Role of fatty acids in cold adaptation of Antarctic psychrophilic Flavobacterium spp.

Cold-loving microorganisms developed numerous adaptation mechanisms allowing them to survive in extremely cold habitats, such as adaptation of the cell membrane. The focus of this study was on the membrane fatty acids of Antarctic Flavobacterium spp., and their adaptation response to cold-stress. Fatty acids and cold-response of Antarctic flavobacteria was also compared to mesophilic and thermophilic members of the genus Flavobacterium. The results showed that the psychrophiles produced more types of major fatty acids than meso- and thermophilic members of this genus, namely C_15:1 iso G, C_15:0 iso, C_15:0 anteiso, C_15:1 ω6c, C_15:0 iso 3OH, C_17:1 ω6c, C_16:0 iso 3OH and C_17:0 iso 3OH, summed features 3 (C_16:1 ω7cand/or C_16:1 ω6c) and 9 (C_16:0 10-methyl and/or C_17:1 iso ω9c). It was shown that the cell membrane of psychrophiles was composed mainly of branched and unsaturated fatty acids. The results also implied that Antarctic flavobacteria mainly used two mechanisms of membrane fluidity alteration in their cold-adaptive response. The first mechanism was based on unsaturation of fatty acids, and the second mechanism on de novo synthesis of branched fatty acids. The alteration of the cell membrane was shown to be similar for all thermotypes of members of the genus Flavobacterium.     

Influence of leaf water status on stomatal response to humidity,hydraulic conductance,and soil drought in Betula occidentalis

Whole-canopy measurements of water flux were used to calculate stomatal conductance (g _s ) and transpiration (E) for seedlings of western water birch (Betula occidentalis Hook.) under various soil-plant hydraulic conductances (k), evaporative driving forces (ΔN; difference in leaf-to-air molar fraction of water vapor), and soil water potentials (Ψ_s). As expected, g _s dropped in response to decreased k or Ψ_S, or increased ΔN(> 0.025). Field data showed a decrease in mid-day g _s with decreasing k from soil-to-petiole, with sapling and adult plants having lower values of both parameters than juveniles. Stomatal closure prevented E and Ψ from inducing xylem cavitation except during extreme soil drought when cavitation occurred in the main stem and probably roots as well. Although all decreases in g _s were associated with approximately constant bulk leaf water potential (ψ_l), this does not logically exclude a feedback response between Ψ_L and g _s . To test the influence of leaf versus root water status on g _s , we manipulated water status of the leaf independently of the root by using a pressure chamber enclosing the seedling root system; pressurizing the chamber alters cell turgor and volume only in the shoot cells outside the chamber. Stomatal closure in response to increased ΔN, decreased k, and decreased Ψ_S was fully or partially reversed within 5 min of pressurizing the soil. Bulk Ψ_L remained constant before and after soil pressurizing because of the increase in E associated with stomatal opening. When ΔN was low (i.e., < 0.025), pressurizing the soil either had no effect on g _s , or caused it to decline; and bulk Ψ_L increased. Increased Ψ_l may have caused stomatal closure via increased backpressure on the stomatal apparatus from elevated epidermal turgor. The stomatal response to soil pressurizing indicated a central role of leaf cells in sensing water stress caused by high ΔN, low k, and low Ψ_S. Invoking a prominent role for feedforward signalling in short-term stomatal control may be premature.     

The ecosystem wilting point defines drought response and recovery of a Quercus-Carya forest

