Genetic Enhancement of Cold Tolerance by Expression of a Gene for Chloroplast [omega]-3 Fatty Acid Desaturase in Transgenic Tobacco

cDNA Microarray Analysis of Rice Anther Genes under Chilling Stress at the Microsporogenesis Stage Revealed Two Genes with DNA TransposonCastawayin the 5′-Flanking Region

Rice is most chilling sensitive at the onset of microspore release. Chilling treatment at this stage causes male sterility. The gene expression profile during the microspore development process under chilling stress was revealed using a microarray that included 8,987 rice cDNAs. As many as 160 cDNAs were up- or down-regulated by chilling during the microspore release stage. RT-PCR analysis of 5 genes confirmed the microarray results. We identified 3 novel genes whose expression levels were remarkably changed by chilling in rice anther. A new cis element that includes a DNA transposon Castaway sequence was found in the 5' upstream region of two genes which were conspicuously down-regulated by chilling temperatures in rice anther.

Deep-sequencing transcriptome analysis of low temperature perception in a desert tree, Populus euphratica

Background

Compared with other Populus species, Populus euphratica Oliv. exhibits better tolerance to abiotic stress, especially those involving extreme temperatures. However, little is known about gene regulation and signaling pathways involved in low temperature stress responses in this species. Recent development of Illumina/Solexa-based deep-sequencing technologies has accelerated the study of global transcription profiling under specific conditions. To understand the gene network controlling low temperature perception in P. euphratica, we performed transcriptome sequencing using Solexa sequence analysis to generate a leaf transcriptome at a depth of 10 gigabases for each sample.

Results

Using the Trinity method, 52,081,238 high-quality trimmed reads were assembled into a non-redundant set and 108,502 unigenes with an average length of 1,047 bp were generated. After performing functional annotations by aligning all-unigenes with public protein databases, 85,584 unigenes were annotated. Differentially expressed genes were investigated using the FPKM method by applying the Benjamini and Hochberg corrections. Overall, 2,858 transcripts were identified as differentially expressed unigenes in at least two samples and 131 were assigned as unigenes expressed differently in all three samples. In 4°C-treated sample and -4°C-treated sample, 1,661 and 866 differently expressed unigenes were detected at an estimated absolute log₂-fold change of > 1, respectively. Among them, the respective number of up-regulated unigenes in C4 and F4 sample was 1,113 and 630, while the respective number of down-regulated ungenes is 548 and 236. To increase our understanding of these differentially expressed genes, we performed gene ontology enrichment and metabolic pathway enrichment analyses. A large number of early cold (below or above freezing temperature)-responsive genes were identified, suggesting that a multitude of transcriptional cascades function in cold perception. Analyses of multiple cold-responsive genes, transcription factors, and some key transduction components involved in ABA and calcium signaling revealed their potential function in low temperature responses in P. euphratica.

Conclusions

Our results provide a global transcriptome picture of P. euphratica under low temperature stress. The potential cold stress related transcripts identified in this study provide valuable information for further understanding the molecular mechanisms of low temperature perception in P. euphratica.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-326) contains supplementary material, which is available to authorized users.

Low temperature alters plasma membrane lipid composition and ATPase activity of pineapple fruit during blackheart development

Plasma membrane (PM) plays central role in triggering primary responses to chilling injury and sustaining cellular homeostasis. Characterising response of membrane lipids to low temperature can provide important information for identifying early causal factors contributing to chilling injury. To this end, PM lipid composition and ATPase activity were assessed in pineapple fruit (Ananas comosus) in relation to the effect of low temperature on the development of blackheart, a form of chilling injury. Chilling temperature at 10 °C induced blackheart development in concurrence with increase in electrolyte leakage. PM ATPase activity was decreased after 1 week at low temperature, followed by a further decrease after 2 weeks. The enzyme activity was not changed during 25 °C storage. Loss of total PM phospholipids was found during postharvest senescence, but more reduction was shown from storage at 10 °C. Phosphatidylcholine and phosphatidylethanolamine were the predominant PM phospholipid species. Low temperature increased the level of phosphatidic acid but decreased the level of phosphatidylinositol. Both phospholipid species were not changed during storage at 25 °C. Postharvest storage at both temperatures decreased the levels of C18:3 and C16:1, and increased level of C18:1. Low temperature decreased the level of C18:2 and increased the level of C14:0. Exogenous application of phosphatidic acid was found to inhibit the PM ATPase activity of pineapple fruit in vitro. Modification of membrane lipid composition and its effect on the functional property of plasma membrane at low temperature were discussed in correlation with their roles in blackheart development of pineapple fruit.

ACYL-LIPID DESATURASE2 is required for chilling and freezing tolerance in Arabidopsis

Fatty acid desaturation of membrane lipids is a strategy for plants to survive chilling or freezing temperature. To further characterize enzymes involved in this stress response pathway, ACYL-LIPID DESATURASE2 (ADS2; Enzyme Commission 1.14.99) was studied using genetic, cell, and biochemical approaches. ads2 mutant plants appear similar to the wild type under standard growth conditions but display a dwarf and sterile phenotype when grown at 6°C and also show increased sensitivity to freezing temperature. Fatty acid composition analysis demonstrated that ads2 mutant plants at 6°C have reduced levels of 16:1, 16:2, 16:3, and 18:3 and higher levels of 16:0 and 18:0 fatty acids compared with the wild type. Lipid profiling revealed that 34C species of phosphatidylglycerol (PG) and monogalactosyl diacylglycerol (MGDG) content in ads2 mutants were lower and phosphatidic acid, phosphatidylinositol, phosphatidylethanolamine, phosphatidylcholine, lyso-phosphatidylcholine, and phosphatidylserine were higher than the wild type. Subcellular localization of C- and N-terminal enhanced fluorescence fusion proteins indicated that ADS2 localized primarily to the endoplasmic reticulum, although signal was also confirmed in Golgi and plastids. A double mutation with a putative plastid ADS3 paralog exacerbates the growth defects of ads2 mutant plants under low temperature. These observations suggest that ADS2 encodes a 16:0 desaturase of MGDG and PG. We hypothesize that a low temperature-induced shift from the plastid to endoplasmic reticulum pathway for membrane lipid biosynthesis is required for the cold stress response in Arabidopsis thaliana, and ADS2 is essential to adjust the acyl composition of organelle membrane lipid composition in response to cold stress.

Improvement of Polyunsaturated Fatty Acid Production in Echium acanthocarpum Transformed Hairy Root Cultures by Application of Different Abiotic Stress Conditions

Fatty acids are of great nutritional, therapeutic, and physiological importance, especially the polyunsaturated n-3 fatty acids, possessing larger carbon chains and abundant double bonds or their immediate precursors. A few higher plant species are able to accumulate these compounds, like those belonging to the Echium genus. Here, the novel E. acanthocarpum hairy root system, which is able to accumulate many fatty acids, including stearidonic and α-linolenic acids, was optimized for a better production. The application of abiotic stress resulted in larger yields of stearidonic and α-linolenic acids, 60 and 35%, respectively, with a decrease in linoleic acid, when grown in a nutrient medium consisting of B5 basal salts, sucrose or glucose, and, more importantly, at a temperature of 15°C. The application of osmotic stress employing sorbitol showed no positive influence on the fatty acid yields; furthermore, the combination of a lower culture temperature and glucose did not show a cumulative boosting effect on the yield, although this carbon source was similarly attractive. The abiotic stress also influenced the lipid profile of the cultures, significantly increasing the phosphatidylglycerol fraction but not the total lipid neither their biomass, proving the appropriateness of applying various abiotic stress in this culture to achieve larger yields.

Deep-sequencing transcriptome analysis of chilling tolerance mechanisms of a subnival alpine plant, Chorispora bungeana

BACKGROUND

The plant tolerance mechanisms to low temperature have been studied extensively in the model plant Arabidopsis at the transcriptional level. However, few studies were carried out in plants with strong inherited cold tolerance. Chorispora bungeana is a subnival alpine plant possessing strong cold tolerance mechanisms. To get a deeper insight into its cold tolerance mechanisms, the transcriptome profiles of chilling-treated C. bungeana seedlings were analyzed by Illumina deep-sequencing and compared with Arabidopsis.

