Changes in physical properties of vacuolar membrane during transformation of protein bodies into vacuoles in germinating pumpkin seeds

Brassinosteroid-lipid membrane interaction under low and high temperature stress in model systems

BACKGROUND

In earlier studies [1], we indicated that applying brassinosteroids (BRs) to lipids that had been isolated from plants altered the physicochemical properties of the monolayers. A continuation of these dependencies using the defined model lipid systems is presented in this paper. The influence of homocastasterone (HCS) and castasterone (CS) (BRs for which the increase in concentration were characteristic of plants grown at low temperatures) on the membrane properties of their polar and the hydrophobic parts were studied.

RESULTS

Changes in the electrokinetic potential indicate that both BRs decreased the negative charge of the surface, which is an important factor in modifying the contacts with the polar substances. This property of BRs has not yet been described. The studies of the interactions that occur in the hydrophobic part of the membrane were investigated using the EPR methods and Langmuir techniques. The physicochemical parameters of the lipid structure were determined, and the excess of Gibbs free energy was calculated.

CONCLUSION

We conclude that examined BRs modify both the hydrophilic and hydrophobic properties of the membranes, but to a greater extent HCS. The consequence of these changes may be the attempt to maintain the stability of the membranes in stressful temperature conditions and / or to the possibility of adsorption of other substances on membranes surfaces. The change of plant metabolism towards increasing the amount of BR, mainly HCS (under cooling) may by an important factor for maintaining optimal structural properties of membranes and their functionality despite temperature changes.

EARLY RESPONSE TO DEHYDRATION 7 Remodels Cell Membrane Lipid Composition during Cold Stress in Arabidopsis

Plants adjust to unfavorable conditions by altering physiological activities, such as gene expression. Although previous studies have identified multiple stress-induced genes, the function of many genes during the stress responses remains unclear. Expression of ERD7 (EARLY RESPONSE TO DEHYDRATION 7) is induced in response to dehydration. Here, we show that ERD7 plays essential roles in both plant stress responses and development. In Arabidopsis, ERD7 protein accumulated under various stress conditions, including exposure to low temperature. A triple mutant of Arabidopsis lacking ERD7 and two closely related homologs had an embryonic lethal phenotype, whereas a mutant lacking the two homologs and one ERD7 allele had relatively round leaves, indicating that the ERD7 gene family has essential roles in development. Moreover, the importance of the ERD7 family in stress responses was evidenced by the susceptibility of the mutant lines to cold stress. ERD7 protein was found to bind to several, but not all, negatively charged phospholipids and was associated with membranes. Lipid components and cold-induced reduction in PIP2 in the mutant line were altered relative to wild type. Furthermore, membranes from the mutant line had reduced fluidity. Taken together, ERD7 and its homologs are important for plant stress responses and development and associated with the modification in membrane lipid composition.

Molecular Dynamics of Chloroplast Membranes Isolated from Wild-Type Barley and a Brassinosteroid-Deficient Mutant Acclimated to Low and High Temperatures

Plants have developed various acclimation strategies in order to counteract the negative effects of abiotic stresses (including temperature stress), and biological membranes are important elements in these strategies. Brassinosteroids (BR) are plant steroid hormones that regulate plant growth and development and modulate their reaction against many environmental stresses including temperature stress, but their role in modifying the properties of the biological membrane is poorly known. In this paper, we characterise the molecular dynamics of chloroplast membranes that had been isolated from wild-type and a BR-deficient barley mutant that had been acclimated to low and high temperatures in order to enrich the knowledge about the role of BR as regulators of the dynamics of the photosynthetic membranes. The molecular dynamics of the membranes was investigated using electron paramagnetic resonance (EPR) spectroscopy in both a hydrophilic and hydrophobic area of the membranes. The content of BR was determined, and other important membrane components that affect their molecular dynamics such as chlorophylls, carotenoids and fatty acids in these membranes were also determined. The chloroplast membranes of the BR-mutant had a higher degree of rigidification than the membranes of the wild type. In the hydrophilic area, the most visible differences were observed in plants that had been grown at 20 °C, whereas in the hydrophobic core, they were visible at both 20 and 5 °C. There were no differences in the molecular dynamics of the studied membranes in the chloroplast membranes that had been isolated from plants that had been grown at 27 °C. The role of BR in regulating the molecular dynamics of the photosynthetic membranes will be discussed against the background of an analysis of the photosynthetic pigments and fatty acid composition in the chloroplasts.