Soil and atmospheric droughts increasingly threaten plant survival and productivity around the world. Yet, conceptual gaps constrain our ability to predict ecosystem-scale drought impacts under climate change. Here, we introduce the ecosystem wilting point (Ψ_EWP), a property that integrates the drought response of an ecosystem's plant community across the soil–plant–atmosphere continuum. Specifically, Ψ_EWP defines a threshold below which the capacity of the root system to extract soil water and the ability of the leaves to maintain stomatal function are strongly diminished. We combined ecosystem flux and leaf water potential measurements to derive the Ψ_EWP of a Quercus-Carya forest from an “ecosystem pressure–volume (PV) curve,” which is analogous to the tissue-level technique. When community predawn leaf water potential (Ψ_pd) was above Ψ_EWP (=−2.0 MPa), the forest was highly responsive to environmental dynamics. When Ψ_pd fell below Ψ_EWP, the forest became insensitive to environmental variation and was a net source of carbon dioxide for nearly 2 months. Thus, Ψ_EWP is a threshold defining marked shifts in ecosystem functional state. Though there was rainfall-induced recovery of ecosystem gas exchange following soaking rains, a legacy of structural and physiological damage inhibited canopy photosynthetic capacity. Although over 16 growing seasons, only 10% of Ψ_pd observations fell below Ψ_EWP, the forest is commonly only 2–4 weeks of intense drought away from reaching Ψ_EWP, and thus highly reliant on frequent rainfall to replenish the soil water supply. We propose, based on a bottom-up analysis of root density profiles and soil moisture characteristic curves, that soil water acquisition capacity is the major determinant of Ψ_EWP, and species in an ecosystem require compatible leaf-level traits such as turgor loss point so that leaf wilting is coordinated with the inability to extract further water from the soil.     

Cell-Wall Interactions and the Selective Bacteriostatic Activity of a Miniature Oligo-Acyl-Lysyl

The oligo-acyl-lysyl, C_12(ω7)K-β₁₂, is comprised of only three Lys residues. Despite its small size, it exhibits potent bacteriostatic activity against Gram-positive bacteria, but it is ∼10-fold less potent against Gram-negative bacteria. We followed the interactions of C_12(ω7)K-β₁₂ from its initial contact with the bacterial surface across the cell wall down to the cytoplasmic membrane. Binding to anionic lipids, as well as to negatively charged LPS and LTA, occurs with very high affinity. The C_12(ω7)K-β₁₂ does not cross the outer membrane of Gram-negative bacteria; rather, it achieves its action by depositing on the LPS layer, promoting surface adhesion and blocking passage of solutes. In Gram-positive bacteria, the thick peptidoglycan layer containing LTA allows passage of C_12(ω7)K-β₁₂ and promotes its accumulation in the small periplasm. From that location it is then driven to the membrane by strong electrostatic interactions. Despite its high potency against Gram-positive bacteria, this agent is not capable of efficiently breaking down the permeability barrier of the cytoplasmic membrane or of reaching an intracellular target, as suggested by the fact that it does not interact with DNA.     

Homeostasis in leaf water potentials on leeward and windward sides of desert shrub crowns: water loss control vs. high hydraulic efficiency

Phenotypic plasticity in morphophysiological leaf traits in response to wind was studied in two dominant shrub species of the Patagonian steppe, used as model systems for understanding effects of high wind speed on leaf water relations and hydraulic properties of small woody plants. Morpho-anatomical traits, hydraulic conductance and conductivity and water relations in leaves of wind-exposed and protected crown sides were examined during the summer with nearly continuous high winds. Although exposed sides of the crowns were subjected to higher wind speeds and air saturation deficits than the protected sides, leaves throughout the crown had similar minimum leaf water potential (Ψ_L). The two species were able to maintain homeostasis in minimum Ψ_L using different physiological mechanisms. Berberis microphylla avoided a decrease in the minimum Ψ_L in the exposed side of the crown by reducing water loss by stomatal control, loss of cell turgor and low epidermal conductance. Colliguaja integerrima increased leaf water transport efficiency to maintain transpiration rates without increasing the driving force for water loss in the wind-exposed crown side. Leaf physiological changes within the crown help to prevent the decrease of minimum Ψ_L and thus contribute to the maintenance of homeostasis, assuring the hydraulic integrity of the plant under unfavorable conditions. The responses of leaf traits that contribute to mechanical resistance (leaf mass per area and thickness) differed from those of large physiological traits by exhibiting low phenotypic plasticity. The results of this study help us to understand the unique properties of shrubs which have different hydraulic architecture compared to trees.     

