Digalactosyldiacylglycerol synthesis in chloroplasts of the Arabidopsis dgd1 mutant

Galactolipid biosynthesis in plants is highly complex. It involves multiple pathways giving rise to different molecular species. To assess the contribution of different routes of galactolipid synthesis and the role of molecular species for growth and photosynthesis, we initiated a genetic approach of analyzing double mutants of the digalactosyldiacylglycerol (DGDG) synthase mutant dgd1 with the acyltransferase mutant, act1, and the two desaturase mutants, fad2 and fad3. The double mutants showed different degrees of growth retardation: act1,dgd1 was most severely affected and growth of fad2,dgd1 was slightly reduced, whereas fad3,dgd1 plants were very similar to dgd1. In act1,dgd1, lipid and chlorophyll content were reduced and photosynthetic capacity was affected. Molecular analysis of galactolipid content, fatty acid composition, and positional distribution suggested that the growth deficiency is not caused by changes in galactolipid composition per se. Chloroplasts of dgd1 were capable of synthesizing monogalactosyldiacylglycerol, DGDG, and tri- and tetragalactosyldiacylglycerol. Therefore, the reduced growth of act1,dgd1 and fad2,dgd1 cannot be explained by the absence of DGDG synthase activity from chloroplasts. Molecular analysis of DGDG accumulating in the mutants during phosphate deprivation suggested that similarly to the residual DGDG of dgd1, this additional lipid is synthesized in association with chloroplast membranes through a pathway independent of the mutations, act1, dgd1, fad2, and fad3. Our data imply that the severe growth defect of act1,dgd1 is caused by a reduced metabolic flux of chloroplast lipid synthesis through the eukaryotic and prokaryotic pathway as well as by the reduction of photosynthetic capacity caused by the destabilization of photosynthetic complexes.     

Disruption of the two digalactosyldiacylglycerol synthase genes DGD1 and DGD2 in Arabidopsis reveals the existence of an additional enzyme of galactolipid synthesis

Two genes (DGD1 and DGD2) are involved in the synthesis of the chloroplast lipid digalactosyldiacylglycerol (DGDG). The role of DGD2 for galactolipid synthesis was studied by isolating Arabidopsis T-DNA insertional mutant alleles (dgd2-1 and dgd2-2) and generating the double mutant line dgd1 dgd2. Whereas the growth and lipid composition of dgd2 were not affected, only trace amounts of DGDG were found in dgd1 dgd2. The growth and photosynthesis of dgd1 dgd2 were affected more severely compared with those of dgd1, indicating that the residual amount of DGDG in dgd1 is crucial for normal plant development. DGDG synthesis was increased after phosphate deprivation in the wild type, dgd1, and dgd2 but not in dgd1 dgd2. Therefore, DGD1 and DGD2 are involved in DGDG synthesis during phosphate deprivation. DGD2 was localized to the outer side of chloroplast envelope membranes. Like DGD2, heterologously expressed DGD1 uses UDP-galactose for galactosylation. Galactolipid synthesis activity for monogalactosyldiacylglycerol (MGDG), DGDG, and the unusual oligogalactolipids tri- and tetragalactosyldiacylglycerol was detected in isolated chloroplasts of all mutant lines, including dgd1 dgd2. Because dgd1 and dgd2 carry null mutations, an additional, processive galactolipid synthesis activity independent from DGD1 and DGD2 exists in Arabidopsis. This third activity, which is related to the Arabidopsis galactolipid:galactolipid galactosyltransferase, is localized to chloroplast envelope membranes and is capable of synthesizing DGDG from MGDG in the absence of UDP-galactose in vitro, but it does not contribute to net galactolipid synthesis in planta.     

Can digalactosyldiacylglycerol substitute for phosphatidylcholine upon phosphate deprivation in leaves and roots of Arabidopsis?

To explore the role of digalactosyldiacylglycerol (DGDG) in plants the dgd1 mutant of Arabidopsis thaliana was grown in the presence and absence of inorganic phosphate. Phosphate deficiency in the dgd1 mutant causes a strong decrease in all phospholipids accompanied by an increase in DGDG and sulpholipid. Moreover, a significant DGDG accumulation was found in roots upon phosphate deprivation as well. Our data indicate that DGDG accumulation upon phosphate deprivation is due to the activation of a specific eukaryotic dgd1-independent biosynthetic pathway. We propose that DGDG may substitute for phosphatidylcholine upon phosphate deprivation.     