RESULTS

Two cDNA libraries constructed from mRNAs of control and chilling-treated seedlings were sequenced by Illumina technology. A total of 54,870 unigenes were obtained by de novo assembly, and 3,484 chilling up-regulated and 4,571 down-regulated unigenes were identified. The expressions of 18 out of top 20 up-regulated unigenes were confirmed by qPCR analysis. Functional network analysis of the up-regulated genes revealed some common biological processes, including cold responses, and molecular functions in C. bungeana and Arabidopsis responding to chilling. Karrikins were found as new plant growth regulators involved in chilling responses of C. bungeana and Arabidopsis. However, genes involved in cold acclimation were enriched in chilling up-regulated genes in Arabidopsis but not in C. bungeana. In addition, although transcription activations were stimulated in both C. bungeana and Arabidopsis, no CBF putative ortholog was up-regulated in C. bungeana while CBF2 and CBF3 were chilling up-regulated in Arabidopsis. On the other hand, up-regulated genes related to protein phosphorylation and auto-ubiquitination processes were over-represented in C. bungeana but not in Arabidopsis.

CONCLUSIONS

We conducted the first deep-sequencing transcriptome profiling and chilling stress regulatory network analysis of C. bungeana, a subnival alpine plant with inherited cold tolerance. Comparative transcriptome analysis suggests that cold acclimation is not a major chilling tolerance mechanism of C. bungeana. Activation of protein phosphorylation and ubiquitination may confer chilling tolerance to C. bungeana in a more rapid and flexible way than cold acclimation. Such differences may have contributed to the differences in cold tolerance between C. bungeana and Arabidopsis. The results presented in this paper will be informative for gene discovery and the molecular mechanisms related to plant cold tolerance.

Genome-wide gene expression profiling of introgressed indica rice alleles associated with seedling cold tolerance improvement in a japonica rice background

Background

Rice in tropical and sub-tropical areas is often subjected to cold stress at the seedling stage, resulting in poor growth and yield loss. Although japonica rice is generally more cold tolerant (CT) than indica rice, there are several favorable alleles for CT exist in indica that can be used to enhance CT in rice with a japonica background. Genome-wide gene expression profiling is an efficient way to decipher the molecular genetic mechanisms of CT enhancement and to provide valuable information for CT improvement in rice molecular breeding. In this study, the transcriptome of the CT introgression line (IL) K354 and its recurrent parent C418 under cold stress were comparatively analyzed to explore the possible CT enhancement mechanisms of K354.

Results

A total of 3184 differentially expressed genes (DEGs), including 195 transcription factors, were identified in both lines under cold stress. About half of these DEGs were commonly regulated and involved in major cold responsive pathways associated with OsDREB1 and OsMyb4 regulons. K354-specific cold-induced genes were functionally related to stimulus response, cellular cell wall organization, and microtubule-based movement processes that may contribute to increase CT. A set of genes encoding membrane fluidity and defensive proteins were highly enriched only in K354, suggesting that they contribute to the inherent CT of K354. Candidate gene prediction based on introgressed regions in K354 revealed genotype-dependent CT enhancement mechanisms, associated with Sir2, OsFAD7, OsWAK112d, and programmed cell death (PCD) related genes, present in CT IL K354 but absent in its recurrent parent C418. In K354, a number of DEGs were co-localized onto introgressed segments associated with CT QTLs, providing a basis for gene cloning and elucidation of molecular mechanisms responsible for CT in rice.

Conclusions

Genome-wide gene expression analysis revealed that genotype-specific cold induced genes and genes with higher basal expression in the CT genotype contribute jointly to CT improvement. The molecular genetic pathways of cold stress tolerance uncovered in this study, as well as the DEGs co-localized with CT-related QTLs, will serve as useful resources for further functional dissection of the molecular mechanisms of cold stress response in rice.

Genome-wide gene expression profiling of introgressed indica rice alleles associated with seedling cold tolerance improvement in a japonica rice background

BACKGROUND

Rice in tropical and sub-tropical areas is often subjected to cold stress at the seedling stage, resulting in poor growth and yield loss. Although japonica rice is generally more cold tolerant (CT) than indica rice, there are several favorable alleles for CT exist in indica that can be used to enhance CT in rice with a japonica background. Genome-wide gene expression profiling is an efficient way to decipher the molecular genetic mechanisms of CT enhancement and to provide valuable information for CT improvement in rice molecular breeding. In this study, the transcriptome of the CT introgression line (IL) K354 and its recurrent parent C418 under cold stress were comparatively analyzed to explore the possible CT enhancement mechanisms of K354.

RESULTS

A total of 3184 differentially expressed genes (DEGs), including 195 transcription factors, were identified in both lines under cold stress. About half of these DEGs were commonly regulated and involved in major cold responsive pathways associated with OsDREB1 and OsMyb4 regulons. K354-specific cold-induced genes were functionally related to stimulus response, cellular cell wall organization, and microtubule-based movement processes that may contribute to increase CT. A set of genes encoding membrane fluidity and defensive proteins were highly enriched only in K354, suggesting that they contribute to the inherent CT of K354. Candidate gene prediction based on introgressed regions in K354 revealed genotype-dependent CT enhancement mechanisms, associated with Sir2, OsFAD7, OsWAK112d, and programmed cell death (PCD) related genes, present in CT IL K354 but absent in its recurrent parent C418. In K354, a number of DEGs were co-localized onto introgressed segments associated with CT QTLs, providing a basis for gene cloning and elucidation of molecular mechanisms responsible for CT in rice.

CONCLUSIONS

Genome-wide gene expression analysis revealed that genotype-specific cold induced genes and genes with higher basal expression in the CT genotype contribute jointly to CT improvement. The molecular genetic pathways of cold stress tolerance uncovered in this study, as well as the DEGs co-localized with CT-related QTLs, will serve as useful resources for further functional dissection of the molecular mechanisms of cold stress response in rice.

Early changes in the fatty acid composition of photosynthetic membrane lipids from Populus nigra grown on a metallurgical landfill

We compared the fatty acid composition of leaves taken from poplars on a metal-contaminated landfill, and on the uncontaminated roadside bordering this site. For the first time, it is shown that the percentage of linolenic acid, which is mainly associated with thylakoid lipids, was significantly lower in tree species within the landfill than within the control area. A correlation study was carried out to investigate relationships between the C18:3/(C18:0 + C18:1 + C18:2) fatty acid ratios and the metal contents in soils and leaves. Lead and chromium leaf contents were significantly negatively correlated to this fatty acid ratio. The impact of each of these metals remains difficult to evaluate, but chromium in leaf likely plays a major role in toxicity. In addition, the decrease in the C18:3/(C18:0 + C18:1 + C18:2) fatty acid ratio occurred at low leaf metal content, and therefore it is shown that this ratio can be used as an early indicator of the effect of metals.

Thermotolerant cyclamen with reduced acrolein and methyl vinyl ketone

Reduced levels of trienoic fatty acids (TAs) in chloroplast membranes induce thermotolerance in several plant species, but the underlying mechanisms remain unclear. TA peroxidation in plant cell membranes generates cytotoxic, TA-derived compounds containing α,β-unsaturated carbonyl groups. The relationship between low TA levels and the amounts of cytotoxic TA-derived compounds was examined using thermotolerant transgenic cyclamen (Cyclamen persicum Mill.) with low TA contents. Changes in the levels of the cytotoxic TA-derived acrolein (ACR), methyl vinyl ketone (MVK), (E)-2-hexenal, 4-hydroxy-2-nonenal, and malondialdehyde were analysed in the leaf tissues of wild-type (WT) and thermotolerant transgenic cyclamen under heat stress. Levels of ACR and MVK in the WT increased in parallel with the occurrence of heat-induced tissue damage, whereas no such changes were observed in the thermotolerant transgenic lines. Furthermore, exogenous ACR and MVK infiltrated into leaves to concentrations similar to those observed in heat-stressed WT leaves caused similar disease symptoms. These results suggest that thermotolerance in transgenic cyclamen depends on reduced production rates of ACR and MVK under heat stress, due to the low level of TAs in these plants.