Population differentiation inSpartina patens: Water potential components and bulk modulus of elasticity

Pressure-volume technique was utilized to evaluate salinity response among three populations ofSpartina patens (Ait.) Muhl. from Louisiana Gulf coast marshes. Plants were subjected to salinities of 85 and 425 mol m^?3 for 77 d in a greenhouse. Ψ_w and Ψ_π decreased in all populations in response to increases in salinity. There were 32% decrease in Ψ_sat, 42% decrease in Ψ_tlp in response to salinity changes from 85 to 425 mol m^?3 in the Ferblanc population. Similarly, there were 35% and 41% decrease in Ψ_sat in the Clovelly and Lake Tambour populations, respectively. All populations showed the ability to adapt to the increased salinity as was evidenced by osmotic adjustment. However, the Lake Tambour population appeared to have superior ability to adapt to high salinity through having a significantly lower osmotic potential at saturation (Ψ_sat), osmotic potential at turgor loss point (Ψ_tlp), and maximum turgor potential (Ψ_P(max)) compared to other populations. Ferblanc and Clovelly populations revealed the ability to adapt to saline environments to a lesser extent as compared to the Lake Tambour population. Results indicate that there is a potential for selection of superior strains ofSpartina patens for use in marsh restoration projects aiming at prevention of wetland loss in certain coastal areas.     

Effects of Hydrostatic Pressure on Energy Metabolism and Osmoregulation in Crab and Fish

This review will focus on the effects of hydrostatic pressure on the oxidative metabolism and on the energy production of the eel Anguilla anguilla, in comparison with the results of investigations conducted on the other powerful euryhaline species, the chinese crab Eriocheir sinensis. Anguilla and Eriocheir were chosen as being both aquatic ectotherms with comparable life modes, the eel being however “preadapted” to high pressure while the crab normally never encounters high levels of pressure during its life cycle. Comparison between both species should lead to better knowledge of the biological effects of hydrostatic pressure per se.Experimental evidence suggests that the oxygen consumption ṀO₂ decrease observed in both animal species during exposure to 101 ATA hydrostatic pressure and which follows a transient increase, likely results from a decrease in O₂ use at the cell level. That idea of an alteration of aerobic metabolism during the first hours under pressure is substantiated by a set of experiments on the eel. However, results indicate that, after some days under pressure, the shallow water fish is quite able to acclimate perfectly to high pressure. The hypothesis that pressure induces a state resembling histotoxic hypoxia during the first hours of exposure is put forward and discussed.The second part of the review focuses on some results showing that osmoregulation is also concerned with hydrostatic pressure. Results obtained on the freshwater eel clearly establish the occurrence of a Na⁺ balance impairment at the tissue level induced by a long-term (30 days) exposure to pressure. It is interesting to point out that this impairment occurs at the same time when a new state of energetic metabolism results from adjustments of intertissue coupling of anaerobic and aerobic metabolisms induced by pressure. It is shown that the physiological processes involved in the control of the hydromineral balance in the chinese crab (which never experiences high-pressure exposure in the course of its life cycle) are outstandingly resistant to pressure by comparison with other crustaceans like the crayfish and the shore crab. Disturbances in hydromineral balance and energetic metabolism in the chinese crab are rapidly resorbed and adjusted to a new state of activity.     

Osmotic and elastic adjustments in cold desert shrubs differing in rooting depth: coping with drought and subzero temperatures

Physiological adjustments to enhance tolerance or avoidance of summer drought and winter freezing were studied in shallow- to deep-rooted Patagonian cold desert shrubs. We measured leaf water potential (Ψ_L), osmotic potential, tissue elasticity, stem hydraulic characteristics, and stomatal conductance (g _S) across species throughout the year, and assessed tissue damage by subzero temperatures during winter. Species behavior was highly dependent on rooting depth. Substantial osmotic adjustment (up to 1.2?MPa) was observed in deep-rooted species exhibiting relatively small seasonal variations in Ψ_L and with access to a more stable water source, but having a large difference between predawn and midday Ψ_L. On the other hand, shallow-rooted species exposed to large seasonal changes in Ψ_L showed limited osmotic adjustment and incomplete stomatal closure, resulting in turgor loss during periods of drought. The bulk leaf tissue elastic modulus (ε) was lower in species with relatively shallow roots. Daily variation in g _S was larger in shallow-rooted species (more than 50?% of its maximum) and was negatively associated with the difference between Ψ_L at the turgor loss point and minimum Ψ_L (safety margin for turgor maintenance). All species increased ε by about 10?MPa during winter. Species with rigid tissue walls exhibited low leaf tissue damage at ?20?°C. Our results suggest that osmotic adjustment was the main water relationship adaptation to cope with drought during summer and spring, particularly in deep-rooted plants, and that adjustments in cell wall rigidity during the winter helped to enhance freezing tolerance.     