The Role of Diglycosyl Lipids in Photosynthesis and Membrane Lipid Homeostasis in Arabidopsis

The galactolipid digalactosyldiacylglycerol (DGD) is an abundant thylakoid lipid in chloroplasts. The introduction of the bacterial lipid glucosylgalactosyldiacylglycerol (GGD) from Chloroflexus aurantiacus into the DGD-deficient Arabidopsis (Arabidopsis thaliana) dgd1 mutant was previously shown to result in complementation of growth, but photosynthetic efficiency was only partially restored. Here, we demonstrate that GGD accumulation in the double mutant dgd1dgd2, which is totally devoid of DGD, also complements growth at normal and high-light conditions, but photosynthetic efficiency in the GGD-containing dgd1dgd2 line remains decreased. This is attributable to an increased susceptibility of photosystem II to photodamage, resulting in reduced photosystem II accumulation already at normal light intensities. The chloroplasts of dgd1 and dgd1dgd2 show alterations in thylakoid ultrastructure, a phenotype that is restored in the GGD-containing lines. These data suggest that the strong growth retardation of the DGD-deficient lines dgd1 and dgd1dgd2 can be primarily attributed to a decreased capacity for chloroplast membrane assembly and proliferation and, to a smaller extent, to photosynthetic deficiency. During phosphate limitation, GGD increases in plastidial and extraplastidial membranes of the transgenic lines to an extent similar to that of DGD in the wild type, indicating that synthesis and transport of the bacterial lipid (GGD) and of the authentic plant lipid (DGD) are subject to the same mechanisms of regulation.     

Rice NTRC is a high-efficiency redox system for chloroplast protection against oxidative damage

One of the mechanisms plants have developed for chloroplast protection against oxidative damage involves a 2-Cys peroxiredoxin, which has been proposed to be reduced by ferredoxin and plastid thioredoxins, Trx x and CDSP32, the FTR/Trx pathway. We show that rice (Oryza sativa) chloroplast NADPH THIOREDOXIN REDUCTASE (NTRC), with a thioredoxin domain, uses NADPH to reduce the chloroplast 2-Cys peroxiredoxin BAS1, which then reduces hydrogen peroxide. The presence of both NTR and Trx-like domains in a single polypeptide is absolutely required for the high catalytic efficiency of NTRC. An Arabidopsis thaliana knockout mutant for NTRC shows irregular mesophyll cell shape, abnormal chloroplast structure, and unbalanced BAS1 redox state, resulting in impaired photosynthesis rate under low light. Constitutive expression of wild-type NTRC in mutant transgenic lines rescued this phenotype. Moreover, prolonged darkness followed by light/dark incubation produced an increase in hydrogen peroxide and lipid peroxidation in leaves and accelerated senescence of NTRC-deficient plants. We propose that NTRC constitutes an alternative system for chloroplast protection against oxidative damage, using NADPH as the source of reducing power. Since no light-driven reduced ferredoxin is produced at night, the NTRC-BAS1 pathway may be a key detoxification system during darkness, with NADPH produced by the oxidative pentose phosphate pathway as the source of reducing power.     

Cold acclimation of the Arabidopsis dgd1 mutant results in recovery from photosystem I-limited photosynthesis

We compared the thylakoid membrane composition and photosynthetic properties of non- and cold-acclimated leaves from the dgd1 mutant (lacking >90% of digalactosyl-diacylglycerol; DGDG) and wild type (WT) Arabidopsis thaliana. In contrast to warm grown plants, cold-acclimated dgd1 leaves recovered pigment-protein pools and photosynthetic function equivalent to WT. Surprisingly, this recovery was not correlated with an increase in DGDG. When returned to warm temperatures the severe dgd1 mutant phenotype reappeared. We conclude that the relative recovery of photosynthetic activity at 5 degrees C resulted from a temperature/lipid interaction enabling the stable assembly of PSI complexes in the thylakoid.     

The Arabidopsis aba4-1 mutant reveals a specific function for neoxanthin in protection against photooxidative stress