Histology, histochemistry and ultrastructure of pre-embryogenic cells determined for direct somatic embryogenesis in the palm tree Syagrus oleracea

Somatic embryogenesis in palm trees is, in general, a slow and highly complex process, with a predominance of the indirect route and, consequently, a lack of knowledge about the direct route. We present new knowledge related to the morphological, histochemical and ultrastructural aspects of the transition from somatic to embryogenic cells and direct formation of somatic embryos from mature zygotic embryos of Syagrus oleracea, a palm tree. The results support the general concept that 2,4-dichlorophenoxyacetic acid plays a critical role for the formation of somatic embryos of direct and multicellular origin. Seven days in medium with auxin were enough for the identification of embryogenic cells. These cells had a set of characteristics corresponding to totipotent stem cells. At 14 days on induction medium, nodular formations were observed in the distal region of inoculated embryos, which evolved into globular somatic embryos. At 120 days on induction medium, the quality of the somatic embryos was compromised. The dynamics of the mobilization of reserve compounds was also demonstrated, with emphasis on starch and protein as energy sources required for the embryogenic process. This study shows for the first time the anatomical and ultrastructural events involved in direct somatic embryogenesis in a palm tree and incites the scientific community to return to the discussion of classical concepts related to direct somatic embryogenesis, especially regarding the characteristics and location of determined pre-embryogenic cells.

Phosphorylation of TIP3 Aquaporins during Phaseolus vulgaris Embryo Development

The membrane phosphoproteome in plant seed changes dynamically during embryo development. We examined the patterns of Phaseolus vulgaris (common bean) seed membrane protein phosphorylation from the mid-maturation stage until two days after germination. Serine and threonine phosphorylation declined during seed maturation while tyrosine phosphorylation remained relatively constant. We discovered that the aquaporin PvTIP3;1 is the primary seed membrane phosphoprotein, and PvTIP3;2 shows a very low level of expression. The level of phosphorylated Ser7 in PvTIP3;1 increased four-fold after seed maturation. Since phosphorylation increases water channel activity, we infer that water transport by PvTIP3;1 is highest in dry and germinating seeds, which would be optimal for seed imbibition. By the use of isoform-specific, polyclonal peptide antibodies, we found that PvTIP3;2 is expressed in a developmental pattern similar to PvTIP3;1. Unexpectedly, PvTIP3;2 is tyrosine phosphorylated following seed maturation, which may suggest a mechanism for the regulation of PvTIP3;2 following seed germination. Analysis of protein secondary structure by circular dichroism spectroscopy indicated that the amino-terminal domain of PvTIP3;1 is generally unstructured, and phosphorylation increases polyproline II (PPII) helical structure. The carboxy-terminal domain also gains PPII character, but in a pH-dependent manner. These structural changes are a first step to understand TIP3 aquaporin regulation.

Diadinoxanthin de-epoxidation as important factor in the short-term stabilization of diatom photosynthetic membranes exposed to different temperatures

The importance of diadinoxanthin (Ddx) de-epoxidation in the short-term modulation of the temperature effect on photosynthetic membranes of the diatom Phaeodactylum tricornutum was demonstrated by electron paramagnetic resonance (EPR), Laurdan fluorescence spectroscopy, and high-performance liquid chromatography. The 5-SASL spin probe employed for the EPR measurements and Laurdan provided information about the membrane area close to the polar head groups of the membrane lipids, whereas with the 16-SASL spin probe, the hydrophobic core, where the fatty acid residues are located, was probed. The obtained results indicate that Ddx de-epoxidation induces a two component mechanism in the short-term regulation of the membrane fluidity of diatom thylakoids during changing temperatures. One component has been termed the "dynamic effect" and the second the "stable effect" of Ddx de-epoxidation. The "dynamic effect" includes changes of the membrane during the time course of de-epoxidation whereas the "stable effect" is based on the rigidifying properties of Dtx. The combination of both effects results in a temporary increase of the rigidity of both peripheral and internal parts of the membrane whereas the persistent increase of the rigidity of the hydrophobic core of the membrane is solely based on the "stable effect."

Olive seed protein bodies store degrading enzymes involved in mobilization of oil bodies

The major seed storage reserves in oilseeds are accumulated in protein bodies and oil bodies, and serve as an energy, carbon, and nitrogen source during germination. Here, the spatio-temporal relationships between protein bodies and several key enzymes (phospholipase A, lipase, and lipoxygenase) involved in storage lipid mobilization in cotyledon cells was analysed during in vitro seed germination. Enzyme activities were assayed in-gel and their cellular localization were determined using microscopy techniques. At seed maturity, phospholipase A and triacylglycerol lipase activities were found exclusively in protein bodies. However, after seed imbibition, these activities were shifted to the cytoplasm and the surface of the oil bodies. The activity of neutral lipases was detected by using α-naphthyl palmitate and it was associated mainly with protein bodies during the whole course of germination. This pattern of distribution was highly similar to the localization of neutral lipids, which progressively appeared in protein bodies. Lipoxygenase activity was found in both the protein bodies and on the surface of the oil bodies during the initial phase of seed germination. The association of lipoxygenase with oil bodies was temporally correlated with the appearance of phospholipase A and lipase activities on the surface of oil bodies. It is concluded that protein bodies not only serve as simple storage structures, but are also dynamic and multifunctional organelles directly involved in storage lipid mobilization during olive seed germination.