The distribution of four Caragana species is related to their differential responses to drought stress

Caragana species are widespread in northwest China. However, species in semiarid areas of Inner Mongolia experience summer and autumn rainfall, whereas morphologically similar close relatives in arid areas in Xinjiang Province experience wet springs, but hot dry summers. We hypothesize that the differences among species in response to soil drought help to explain their distributions. A comparison of the closely related species C. intermedia and C. microphylla from Inner Mongolia (semiarid species) and C. pruinosa and C. spinosa from Xinjiang (arid species) was conducted to examine whether responses to a water deficit in a pot experiment were associated with the environments of their habitat. In the two semiarid species, the stomatal conductance was relatively insensitive to the decrease in predawn leaf water potential (Ψ_leaf), and leaves did not abscise or die even when the Ψ_leaf decreased below ?6.0 MPa, while in seedlings of the two arid species the stomata closed at a relatively high Ψ_leaf, and the leaves abscised gradually when Ψ_leaf fell below about ?3.0 MPa. Furthermore, major vein density, minor vein density and the cubed ratio of the conduit wall thickness to the conduit lumen breadth [(t/b)³] of minor veins increased by an average of 60, 150 and 220 %, respectively, in the semiarid species compared with the arid species. The results indicated that semiarid species have typical anisohydric properties, while arid species have typical isohydric properties. We suggest that the divergence in water-use strategies in the genus may be associated with variation in vein architecture, and may possibly play an important role in determining the distribution of these species in the different environments of northwest China.     

The FgHOG1 Pathway Regulates Hyphal Growth,Stress Responses,and Plant Infection in Fusarium graminearum

Fusarium head blight (FHB) caused by Fusarium graminearum is a destructive disease of wheat and barley worldwide. In a previous study of systematic characterization of protein kinase genes in F. graminearum, mutants of three putative components of the osmoregulation MAP kinase pathway were found to have distinct colony morphology and hyphal growth defects on PDA plates. Because the osmoregulation pathway is not known to regulate aerial hyphal growth and branching, in this study we further characterized the functions of the FgHog1 pathway in growth, pathogenesis, and development. The Fghog1, Fgpbs2, and Fgssk2 mutants were all reduced in growth rate, aerial hyphal growth, and hyphal branching angle. These mutants were not only hypersensitive to osmotic stress but also had increased sensitivity to oxidative, cytoplasm membrane, and cell wall stresses. The activation of FgHog1 was blocked in the Fgpbs2 and Fgssk2 mutants, indicating the sequential activation of FgSsk2-FgPbs2-FgHog1 cascade. Interestingly, the FgHog1 MAPK pathway mutants appeared to be sensitive to certain compounds present in PDA. They were female sterile but retained male fertility. We also used the metabolomics profiling approach to identify compatible solutes that were accumulated in the wild type but not in the Fghog1 deletion mutant. Overall, our results indicate that the FgSsk2-FgPbs2-FgHog1 MAPK cascade is important for regulating hyphal growth, branching, plant infection, and hyperosmotic and general stress responses in F. graminearum.     

Determination of the hydraulic conductivity and of reflection coefficients in Nitella flexilis by means of direct cell-turgor pressure measurements

From direct and continuous measurements of the internal hydrostatic pressure (P) in the internodes of Nitella flexilis, the reflection coefficients (σ_s) of some non-electrolytes were determined, using a zero-flow method, and were compared with those found previously on Valonia utricularis and with those obtained by Dainty and Ginzburg on other Characean internodes from transcellular osmosis experiments. The hydraulic conductivities (Lp) of the cell membranes were determined by two independent methods, that is, using hydrostatically or osmotically induced flows. From the exponential time course of P in such experiments and from the volumetric elastic modulus (ε) of the cell wall, Lp was calculated. The effect of unstirred layers in the methods described was negligibly small.In osmotic experiments with different non-plasmolysing external sucrose concentrations (20–200 mM) the exosmotic hydraulic conductivity (Lp_ex) decreases markedly with increasing concentration, while the endosmotic hydraulic conductivity (Lp_en) shows only a weak dependence. In the hydrostatic experiments the hydraulic conductivities for single cells were constant in the pressure range for P from 2 to 7 atm. In this pressure range Lp_en and Lp_ex varied for different cells from 2.2·10^?5 to 2.8·10^?5 and from 1.8·10^?5 to 2.5·10^?5 cm·s^?1·atm^?1, respectively, with an average ratio Lp_en to Lp_ex of 1.1, which indicates a polarity in water movement.These values were the same as those obtained in the osmotic experiments from extrapolation to zero sucrose concentration. At internal pressures below 2 atm the Lp-values markedly increase on approaching the plasmolytic point.The results are discussed in terms of a dehydration of the membranes (or the cytoplasm) at increased solute concentrations. In addition, the strong dependence of Lp at low internal hydrostatic pressures points to a direct influence of P on the water permeability of the membranes.     

