Micro-heterogeneity of pectins and calcium distribution in the epidermal and cortical parenchyma cell walls of flax hypocotyl

Summary Pectic polysaccharides are major components of the plant cell wall matrix and are known to perform many important functions for the plant. In the course of our studies on the putative role of pectic polysaccharides in the control of cell elongation, we have examined the distribution of polygalacturonans in the epidermal and cortical parenchyma cell walls of flax seedling hypocotyls. Pectic components have been detected with (1) the nickel (Ni²⁺) staining method to visualize polygalacturonates, (2) monoclonal antibodies specific to low (JIM5) and highly methylesterified (JIM7) pectins and (3) a combination of subtractive treatment and PATAg (periodic acid-thiocarbohydrazide-silver proteinate) staining. In parallel, calcium (Ca²⁺) distribution has been imaged using SIMS microscopy (secondary ion mass spectrometry) on cryo-prepared samples and TEM (transmission electron microscopy) after precipitation of calcium with potassium pyroantimonate. Our results show that, at the tissular level, polygalacturonans are mainly located in the epidermal cell walls, as revealed by the Ni²⁺ staining and immunofluorescence microscopy with JIM5 and JIM7 antibodies. In parallel, Ca²⁺ distribution points to a higher content of this cation in the epidermal walls compared to cortical parenchyma walls. At the ultrastructural level, immunogold labeling with JIM5 and JIM7 antibodies shows a differential distribution of pectic polysaccharides within cell walls of both tissues. The acidic polygalacturonans (recognized by JIM5) held through calcium bridges are mainly found in the outer part of the external wall of epidermal cells. In contrast, the labeling of methylesterified pectins with JIM7 is slightly higher in the inner part than in the outer part of the wall. In the cortical parenchyma cells, acidic pectins are restricted to the cell junctions and the wall areas in contact with the air-spaces, whereas methylesterified pectins are evenly distributed all over the wall. In addition, the pyroantimonate precipitation method reveals a clear difference in the Ca²⁺ distribution in the epidermal wall, suggesting that this cation is more tightly bound to acidic pectins in the outer part than in the inner part of that wall. Our findings show that the distribution of pectic polysaccharides and the nature of their linkages differ not only between tissues, but also within a single wall of a given cell in flax hypocotyls. The differential distribution of pectins and Ca²⁺ in the external epidermal wall suggests a specific control of the demethylation of pectins and a central role for Ca²⁺ in this regulation.Abbreviations Cdta diamino-1,2-cyclohexane tetra-acetic acid - PATAg periodic acid-thiocarbohydrazide-silver proteinate - PGA polygalacturonic acid - PME pectin methylesterase - RG I rhamnogalacturonan I - SIMS secondary ion mass spectrometry - TEM transmission electron microscopy     

Properties of Neurons in External Globus Pallidus Can Support Optimal Action Selection

The external globus pallidus (GPe) is a key nucleus within basal ganglia circuits that are thought to be involved in action selection. A class of computational models assumes that, during action selection, the basal ganglia compute for all actions available in a given context the probabilities that they should be selected. These models suggest that a network of GPe and subthalamic nucleus (STN) neurons computes the normalization term in Bayes’ equation. In order to perform such computation, the GPe needs to send feedback to the STN equal to a particular function of the activity of STN neurons. However, the complex form of this function makes it unlikely that individual GPe neurons, or even a single GPe cell type, could compute it. Here, we demonstrate how this function could be computed within a network containing two types of GABAergic GPe projection neuron, so-called ‘prototypic’ and ‘arkypallidal’ neurons, that have different response properties in vivo and distinct connections. We compare our model predictions with the experimentally-reported connectivity and input-output functions (f-I curves) of the two populations of GPe neurons. We show that, together, these dichotomous cell types fulfil the requirements necessary to compute the function needed for optimal action selection. We conclude that, by virtue of their distinct response properties and connectivities, a network of arkypallidal and prototypic GPe neurons comprises a neural substrate capable of supporting the computation of the posterior probabilities of actions.     

Non-contact micro-cantilevers detect photothermally induced vibrations that can segregate different categories of exfoliative cervical cytology

We implemented a non-contact photo-thermo-mechanical recording method whereby a silicon nitride atomic force microscopy cantilever is placed several micrometer above the surface of samples. Samples were illuminated with infrared (IR) radiation after which, cantilever mechanical vibrations were optically sensed. Following spectrometric acquisition and Fourier transformation, true IR absorption spectra were obtained. With multivariate analysis, segregation between different categories of exfoliative cervical cytology was obtained. This approach points towards the implementation of a novel near-field system that allows IR spectral analysis without probe contamination.     