Loss of function of FATTY ACID DESATURASE7 in tomato enhances basal aphid resistance in a salicylate-dependent manner

We report here that disruption of function of the ω-3 FATTY ACID DESATURASE7 (FAD7) enhances plant defenses against aphids. The suppressor of prosystemin-mediated responses2 (spr2) mutation in tomato (Solanum lycopersicum), which eliminates the function of FAD7, reduces the settling behavior, survival, and fecundity of the potato aphid (Macrosiphum euphorbiae). Likewise, the antisense suppression of LeFAD7 expression in wild-type tomato plants reduces aphid infestations. Aphid resistance in the spr2 mutant is associated with enhanced levels of salicylic acid (SA) and mRNA encoding the pathogenesis-related protein P4. Introduction of the Naphthalene/salicylate hydroxylase transgene, which suppresses SA accumulation, restores wild-type levels of aphid susceptibility to spr2. Resistance in spr2 is also lost when we utilize virus-induced gene silencing to suppress the expression of NONEXPRESSOR OF PATHOGENESIS-RELATED PROTEINS1 (NPR1), a positive regulator of many SA-dependent defenses. These results indicate that FAD7 suppresses defenses against aphids that are mediated through SA and NPR1. Although loss of function of FAD7 also inhibits the synthesis of jasmonate (JA), the effects of this desaturase on aphid resistance are not dependent on JA; other mutants impaired in JA synthesis (acx1) or perception (jai1-1) show wild-type levels of aphid susceptibility, and spr2 retains aphid resistance when treated with methyl jasmonate. Thus, FAD7 may influence JA-dependent defenses against chewing insects and SA-dependent defenses against aphids through independent effects on JA synthesis and SA signaling. The Arabidopsis (Arabidopsis thaliana) mutants Atfad7-2 and Atfad7-1fad8 also show enhanced resistance to the green peach aphid (Myzus persicae) compared with wild-type controls, indicating that FAD7 influences plant-aphid interactions in at least two plant families.

Arabidopsis fatty acid desaturase FAD2 is required for salt tolerance during seed germination and early seedling growth

Fatty acid desaturases play important role in plant responses to abiotic stresses. However, their exact function in plant resistance to salt stress is unknown. In this work, we provide the evidence that FAD2, an endoplasmic reticulum localized ω-6 desaturase, is required for salt tolerance in Arabidopsis. Using vacuolar and plasma membrane vesicles prepared from the leaves of wild-type (Col-0) and the loss-of-function Arabidopsis mutant, fad2, which lacks the functional FAD2, we examined the fatty acid composition and Na+-dependent H+ movements of the isolated vesicles. We observed that, when compared to Col-0, the level of vacuolar and plasma membrane polyunsaturation was lower, and the Na+/H+ exchange activity was reduced in vacuolar and plasma membrane vesicles isolated from fad2 mutant. Consistent with the reduced Na+/H+ exchange activity, fad2 accumulated more Na+ in the cytoplasm of root cells, and was more sensitive to salt stress during seed germination and early seedling growth, as indicated by CoroNa-Green staining, net Na+ efflux and salt tolerance analyses. Our results suggest that FAD2 mediated high-level vacuolar and plasma membrane fatty acid desaturation is essential for the proper function of membrane attached Na+/H+ exchangers, and thereby to maintain a low cytosolic Na+ concentration for salt tolerance during seed germination and early seedling growth in Arabidopsis.

Engineering cold stress tolerance in crop plants

Plants respond with changes in their pattern of gene expression and protein products when exposed to low temperatures. Thus ability to adapt has an impact on the distribution and survival of the plant, and on crop yields. Many species of tropical or subtropical origin are injured or killed by non-freezing low temperatures, and exhibit various symptoms of chilling injury such as chlorosis, necrosis, or growth retardation. In contrast, chilling tolerant species are able to grow at such cold temperatures. Conventional breeding methods have met with limited success in improving the cold tolerance of important crop plants involving inter-specific or inter-generic hybridization. Recent studies involving full genome profiling/ sequencing, mutational and transgenic plant analyses, have provided a deep insight of the complex transcriptional mechanism that operates under cold stress. The alterations in expression of genes in response to cold temperatures are followed by increases in the levels of hundreds of metabolites, some of which are known to have protective effects against the damaging effects of cold stress. Various low temperature inducible genes have been isolated from plants. Most appear to be involved in tolerance to cold stress and the expression of some of them is regulated by C-repeat binding factor/ dehydration-responsive element binding (CBF/DREB1) transcription factors. Numerous physiological and molecular changes occur during cold acclimation which reveals that the cold resistance is more complex than perceived and involves more than one pathway. The findings summarized in this review have shown potential practical applications for breeding cold tolerance in crop and horticultural plants suitable to temperate geographical locations.

Cold affects the transcription of fatty acid desaturases and oil quality in the fruit of Olea europaea L. genotypes with different cold hardiness

The olive tree lacks dormancy and is low temperature sensitive, with differences in cold tolerance and oil quality among genotypes. The oil is produced in the drupe, and the unsaturated fatty acids contribute to its quality. The aim of the present research was to investigate the relationship among development, cold response, expression of fatty acid desaturase (FAD) genes, and unsaturated fatty acid composition in drupes belonging to genotypes differing in leaf cold tolerance, but producing good oil (i.e. the non-hardy Moraiolo, the semi-hardy Frantoio, and the hardy Canino). In all genotypes, cold sensitivity, evaluated by cold-induced transient increases in cytosolic calcium, was high in the epi-mesocarp cells before oil body formation, and decreased during oil biogenesis. However, genotype-dependent differences in cold sensitivity appeared at the end of oil production. Genotype-dependent differences in FAD2.1, FAD2.2, FAD6, and FAD7 expression levels occurred in the epi-mesocarp cells during the oleogenic period. However, FAD2.1 and FAD7 were always the highest in the first part of this period. FAD2.2 and FAD7 increased after cold applications during oleogenesis, independently of the genotype. Unsaturated fatty acids increased in the drupes of the non-hardy genotype, but not in those of the hardy one, after cold exposure at the time of the highest FAD transcription. The results show a direct relationship between FAD expression and lipid desaturation in the drupes of the cold-sensitive genotype, and an inverse relationship in those of the cold-resistant genotype, suggesting that drupe cold acclimation requires a fine FAD post-transcriptional regulation. Hypotheses relating FAD desaturation to storage and membrane lipids, and genotype cold hardiness are discussed.

Nonsymbiotic hemoglobin-2 leads to an elevated energy state and to a combined increase in polyunsaturated fatty acids and total oil content when overexpressed in developing seeds of transgenic Arabidopsis plants

Nonsymbiotic hemoglobins are ubiquitously expressed in plants and divided into two different classes based on gene expression pattern and oxygen-binding properties. Most of the published research has been on the function of class 1 hemoglobins. To investigate the role of class 2 hemoglobins, transgenic Arabidopsis (Arabidopsis thaliana) plants were generated overexpressing Arabidopsis hemoglobin-2 (AHb2) under the control of a seed-specific promoter. Overexpression of AHb2 led to a 40% increase in the total fatty acid content of developing and mature seeds in three subsequent generations. This was mainly due to an increase in the polyunsaturated C18:2 (ω-6) linoleic and C18:3 (ω-3) α-linolenic acids. Moreover, AHb2 overexpression led to an increase in the C18:2/C18:1 and C18:3/C18:2 ratios as well as in the C18:3 content in mol % of total fatty acids and in the unsaturation/saturation index of total seed lipids. The increase in fatty acid content was mainly due to a stimulation of the rate of triacylglycerol synthesis, which was attributable to a 3-fold higher energy state and a 2-fold higher sucrose content of the seeds. Under low external oxygen, AHb2 overexpression maintained an up to 5-fold higher energy state and prevented fermentation. This is consistent with AHb2 overexpression results in improved oxygen availability within developing seeds. In contrast to this, overexpression of class 1 hemoglobin did not lead to any significant increase in the metabolic performance of the seeds. These results provide evidence for a specific function of class 2 hemoglobin in seed oil production and in promoting the accumulation of polyunsaturated fatty acids by facilitating oxygen supply in developing seeds.