The aba4-1 mutant completely lacks neoxanthin but retains all other xanthophyll species. The missing neoxanthin in light-harvesting complex (Lhc) proteins is compensated for by higher levels of violaxanthin, albeit with lower capacity for photoprotection compared with proteins with wild-type levels of neoxanthin. Detached leaves of aba4-1 were more sensitive to oxidative stress than the wild type when exposed to high light and incubated in a solution of photosensitizer agents. Both treatments caused more rapid pigment bleaching and lipid oxidation in aba4-1 than wild-type plants, suggesting that neoxanthin acts as an antioxidant within the photosystem II (PSII) supercomplex in thylakoids. While neoxanthin-depleted Lhc proteins and leaves had similar sensitivity as the wild type to hydrogen peroxide and singlet oxygen, they were more sensitive to superoxide anions. aba4-1 intact plants were not more sensitive than the wild type to high-light stress, indicating the existence of compensatory mechanisms of photoprotection involving the accumulation of zeaxanthin. However, the aba4-1 npq1 double mutant, lacking zeaxanthin and neoxanthin, underwent stronger PSII photoinhibition and more extensive oxidation of pigments than the npq1 mutant, which still contains neoxanthin. We conclude that neoxanthin preserves PSII from photoinactivation and protects membrane lipids from photooxidation by reactive oxygen species. Neoxanthin appears particularly active against superoxide anions produced by the Mehler's reaction, whose rate is known to be enhanced in abiotic stress conditions.     

Necessity of OxyR for the hydrogen peroxide stress response and full virulence in Ralstonia solanacearum

The plant pathogen Ralstonia solanacearum, which causes bacterial wilt disease, is exposed to reactive oxygen species (ROS) during tomato infection and expresses diverse oxidative stress response (OSR) genes during midstage disease on tomato. The R. solanacearum genome predicts that the bacterium produces multiple and redundant ROS-scavenging enzymes but only one known oxidative stress response regulator, OxyR. An R. solanacearum oxyR mutant had no detectable catalase activity, did not grow in the presence of 250 μM hydrogen peroxide, and grew poorly in the oxidative environment of solid rich media. This phenotype was rescued by the addition of exogenous catalase, suggesting that oxyR is essential for the hydrogen peroxide stress response. Unexpectedly, the oxyR mutant strain grew better than the wild type in the presence of the superoxide generator paraquat. Gene expression studies indicated that katE, kaG, ahpC1, grxC, and oxyR itself were each differentially expressed in the oxyR mutant background and in response to hydrogen peroxide, suggesting that oxyR is necessary for hydrogen peroxide-inducible gene expression. Additional OSR genes were differentially regulated in response to hydrogen peroxide alone. The virulence of the oxyR mutant strain was significantly reduced in both tomato and tobacco host plants, demonstrating that R. solanacearum is exposed to inhibitory concentrations of ROS in planta and that OxyR-mediated responses to ROS during plant pathogenesis are important for R. solanacearum host adaptation and virulence.     

Down-regulation of metallothionein, a reactive oxygen scavenger, by the small GTPase OsRac1 in rice

Metallothioneins are small, ubiquitous Cys-rich proteins known to be involved in reactive oxygen species (ROS) scavenging and metal homeostasis. We found that the expression of a metallothionein gene (OsMT2b) was synergically down-regulated by OsRac1 and rice (Oryza sativa) blast-derived elicitors. Transgenic plants overexpressing OsMT2b showed increased susceptibility to bacterial blight and blast fungus. OsMT2b-overexpressing cells showed reduced elicitor-induced hydrogen peroxide production. In contrast, homozygous OsMT2b::Tos17-inserted mutant and OsMT2b-RNAi-silenced transgenic cells showed significantly higher elicitor-induced hydrogen peroxide production than the wild-type cells. In vitro assay showed that recombinant OsMT2b protein possessed superoxide- and hydroxyl radical-scavenging activities. Taken together, these results showed that OsMT2b is an ROS scavenger and its expression is down-regulated by OsRac1, thus potentiating ROS, which function as signals in resistance response. The results suggest that OsRac1 plays a dual role as an inducer of ROS production and a suppressor of ROS scavenging.     

Photo-oxidative stress in a xanthophyll-deficient mutant of Chlamydomonas

When there is an imbalance between the light energy absorbed by a photosynthetic organism and that which can be utilized in photosynthesis, photo-oxidative stress can damage pigments, proteins, lipids, and nucleic acids. In this work we compared the wild type and a xanthophyll-deficient mutant of Chlamydomonas reinhardtii in their response to high amounts of light. Wild-type Chlamydomonas cells were able to acclimate to high amounts of light following transfer from low light conditions. In contrast, the npq1 lor1 double mutant, which lacks protective xanthophylls (zeaxanthin and lutein) in the chloroplast, progressively lost viability and photosynthetic capacity along with destruction of thylakoid membrane protein-pigment complexes and accumulation of reactive oxygen species and membrane lipid peroxides. Loss of viability was partially rescued by lowered oxygen tension, suggesting that the high sensitivity of the mutant to light stress is caused by the production of reactive oxygen species in the chloroplast. Cell death was not prevented by the addition of an organic carbon source to the growth medium, demonstrating that the photo-oxidative damage can target other essential chloroplast processes besides photosynthesis. From the differential sensitivity of the mutant to exogenously added pro-oxidants, we infer that the reactive oxygen species produced during light stress in npq1 lor1 may be singlet oxygen and/or superoxide but not hydrogen peroxide. The bleaching phenotype of npq1 lor1 was not due to enhanced photodamage to photosystem II but rather to a less localized phenomenon of accumulation of photo-oxidation products in chloroplast membranes.     