Cell wall biochemical alterations during Agrobacterium‐mediated expression of haemagglutinin‐based influenza virus‐like vaccine particles in tobacco

Influenza virus‐like particles (VLPs) have been shown to induce a safe and potent immune response through both humoral and cellular responses. They represent promising novel influenza vaccines. Plant‐based biotechnology allows for the large‐scale production of VLPs of biopharmaceutical interest using different model organisms, including Nicotiana benthamiana plants. Through this platform, influenza VLPs bud from the plasma membrane and accumulate between the membrane and the plant cell wall. To design and optimize efficient production processes, a better understanding of the plant cell wall composition of infiltrated tobacco leaves is a major interest for the plant biotechnology industry. In this study, we have investigated the alteration of the biochemical composition of the cell walls of N. benthamiana leaves subjected to abiotic and biotic stresses induced by the Agrobacterium‐mediated transient transformation and the resulting high expression levels of influenza VLPs. Results show that abiotic stress due to vacuum infiltration without Agrobacterium did not induce any detectable modification of the leaf cell wall when compared to non infiltrated leaves. In contrast, various chemical changes of the leaf cell wall were observed post‐Agrobacterium infiltration. Indeed, Agrobacterium infection induced deposition of callose and lignin, modified the pectin methylesterification and increased both arabinosylation of RG‐I side chains and the expression of arabinogalactan proteins. Moreover, these modifications were slightly greater in plants expressing haemagglutinin‐based VLP than in plants infiltrated with the Agrobacterium strain containing only the p19 suppressor of silencing.     

Tracking cell wall changes in wine and table grapes undergoing Botrytis cinerea infection using glycan microarrays

Background and AimsThe necrotrophic fungus Botrytis cinerea infects a broad range of fruit crops including domesticated grapevine Vitis vinifera cultivars. Damage caused by this pathogen is severely detrimental to the table and wine grape industries and results in substantial crop losses worldwide. The apoplast and cell wall interface is an important setting where many plant–pathogen interactions take place and where some defence-related messenger molecules are generated. Limited studies have investigated changes in grape cell wall composition upon infection with B. cinerea, with much being inferred from studies on other fruit crops.MethodsIn this study, comprehensive microarray polymer profiling in combination with monosaccharide compositional analysis was applied for the first time to investigate cell wall compositional changes in the berries of wine (Sauvignon Blanc and Cabernet Sauvignon) and table (Dauphine and Barlinka) grape cultivars during Botrytis infection and tissue maceration. This was used in conjunction with scanning electron microscopy (SEM) and X-ray computed tomography (CT) to characterize infection progression.Key ResultsGrapes infected at veraison did not develop visible infection symptoms, whereas grapes inoculated at the post-veraison and ripe stages showed evidence of significant tissue degradation. The latter was characterized by a reduction in signals for pectin epitopes in the berry cell walls, implying the degradation of pectin polymers. The table grape cultivars showed more severe infection symptoms, and corresponding pectin depolymerization, compared with wine grape cultivars. In both grape types, hemicellulose layers were largely unaffected, as was the arabinogalactan protein content, whereas in moderate to severely infected table grape cultivars, evidence of extensin epitope deposition was present.ConclusionsSpecific changes in the grape cell wall compositional profiles appear to correlate with fungal disease susceptibility. Cell wall factors important in influencing resistance may include pectin methylesterification profiles, as well as extensin reorganization.     

A role for pectin-associated arabinans in maintaining the flexibility of the plant cell wall during water deficit stress