A mutation in the LPAT1 gene suppresses the sensitivity of fab1 plants to low temperature

The Arabidopsis (Arabidopsis thaliana) fatty acid biosynthesis1 (fab1) mutant grows as well as wild type at 22 degrees C, but after transfer to 2 degrees C fab1 plants cannot maintain photosynthetic function and die after 5 to 7 weeks at 2 degrees C. A fab1 suppressor line, S7, was isolated in a screen that identified mutants that remained alive after 16 weeks at 2 degrees C and were able to flower and produce seed after return to 22 degrees C. Relative to wild type, S7 plants had reduced levels of 16:3 fatty acid in leaf galactolipids, indicating reduced synthesis of chloroplast glycerolipids by the prokaryotic pathway of lipid metabolism. The suppressor mutation was identified, by map-based and candidate-gene approaches, as a hypomorphic allele of lysophosphatidic acid acyltransferase1 (lpat1), lpat1-3. LPAT1 encodes the enzyme that catalyzes the second reaction in the prokaryotic pathway. Several lines of evidence indicate that damage and death of fab1 plants at 2 degrees C may be a result of the increased proportion of phosphatidylglycerol (PG) in fab1 that are high-melting-point molecular species (containing only 16:0, 18:0, and 16:1,Delta3-trans fatty acids). Consistent with this proposal, the lpat1-3 mutation strongly affects the fatty acid composition of PG. The proportion of high-melting-point molecular species in PG is reduced from 48.2% in fab1 to 10.7% in fab1 lpat1-3 (S7), a value close to the 7.6% found in wild type.

Increasing omega-3 desaturase expression in tomato results in altered aroma profile and enhanced resistance to cold stress

One of the drawbacks in improving the aroma properties of tomato (Solanum lycopersicum) fruit is the complexity of this organoleptic trait, with a great variety of volatiles contributing to determine specific quality features. It is well established that the oxylipins hexanal and (Z)-hex-3-enal, synthesized through the lipoxygenase pathway, are among the most important aroma compounds and impart in a correct proportion some of the unique fresh notes in tomato. Here, we confirm that all enzymes responsible for the synthesis of these C6 compounds are present and active in tomato fruit. Moreover, due to the low odor threshold of (Z)-hex-3-enal, small changes in the concentration of this compound could modify the properties of the tomato fruit aroma. To address this possibility, we have overexpressed the omega-3 fatty acid desaturases FAD3 and FAD7 that catalyze the conversion of linoleic acid (18:2) to linolenic acid (18:3), the precursor of hexenals and its derived alcohols. Transgenic OE-FAD tomato plants exhibit altered fatty acid composition, with an increase in the 18:3/18:2 ratio in leaves and fruits. These changes provoke a clear variation in the C6 content that results in a significant alteration of the (Z)-hex-3-enal/hexanal ratio that is particularly important in ripe OE-FAD3FAD7 fruits. In addition to this effect on tomato volatile profile, OE-FAD tomato plants are more tolerant to chilling. However, the different behaviors of OE-FAD plants underscore the existence of separate fatty acid fluxes to ensure plant survival under adverse conditions.

Loss of sense transgene-induced post-transcriptional gene silencing by sequential introduction of the same transgene sequences in tobacco

RNA silencing is an epigenetic inhibition of gene expression and is guided by small interfering RNAs. Sense transgene-induced post-transcriptional gene silencing (S-PTGS) occurs in a portion of a transgenic plant population. When a sense transgene encoding a tobacco endoplasmic reticulum omega-3 fatty acid desaturase (NtFAD3) was introduced into tobacco plants, an S-PTGS line, S44, was obtained. Introduction of another copy of the NtFAD3 transgene into S44 plants caused a phenotypic change from S-PTGS to overexpression. Because this change was associated with the methylation of the promoter sequences of the transgene, reduced transcriptional activity may abolish S-PTGS and residual transcription of the sense transgene may account for the overexpression. To clarify whether RNA-directed DNA methylation (RdDM) can repress the transcriptional activity of the S44 transgene locus, we introduced several RdDM constructs targeting the transgene promoter. An RdDM construct harboring a 200-bp-long fragment of promoter sequences efficiently abrogated the generation of NtFAD3 small interfering RNAs in S44 plants. Transcription of the transgene was partially repressed, but the resulting NtFAD3 mRNAs successfully accumulated and an overexpressed phenotype was established. Our results indicate an example in which overexpression of the transgene is established by complex epigenetic interactions among the transgenic loci.

Omega-3 fatty acid desaturase genes isolated from purslane (Portulaca oleracea L.): expression in different tissues and response to cold and wound stress

Two full-length cDNA clones PoleFAD7 and PoleFAD8, encoding plastidial omega-3 fatty acid desaturases were isolated from purslane (Portulaca oleracea). The encoded enzymes convert linoleic to alpha-linolenic acid (C18:3n-3). Three histidine clusters characteristic of fatty acid desaturases, a putative chloroplast transit peptide in the N-terminal, and three putative transmembrane domains were identified in the sequence. Both genes were expressed in all analyzed tissues showing different levels of expression. PoleFAD7 was up-regulated by wounding but not by low temperature. PoleFAD8 was up-regulated by cold stress but not by wounding. Total fatty acid and linolenic acid content were higher both, in wounded and intact leaves of plants exposed to low temperature.

Changes in freezing tolerance in hybrid poplar caused by up- and down-regulation of PtFAD2 gene expression

In plant species, the level of polyunsaturated fatty acids (PUFAs) is essential for cold acclimation. To test whether changes in PUFA levels can lead to the alteration of freezing tolerance in poplar trees, we up- and down-regulated a Populus tomentosa Delta-12 fatty acid desaturase gene (PtFAD2) in the hybrid poplar (P. alba x P. glandulosa) clone 84 K. Real-time PCR results demonstrated that compared to untransformed control lines, the transcriptional level of PtFAD2 increased by up to 90% in over-expressing poplar lines (line OE-1) and decreased in down-regulated RNAi lines by up to 64% (line DR-1). As a result, the content of linoleic (C18:2) and linolenic (C18:3) unsaturated fatty acids (FAs) in total FAs increased by 7.5 and 3.9%, respectively, in the OE-1 line and decreased by 14.4 and 5.4% in the DR-2 line when compared to non-transgenic lines. After freezing treatment at -4 degrees C for 3 h without pre-cold acclimation, the survival rates of the PtFAD2-over-expressing cuttings were significantly higher (60% for OE-1) than those of non-transgenic plants (36.7%) and down-regulated lines (10% for DR-2). These results clearly demonstrate that the expression level of PUFAs substantially affected the freezing tolerance of hybrid poplar cuttings and could thus be utilized as an effective strategy to improve poplar anti-freezing traits through genetic engineering biotechnology.

Membrane protein dynamics: limited lipid control

Correlation of lipid disorder with membrane protein dynamics has been studied with infrared spectroscopy, by combining data characterizing lipid phase, protein structure and, via hydrogen-deuterium (H/D) exchange, protein dynamics. The key element was a new measuring scheme, by which the combined effects of time and temperature on the H/D exchange could be separated. Cyanobacterial and plant thylakoid membranes, mammalian mitochondria membranes, and for comparison, lysozyme were investigated. In dissolved lysozyme, as a function of temperature, H/D exchange involved only reversible movements (the secondary structure did not change considerably); heat-denaturing was a separate event at much higher temperature. Around the low-temperature functioning limit of the biomembranes, lipids affected protein dynamics since changes in fatty acyl chain disorders and H/D exchange exhibited certain correlation. H/D exchange remained low in all membranes over physiological temperatures. Around the high-temperature functioning limit of the membranes, the exchange rates became higher. When temperature was further increased, H/D exchange rates went over a maximum and afterwards decreased (due to full H/D exchange and/or protein denaturing). Maximal H/D exchange rate temperatures correlated neither with the disorder nor with the unsaturation of lipids. In membrane proteins, in contrast to lysozyme, the onsets of sizable H/D exchange rates were the onsets of irreversible denaturing as well. Seemingly, at temperatures where protein self-dynamics allows large-scale H/D exchange, lipid-protein coupling is so weak that proteins prefer aggregating to limit the exposure of their hydrophobic surface regions to water. In all membranes studied, dynamics seemed to be governed by lipids around the low-temperature limit, and by proteins around the high-temperature limit of membrane functionality.PACS codes: 87.14.ep, 87.14.cc, 87.16.D.

