Arbuscular mycorrhiza formation with fungi of the Glomeromycota represents a widespread symbiotic interaction of vascular plants. Different signaling events and metabolic adaptations are required for ...the close interaction between the two partners. Membrane lipid synthesis is a prerequisite for symbiosis, and membrane properties depend on lipid composition. Lipid profiling was performed by liquid chromatography mass spectrometry to study the role of triacylglycerol, diacylglycerol, phospholipids, galactolipids, sterols and sphingolipids during the colonization of Lotus japonicus roots with Rhizophagus irregularis (syn. Glomus intraradices). Mycorrhization leads to an increased phosphate supply and suppresses the increase in galactolipids commonly observed in phosphate‐deprived plants. In addition to free sterols and sterol esters, R. irregularis contains sterol glucosides and acylated sterol glucosides. Glycosylated sphingolipids (glucosylceramide, dihexosylceramide) and inositolphosphorylceramide were detected in the fungus. Lyso‐phosphatidylcholine, a lipid previously implicated in mycorrhiza signaling, is present in low amounts in mock‐infected and mycorrhized roots. The composition of fungal phospholipids changes after mycorrhization because molecular species with palmitvaccenic (di‐16:1) or tetracosenoic (24:1) acyl groups decrease in intraradical mycelium. This adaptation of lipid metabolism during intraradical growth is likely a prerequisite for symbiosis, achieving functional compatibility between the fungal and the periarbuscular membrane. Data mining in genomic and transcript databases revealed the presence of genes encoding enzymes of lipid biosynthesis in R. irregularis. However, no gene encoding multidomain fatty acid de novo synthase was detected in the genome sequence of this obligate biotrophic fungus.
During arbuscular mycorrhizal symbiosis (AMS), considerable amounts of lipids are generated, modified and moved within the cell to accommodate the fungus in the root, and it has also been suggested ...that lipids are delivered to the fungus. To determine the mechanisms by which root cells redirect lipid biosynthesis during AMS we analyzed the roles of two lipid biosynthetic enzymes (FatM and RAM2) and an ABC transporter (STR) that are required for symbiosis and conserved uniquely in plants that engage in AMS.
Complementation analyses indicated that the biochemical function of FatM overlaps with that of other Fat thioesterases, in particular FatB. The essential role of FatM in AMS was a consequence of timing and magnitude of its expression.
Lipid profiles of fatm and ram2 suggested that FatM increases the outflow of 16:0 fatty acids from the plastid, for subsequent use by RAM2 to produce 16:0 β-monoacylglycerol.
Thus, during AMS, high-level, specific expression of key lipid biosynthetic enzymes located in the plastid and the endoplasmic reticulum enables the root cell to fine-tune lipid biosynthesis to increase the production of β-monoacylglycerols. We propose a model in which β-monoacylglycerols, or a derivative thereof, are exported out of the root cell across the periarbuscular membrane for ultimate use by the fungus.
Vitamin E is considered a major antioxidant in biomembranes, but little evidence exists for this function in plants under photooxidative stress. Leaf discs of two vitamin E mutants, a tocopherol ...cyclase mutant (vte1) and a homogentisate phytyl transferase mutant (vte2), were exposed to high light stress at low temperature, which resulted in bleaching and lipid photodestruction. However, this was not observed in whole plants exposed to long-term high light stress, unless the stress conditions were extreme (very low temperature and very high light), suggesting compensatory mechanisms for vitamin E deficiency under physiological conditions. We identified two such mechanisms: nonphotochemical energy dissipation (NPQ) in photosystem II (PSII) and synthesis of zeaxanthin. Inhibition of NPQ in the double mutant vte1 npq4 led to a marked photoinhibition of PSII, suggesting protection of PSII by tocopherols. vte1 plants accumulated more zeaxanthin in high light than the wild type, and inhibiting zeaxanthin synthesis in the vte1 npq1 double mutant resulted in PSII photoinhibition accompanied by extensive oxidation of lipids and pigments. The single mutants npq1, npq4, vte2, and vte1 showed little sensitivity to the stress treatments. We conclude that, in cooperation with the xanthophyll cycle, vitamin E fulfills at least two different functions in chloroplasts at the two major sites of singlet oxygen production: preserving PSII from photoinactivation and protecting membrane lipids from photooxidation.