One of the main components of pectin, a primary constituent of higher plant cell walls, is rhamnogalacturonan I. This polymer comprised of linked alternating rhamnose and galacturonic acid residues is decorated with side chains composed of arabinose and galactose residues. At present, the function of these side chains is not fully understood. Our research on Southern African resurrection plants, plants that are capable of surviving severe dehydration (desiccation), has revealed that their cell walls are capable of extreme flexibility in response to water loss. One species, Myrothamnus flabellifolia, has evolved a constitutively protected leaf cell wall, composed of an abundance of arabinose polymer side chains, suggested to be arabinans and/or arabinogalactans, associated with the pectin matrix. In this article, we propose a hypothetical model that explains how the arabinan rich pectin found in the leaves of this desiccation-tolerant plant permits almost complete water loss without deleterious consequences, such as irreversible polymer adhesion, from occurring. Recent evidence suggesting a role for pectin-associated arabinose polymers in relation to water dependent processes in other plant species is also discussed.Key words: arabinans, cell wall, desiccation, resurrection, rehydration, rhamnogalacturonan IThe flowering plant cell wall is a composite structure consisting of a skeletal framework of cellulose and hemicellulose embedded within a matrix of pectin polysaccharides and cell wall glycoproteins.^1,2 The pectin matrix, in turn, is composed of three primary types of polysaccharides, these being rhamnogalacturonan I (RGI), rhamnogalacturonan II (RGII) and homogalacturonan (HG).¹ RGII is a complex polysaccharide, consisting of many unusual sugar moieties, and is not present in large amounts in the wall.³ HG is effectively a linear homopolymer of galacturonic acid and is believed to facilitate the formation of tight junctions, ‘egg boxes’, by complexing with calcium ions present in the cell wall.¹ RGI is a polymer composed of a backbone of alternating glycosidically linked rhamnose and galacturonic acid residues.¹ Side chains, consisting of either arabinogalactan polymers or linear chains of arabinans and/or galactans, are then attached to the rhamnose residues of the RGI backbone.¹ The manner with which these polymers are attached or become entangled with each other and cellulosic polymers to form the pectin matrix has been a matter of debate. The classical theory is that the RGI and HG polymers alternate with each other as block polymers and that the side chains interact with neighbouring polysaccharide chains. Recently, this standard theory has been questioned and an argument whereby the HG polymers are actually side chains of a RGI backbone polymer has been advanced.⁴ Nevertheless, the complexity of pectin polysaccharides is such that ascribing definitive functions to this matrix of polysaccharides has proven quite difficult. The physical properties of the pectin matrix suggest a number of possible functions. The water binding properties of the galacturonic acid residues indicate that polymers containing these groups have the capacity to hydrate and swell and so possibly help maintain polymer separation in the wall.⁵ The side chains of RGI include arabinan and galactan polymers which have been shown to be highly mobile^6,7,8 with the potential to interact with each other forming a temporally entangled matrix.⁹ It is also believed that arabinan chains, which have been shown to contain ferulate residues attached to terminal arabinose groups, are able to oxidatively cross-link via the formation of diferulate bridges between arabinan chains that originate on separate RGI polysaccharides.¹⁰ The pectin matrix is now believed to contain sub-domains of RGI, HG and RGII which may interact with different polysaccharide components of the cell wall such as cellulose or xyloglucan.^11,12 Hence, it is possible that the pectin matrix may form these associations with other polysaccharides via covalent⁹ and/or non-covalent¹¹ (e.g., H-bonding) interactions and in so doing ensure the integrity of the wall and its polymer organisation. Although a number of general functions, such as hydration and ion binding, have been proposed for the pectin matrix, in particular the RGI polymer and its neutral side chains, there has been difficulty in elucidating specific functions for these polysaccharides. A number of molecular genetic studies have been performed with the aim of establishing specific functions for the RGI side chains. A recent study showed that genetic removal of the arabinan side chains in the cell walls of Nicotiana plumbaginfolia results in the formation of a non-organogenic callus culture with loosely attached cells.¹³ Furthermore, it has been shown that ‘in muro’ fragmentation of the RG1 backbone in Solanum tuberosum results in abnormal development of the periderm.¹⁴ This suggests that these side chains may play at least some role in normal cell attachment and cell development. However, the real problem is that no obvious phenotypic differences between wild type and mutant plants (in which neutral side chains have been modified) have been observed.^15,16,17 It may be that the conditions under which phenotypic differences between wild type and mutant plants would arise have not yet been investigated. We believe the water binding and attachment properties of the pectin matrix are particularly important. This is especially so given the role pectin plays in the middle lamella ensuring attachment of cells to each other and in the formation of the apoplast where water mediated transport of solutes occurs.¹ Our research has focused on a group of Southern African plants termed ‘Resurrection plants’ because of their unique ability to survive severe dehydration (desiccation) to an almost air-dry state.¹⁸ We have been interested in how the cell walls of angiosperm resurrection plants such as Craterostigma wilmsii^19,20 and Myrothamnus flabellifolia^21,22 may have become adapted to survive this extreme water deficit stress (desiccation). We have shown that in the case of the Myrothamnus flabellifolia leaf cell wall, which becomes considerably folded when dried, does not undergo dramatic changes in composition or polymer location in response to desiccation.²¹ Rather we propose that this plant has evolved a constitutively protected cell wall which is able to undergo repeated cycles of desiccation and rehydration.^21,22 We have observed that the pectin component of the leaf cell wall in this species was unusually rich in arabinose polymers, most likely arabinan and arabinogalactan in nature, which we advanced was the reason that the cell wall of this species was able to tolerate desiccation.²¹ Here we provide a simple model (Fig. 1) whereby the arabinan side chains of the pectin polysaccharides are responsible for possibly buffering/replacing the lost water during desiccation and in so doing prevent the formation of tight junctions (e.g., egg boxes) or strong H-bonding interactions between the normally separate ‘skeletal’ polysaccharides (e.g., cellulose microfibrils and xyloglucan tethers) embedded in the pectin matrix. Our model is supported by the observation that cell wall arabinans play a crucial role in the response of guard cells to turgor pressure.²³ It was shown that removal of arabinans by enzymatic digestion of leaf strips of Commelina communis resulted in locking of the guard cell walls in either the open or closed position.²³ Additional roles for arabinan polymers in cell walls have recently been implied with respect to the salt tolerance of Mesembryanthemum crystallinum,²⁴ ensuring hydration of the seed endosperm of Gleditsia triacanthos during germination²⁵ and the tolerance of tropical legume seeds to dehydration.²⁶ We believe that the arabinan side chains of RGI play a critical role in the ability of cell walls to remain flexible during plant growth and may have important functions in relation to the water content of the cell. Further studies aimed at determining the relationship between wall water content, RGI side chains and cell wall flexibility may reveal hitherto unsuspected functions for these polysaccharides in the life of the plant.Open in a separate windowFigure 1A model proposing the role of arabinose rich pectin polymers in stabilising the cell wall against water loss. (A) Pectin consisting of short arabinan chains in the hydrated state, (B) pectin consisting of short arabinan chains in the dehydrated state; (C) pectin consisting of long arabinan chains in the hydrated state; (D) pectin consisting of long arabinan chains in the dehydrated state. The likelihood of irreversible tight junctions (e.g., egg boxes) forming in arabinan poor cell walls during dehydration is demonstrated in (B) while the reversible buffering effect of arabinan rich cell walls is proposed in (D) as would possibly occur in Myrothamnus flabellifolia. For simplicity arabinan chains not participating in the buffering interactions between the RGI backbone chains have been shortened to two arabinose residues in length. Note in (A) and (B) all arabinan chains are two arabinose residues in length.     