Transplastomic tobacco plants expressing a fatty acid desaturase gene exhibit altered fatty acid profiles and improved cold tolerance

The possibility of altering the unsaturation level of fatty acids in plant lipids by genetic transformation has implications for the stress tolerance of higher plants as well as for their nutritional value and industrial utilisation. While the integration and expression of transgenes in the plastome has several potential advantages over nuclear transformation, very few attempts have been made to manipulate fatty acid biosynthesis using plastid transformation. We produced transplastomic tobacco plants that express a Delta(9) desaturase gene from either the wild potato species Solanum commersonii or the cyanobacterium Anacystis nidulans, using PEG-mediated DNA uptake by protoplasts. Incorporation of chloroplast antibiotic-insensitive point mutations in the transforming DNA was used to select transformants. The presence of the transcript and the Delta(9) desaturase protein in transplastomic plants was confirmed by northern and western blot analyses. In comparison with control plants, transplastomic plants showed altered fatty acid profiles and an increase in their unsaturation level both in leaves and seeds. The two transgenes produced comparable results. The results obtained demonstrate the feasibility of using plastid transformation to engineer lipid metabolic pathways in both vegetative and reproductive tissues and suggest an increase of cold tolerance in transplastomic plants showing altered leaf fatty acid profiles. This is the first example of transplastomic plants expressing an agronomically relevant gene produced with the "binding-type" vectors, which do not contain a heterologous marker gene. In fact, the transplastomic plants expressing the S. commersonii gene contain only plant-derived sequences, a clear attraction from a public acceptability perspective.

Overexpression of the Arabidopsis 10-kilodalton acyl-coenzyme A-binding protein ACBP6 enhances freezing tolerance

Small 10-kD acyl-coenzyme A-binding proteins (ACBPs) are highly conserved proteins that are prevalent in eukaryotes. In Arabidopsis (Arabidopsis thaliana), other than the 10-kD ACBP homolog (designated Arabidopsis ACBP6), there are five larger forms of ACBPs ranging from 37.5 to 73.1 kD. In this study, the cytosolic subcellular localization of Arabidopsis ACBP6 was confirmed by analyses of transgenic Arabidopsis expressing autofluorescence-tagged ACBP6 and western-blot analysis of subcellular fractions using ACBP6-specific antibodies. The expression of Arabidopsis ACBP6 was noticeably induced at 48 h after 4 degrees C treatment by northern-blot analysis and western-blot analysis. Furthermore, an acbp6 T-DNA insertional mutant that lacked ACBP6 mRNA and protein displayed increased sensitivity to freezing temperature (-8 degrees C), while ACBP6-overexpressing transgenic Arabidopsis plants were conferred enhanced freezing tolerance. Northern-blot analysis indicated that ACBP6-associated freezing tolerance was not dependent on the induction of cold-regulated COLD-RESPONSIVE gene expression. Instead, ACBP6 overexpressors showed increased expression of mRNA encoding phospholipase Ddelta. Lipid profiling analyses of rosettes from cold-acclimated, freezing-treated (-8 degrees C) transgenic Arabidopsis plants overexpressing ACBP6 showed a decline in phosphatidylcholine (-36% and -46%) and an elevation of phosphatidic acid (73% and 67%) in comparison with wild-type plants. From our comparison, the gain in freezing tolerance in ACBP6 overexpressors that was accompanied by decreases in phosphatidylcholine and an accumulation of phosphatidic acid is consistent with previous findings on phospholipase Ddelta-overexpressing transgenic Arabidopsis. In vitro filter-binding assays indicating that histidine-tagged ACBP6 binds phosphatidylcholine, but not phosphatidic acid or lysophosphatidylcholine, further imply a role for ACBP6 in phospholipid metabolism in Arabidopsis, including the possibility of ACBP6 in the cytosolic trafficking of phosphatidylcholine.

Green corolla segments in a wild Petunia species caused by a mutation in FBP2, a SEPALLATA-like MADS box gene

A Petunia inflata isolate with a novel phenotype of a purple corolla limb with green corolla segments (GCS) was characterized. The GCS have stomata and trichomes on the adaxial side, and resemble calyx segments in epidermal morphology. The GCS phenotype was inherited in a recessive manner. In the GCS plant, a novel inhibitor/defective spm-like transposable element (dPifTp1) was inserted in the second intron of the Floral Binding Protein 2 (FBP2) gene. The sequence of the resulting transcript contained five silent mutations as compared the corresponding open reading frame of P. x hybrida FBP2 mRNA. The GCS phenotype co-segregated with an FBP2 fragment containing a dPifTp1 insertion. The transcript level of the FBP2 gene in GCS flowers was markedly lower than that in wild-type (WT) flowers, suggesting that partially inhibited FBP2 gene expression caused the morphogenesis of calyx-like tissue in the corolla segments of GCS flowers. Gene expression pattern analysis using a full-length Petunia floral cDNA microarray indicated that some photosynthesis-related genes were expressed at significantly higher levels in the GCS of GCS flowers, but the mRNA levels of most other genes in the GCS were similar to those in the WT corolla. Taken together, these data suggest that the partial loss of FBP2 expression does not shift global gene expression in the corolla segments of the GCS flower toward that of calyx, even though calyx-like morphogenesis was established in the corolla segments.

Heterologous microarray experiments allow the identification of the early events associated with potato tuber cold sweetening

Background

Since its discovery more than 100 years ago, potato (Solanum tuberosum) tuber cold-induced sweetening (CIS) has been extensively investigated. Several carbohydrate-associated genes would seem to be involved in the process. However, many uncertainties still exist, as the relative contribution of each gene to the process is often unclear, possibly as the consequence of the heterogeneity of experimental systems. Some enzymes associated with CIS, such as β-amylases and invertases, have still to be identified at a sequence level. In addition, little is known about the early events that trigger CIS and on the involvement/association with CIS of genes different from carbohydrate-associated genes. Many of these uncertainties could be resolved by profiling experiments, but no GeneChip is available for the potato, and the production of the potato cDNA spotted array (TIGR) has recently been discontinued. In order to obtain an overall picture of early transcriptional events associated with CIS, we investigated whether the commercially-available tomato Affymetrix GeneChip could be used to identify which potato cold-responsive gene family members should be further studied in detail by Real-Time (RT)-PCR (qPCR).

Results

A tomato-potato Global Match File was generated for the interpretation of various aspects of the heterologous dataset, including the retrieval of best matching potato counterparts and annotation, and the establishment of a core set of highly homologous genes. Several cold-responsive genes were identified, and their expression pattern was studied in detail by qPCR over 26 days. We detected biphasic behaviour of mRNA accumulation for carbohydrate-associated genes and our combined GeneChip-qPCR data identified, at a sequence level, enzymatic activities such as β-amylases and invertases previously reported as being involved in CIS. The GeneChip data also unveiled important processes accompanying CIS, such as the induction of redox- and ethylene-associated genes.

Conclusion

Our Global Match File strategy proved critical for accurately interpretating heterologous datasets, and suggests that similar approaches may be fruitful for other species. Transcript profiling of early events associated with CIS revealed a complex network of events involving sugars, redox and hormone signalling which may be either linked serially or act in parallel. The identification, at a sequence level, of various enzymes long known as having a role in CIS provides molecular tools for further understanding the phenomenon.