The behaviour of beta-carotene in the phosphatidylcholine bilayer as revealed by a molecular simulation study

A molecular dynamics (MD) simulation of the fully hydrated bilayer made of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) and containing beta-carotene (beta-Car) molecules was carried out as a complementary approach to experimental techniques to investigate the orientation of beta-Car in the lipid membrane as well as its influence on the bilayer properties. The bilayer reached thermal equilibrium after 1200 ps of MD simulation and the productive run was carried out for 2800 ps. The results indicate that the carotene rings are located in the region occupied by the carbonyl groups of the POPC gamma-chain with no trace of penetration towards the centre of the bilayer. Carotene exhibits an ordering effect on both the beta- and the gamma-chain. While the fully saturated gamma-chain is affected evenly along, the order of the mono-unsaturated beta-chain is modified mainly below the double bond. In general, a high value of the order parameter and the chain tilt in the range from 11.4 degrees to 26.7 degrees were observed for the beta-Car molecules. However, for chain segment adjacent to methyl groups the value of the order parameter is low and the tilt angle is close to 75 degrees . Moreover, the probability of trans conformation being generally close to 1.0 along the beta-Car chain is reduced for these segments. Our MD simulation study suggests two pools of the preferential orientation of beta-Car: a slightly bent structure corresponding to a small chain tilt angle and a rather stretched structure that corresponds to a higher chain tilt. The results are discussed in the light of experimental findings.

A complex and mobile structure forms a distinct subregion within the continuous vacuolar membrane in young cotyledons of Arabidopsis

The plant vacuole is a multifunctional organelle which is essential for growth and development. To visualize the dynamics of plant vacuolar membranes, gamma-TIP (tonoplast intrinsic protein) was fused to GFP and expressed in Arabidopsis thaliana. The marker molecule was targeted to the vacuolar membranes in most tissues, as expected. In rapidly expanding cells, some additional spherical structures were often observed within the lumen of vacuoles, which emitted strong fluorescence. To confirm their normal presence, we examined wild-type Arabidopsis cotyledons by transmission electron microscopy. The metal-contact rapid-freezing method revealed that the vacuolar lumen of epidermal cells contained many cytoplasmic projections, which often formed spherical structures (1-3 microm diameter) consisting of double membranes. Thus we concluded that these structures are authentic and named them 'bulbs'. Three-dimensional reconstruction from serial electron microscopic images demonstrates that bulbs are very intricately folded, but are continuous with the limiting vacuolar membrane. The fluorescence intensity of bulbs is about threefold higher than that of vacuolar membrane. GFP-AtRab75c, another marker of the vacuole, did not give fluorescent signals of bulbs in transgenic plants, but the existence of bulbs was still confirmed by electron microscopy. These results suggest that bulbs define a subregion in the continuous vacuolar membrane, where some proteins are concentrated and others segregated.

The AtVAM3 encodes a syntaxin-related molecule implicated in the vacuolar assembly in Arabidopsis thaliana

The vacuole constitutes a large compartment in plant and fungal cells. The VAM3 gene of Saccharomyces cerevisiae encodes a syntaxin-related protein required for vacuolar assembly. An Arabidopsis thaliana cDNA library, designed for expression in S. cerevisiae, was screened for cDNAs able to complement defective vacuolar assembly of the Deltavam3 mutation. One cDNA, encoding a 33-kDa protein with structural similarities to the other syntaxins, was identified. The product of AtVAM3 (AtVam3p) was expressed in various tissues including roots, leaves, inflorescence stems, flower buds, and young siliques. The AtVAM3 transcripts were abundant in undifferentiated cells in the meristematic region. AtVam3p fractionated predominantly to an 8,000 x g pellet fraction where a vacuolar membrane protein H+-translocating inorganic pyrophosphatase (H+-PPase) also fractionated. Immunoelectron microscopy showed that AtVam3p was localized to restricted regions on the vacuolar membranes. We propose that AtVam3p provides the t-SNARE function in the vacuolar assembly in A. thaliana.

AQUAPORINS AND WATER PERMEABILITY OF PLANT MEMBRANES

The mechanisms of plant membrane water permeability have remained elusive until the recent discovery in both vacuolar and plasma membranes of a class of water channel proteins named aquaporins. Similar to their animal counterparts, plant aquaporins have six membrane-spanning domains and belong to the MIP superfamily of transmembrane channel proteins. Their very high efficiency and selectivity in transporting water molecules have been mostly characterized using heterologous expression in Xenopus oocytes. However, techniques set up to measure the osmotic water permeability of plant membranes such as transcellular osmosis, pressure probe measurements, or stopped-flow spectrophotometry are now being used to analyze the function of plant aquaporins in their native membranes. Multiple mechanisms, at the transcriptional and posttranslational levels, control the expression and activity of the numerous aquaporin isoforms found in plants. These studies suggest a general role for aquaporins in regulating transmembrane water transport during the growth, development, and stress responses of plants. Future research will investigate the integrated function of aquaporins in long-distance water transport and cellular osmoregulation.