Cardiolipin (CL), an anionic phospholipid of the inner mitochondrial membrane, provides essential functions for stabilizing respiratory complexes and is involved in mitochondrial morphogenesis and ...programmed cell death in animals. The role of CL and its metabolism in plants are less well understood. The measurement of CL in plants, including its molecular species composition, is hampered by the fact that CL is of extremely low abundance, and that plants contain large amounts of interfering compounds including galactolipids, neutral lipids, and pigments. We used solid phase extraction by anion exchange chromatography to purify CL from crude plant lipid extracts. LC/MS was used to determine the content and molecular species composition of CL. Thus, up to 23 different molecular species of CL were detected in different plant species, including Arabidopsis, mung bean, spinach, barley, and tobacco. Similar to animals, plant CL is dominated by highly unsaturated species, mostly containing linoleic and linolenic acid. During phosphate deprivation or exposure to an extended dark period, the amount of CL decreased in Arabidopsis, accompanied with an increased degree in unsaturation. The mechanism of CL remodeling during stress, and the function of highly unsaturated CL molecular species, remains to be defined.
Arbuscular mycorrhiza (AM) fungi deliver mineral nutrients to the plant host in exchange for reduced carbon in the form of sugars and lipids. Colonization with AM fungi upregulates a specific host ...lipid synthesis pathway resulting in the production of fatty acids. Predominantly palmitic acid (16:0) and the unusual palmitvaccenic acid (16:1Δ11cis) accumulate in the fungus Rhizophagus irregularis. Here, we present the isolation and characterization of RiOLE1-LIKE, the desaturase involved in palmitvaccenic acid synthesis, by heterologous expression in yeast and plants. Results are in line with the scenario in which RiOLE1-LIKE encodes an acyl-CoA desaturase with substrate specificity for C15–C18 acyl groups, in particular C16. Phylogenetic analysis of RiOLE1-LIKE-related sequences revealed that this gene is conserved in AM fungi from the Glomales and Diversisporales but is absent from nonsymbiotic Mortierellaceae and Mucoromycotina fungi, suggesting that 16:1Δ11cis provides a specific function during AM colonization.
Summary
Dehydration leads to different physiological and biochemical responses in plants. We analysed the lipid composition and the expression of genes involved in lipid biosynthesis in the ...desiccation‐tolerant plant Craterostigma plantagineum. A comparative approach was carried out with Lindernia brevidens (desiccation tolerant) and two desiccation‐sensitive species, Lindernia subracemosa and Arabidopsis thaliana. In C. plantagineum the total lipid content remained constant while the lipid composition underwent major changes during desiccation. The most prominent change was the removal of monogalactosyldiacylglycerol (MGDG) from the thylakoids. Analysis of molecular species composition revealed that around 50% of 36:x (number of carbons in the acyl chains: number of double bonds) MGDG was hydrolysed and diacylglycerol (DAG) used for phospholipid synthesis, while another MGDG fraction was converted into digalactosyldiacylglycerol via the DGD1/DGD2 pathway and subsequently into oligogalactolipids by SFR2. 36:x‐DAG was also employed for the synthesis of triacylglycerol. Phosphatidic acid (PA) increased in C. plantagineum, L. brevidens, and L. subracemosa, in agreement with a role of PA as an intermediate of lipid turnover and of phospholipase D in signalling during desiccation. 34:x‐DAG, presumably derived from de novo assembly, was converted into phosphatidylinositol (PI) in C. plantagineum and L. brevidens, but not in desiccation‐sensitive plants, suggesting that PI is involved in acquisition of desiccation tolerance. The accumulation of oligogalactolipids and PI in the chloroplast and extraplastidial membranes, respectively, increases the concentration of hydroxyl groups and enhances the ratio of bilayer‐ to non‐bilayer‐forming lipids, thus contributing to protein and membrane stabilization.
