal from 13-hydroperoxylinoleic acid. Expression levels were measured in different tissues of Picual and Arbequina varieties, including mesocarp and seed during development and ripening of olive fruits. The involvement of this olive hydroperoxide lyase gene in the biosynthesis of virgin olive oil aroma compounds is discussed.

Functional characterization of two 13-lipoxygenase genes from olive fruit in relation to the biosynthesis of volatile compounds of virgin olive oil

Two LOX cDNA clones, Oep1LOX2 and Oep2LOX2, have been isolated from olive ( Olea europaea cv. Picual). Both deduced amino acid sequences showed significant similarity to known plant LOX2, and they contain an N-terminal chloroplastic transit peptide. Genomic Southern blot analyses suggest that at least three copies of Oep1LOX2 and one copy of Oep2LOX2 should be present in the olive genome. Linolenic acid proved to be the preferred substrate for both olive recombinant LOXs, and analyses of reaction products revealed that both enzymes produce primarily 13-hydroperoxides from linoleic and linolenic acids. Expression levels of both genes were measured in the mesocarp and seeds during development and ripening of Picual and Arbequina olive fruit along with the level of volatile compounds in the corresponding virgin olive oils. Biochemical and gene expression data suggest a major involvement of the Oep2LOX2 gene in the biosynthesis of virgin olive oil aroma compounds.

Expression analysis identifies FAD2-2 as the olive oleate desaturase gene mainly responsible for the linoleic acid content in virgin olive oil

The effect of ripening stage and water regimen on oleate desaturase gene expression levels in the fruit of different olive ( Olea europaea L.) varieties was investigated to elucidate the contribution of each to the linoleic acid content in virgin olive oil. To this end, fatty acid analysis and quantitative real time PCR were performed using distinct olive tissues and different developmental stages from the Picual and Arbequina cultivars. The results showed that the olive FAD2-1, FAD2-2, and FAD6 genes were spatial and temporally regulated. In addition, the data indicated that FAD2-2 seems to be the main gene responsible for the linoleic acid content in the olive fruit mesocarp tissue. This conclusion was also confirmed when the study was extended to Hojiblanca, Picudo, and Manzanilla varieties. With regard to the water regimen, unlike the Picual cultivar, a small increase of linoleic acid was observed in the Arbequina variety cultivated with irrigation, which correlated well with the increase detected for the FAD2-2 gene expression level. All of these data strongly suggest that FAD2-2 is the main gene that determines the linoleic acid content in the virgin olive oil.

The utilization and desaturation of oleate and linoleate during glycerolipid biosynthesis in olive (Olea europaea L.) callus cultures

Callus cultures from olive (Olea europaea L.) were used to study characteristics of desaturation in this oil-rich tissue. The incorporation of [1-(14)C]oleate and [1-(14)C]linoleate into complex lipids and their further desaturation was followed in incubations of up to 48 h. Both radiolabelled fatty acids were rapidly incorporated into lipids, especially phosphatidylcholine and triacylglycerol. Radiolabelling of these two lipids peaked after 1-4 h, after which it fell. In contrast, other phosphoglycerides and the galactosylglycerides were labelled in a more sustained manner. [1-(14)C]Linoleate was almost exclusively found in the galactolipids. With [1-(14)C]linoleate as a precursor, the only significant desaturation to linolenate was in the galactolipids. Monogalactosyldiacylglycerol was the first lipid in which [1-(14)C]linoleate and [1-(14)C]linolenate appeared after incubation of the calli with [1-(14)C]oleate and [1-(14)C]linoleate, respectively. The presence of radioactivity in the plastidial lipids shows that both [1-(14)C]oleate and [1-(14)C]linoleate can freely enter the chloroplast. Two important environmental effects were also examined. Raised incubation temperatures (30-35 degrees C) reduced oleate desaturation and this was also reflected in the endogenous fatty acid composition. Low light also caused less oleate desaturation. The data indicate that lysophosphatidylcholine acyltransferase is important for the entry of oleate and linoleate into olive callus lipid metabolism and phospholipid:diacylglycerol acyltransferase may be involved in triacylglycerol biosynthesis. In addition, it is shown that plastid desaturases are mainly responsible for the production of polyunsaturated fatty acids. Individual fatty acid desaturases were differently susceptible to environmental stresses with FAD2 being reduced by both high temperature and low light, whereas FAD7 was only affected by high temperature.

Temperature-dependent endogenous oxygen concentration regulates microsomal oleate desaturase in developing sunflower seeds

Oleoyl-phosphatidylcholine desaturase (FAD2) is a key enzyme involved in fatty acid desaturation in oilseeds, which is affected by environmental temperature. The results of this study show that FAD2 is regulated in vivo via temperature-dependent endogenous oxygen concentrations in developing sunflower (Helianthus annuus L.) seeds. By combining in vivo oxygen profiling, in situ hybridization of FAD2 genes, an assay of energy status, fatty acid analysis, and an in vitro FAD2 enzyme activity assay, it is shown that: (i) the oil-storing embryo is characterized by a very low oxygen level that is developmentally regulated. Oxygen supply is mainly limited by the thin seed coat. (ii) Elevations of external oxygen supply raised the energy status of seed and produced a dramatic increase of the FAD2 enzyme activity as well as the linoleic acid content. (iii) A clear negative correlation exists between temperature and internal oxygen concentration. The changes occurred almost instantly and the effect was fully reversible. The results indicate that the internal oxygen level acts as a key regulator for the activity of the FAD2 enzyme. It is concluded that a major mechanism by which temperature modifies the unsaturation degree of the sunflower oil is through its effect on dissolved oxygen levels in the developing seed.