Two new Fe(II) spin crossover complexes with tetrazol-1-yl-cycloalkane ligands

Two new 1-(tetrazol-1-yl)cycloalkanes [C_ntz] with n = 5 and 6 were synthesised as ligands for iron(II) spin crossover complexes. Just recently, the [Fe(C₃tz)₆](BF₄)₂ showed that the rigid cyclopropyl-substituent of the tetrazole yielded a rather abrupt and complete spin transition at T_½ ≈ 190 K [1]. Aiming for a deeper insight into the factors governing the spin transition behavior such as abruptness and spin transition temperature we synthesized the two new homologous complexes [Fe(C₅tz)₆](BF₄)₂ and [Fe(C₆tz)₆](BF₄)₂ which were characterized by XRPD, magnetic susceptibility measurements, DSC, ⁵⁷Fe-Mössbauer, UV-Vis-NIR and MIR spectroscopy. The magnetic and structural properties of both [Fe(C_ntz)₆](BF₄)₂ with n = 5 and 6 are also compared with the [Fe(C₃tz)₆](BF₄)₂ and its structural peculiarities are discussed.     

Root extracellular traps versus neutrophil extracellular traps in host defence,a case of functional convergence?

The root cap releases cells that produce massive amounts of mucilage containing polysaccharides, proteoglycans, extracellular DNA (exDNA) and a variety of antimicrobial compounds. The released cells – known as border cells or border‐like cells – and mucilage secretions form networks that are defined as root extracellular traps (RETs). RETs are important players in root immunity. In animals, phagocytes are some of the most abundant white blood cells in circulation and are very important for immunity. These cells combat pathogens through multiple defence mechanisms, including the release of exDNA‐containing extracellular traps (ETs). Traps of neutrophil origin are abbreviated herein as NETs. Similar to phagocytes, plant root cap‐originating cells actively contribute to frontline defence against pathogens. RETs and NETs are thus components of the plant and animal immune systems, respectively, that exhibit similar compositional and functional properties. Herein, we describe and discuss the formation, molecular composition and functional similarities of these similar but different extracellular traps.     

An architecture for P2P bag-of-tasks execution with multiple task allocation policies in desktop grids

In this paper, we propose and evaluate a flexible architecture for desktop grids that supports multiple task allocation policies on top of a structured P2P overlay. In our proposal, a?Bag-of-Tasks application is submitted to random nodes and placed in their local queue, that is processed in a FIFO way. When a node becomes idle, a task allocation policy is executed that fetches tasks from remote nodes. The proposed architecture is flexible since it is decoupled from both the P2P middleware and the P2P overlay. A?prototype of the proposed architecture was implemented on top of the JXTA middleware, using the Chord P2P search overlay. The results obtained in a 16-machine heterogeneous desktop grid show that very good performance gains are obtained with multiple task allocation policies. Also, a speedup of 9.85 was achieved for an application composed of 270 network flow balancing tasks, reducing its wallclock execution time from 32.51?min to 3.3?min.