Water Potential Modulates Extensibility of Rye Coleoptile Cell Walls

Extensibility of walls of frozen/thawed rye (Secale cereale) coleoptile segments as a function of the water potential of the incubation solution (Ψ₀) was analyzed employing the creep test method. Negative Ψ₀ exerts an inhibiting effect on extension of isolated walls. The lower the Ψ₀ of polyethylene glycol 6000 (PEG), the less the walls of frozen/thawed segments extended under load. This inhibiting effect of Ψ₀ on wall creep was reversible and independent of the preincubation temperature of the segments. An increase in Ψ₀ resulted in increased extension rate within 2–4 min, whereas a decrease in Ψ₀ resulted in gradually decreasing extension rate after 8–12 min. This finding implies that wall extension changes during growth induced by changes of Ψ₀ in vivo are not only due to changes of turgor pressure but also due to a direct influence by negative Ψ₀ on physical wall properties. The results are discussed with respect to the regulation of extension growth during conditions of water stress.     

Xylem cavitation vulnerability influences tree species’ habitat preferences in miombo woodlands

Although precipitation plays a central role in structuring Africa’s miombo woodlands, remarkably little is known about plant-water relations in this seasonally dry tropical forest. Therefore, in this study, we investigated xylem vulnerability to cavitation for nine principal tree species of miombo woodlands, which differ in habitat preference and leaf phenology. We measured cavitation vulnerability (Ψ₅₀), stem-area specific hydraulic conductivity (K _S), leaf specific conductivity (K _L), seasonal variation in predawn water potential (Ψ_PD) and xylem anatomical properties [mean vessel diameter, mean hydraulic diameter, mean hydraulic diameter accounting for 95 % flow, and maximum vessel length (V _L)]. Results show that tree species with a narrow habitat range (mesic specialists) were more vulnerable to cavitation than species with a wide habitat range (generalists). Ψ₅₀ for mesic specialists ranged between ?1.5 and ?2.2 MPa and that for generalists between ?2.5 and ?3.6 MPa. While mesic specialists exhibited the lowest seasonal variation in Ψ_PD, generalists displayed significant seasonal variations in Ψ_PD suggesting that the two miombo habitat groups differ in their rooting depth. We observed a strong trade-off between K _S and Ψ₅₀ suggesting that tree hydraulic architecture is one of the decisive factors setting ecological boundaries for principal miombo species. While vessel diameters correlated weakly (P > 0.05) with Ψ₅₀, V _L was positively and significantly correlated with Ψ₅₀. Ψ_PD was significantly correlated with Ψ₅₀ further reinforcing the conclusion that tree hydraulic architecture plays a significant role in species’ habitat preference in miombo woodlands.     

Temperature responses of photosynthesis and leaf hydraulic conductance in rice and wheat

Studies on the temperature (T) responses of photosynthesis and leaf hydraulic conductance (K_leaf) are important to plant gas exchange. In this study, the temperature responses of photosynthesis and K_leaf were studied in Shanyou 63 (Oryza sativa) and Yannong 19 (Triticum aestivum). Leaf water potential (Ψ_leaf) was insensitive to T in Shanyou 63, while it significantly decreased with T in Yannong 19. The differential Ψ_leaf − T relationship partially accounted for the differing g_m–T relationships, where g_m was less sensitive to T in Yannong 19 than in Shanyou 63. With different g_m–T and Ψ_leaf–T relationships, the temperature responses of photosynthetic limitations were surprisingly similar between the two lines, and the photosynthetic rate was highly correlated with g_m. With the increasing T, K_leaf increased in Shanyou 63 while it decreased in Yannong 19. The different K_leaf–T relationships were related to different Ψ_leaf–T relationships. When excluding the effects of water viscosity and Ψ_leaf, K_leaf was insensitive to T in both lines. g_m and K_leaf were generally not coordinated across different temperatures. This study highlights the importance of Ψ_leaf on leaf carbon and water exchanges, and the mechanisms for the g_m–T and K_leaf–T relationships were discussed.     