Molecular cloning and characterization of genes encoding two microsomal oleate desaturases (FAD2) from olive

Two different cDNA sequences, designated OepFAD2-1 and OepFAD2-2, encoding two microsomal oleate desaturases (FAD2) have been isolated from olive (Olea europaea cv. Picual) using a PCR approach. Both deduced amino acid sequences showed the three histidine boxes characteristic of all membrane-bound desaturases, and possess a C-terminal endoplasmic reticulum retention signal. Phylogenetic analysis shows that OepFAD2-1 and OepFAD2-2 are grouped with other plant FAD2 sequences. Functional expression of the corresponding FAD2 cDNAs in yeast confirmed that they encode microsomal oleate desaturases. Genomic Southern blot analysis is consistent with the presence of at least two copies of each OepFAD2 gene in the olive genome. OepFAD2-1 transcript was strongly detected in very young seeds and in leaves, showing low levels in mesocarps, while the transcript of the OepFAD2-2 gene was moderately expressed in developing seeds, ripening mesocarp and leaves. These expression data suggest differential functions for the two olive microsomal oleate desaturase genes, with FAD2-1 possibly responsible for the desaturation of reserve lipids in the young seed, while FAD2-2 may be mainly involved in storage lipid desaturation in the mature seeds and the mesocarp.

Growth temperature control of the linoleic acid content in safflower (Carthamus tinctorius) seed oil

The temperature and oxygen regulation of the microsomal oleate desaturase (FAD2) from safflower (Carthamus tinctorius L.) seeds was investigated. Heat-resistance profiles obtained in vivo and in vitro showed that the FAD2 enzyme maintained its maximal activity until 30 degrees C. A temperature increase from 10 to 40 degrees C caused a decrease of the FAD2 activity. However, when the temperature was decreased from 40 to 10 degrees C, no increase in the activity level was detected. The removal of hulls from safflower seeds followed by incubation in air did not change the FAD2 activity level, whereas incubation under nitrogen caused a strong decrease. Air replacement brought about the recovery of the initials levels. Oxygen concentrations less than 3% produced the inactivation of the enzyme. These data indicate that the higher thermal stability and the lower dependence on oxygen availability of the safflower FAD2 enzyme, compared with that of sunflower, could be the main factors to explain why the linoleate content of safflower seeds is more independent of growth temperature than that of sunflower seeds.

Temperature and oxygen regulation of oleate desaturation in developing sunflower (Helianthus annuus) seeds

The effect of low (10 degrees C) and high (30 degrees C) temperature on in vivo oleate desaturation has been studied in developing sunflower (Helianthus annuus L.) seeds under conditions of different oxygen availability (capitulum, detached achenes or peeled seeds). In seeds remaining in the capitulum, only a part of the oleate newly synthesized at high temperature was desaturated to linoleate, whereas more oleate than that synthesized de novo was desaturated at low temperature. Achenes were only able to significantly desaturate oleate at low temperatures. In contrast, oleate desaturation was detected in peeled seeds incubated at low and high temperatures, showing the highest rate at 20 degrees C. Hull removing dramatically increased the activity of the microsomal oleate desaturase (FAD2, EC 1.3.1.35) at all studied temperatures, although a long-term inactivation of the enzyme was observed at high temperatures. Low oxygen concentration (1-2%) obtained by respiration of peeled seeds incubated in sealed vials, brought about the inactivation of the enzyme. All these data suggest that temperature regulates oleate desaturation controlling the amount of oleate and the FAD2 activity. In addition, this enzyme seems to be also regulated by the availability of oxygen, which is affected inside the achene by its diffusion through the hull, and the competition with respiration, both factors being temperature-dependent.

Temperature and oxygen regulation of microsomal oleate desaturase (FAD2) from sunflower

The effect of temperature and oxygen on the in vivo oleate desaturation and microsomal oleate desaturase (FAD2) activity was studied in peeled developing sunflower seeds. Using an oxygen concentration that was saturating for FAD2 enzyme, the amount of linoleic acid increased for all studied temperatures, being maximal at 20 degrees C. Under these conditions, FAD2 activity increased at the beginning of the incubation, remaining constant for the rest of the time, but reaching a lower level at 30 degrees C. Anoxia brought about a decrease in the FAD2 activity for all studied temperatures, becoming faster as the temperature increased. All these data suggest that temperature and oxygen control the level of FAD2 activity by separate mechanisms.

Effect of culture conditions on the isocitrate dehydrogenase and isocitrate lyase activities in Chlamydomonas reinhardtii

In the green alga Chlamydomonas reinhardtii, nitrogen staravation induced a reversible increase (2-fold) in NAD-isocitrate dehydrogenase (NAD-IDH; EC 1.1.1.41) and NADP-isocitrate dehydrogenase (NADP-IDH; EC 1.1.1.42) activities. Both enzymes were not affected by the concentration of CO₂ , the dark or the nature of the nitrogen source (nitrate, nitrite, or ammonium). When cells growing autotrophically were transferred to heterotrophic conditions, a 40% reduction of the NAD-IDH activity was detected, a 2-fold increase of NADP-IDH was observed and isocitrate lyase (ICL; EC 4.1.3.1) activity was induced. The replacement of autotrophic conditions led to the initial activity levels. NAD- and NADP-IDH activities showed markedly different patterns of increase in synchronous cultures of this alga obtained by 12 h light/12 h dark transitions. While NAD-IDH increased in the last 4 h of the dark period, NADP-IDH increased during the last 4 h of the light period, remaining constant for the rest of the cycle.