Phosphatidylcholine levels of peanut-nodulatingBradyrhizobiumsp. SEMIA 6144 affect cell size and motility

Three separate pathways in Rhizobium leguminosarum maintain phosphatidylcholine biosynthesis, which is required for symbiotic nitrogen fixation with clover

Phosphatidylcholine (PC) is critical for the nitrogen-fixing symbiosis between rhizobia and legumes. We characterized three PC biosynthesis pathways in Rhizobium leguminosarum and evaluated their impact on nitrogen fixation in clover nodules. In the presence of choline, a PC synthase catalyzes the condensation of cytidine diphosphate-diacylglycerol with choline to produce PC. In the presence of lyso-PC, acyltransferases acylate this mono-acylated phospholipid to PC. The third pathway relies on phospholipid N-methyltransferases (Pmts), which sequentially methylate phosphatidylethanolamine (PE) through three rounds of methylation, yielding PC via the intermediates monomethyl-PE and dimethyl-PE. In R. leguminosarum, at least three Pmts participate in this methylation cascade. To elucidate the functions of these enzymes, we recombinantly produced and biochemically characterized them. We moved on to determine the phospholipid profiles of R. leguminosarum mutant strains harboring single and combinatorial deletions of PC biosynthesis genes. The cumulative results show that PC production occurs through the combined action of multiple enzymes, each with distinct substrate and product specificities. The methylation pathway emerges as the dominant PC biosynthesis route, and we pinpoint PmtS2, which catalyzes all three methylation steps, as the enzyme responsible for providing adequate PC amounts for a functional nitrogen-fixing symbiosis with clover.

IMPORTANCE

Understanding the molecular mechanisms of symbiotic nitrogen fixation has important implications for sustainable agriculture. The presence of the phospholipid phosphatidylcholine (PC) in the membrane of rhizobia is critical for the establishment of productive nitrogen-fixing root nodules on legume plants. The reasons for the PC requirement are unknown. Here, we employed Rhizobium leguminosarum and clover as model system for a beneficial plant-microbe interaction. We found that R. leguminosarum produces PC by three distinct pathways. The relative contribution of these pathways to PC formation was determined in an array of single, double, and triple mutant strains. Several of the PC biosynthesis enzymes were purified and biochemically characterized. Most importantly, we demonstrated the essential role of PC formation by R. leguminosarum in nitrogen fixation and pinpointed a specific enzyme indispensable for plant-microbe interaction. Our study offers profound insights into bacterial PC biosynthesis and its pivotal role in biological nitrogen fixation.

Recombinant and endogenous ways to produce methylated phospholipids in Escherichia coli

Escherichia coli is the daily workhorse in molecular biology research labs and an important platform microorganism in white biotechnology. Its cytoplasmic membrane is primarily composed of the phospholipids phosphatidylethanolamine (PE), phosphatidylglycerol (PG), and cardiolipin (CL). As in most other bacteria, the typical eukaryotic phosphatidylcholine (PC) is not a regular component of the E. coli membrane. PC is known to act as a substrate in various metabolic or catabolic reactions, to affect protein folding and membrane insertion, and to activate proteins that originate from eukaryotic environments. Options to manipulate the E. coli membrane to include non-native lipids such as PC might make it an even more powerful and versatile tool for biotechnology and protein biochemistry. This article outlines different strategies how E. coli can be engineered to produce PC and other methylated PE derivatives. Several of these approaches rely on the ectopic expression of genes from natural PC-producing organisms. These include PC synthases, lysolipid acyltransferases, and several phospholipid N-methyltransferases with diverse substrate and product preferences. In addition, we show that E. coli has the capacity to produce PC by its own enzyme repertoire provided that appropriate precursors are supplied. Screening of the E. coli Keio knockout collection revealed the lysophospholipid transporter LplT to be responsible for the uptake of lyso-PC, which is then further acylated to PC by the acyltransferase-acyl carrier protein synthetase Aas. Overall, our study shows that the membrane composition of the most routinely used model bacterium can readily be tailored on demand.Key points• Escherichia coli can be engineered to produce non-native methylated PE derivatives.• These lipids can be produced by foreign and endogenous proteins.• Modification of E. coli membrane offers potential for biotechnology and research.