Plasmodesmata (PD) are nano-sized membrane-lined channels controlling intercellular communication in plants. Although progress has beenmade in identifying PD proteins, the role played by ...majormembrane constituents, such as the lipids, in defining specialized membrane domains in PD remains unknown. Through a rigorous isolation of “native” PD membrane fractions and comparative mass spectrometry-based analysis, we demonstrate that lipids are laterally segregated along the plasma membrane (PM) at the PD cell-to-cell junction in Arabidopsis thaliana. Remarkably, our results show that PD membranes display enrichment in sterols and sphingolipids with very long chain saturated fatty acids when compared with the bulk of the PM. Intriguingly, this lipid profile is reminiscent of detergent-insoluble membrane microdomains, although our approach is valuably detergent-free. Modulation of the overall sterol composition of young dividing cells reversibly impaired the PD localization of the glycosylphosphatidylinositolanchored proteins Plasmodesmata Callose Binding 1 and the β-1,3-glucanase PdBG2 and altered callose-mediated PD permeability. Altogether, this study not only provides a comprehensive analysis of the lipid constituents of PD but also identifies a role for sterols in modulating cell-to-cell connectivity, possibly by establishing and maintaining the positional specificity of callosemodifying glycosylphosphatidylinositol proteins at PD. Our work emphasizes the importance of lipids in defining PD membranes.
In a changing environment, plants need to cope with the impact of rising temperatures together with high light intensity. Here, we used lipidomics in the tomato model system to identify lipophilic ...molecules that enhance tolerance to combined high-temperature and high-light stress. Among several hundred metabolites, the two most strongly up-regulated compounds were α-tocopherol and plastoquinone/plastoquinol. Both are well-known lipid antioxidants and contribute to the protection of photosystem II (PSII) against photodamage under environmental stress. To address the protective function of tocopherol, an RNAi line (vte5) with decreased expression of VTE5 and reduced levels of α-tocopherol was selected. VTE5 encodes phytol kinase, which acts in the biosynthetic pathway of tocopherols. vte5 suffered strong photoinhibition and photobleaching when exposed to combined high-light and high-temperature stress, but neither stress alone produced a visible phenotype. As vte5 had plastoquinone levels similar to those of the wild type under combined stress, the strong phenotype could be attributed to the lack of α-tocopherol. These findings suggest that VTE5 protects against combined high-light and high-temperature stress and does so by supporting α-tocopherol production.
Efficient acclimation to different growth light intensities is essential for plant fitness. So far, most studies on light acclimation have been conducted with plants grown under different constant ...light regimes, but more recent work indicated that acclimation to fluctuating light or field conditions may result in different physiological properties of plants. Thale cress (
) was grown under three different constant light intensities (LL: 25 μmol photons m
s
; NL: 100 μmol photons m
s
; HL: 500 μmol photons m
s
) and under natural fluctuating light (NatL) conditions. We performed a thorough characterization of the morphological, physiological, and biochemical properties focusing on photo-protective mechanisms. Our analyses corroborated the known properties of LL, NL, and HL plants. NatL plants, however, were found to combine characteristics of both LL and HL grown plants, leading to efficient and unique light utilization capacities. Strikingly, the high energy dissipation capacity of NatL plants correlated with increased dynamics of thylakoid membrane reorganization upon short-term acclimation to excess light. We conclude that the thylakoid membrane organization and particularly the light-dependent and reversible unstacking of grana membranes likely represent key factors that provide the basis for the high acclimation capacity of NatL grown plants to rapidly changing light intensities.
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