Disease resistance against Magnaporthe grisea is enhanced in transgenic rice with suppression of omega-3 fatty acid desaturases

Linolenic acid (18:3) is the most abundant fatty acid in plant membrane lipids and is a source for various oxidized metabolites, called oxylipins. 18:3 and oxylipins play important roles in the induction of defense responses to pathogen infection and wound stress in Arabidopsis. However, in rice, endogenous roles for 18:3 and oxylipins in disease resistance have not been confirmed. We generated 18:3-deficient transgenic rice plants (F78Ri) with co-suppression of two omega-3 fatty acid desaturases, OsFAD7 and OsFAD8. that synthesize 18:3. The F78Ri plants showed enhanced resistance to the phytopathogenic fungus Magnaporthe grisea. A typical 18:3-derived oxylipin, jasmonic acid (JA), acts as a signaling molecule in defense responses to fungal infection in Arabidopsis. However, in F78Ri plants, the expression of JA-responsive pathogenesis-related genes, PBZ1 and PR1b, was induced after inoculation with M. grisea, although the JA-mediated wound response was suppressed. Furthermore, the application of JA methyl ester had no significant effect on the enhanced resistance in F78Ri plants. Taken together, our results indicate that, although suppression of fatty acid desaturases involves the concerted action of varied oxylipins via diverse metabolic pathways, 18:3 or 18:3-derived oxylipins, except for JA, may contribute to signaling on defense responses of rice to M. grisea infection.

The effects of spacer sequences on silencing efficiency of plant RNAi vectors

RNA interference (RNAi) has been used to suppress gene expression in various eukaryotic organisms. In plants, RNAi can be induced by introduction of an RNAi vector that transcribes a self-complementary hairpin RNA. Most basic RNAi constructs have an inverted repeat interrupted with a spacer sequence. To test silencing capability of RNAi constructs, we developed an in vivo assay that is based on the RNAi-mediated changes of the alpha-linolenic acid content in hairy roots. A tobacco endoplasmic reticulum omega-3 fatty acid desaturase (NtFAD3) is the main enzyme for production of alpha-linolenic acid of root membrane lipids. Tobacco hairy roots transformed with the RNAi vectors against the NtFAD3 gene showed a decrease in alpha-linolenic acid content. The frequency of RNA silencing was more affected by spacer sequence than by spacer length, at least between 100 and 1800 bp. Since significant amounts of hairpin RNA against the NtFAD3 gene remained in the transgenic plants displaying a weak silencing phenotype, low degree of silencing was attributed to low efficiency of hairpin RNA processing mediated by Dicer-like proteins. Our results show the possibility of producing a broad range of the RNAi-induced silencing phenotypes by replacing the spacer sequence of RNAi construct.

Generation of secondary small interfering RNA in cell-autonomous and non-cell autonomous RNA silencing in tobacco

Small interfering RNA (siRNA) species with 21-25 nucleotides in length guide mRNA cleavage, translational arrest, and heterochromatin formation in RNA interference (RNAi). To delineate the target region of RNAi, a construct harboring a transcriptional fusion between parts of the target mRNA and the beta-glucuronidase gene was biolistically delivered into tobacco leaves showing an RNAi phenotype and the assay sequence was transiently expressed. The RNAi effect was monitored by amplification of this chimeric transcript. By using this assay method, we addressed the transitive RNA silencing of a tobacco endoplasmic reticulum omega-3 fatty acid desaturase gene (NtFAD3). In the NtFAD3 RNAi plants, the target region of RNAi was restricted in the inducer region corresponding to a stem sequence of the hairpin double-stranded RNA, indicating that endogenous NtFAD3 mRNA was not a template for an RNA-dependent RNA polymerase. The secondary NtFAD3 siRNAs were produced in the crossbred plants between the NtFAD3 overexpressed plant and the NtFAD3 RNAi plant. Similarly, the secondary siRNAs were generated in the systemically silenced scion. Although these secondary siRNAs originated preferentially from the 3' region downstream of the inducer region, the secondary siRNAs produced in the silenced scion (non-cell autonomous secondary siRNAs) resulted in the strong degradation of the target mRNA, but the secondary siRNAs in the crossbred plants (cell-autonomous secondary siRNAs) showed limited RNA degradation activity. These results showed that this in vivo assay for determination of RNAi efficiency is a useful tool to delineate RNAi mechanisms.

Warm and cold parental reproductive environments affect seed properties, fitness, and cold responsiveness in Arabidopsis thaliana progenies

Conditions in the parental environment during reproduction can affect the performance of the progenies. The goals of this study were to investigate whether warm or cold temperatures in the parental environment during flowering and seed development affect Arabidopsis thaliana seed properties, growth performance, reproduction and stress tolerance of the progenies, and to find candidate genes for progeny-related differences in stress responsiveness. Parental plants were raised at 20 degrees C and maintained from bolting to seed maturity at warm (25 degrees C) or cold (15 degrees C) temperatures. Analysis of seed properties revealed significant increases in nitrogen in seeds from warm temperature and significant increases in lipids and in the ratio of alpha-linolenic to oleic acid in seeds from the cold parental environment. Progenies of the warm parental environment showed faster germination rates, faster root elongation growth, higher leaf biomass and increased seed production at various temperatures compared with those from the cold parental environment. This indicates that under stable environmental conditions, progenies from warm parental environments had a clear adaptive advantage over those from cold parental environments. This parental effect was presumably transmitted by the higher nitrogen content of the seeds developed in warm conditions. When offspring from parents grown at different temperatures were exposed to chilling or freezing stress, photosynthetic yield recovered faster in progenies originating from cold parental environments. Cold acclimation involved up-regulation of transcripts of flavanone 3-hydroxylase (F3H) and pseudo response regulator 9 (PRR9) and down-regulation of growth-associated transcription factors (TFs) NAP and AP2domain containing RAP2.3. NAP, a regulator of senescence, and PRR9, a temperature-sensitive modulator of the circadian clock, were probably involved in mediating parent-of-origin effects, because they showed progeny-related expression differences under chilling. Because low temperatures also delay senescence, cold responsiveness of NAP suggests that this factor is linked with the regulatory network that is important for environmental acclimation of plants.

Beyond the lipid hypothesis: mechanisms underlying phenotypic plasticity in inducible cold tolerance

The physiological adjustment of organisms in response to temperature variation is a crucial part of coping with environmental stress. An important component of the cold response is the increase in membrane lipid unsaturation, and this has been linked to an enhanced resistance to the debilitating or lethal effects of cold. Underpinning the lipid response is the upregulation of fatty acid desaturases (des), particularly those introducing double bonds at the 9-10 position of saturated fatty acids. For plants and microbes there is good genetic evidence that regulation of des genes, and the consequent changes in lipid saturation, are causally linked to generation of a cold-tolerant phenotype. In animals, however, supporting evidence is almost entirely limited to correlations of saturation with cold conditions. We describe our recent attempts to provide a direct test of this relationship by genetic manipulation of the nematode Caenorhabditis elegans. We show that this species displays a strong cold tolerant phenotype induced by prior conditioning to cold, and that this is directly linked to upregulated des activity. However, whilst genetic disruption of des activity and lipid unsaturation significantly reduced cold tolerance, animals retained a substantial component of their stress tolerant phenotype produced by cold conditioning. This indicates that mechanisms other than lipid unsaturation play an important role in cold adaptation.

Functional characterization of a plastidial omega-3 desaturase from sunflower (Helianthus annuus) in cyanobacteria

Fatty acid desaturases (FAD) play an important role in plant lipid metabolism and they can be found in several subcellular compartments such as the plastids and endoplasmic reticulum. Lipids are critical components of the cell membrane and, as a consequence, they are fundamental for the proper growth and development of all living organisms. We have used sequences from the conserved regions of known omega-3-desaturases to design degenerated oligonucleotides and clone a cDNA encoding a plastidial omega-3 desaturase from sunflower (HaFAD7). From its presumed full-length sequence, we predict that Hafad7 encodes a protein of 443 amino acids with a molecular mass of 50.8 kDa, and that it contains a putative chloroplast transit peptide of 51 amino acids. The predicted hydrophobicity of the protein identifies four potential membrane-spanning regions and, according to the TargetP algorithm, the protein should be targeted to the plastid/chloroplast membrane. RT-PCR analysis of its expression shows the transcript is preferentially expressed in photosynthetically active tissues. Heterologous expression of this protein in the unicellular cyanobacterium Synechocystis sp. PCC 6803 confirmed that the protein produced from this cDNA has omega-3 desaturase activity.