ExoS/ChvI Two-Component Signal-Transduction System Activated in the Absence of Bacterial Phosphatidylcholine

Sinorhizobium meliloti contains the negatively charged phosphatidylglycerol and cardiolipin as well as the zwitterionic phosphatidylethanolamine (PE) and phosphatidylcholine (PC) as major membrane phospholipids. In previous studies we had isolated S. meliloti mutants that lack PE or PC. Although mutants deficient in PE are able to form nitrogen-fixing nodules on alfalfa host plants, mutants lacking PC cannot sustain development of any nodules on host roots. Transcript profiles of mutants unable to form PE or PC are distinct; they differ from each other and they are different from the wild type profile. For example, a PC-deficient mutant of S. meliloti shows an increase of transcripts that encode enzymes required for succinoglycan biosynthesis and a decrease of transcripts required for flagellum formation. Indeed, a PC-deficient mutant is unable to swim and overproduces succinoglycan. Some suppressor mutants, that regain swimming and form normal levels of succinoglycan, are altered in the ExoS sensor. Our findings suggest that the lack of PC in the sinorhizobial membrane activates the ExoS/ChvI two-component regulatory system. ExoS/ChvI constitute a molecular switch in S. meliloti for changing from a free-living to a symbiotic life style. The periplasmic repressor protein ExoR controls ExoS/ChvI function and it is thought that proteolytic ExoR degradation would relieve repression of ExoS/ChvI thereby switching on this system. However, as ExoR levels are similar in wild type, PC-deficient mutant and suppressor mutants, we propose that lack of PC in the bacterial membrane provokes directly a conformational change of the ExoS sensor and thereby activation of the ExoS/ChvI two-component system.

Phospholipid N-methyltransferases produce various methylated phosphatidylethanolamine derivatives in thermophilic bacteria

One of the most common pathways for the biosynthesis of the phospholipid phosphatidylcholine (PC) in bacteria is the successive three-fold N-methylation of phosphatidylethanolamine (PE) catalyzed by phospholipid N-methyltransferases (Pmts). Pmts with different activities have been described in a number of mesophilic bacteria. In the present study, we identified and characterized the substrate and product spectrum of four Pmts from thermophilic bacteria. Three of these enzymes were purified in an active form. The Pmts from Melghirimyces thermohalophilus, Thermochromogena staphylospora and Thermobifida fusca produce monomethyl-PE (MMPE) and dimethyl-PE (DMPE). T. fusca encodes two Pmt candidates, one is mutationally inactivated and the other is responsible for the accumulation of large amounts of MMPE. The Pmt enzyme from Rubellimicrobium thermophilum catalyzes all three methylation reactions to synthesize PC. Moreover, we show that PE, previously reported to be absent in R. thermophilum, is in fact produced and serves as precursor for the methylation pathway. In an alternative route, the strain is able to produce PC by the PC synthase pathway when choline is available. The activity of all purified thermophilic Pmt enzymes was stimulated by anionic lipids suggesting membrane recruitment of these cytoplasmic proteins via electrostatic interactions. Our study provides novel insights into the functional characteristics of phospholipid N-methyltransferases in a previously unexplored set of thermophilic environmental bacteria. Importance In recent years, the presence of phosphatidylcholine (PC) in bacterial membranes has gained increasing attention, partly due to its critical role in the interaction with eukaryotic hosts. PC biosynthesis via a three-step methylation of phosphatidylethanolamine, catalyzed by phospholipid N-methyltransferases (Pmts), has been described in a range of mesophilic bacteria. Here, we expand our knowledge on bacterial PC formation by the identification, purification and characterization of Pmts from phylogenetically diverse thermophilic bacteria, and thereby provide insights into the functional characteristics of Pmt enzymes in thermophilic actinomycetes and proteobacteria.