Temperature effects on the reactive oxygen species formation and antioxidant defence in roots of two cucurbit species with contrasting root zone temperature optima

Suboptimal root zone temperature (14°C) was imposed on chilling-sensitive cucumber (Cucumis sativus L.) and chilling-tolerant figleaf gourd (Cucurbita ficifolia Bouché) plants. Exposure of roots to low temperature for up to 10 days caused a strong growth inhibition in cucumber compared with figleaf gourd. Physiological analysis showed that generation of reactive oxygen species (ROS) such as hydrogen peroxide and superoxide anion was significantly induced in cucumber plants as fast as 1 day after low root zone temperature treatment. In addition to the significant induction of antioxidant superoxide dismutase activity, low root zone temperature also increased the mitochondrial electron transport allocated to alternative pathway while decreased cytochrome pathway salicylhydroxamic acid-resistant respiration. However, these defense responses could not compensate for the ROS production, resulting in membrane lipid peroxidation and loss of root cell viability in the low root zone temperature treated cucumber roots. In contrast, 14°C root zone temperature had no significant effects on figleaf gourd plant growth, antioxidant enzymes, ROS levels and alternative respiratory pathway. Hence, difference in ROS metabolism would be associated with the remarkable difference in adaptability of cucumber and figleaf gourd plants in response to suboptimal root zone temperature condition.     

Isolation and characterization of an Arabidopsis mutant deficient in the thylakoid lipid digalactosyl diacylglycerol.

The galactolipids monogalactosyl and digalactosyl diacylglycerol occur in all higher plants and are the predominant lipid components of chloroplast membranes. They are thought to be of major importance to chloroplast morphology and physiology, although direct experimental evidence is still lacking. The enzymes responsible for final assembly of galactolipids are associated with the envelope membranes of plastids, and their biochemical analysis has been notoriously difficult. Therefore, we have chosen a genetic approach to study the biosynthesis and function of galactolipids in higher plants. We isolated a mutant of Arabidopsis that is deficient in digalactosyl diacylglycerol by directly screening a mutagenized M2 population for individuals with altered leaf lipid composition. This mutant carries a recessive nuclear mutation at a single locus designated dgd1. Backcrossed mutants show stunted growth, pale green leaf color, reduced photosynthetic capability, and altered thylakoid membrane ultrastructure.     

Green light for galactolipid trafficking

Galactolipids not only play a crucial role in photosynthesis but are also important for the adaptation of membrane-lipid composition in plants to phosphate-limiting conditions. The enzymes of galactolipid assembly have been localised to the envelope membranes of chloroplasts. Lipid trafficking is essential for galactolipid synthesis and redistribution because lipid precursors originate from two compartments, the endoplasmic reticulum (ER) and the plastid, and because galactolipids have to be transported to extraplastidial membranes during phosphate deprivation. Analysis of Arabidopsis mutants that are impaired in galactolipid synthesis (i.e. dgd1 and dgd2) or in ER-to-plastid lipid transport (i.e. tgd1) has resulted in the identification of a processive galactosyltransferase whose function is still enigmatic.     

Characterization of the Arabidopsis thermosensitive mutant atts02 reveals an important role for galactolipids in thermotolerance