Changes in physiological,biochemical and antioxidant enzyme activities of green gram (<Emphasis Type="Italic">Vigna radiata</Emphasis> L.) genotypes under drought

Water deficit is one of the major factors affecting the growth and productivity field crops. This study was conducted to characterize four popularly genotypes, viz. SGC 16, TMB 37, SG 21-5, and Pratap, of green gram (Vigna radiata L.) on physiological, biochemical and morphological basis under water deficit at vegetative, flowering and pod filling stages. A substantial decrease in the mid-day leaf water potential (Ψ_L), net photosynthesis (P _N), total soluble protein (TSP) and membrane stability index (MSI) were recorded under water deficit. However, the membrane lipid peroxidation (MDA), total free amino acid, and the activities of super oxide dismutase and catalase increased significantly (p ≤ 0.05). Percentage reduction of grain yield, under water deficit, was the maximum in the genotype TMB 37 (50.05–63.80 %) indicating its sensitivity towards water deficit. In contrast, the genotype Pratap was the most tolerant to water deficit as it had the minimum reduction (20.76–34.87 %) in grain yield. Flowering was the most critical crop growth stage to water deficit. Among the studied parameters, Ψ_L, P _N, TSP, MSI and MDA were identified as the marker parameters for explaining the response mechanism of green gram genotypes to water deficit.     

A Novel Pyrimidin-Like Plant Activator Stimulates Plant Disease Resistance and Promotes Growth

Plant activators are chemicals that induce plant defense responses to a broad spectrum of pathogens. Here, we identified a new potential plant activator, 5-(cyclopropylmethyl)-6-methyl-2-(2-pyridyl)pyrimidin-4-ol, named PPA (pyrimidin-type plant activator). Compared with benzothiadiazole S-methyl ester (BTH), a functional analog of salicylic acid (SA), PPA was fully soluble in water and increased fresh weight of rice (Oryza sativa) and Arabidopsis plants at low concentrations. In addition, PPA also promoted lateral root development. Microarray data and real-time PCR revealed that PPA-treated leaves not challenged with pathogen showed up-regulation of genes related to reactive oxygen species (ROS), defenses and SA. During bacterial infection, Arabidopsis plants pretreated with PPA showed dramatically decreased disease symptoms and an earlier and stronger ROS burst, compared with plants pretreated with BTH. Microscopy revealed that H₂O₂ accumulated in the cytosol, plasma membrane and cell wall around intracellular bacteria, and also on the bacterial cell wall, indicating that H₂O₂ was directly involved in killing bacteria. The increase in ROS-related gene expression also supported this observation. Our results indicate that PPA enhances plant defenses against pathogen invasion through the plant redox system, and as a water-soluble compound that can promote plant growth, has broad potential applications in agriculture.     

Early plant growth and biochemical responses induced by <Emphasis Type="Italic">Azospirillum brasilense</Emphasis> Sp245 lipopolysaccharides in wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) seedlings are attenuated by procyanidin B2

This study analyzes the effects of procyanidin B2 on early wheat plant growth and plant biochemical responses promoted by lipopolysaccharides (LPS) derived from the rhizobacteria Azospirillum brasilense Sp245. Measurements of leaf, root length, fresh weight, and dry weight showed in vitro plant growth stimulation 4 days after treatment with A. brasilense as well as LPS. Superoxide anion (O₂ ^·?) and hydrogen peroxide (H₂O₂) levels increased in seedling roots treated with LPS (100 μg mL^?1). The chlorophyll content in leaf decreased while the starch content increased 24 h after treatment in seedling roots. The LPS treatment induced a high increase in total peroxidase (POX) (EC 1.11.1.7) activity and ionically bound cell wall POX content in roots, when compared to respective controls. Early plant growth and biochemical responses observed in wheat seedlings treated with LPS were inhibited by the addition of procyanidin B2 (5 μg mL^?1), a B type proanthocyanidin (PAC), plant-derived polyphenolic compound with binding properties of LPS. All results suggest first that the ionically bound cell wall POX enzymes could be a molecular target of A. brasilense LPS, and second that the recognition or association of LPS by plant cells is required to activate plant responses. This last event could play a critical role during plant growth regulation by A. brasilense LPS.     