Immobilization of Bradyrhizobium and Azospirillum in alginate matrix for long time of storage maintains cell viability and interaction with peanut

Immobilizarion of PGPR for agricultural applications aims to provide temporary physical protection from stressful environmental conditions and the gradual release of cells for successful root colonization, release the cells gradually. In this work, we immobilized Bradyrhizobium sp. SEMIA6144 or Azospirillum brasilense Az39 cells in 2% alginate beads prepared by ionic gelation process, and then stored up to 12 months at 4 °C. Alginate matrix showed interaction with the immobilized bacteria (FTIR), allowed a constant release of cells, and improved their viability and capability to interact with Arachis hypogaea. Cell number into beads reached 10⁷ CFU.bead^-1; however, viability decreased from 4 months of storage for Az39, while it was maintained up to 12 months for SEMIA6144, showing a low metabolic activity measured by the MTT assay. Adhesion of SEMIA6144 and Az39 from new beads to peanut root was 11.5% and 16%, respectively, higher than non-immobilized bacteria. Peanut inoculation with 12 months storage SEMIA6144 beads significantly increased root length and biomass at 30 days of growth, and under restrictive water condition (RWC), nodulation and total plant N content increased compared with liquid inoculation. Our results demonstrate that immobilization of SEMIA6144 and Az39 in alginate matrix is a potential alternative to enhance peanut growth even under RWC. KEY POINTS: • Alginate encapsulation enhances viability of SEMIA6144 or Az39 under storage at 4 °C for 1 year. • Alginate beads 2% ensure the gradual release of the microorganisms. • Cells from beads stored for long periods present chemotaxis and adhesion to peanut root. • Peanut inoculation with 1-year-old SEMIA6144 beads improves nodulation and growth in RWC.

Restrictive water condition modifies the root exudates composition during peanut-PGPR interaction and conditions early events, reversing the negative effects on plant growth

Water deficit is one of the most serious environmental factors that affect the productivity of crops in the world. Arachis hypogaea is a legume with a high nutritional value and 70% is cultivated in semi-arid regions. This research aimed to study the effect of water deficit on peanut root exudates composition, analyzing the importance of exudates on peanut-PGPR interaction under restrictive water condition. Peanut seedlings were subjected to six treatments: 0 and 15 mM PEG, in combination with non-inoculated, Bradyrhizobium sp. and Bradyrhizobium-Azospirillum brasilense inoculated treatments. We analyzed the 7-day peanut root exudate in response to a water restrictive condition and the presence of bacterial inocula. Molecular analysis was performed by HPLC, UPLC and GC. Bacteria motility, chemotaxis, bacterial adhesion to peanut roots and peanut growth parameters were analyzed. Restrictive water condition modified the pattern of molecules exuded by roots, increasing the exudation of Naringenin, oleic FA, citric and lactic acid, and stimulation the release of terpenes of known antioxidant and antimicrobial activity. The presence of microorganisms modified the composition of root exudates. Water deficit affected the first events of peanut-PGPR interaction and the root exudates favored bacterial mobility, the chemotaxis and attachment of bacteria to peanut roots. Changes in the profile of molecules exuded by roots allowed A. hypogaea-Bradyrhizobium and A.hypogaea-Bradyrhizobium-Azospirillum interaction thus reversing the negative effects of restrictive water condition on peanut growth. These findings have a future potential application to improve plant-PGPR interactions under water deficit by formulating inoculants containing key molecules exuded during stress.

Reorganization of Azospirillum brasilense cell membrane is mediated by lipid composition adjustment to maintain optimal fluidity during water deficit

AIMS

We study the Azospirillum brasilense tolerance to water deficit and the dynamics of adaptive process at the level of the membrane.