Plants are constantly challenged with various abiotic stresses in their natural environment. Elevated temperatures have a detrimental impact on overall plant growth and productivity. Many plants increase their tolerance to high temperatures through an adaptation response known as acquired thermotolerance. To identify the various mechanisms that plants have evolved to cope with high temperature stress, we have isolated a series of Arabidopsis mutants that are defective in the acquisition of thermotolerance after an exposure to 38 degrees C, a treatment that induces acquired thermotolerance in wild-type plants. One of these mutants, atts02, was not only defective in acquiring thermotolerance after the treatment, but also displayed a reduced level of basal thermotolerance in a 30 degrees C growth assay. The affected gene in atts02 was identified by positional cloning and encodes digalactosyldiacylglycerol synthase 1 (DGD1) (the atts02 mutant was, at that point, renamed dgd1-2). An additional dgd1 allele, dgd1-3, was identified in two other mutant lines displaying altered acquired thermotolerance, atts100 and atts104. Expression patterns of several heat shock proteins (HSPs) in heat-treated dgd1-2 homozygous plants were similar to those from identically treated wild-type plants, suggesting that the thermosensitivity in the dgd1-2 mutant was not caused by a defect in HSP induction. Lipid analysis of wild-type and mutant plants indicated a close correlation between the ability to acquire thermotolerance and the increases in digalactosyldiacylglycerol (DGDG) level and in the ratio of DGDG to monogalactosyldiacylglycerol (MGDG). Thermosensitivity in dgd1-2 and dgd1-3 was associated with (1) a decreased DGDG level and (2) an inability to increase the ratio of DGDG to MGDG upon exposure to a 38 degrees C sublethal temperature treatment. Our results suggest that the DGDG level and/or the ratio of DGDG to MGDG may play an important role in basal as well as acquired thermotolerance in Arabidopsis.     

The Functional Significance of the Pentose Phosphate Pathway in Synaptosomes: Protection Against Peroxidative Damage by Catecholamines and Oxidants

Abstract: Catecholamines added in vitro in rat brain synaptosomes activate the decarboxylation of glucose radioactively labelled on carbon 1, suggesting an effective activation of the pentose phosphate pathway. Stimulation also occurred with phenazine methosulphate, reduced glutathione and hydrogen peroxide. The activation of the pentose phosphate pathway by 5-hydroxytryptamine, noradrenaline and dopamine is ascribed to the activation of monoamine oxidase, producing both the respective biogenic aldehyde and hydrogen peroxide. Evidence is presented that the further metabolism of the aldehyde by aldehyde reductase and the removal of hydrogen peroxide by glutathione peroxidase both release the limitation of N ADP⁺ availability for the pentose phosphate pathway by leading to the oxidation of NADPH. The relevance of the maintenance of reduced NADP⁺ on brain is discussed in relation to the metabolism of glutathione and to lipid peroxidation.     

Hydrogen peroxide inhibits non-small cell lung cancer cell anoikis through the inhibition of caveolin-1 degradation

Anoikis or detachment-induced apoptosis plays an essential role in the regulation of cancer cell metastasis. Caveolin-1 (Cav-1) is a key protein involved in tumor metastasis, but its role in anoikis and its regulation during cell detachment are unclear. We report here that Cav-1 plays a key role as a negative regulator of anoikis through a reactive oxygen species (ROS)-dependent mechanism in human lung carcinoma H460 cells. During cell detachment, Cav-1 is downregulated, whereas ROS generation is upregulated. Hydrogen peroxide and hydroxyl radical are two key ROS produced by cells during detachment. Treatment of the cells with hydrogen peroxide scavengers, catalase and N-acetylcysteine, promoted Cav-1 downregulation and anoikis during cell detachment, indicating that produced hydrogen peroxide plays a primary role in preventing anoikis by stabilizing Cav-1 protein. Catalase and N-acetylcysteine promoted ubiquitination and proteasomal degradation of Cav-1, which is a major pathway of its downregulation during cell anoikis. Furthermore, addition of hydrogen peroxide exogenously to the cells inhibited Cav-1 downregulation by preventing the formation of Cav-1-ubiquitin complex, supporting the inhibitory role of endogenous hydrogen peroxide in Cav-1 degradation during cell detachment. Together, these results indicate a novel role of hydrogen peroxide as an endogenous suppressor of cell anoikis through its stabilizing effect on Cav-1.     

Galactoglycerolipid metabolism under stress: a time for remodeling

Galactoglycerolipids are the predominant lipid building blocks of chloroplast membranes and are essential for plant growth. Plant chloroplasts harbor a constitutive set of UDP-Gal-dependent lipid galactosyltransferases that are responsible for the bulk of galactoglycerolipid biosynthesis. A set of paralogs is induced in response to phosphate deprivation, which leads to the remodeling of extraplastidic membranes with a partial replacement of phosphoglycerolipid by digalactosyldiacylglycerol. A third type of galactoglycerolipid biosynthetic enzyme, a UDP-Gal-independent galactoglycerolipid galactosyltransferase, was recently shown to be involved in freezing tolerance. Here, we look at how understanding of the regulation of galactoglycerolipid biosynthesis in chloroplasts by these multiple enzyme sets is rapidly evolving and discuss the increasingly recognized role of lipid remodeling in response to diverse abiotic stresses.