Salt leaching leads to drier soils in disturbed semiarid woodlands of central Argentina

Disturbances in semiarid environments have revealed a strong connection between water, salt and vegetation dynamics highlighting how the alteration of water fluxes can drive salt redistribution process and long-term environmental degradation. Here, we explore to what extent the reciprocal effect, that of salt redistribution on water fluxes, may play a role in dictating environmental changes following disturbance in dry woodlands. We assessed salt and water dynamics comparing soil-solution electrical conductivity, chloride concentration, soil water content (SWC) and soil matric and osmotic water potential (Ψ_m, Ψ_os) between disturbed and undisturbed areas. A large pool of salts and chlorides present in undisturbed areas was absent in disturbed plots, suggesting deep leaching. Unexpectedly, this was associated with slight but consistently lower SWC in disturbed versus undisturbed situations during two growing seasons. The apparent paradox of increased leaching but diminishing SWC after disturbance can be explained by the effect of native salt lowering Ψ_os enough to prevent full soil drying. Under disturbed conditions, the onset of deep drainage and salt leaching would raise Ψ_os allowing a decline of Ψ_m and SWC. Soil water storage seems to be modulated by the presence (under natural conditions) and partial leaching (following selective shrub disturbance) of large salt pools. This counterintuitive effect of disturbances may be important in semiarid regions where deep soil salt accumulation is a common feature. Our results highlight the importance of water–salt–vegetation coupling for the understanding and management of these systems.     

Changes in cucumber hypocotyl cell wall dynamics caused by Azospirillum brasilense inoculation

We previously reported that Azospirillum brasilense induced a more elastic cell wall and a higher apoplastic water fraction in both wheat coleoptile and flag leaf. These biophysical characteristics could permit increased growth. Knowledge of the biochemical effects the bacteria could elicit in plant cell walls and how these responses change plant physiology is still scarce. The objective of this work was to analyze whether A. brasilense Sp245 inoculation affected elongation and extensibility of growing cucumber (Cucumis sativus) hypocotyls and ionically bound cell wall peroxidase activities. Hypocotyl tip and basal segments were excised from A. brasilense Sp245-inoculated cucumber seedlings growing in darkness under hydroponic conditions. Elongation, cell wall extensibility, cell wall peroxidase activities against ferulic acid and guaiacol and NADH oxidase activities were analyzed. Azospirillum-inoculated cucumber seedlings grew bigger than non-inoculated ones. Dynamic cell wall differences were detected between inoculated and non-inoculated hypocotyls. They included greater acid-induced cell wall extension and in vivo elongation when incubated in distilled water. Although there was no difference between treatments in either region of the hypocotyl NADH oxidase and ferulic acid peroxidase activities were lower in both regions in inoculated seedlings. These lesser activities could be delaying the stiffening of cell wall in inoculated seedlings. These results showed that the cell wall is a target for A. brasilense growth promotion.     

Dynamical Organization of Compositionally Distinct Inner and Outer Membrane Lipids of Mycobacteria

Mycobacterium species, including Mycobacterium tuberculosis, employs atypical long (C_60–90) and branched lipids to produce a complex cell wall and localizes these toward distinct spatial locations, inner membrane (IM) and outer membrane (OM), thus forming a robust permeability barrier. The properties and functional roles of these spatially orchestrated membrane platforms remain unknown. Herein, we report the distinctive lateral organization, fluidity, and lipid domain architecture of protein-free membranes reconstituted from IM and OM lipids in vitro from M. smegmatis (Msm) underscored by their lipid packing and lipid dynamics. We show that Msm OM, against common notion, is more dynamic and fluid compared with IM and reveal the role of cell wall-associated peptidoglycans and lipoarabinomannan on the Msm OM organization. Overall, these studies indicate that mycobacterial species may regulate their overall membrane functionality by regulating the synthesis of these complex arrays of lipids. Based on the structure-function relationship drawn here, documented alteration in the mycobacterial lipidome during cellular infection and/or drug treatment could reflect a mechanism to fine-tune M. tuberculosis membrane properties to its advantage. These findings are expected to inspire development of lipid-centric therapeutic approaches targeted toward its membrane.