METHODS AND RESULTS

Azospirillum brasilense was exposed to polyethylene glycol (PEG) growth and PEG shock. Tolerance, phospholipids and fatty acid (FA) composition and membrane fluidity were determined. Azospirillum brasilense was able to grow in the presence of PEG; however, its viability was reduced. Cells grown with PEG showed membrane fluidity similar to those grown without, the lipid composition was modified, increasing phosphatidylcholine and decreasing phosphatidylethanolamine amounts. The unsaturation FAs degree was reduced. The dynamics of the adaptive response revealed a decrease in fluidity 20 min after the addition of PEG, indicating that the PEG has a fluidizing effect on the hydrophobic region of the cell membrane. Fluidity returned to initial values after 60 min of PEG exposure.

CONCLUSION

Azospirillum brasilense is able to perceive osmotic changes by changing the membrane fluidity. This effect is offset by changes in the composition of membrane phospholipid and FA, contributing to the homeostasis of membrane fluidity under water deficit.

SIGNIFICANCE AND IMPACT OF THE STUDY

This knowledge can be used to develop new Azospirillum brasilense formulations showing an adapted membrane to water deficit.

Lipids in plant-microbe interactions

Bacteria and fungi can undergo symbiotic or pathogenic interactions with plants. Membrane lipids and lipid-derived molecules from the plant or the microbial organism play important roles during the infection process. For example, lipids (phospholipids, glycolipids, sphingolipids, sterol lipids) are involved in establishing the membrane interface between the two organisms. Furthermore, lipid-derived molecules are crucial for intracellular signaling in the plant cell, and lipids serve as signals during plant-microbial communication. These signal lipids include phosphatidic acid, diacylglycerol, lysophospholipids, and free fatty acids derived from phospholipase activity, apocarotenoids, and sphingolipid breakdown products such as ceramide, ceramide-phosphate, long chain base, and long chain base-phosphate. Fatty acids are the precursors for oxylipins, including jasmonic acid, and for azelaic acid, which together with glycerol-3-phosphate are crucial for the regulation of systemic acquired resistance. This article is part of a Special Issue titled "Plant Lipid Biology," guest editors Kent Chapman and Ivo Feussner.

Phosphatidylcholine biosynthesis and function in bacteria

Phosphatidylcholine (PC) is the major membrane-forming phospholipid in eukaryotes and is estimated to be present in about 15% of the domain Bacteria. Usually, PC can be synthesized in bacteria by either of two pathways, the phospholipid N-methylation (Pmt) pathway or the phosphatidylcholine synthase (Pcs) pathway. The three subsequent enzymatic methylations of phosphatidylethanolamine are performed by a single phospholipid N-methyltransferase in some bacteria whereas other bacteria possess multiple phospholipid N-methyltransferases each one performing one or several distinct methylation steps. Phosphatidylcholine synthase condenses choline directly with CDP-diacylglycerol to form CMP and PC. Like in eukaryotes, bacterial PC also functions as a biosynthetic intermediate during the formation of other biomolecules such as choline, diacylglycerol, or diacylglycerol-based phosphorus-free membrane lipids. Bacterial PC may serve as a specific recognition molecule but it affects the physicochemical properties of bacterial membranes as well. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Phosphatidylcholine biosynthesis and its significance in bacteria interacting with eukaryotic cells

Phosphatidylcholine (PC), a typical eukaryotic membrane phospholipid, is present in only about 10% of all bacterial species, in particular in bacteria interacting with eukaryotes. A number of studies revealed that PC plays a fundamental role in symbiotic and pathogenic microbe-host interactions. Agrobacterium tumefaciens mutants lacking PC are unable to elicit plant tumors. The human pathogens Brucella abortus and Legionella pneumophila require PC for full virulence. The plant symbionts Bradyrhizobium japonicum and Sinorhizobium meliloti depend on wild-type levels of PC to establish an efficient root nodule symbiosis. Two pathways for PC biosynthesis are known in bacteria, the methylation pathway and the phosphatidylcholine synthase (Pcs) pathway. The methylation pathway involves a three-step methylation of phosphatidylethanolamine by at least one phospholipid N-methyltransferase to yield phosphatidylcholine. In the Pcs pathway, choline is condensed directly with CDP-diacylglycerol to form PC. This review focuses on the biosynthetic pathways and the significance of PC in bacteria with an emphasis on plant-microbe interactions.