A suppressor of fab1 challenges hypotheses on the role of thylakoid unsaturation in photosynthetic function

Leaf membrane lipids of the Arabidopsis (Arabidopsis thaliana) fatty acid biosynthesis 1 (fab1) mutant contain a 35% to 40% increase in the predominant saturated fatty acid 16:0, relative to wild type. This increase in membrane saturation is associated with loss of photosynthetic function and death of mutant plants at low temperatures. We have initiated a suppressor screen for mutations that allow survival of fab1 plants at 2 degrees C. Five suppressor mutants identified in this screen all rescued the collapse of photosynthetic function observed in fab1 plants. While fab1 plants died after 5 to 7 weeks at 2 degrees C, the suppressors remained viable after 16 weeks in the cold, as judged by their ability to resume growth following a return to 22 degrees C and to subsequently produce viable seed. Three of the suppressors had changes in leaf fatty acid composition when compared to fab1, indicating that one mechanism of suppression may involve compensating changes in thylakoid lipid composition. Surprisingly, the suppressor phenotype in one line, S31, was associated with a further substantial increase in lipid saturation. The overall leaf fatty acid composition of S31 plants contained 31% 16:0 compared with 23% in fab1 and 17% in wild type. Biochemical and genetic analysis showed that S31 plants contain a new allele of fatty acid desaturation 5 (fad5), fad5-2, and are therefore partially deficient in activity of the chloroplast 16:0 Delta7 desaturase. A double mutant produced by crossing fab1 to the original fad5-1 allele also remained alive at 2 degrees C, indicating that the fad5-2 mutation is the suppressor in the S31 (fab1 fad5-2) line. Based on the biophysical characteristics of saturated and unsaturated fatty acids, the increased 16:0 in fab1 fad5-2 plants would be expected to exacerbate, rather than ameliorate, low-temperature damage. We propose instead that a change in shape of the major thylakoid lipid, monogalactosyldiacylglycerol, mediated by the fad5-2 mutation, may compensate for changes in lipid structure resulting from the original fab1 mutation. Our identification of mutants that suppress the low-temperature phenotype of fab1 provides new tools to understand the relationship between thylakoid lipid structure and photosynthetic function.

Gene expression of stearoyl-ACP desaturase and Δ12 fatty acid desaturase 2 is modulated during seed development of flax (Linum usitatissimum)

Flax's recent popularity in human and animal foods is mostly due to its desirable FA composition. Flax is an excellent source of omega-3 FA, which have been shown to have many health benefits. To date, little is known about the genetic and environmental factors that control the FA composition of flax seeds. To elucidate some of the important genetic components, reverse transcriptase (RT)-PCR and real-time PCR were used to determine the expression profiles of two key FA biosynthetic genes during seed development. Plants of flax cultivar AC McDuff were grown under field conditions, and RNA was extracted from ovaries and developing bolls collected from 2 d after anthesis (DAA) to maturity. Desaturation enzymes stearoyl-ACP desaturase (SAD) and delta12 FA desaturase 2 (FAD2) were both expressed in ovaries, and their expression was differentially modulated throughout seed development. SAD was most highly expressed in ovaries. Its expression quickly decreased until 4 DAA; this was followed by a slight peak at 8 DAA, only to return to relatively low levels of expression in maturing bolls, ranging from 2.1% to 4.5% relative to the level observed in ovaries. FAD2 expression displayed a different temporal pattern. While expression of FAD2 did decrease in the early stages of seed development, expression increased starting at 8 DAA, peaking at 16 DAA, when it was 158% relative to the level observed in ovaries. FAD2, which desaturates oleic acid (18:1cisdelta9) into linoleic acid (18:2cisdelta9,12), is therefore controlled at the transcription level. To relate enzyme expression with FA profile, GC was performed on the same subsamples used for RT-PCR and real-time PCR, and proportions of palmitic, stearic, oleic, linoleic, and linolenic acids were determined for the same developmental stages. Although FAD2 expression increased from 8 to 16 DAA, relative changes in linoleic acid (18:2cis delta9,12) were not observed. However, linolenic acid (ALA; alpha-18:3; 18:3cisdelta9,12,15) levels increased steadily, meaning that linoleic acid (18:2cisdelta9,12) is a transient substrate converted by FAD3 as quickly as it is produced by FAD2. Phenotypes are the result of genotypes, environment, and the interaction of the two. To evaluate the environmental impact on the production of FA in flax, FA profiles were assessed in a total of four environments (two locations, two years). Warm and dry environmental conditions resulted in lower levels of PUFA 18:2cisdelta9,12 and 18:3cisdelta9,12,15, and higher levels of 18:1 cisdelta9. FAD2 expression and/or activity may therefore be affected by the environment.

Reduction of trienoic fatty acid content by expression of a double-stranded RNA of a plastid omega-3 fatty acid desaturase gene in transgenic tobacco

Plastid omega-3 fatty acid desaturase catalyzes the conversion of dienoic fatty acids (16:2 and 18:2) to trienoic fatty acids (16:3 and alpha-18:3) in glycerolipids which are the main constituents of chloroplast membranes. We produced transgenic tobacco plants that express the transcript of a double-stranded RNA (dsRNA) of tobacco plastid omega-3 fatty acid desaturase gene, NtFAD7. In these transgenic plants, 16:3 and alpha-18:3 content in leaves decreased to less than 2.7% and 7.5-10.4%, respectively, when compared with the control plant. The steady-state NtFAD7 mRNA was not detected in the transgenic plants. These results indicate that down-regulation of the transcript level in the NtFAD7 by introduction of NtFAD7 dsRNA constructs is useful to decrease the trienoic fatty acid contents of the vegetative tissues in higher plants.

Induced Expression of DREB Transcriptional Factor and Study on Its Physiological Effects of Drought Tolerance in Transgenic Wheat

Expression vector pBAC128F, which carries DREB transcriptional factor gene driven by drought inducing promoter rd29B and bar gene driven by CaMV 35S promoter and maize Adh1 gene first intron, was transferred into the explants of immature inflorescence and immature embryos of hexaploid winter wheat cv. 8901, 5-98, 99-92 and 104 by particle bombardment. More than 70 resistant transgenic plants were obtained. Genomic PCR and RNA dot blotting analyses showed that DREB gene had been integrated into wheat genome of the transgenic plants (T0 and T1) and was well expressed in offspring seed of different transgenic lines. The content of proline in leaves and seeds of T2 transgenic lines was analyzed. Among 16 tested transgenic lines, 10 transgenic lines exhibited more than two fold of proline level in leaves as compared with CK plants. Under drought condition, after stopping water for 15 days the leaves of transgenic lines were still green, while CK were faded. After rewatering for 10 days, the leaves of transgenic lines maintained their green, while all CK plants were dead. Our research suggested that introducing a novel DREB transcriptional factor into wheat is an effective way to improve its drought-tolerance ability.

Enhanced cold tolerance in transgenic tobacco expressing a chloroplast omega-3 fatty acid desaturase gene under the control of a cold-inducible promoter

A new cold-inducible genetic construct was cloned using a chloroplast-specific omega-3-fatty acid desaturase gene (FAD7) under the control of a cold-inducible promoter (cor15a) from Arabidopsis thaliana. RT-PCR confirmed a marked increase in FAD7 expression, in young Nicotiana tabacum (cv. Havana) plants harboring cor15a-FAD7, after a short-term exposure to cold. When young, cold-induced tobacco seedlings were exposed to low-temperature (0.5, 2 or 3.5 degrees C) for up to 44 days, survival within independent cor15a-FAD7 transgenic lines (40.2-96%) was far superior to the wild type (6.7-10.2%). In addition, the major trienoic fatty acid species remained stable in cold-induced cor15a-FAD7 N. tabacum plants under prolonged cold storage while the levels of hexadecatrienoic acid (16:3) and octadecatrienoic acid (18:3) declined in wild type plants under the same conditions (79 and 20.7% respectively). Electron microscopy showed that chloroplast membrane ultrastructure in cor15a-FAD7 transgenic plants was unaffected by prolonged exposure to cold temperatures. In contrast, wild type plants experienced a loss of granal stacking and disorganization of the thylakoid membrane under the same conditions. Changes in membrane integrity coincided with a precipitous decline in leaf chlorophyll concentration and low survival rates in wild type plants. Cold-induced double transgenic N. alata (cv. Domino Mix) plants, harboring both the cor15a-FAD7 cold-tolerance gene and a cor15a-IPT dark-tolerance gene, exhibited dramatically higher survival rates (89-90%) than wild type plants (2%) under prolonged cold storage under dark conditions (2 degrees C for 50 days).

