Behavior of plant plasma membranes under hydrostatic pressure as monitored by fluorescent environment-sensitive probes

Lipid-Surrounding Water Molecules Probed by Time-Resolved Emission Spectra of Laurdan

The hydration states of the interfacial region of lipid bilayers were investigated on the basis of the time-resolved emission spectra (TRES) analysis of 6-lauroyl-2-dimethylamino naphthalene (Laurdan), a common fluorescence probe used to analyze membrane hydration. TRES derived from long and short lifetime components were extracted from samples of different lipid species: 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC), 1,2-dioleoyl- sn-glycero-3-phosphocholine (DOPC), d- erythro- N-palmitoyl-sphingosylphosphorylcholine (PSM), and a DOPC/PSM binary bilayer system. Neither lifetime component (short or long) corresponded with the hydration properties; the short lifetime component of DOPC (1.97 ns) exhibited a peak at 440 nm, and the long lifetime components of DPPC and PSM (7.76 and 7.77 ns, respectively) exhibited peaks at the same wavelength. This similarity arose from the competition between the collisional quenching and the hydration effects of water molecules. Herein, this phenomenon was investigated using a plot of the lifetime τ and the peak position λ (τ vs λ plot), simultaneously visualizing both effects by deconvoluting the TRES. On the basis of collisional quenching theory, the distribution of the water population per lipid (water map) was generated. According to this theory, the τ vs λ plot was applied to the water map and the calculation of the number of water molecules per lipid, which is consistent with previous reports. This approach provides novel insights for the analysis of molecular hydration states using the fluorescence of Laurdan.

Plasma membrane order and fluidity are diversely triggered by elicitors of plant defence

Although plants are exposed to a great number of pathogens, they usually defend themselves by triggering mechanisms able to limit disease development. Alongside signalling events common to most such incompatible interactions, modifications of plasma membrane (PM) physical properties could be new players in the cell transduction cascade. Different pairs of elicitors (cryptogein, oligogalacturonides, and flagellin) and plant cells (tobacco and Arabidopsis) were used to address the issue of possible modifications of plant PM biophysical properties induced by elicitors and their links to other events of the defence signalling cascade. We observed an increase of PM order whatever the elicitor/plant cell pair used, provided that a signalling cascade was induced. Such membrane modification is dependent on the NADPH oxidase-mediated reactive oxygen species production. Moreover, cryptogein, which is the sole elicitor able to trap sterols, is also the only one able to trigger an increase in PM fluidity. The use of cryptogein variants with altered sterol-binding properties confirms the strong correlation between sterol removal from the PM and PM fluidity enhancement. These results propose PM dynamics as a player in early signalling processes triggered by elicitors of plant defence.

Differential effect of plant lipids on membrane organization: specificities of phytosphingolipids and phytosterols

The high diversity of the plant lipid mixture raises the question of their respective involvement in the definition of membrane organization. This is particularly the case for plant plasma membrane, which is enriched in specific lipids, such as free and conjugated forms of phytosterols and typical phytosphingolipids, such as glycosylinositolphosphoceramides. This question was here addressed extensively by characterizing the order level of membrane from vesicles prepared using various plant lipid mixtures and labeled with an environment-sensitive probe. Fluorescence spectroscopy experiments showed that among major phytosterols, campesterol exhibits a stronger ability than β-sitosterol and stigmasterol to order model membranes. Multispectral confocal microscopy, allowing spatial analysis of membrane organization, demonstrated accordingly the strong ability of campesterol to promote ordered domain formation and to organize their spatial distribution at the membrane surface. Conjugated sterol forms, alone and in synergy with free sterols, exhibit a striking ability to order membrane. Plant sphingolipids, particularly glycosylinositolphosphoceramides, enhanced the sterol-induced ordering effect, emphasizing the formation and increasing the size of sterol-dependent ordered domains. Altogether, our results support a differential involvement of free and conjugated phytosterols in the formation of ordered domains and suggest that the diversity of plant lipids, allowing various local combinations of lipid species, could be a major contributor to membrane organization in particular through the formation of sphingolipid-sterol interacting domains.

Modification of plasma membrane organization in tobacco cells elicited by cryptogein

Lipid mixtures within artificial membranes undergo a separation into liquid-disordered and liquid-ordered phases. However, the existence of this segregation into microscopic liquid-ordered phases has been difficult to prove in living cells, and the precise organization of the plasma membrane into such phases has not been elucidated in plant cells. We developed a multispectral confocal microscopy approach to generate ratiometric images of the plasma membrane surface of Bright Yellow 2 tobacco (Nicotiana tabacum) suspension cells labeled with an environment sensitive fluorescent probe. This allowed the in vivo characterization of the global level of order of this membrane, by which we could demonstrate that an increase in its proportion of ordered phases transiently occurred in the early steps of the signaling triggered by cryptogein and flagellin, two elicitors of plant defense reactions. The use of fluorescence recovery after photobleaching revealed an increase in plasma membrane fluidity induced by cryptogein, but not by flagellin. Moreover, we characterized the spatial distribution of liquid-ordered phases on the membrane of living plant cells and monitored their variations induced by cryptogein elicitation. We analyze these results in the context of plant defense signaling, discuss their meaning within the framework of the "membrane raft" hypothesis, and propose a new mechanism of signaling platform formation in response to elicitor treatment.

Lipids of plant membrane rafts

Lipids tend to organize in mono or bilayer phases in a hydrophilic environment. While they have long been thought to be incapable of coherent lateral segregation, it is now clear that spontaneous assembly of these compounds can confer microdomain organization beyond spontaneous fluidity. Membrane raft microdomains have the ability to influence spatiotemporal organization of protein complexes, thereby allowing regulation of cellular processes. In this review, we aim at summarizing briefly: (i) the history of raft discovery in animals and plants, (ii) the main findings about structural and signalling plant lipids involved in raft segregation, (iii) imaging of plant membrane domains, and their biochemical purification through detergent-insoluble membranes, as well as the existing debate on the topic. We also discuss the potential involvement of rafts in the regulation of plant physiological processes, and further discuss the prospects of future research into plant membrane rafts.

Eicosapentaenoic acid plays a role in stabilizing dynamic membrane structure in the deep-sea piezophile Shewanella violacea: a study employing high-pressure time-resolved fluorescence anisotropy measurement

Shewanella violacea DSS12 is a psychrophilic piezophile that optimally grows at 30MPa. It contains a substantial amount of eicosapentaenoic acid (EPA) in the membrane. Despite evidence linking increased fatty acid unsaturation and bacterial growth under high pressure, little is known of how the physicochemical properties of the membrane are modulated by unsaturated fatty acids in vivo. By means of the newly developed system performing time-resolved fluorescence anisotropy measurement under high pressure (HP-TRFAM), we demonstrate that the membrane of S. violacea is highly ordered at 0.1MPa and 10°C with the order parameter S of 0.9, and the rotational diffusion coefficient D(w) of 5.4μs(-1) for 1-[4-(trimethylamino)pheny]-6-phenyl-1,3,5-hexatriene in the membrane. Deletion of pfaA encoding the omega-3 polyunsaturated fatty acid synthase caused disorder of the membrane and enhanced the rotational motion of acyl chains, in concert with a 2-fold increase in the palmitoleic acid level. While the wild-type membrane was unperturbed over a wide range of pressures with respect to relatively small effects of pressure on S and D(w), the ΔpfaA membrane was disturbed judging from the degree of increased S and decreased D(w). These results suggest that EPA prevents the membrane from becoming hyperfluid and maintains membrane stability against significant changes in pressure. Our results counter the generally accepted concept that greater fluidity is a membrane characteristic of microorganisms that inhabit cold, high-pressure environments. We suggest that retaining a certain level of membrane physical properties under high pressure is more important than conferring membrane fluidity alone.