Characterization of a rice (Oryza sativa L.) gene encoding a temperature-dependent chloroplast omega-3 fatty acid desaturase

A cDNA, designated Osfad8, encoding a chloroplast omega-3 fatty acid desaturase responsible for trienoic fatty acid formation, was isolated from the leaves of Oryza sativa L. by RT-PCR. Southern blot hybridization indicated that a small gene family composed of two copies or closely linked genes exists. RNA in situ hybridization showed that the accumulation of Osfad8 mRNA was abundant in leaves but hardly detectable in roots. The Osfad8 transcript level in leaves was much higher at 15 degrees C than at normal temperature (25 degrees C). In situ hybridization also showed particularly prominent expression of Osfad8 in the palisade layer and spongy parenchyma cells of leaves when exposed to 15 degrees C conditions for 5 days and 10 days. Two transgenic lines (8S-52 and 8S-101) harboring the Osfad8 ORF in sense orientation under the control of the CaMV 35S promoter contained increased levels of hexadecatrienoic (16:3) and linolenic (18:3) fatty acids. When exposed to 2 degrees C for 7 days, the damage observed to the control plants was significantly alleviated in the 8S-52 and 8S-101 lines. The amounts of trienoic fatty acids in an Osfad8 antisense line (8A-35) declined 40.2% compared to the control plants. The 8A-35 plants survived after growth at 44 degrees C for 3 days while the control plants died. These data suggest that Osfad8 encodes a temperature-dependent chloroplast omega-3 fatty acid desaturase.

Contribution of omega-3 fatty acid desaturase and 3-ketoacyl-ACP synthase II (KASII) genes in the modulation of glycerolipid fatty acid composition during cold acclimation in birch leaves

Temperate and boreal tree species respond to low positive temperatures (LT) or a shortening of the photoperiod (SD) by inducing cold acclimation. One of the metabolic consequences of cold acclimation is an increase in fatty acid (FA) desaturation in membrane lipids, which allows functional membrane fluidity to be maintained at LT. The molecular mechanisms of FA desaturation were investigated in leaves of birch seedlings (Betula pendula) during cold acclimation. Four genes involved in FA biosynthesis were isolated: a 3-ketoacyl-ACP synthase II gene (BpKASII) involved in the elongation of palmitoyl-ACP to stearoyl-ACP, and three omega-3 FA desaturase genes (BpFAD3, BpFAD7, and BpFAD8) involved in the desaturation of linoleic acid (18:2) to alpha-linolenic acid (18:3). BpFAD7 was the main omega-3 FAD gene expressed in birch leaves, and it was down-regulated by LT under SD conditions. LT induced the expression of BpFAD3 and BpFAD8 and a synchronous increase in 18:3 occurred in glycerolipids. Changes in the photoperiod did not affect the LT-induced increase in 18:3 in chloroplast lipids (MGDG, DGDG, PG), but it modulated the LT response detected in extra-chloroplastic lipids (PC, PE, PI, PS). A decrease in the proportion of the 16-carbon FAs in lipids occurred at LT, possibly in relation to the regulation of BpKASII expression at LT. These results suggest that LT affects the whole FA biosynthesis pathway. They support a co-ordinated action of microsomal (BpFAD3) and chloroplast enzymes (BpFAD7, BpFAD8) in determining the level of 18:3 in extra-chloroplastic membranes, and they highlight the importance of dynamic lipid trafficking.

Modulated fatty acid desaturation via overexpression of two distinct omega-3 desaturases differentially alters tolerance to various abiotic stresses in transgenic tobacco cells and plants

Changes in the degree of fatty acid (FA) desaturation are implicated in plant responses to various abiotic stresses, including heat, salt and drought. However, it is still not known whether decreased levels of linolenic acid, found in many plants subjected to salt and drought stress, reflect a mechanism of defence or damage. We addressed this question by generating tobacco cells and plants ectopically overexpressing two FA desaturases: the cytosolic FAD3 or the plastidic FAD8. A remarkable increase in the ratio of total linolenic to linoleic acids resulted from overexpression of FAD3, whereas ectopic overexpression of FAD8 induced an increased ratio mainly in the plastidic lipids. Here we present evidence that overexpressing FAD8 imposes much greater heat sensitivity than does FAD3 overexpression, in both cultured cells and whole plants. Overexpression of either FAD3 or FAD8 increases tolerance to drought in tobacco plants and to osmotic stress in cultured cells. These findings suggest that a drought-induced decreased level of linolenic acid reflects damage. Our results point to the potential of exploiting FAD overexpression as a tool to ameliorate drought tolerance.

A Temperature-sensitive Mechanism That Regulates Post-translational Stability of a Plastidial ω-3 Fatty Acid Desaturase (FAD8) in Arabidopsis Leaf Tissues

Trienoic fatty acids (TAs) are the major constituents in plant membrane lipids. In Arabidopsis, two plastidial isozymes of omega-3 fatty acid desaturase, FAD7 and FAD8, are the major contributors for TA production in leaf tissues. Despite a high degree of structural relatedness, activities of these two isozymes are regulated differentially in response to temperature. Elevated temperatures lead to decreases in leaf TA level due to temperature sensitivity of FAD8 activity. A series of FAD7-FAD8 chimeric genes, each encoding a functional plastidial omega-3 desaturase, were introduced into the Arabidopsis fad7fad8 double mutant. Constructs with or without a c-Myc epitope tag were tested. Functionality of each chimeric gene in response to temperature was assayed by Northern and Western analyses and by examining the fatty acid composition. All transformants harboring a chimeric gene containing the FAD8-derived C-terminal coding region (44 amino acids) showed a marked decrease in TA level when exposed to high temperature, similarly as transgenic lines complemented with the native form of FAD8. The reduction of TA level was accompanied by a decrease in the amount of omega-3 desaturase protein but not necessarily by a decrease in its transcript level. Analysis of the decay of c-Myc-tagged products after inhibiting protein synthesis revealed that the FAD8-derived C-terminal region acts in an autoregulatory fashion to destabilize the protein at high temperature. This suggests that the regulation of post-translational stability of FAD8 provides an important regulatory mechanism for modifying its activity in response to temperature, mediating a decrease in TA level at elevated temperatures.

Role of chloroplast trienoic fatty acids in plant disease defense responses

Trienoic fatty acids (TAs) are the major polyunsaturated fatty acid species in the membrane lipids in plant cells. TAs are crucial for the adaptation to abiotic stresses, especially low- or high-temperature stress. We show that TAs in chloroplast membrane lipids are involved in defense responses against avirulent bacterial pathogens. Avirulent pathogen invasion of plants induces a transient production of reactive oxygen intermediates (ROI), programmed cell death and subsequent disease resistance. The Arabidopsis fad7fad8 mutation, which prevents the synthesis of TAs in chloroplast lipids, caused the reduction in ROI accumulation in leaves inoculated with Pseudomonas syringae pv. tomato DC3000 (avrRpm1). Linolenic acid, the most abundant TA, activated the NADPH oxidase that is responsible for ROI generation. TAs were transferred from chloroplast lipids to extrachloroplast lipids coincident with ROI accumulation after inoculation with Pst DC3000 (avrRpm1). Furthermore, the fad7fad8 mutant exhibited reduced cell death and was compromised in its resistance to several avirulent P. syringae strains. These results suggest that TAs derived from chloroplast lipids play an important role in the regulation of plant defense responses.

Amplification of plant genomic DNA by Phi29 DNA polymerase for use in physical mapping of the hypermethylated genomic region

Plant genomes contain a heavily methylated region in which cytosines are methylated in both the symmetrical and asymmetrical sequences. The physical mapping of such a hypermethylated region is difficult because many restriction enzymes are sensitive to methylated cytosine residues in their recognition sites. The Phi29 DNA polymerase provides an efficient and representative amplification of the genomic DNA that is methylation-free. Using this amplified genomic DNA, we were able to show that a heavily methylated genomic DNA region becomes amenable to physical mapping with any restriction enzymes. This protocol will be especially useful for analysis of the heavily methylated region of plant genomes.