Analyses and localization of pectin-like carbohydrates in cell wall and mucilage of the green alga Netrium digitus

Effect of acetochlor on the symbiotic relationship between microalgae and bacteria

In this study, two strains of symbiotic bacteria (SOB-1 and SOB-2) were isolated from Scenedesmus obliquus, and various algal-bacterial mutualistic systems were established under acetochlor (ACT) stress conditions. Following exposure to varying ACT concentrations from 2.0 to 25.0 μg/L, the capacity for co-cultured bacteria to degrade ACT was enhanced in 7 days by up to 226.9% (SOB-1) and 193.0% (SOB-2), compared with axenic algae, although bacteria exposed to higher ACT concentrations exacerbated algal metabolic stress, oxidative states, apoptosis and cellular lysis. ACT reduced carbohydrates in the phycosphere by up to 31.5%; compensatory nutrient plunder and structural damage by bacteria were the potential exploitation pathways determined based on the inhibition of bacterial infection using a glucanase inhibitor. The ACT-induced reduction in algal antimicrobial substances, including fatty acids and phenolics (by up to 58.1% and 56.6%, respectively), also facilitated bacterial exploitation of algae. ACT-dependent interspecific interaction coefficients between algae and bacteria generated from long-term symbiosis cultures implied that bacteria moved from mutualism (0 and 2.0 μg/L ACT) to exploitation (7.9 and 25.0 μg/L ACT). The population dynamic model under incremental ACT-concentration scenarios inferred that theoretical systematic extinction may occur in algal-bacterial systems earlier than in axenic algae. These outcomes provide interspecific insights into the distortion of algal-bacterial reciprocity due to the ecotoxicological effects of ACT.

The extracellular matrix of green algae

Green algae display a wide range of extracellular matrix (ECM) components that include various types of cell walls (CW), scales, crystalline glycoprotein coverings, hydrophobic compounds, and complex gels or mucilage. Recently, new information derived from genomic/transcriptomic screening, advanced biochemical analyses, immunocytochemical studies, and ecophysiology has significantly enhanced and refined our understanding of the green algal ECM. In the later diverging charophyte group of green algae, the CW and other ECM components provide insight into the evolution of plants and the ways the ECM modulates during environmental stress. Chlorophytes produce diverse ECM components, many of which have been exploited for various uses in medicine, food, and biofuel production. This review highlights major advances in ECM studies of green algae.

Same but different - pseudo-pectin in the charophytic alga Chlorokybus atmophyticus

All land-plant cell walls possess hemicelluloses, cellulose and anionic pectin. The walls of their cousins, the charophytic algae, exhibit some similarities to land plants' but also major differences. Charophyte 'pectins' are extractable by conventional land-plant methods, although they differ significantly in composition. Here, we explore 'pectins' of an early-diverging charophyte, Chlorokybus atmophyticus, characterising the anionic polysaccharides that may be comparable to 'pectins' in other streptophytes. Chlorokybus 'pectin' was anionic and upon acid hydrolysis gave GlcA, GalA and sulphate, plus neutral sugars (Ara≈Glc>Gal>Xyl); Rha was undetectable. Most Gal was the l-enantiomer. A relatively acid-resistant disaccharide was characterised as β-d-GlcA-(1→4)-l-Gal. Two Chlorokybus 'pectin' fractions, separable by anion-exchange chromatography, had similar sugar compositions but different sulphate-ester contents. No sugars were released from Chlorokybus 'pectin' by several endo-hydrolases [(1,5)-α-l-arabinanase, (1,4)-β-d-galactanase, (1,4)-β-d-xylanase, endo-polygalacturonase] and exo-hydrolases [α- and β-d-galactosidases, α-(1,6)-d-xylosidase]. 'Driselase', which hydrolyses most land-plant cell wall polysaccharides to mono- and disaccharides, released no sugars except traces of starch-derived Glc. Thus, the Ara, Gal, Xyl and GalA of Chlorokybus 'pectin' were not non-reducing termini with configurations familiar from land-plant polysaccharides (α-l-Araf, α- and β-d-Galp, α- and β-d-Xylp and α-d-GalpA), nor mid-chain residues of α-(1→5)-l-arabinan, β-(1→4)-d-galactan, β-(1→4)-d-xylan or α-(1→4)-d-galacturonan. In conclusion, Chlorokybus possesses anionic 'pectic' polysaccharides, possibly fulfilling pectic roles but differing fundamentally from land-plant pectin. Thus, the evolution of land-plant pectin since the last common ancestor of Chlorokybus and land plants is a long and meandering path involving loss of sulphate, most l-Gal and most d-GlcA; re-configuration of Ara, Xyl and GalA; and gain of Rha.

The cell walls of different Chara species are characterized by branched galactans rich in 3-O-methylgalactose and absence of AGPs

Streptophyte algae are the closest relatives to land plants; their latest common ancestor performed the most drastic adaptation in plant evolution around 500 million years ago: the conquest of land. Besides other adaptations, this step required changes in cell wall composition. Current knowledge on the cell walls of streptophyte algae and especially on the presence of arabinogalactan-proteins (AGPs), important signalling molecules in all land plants, is limited. To get deeper insights into the cell walls of streptophyte algae, especially in Charophyceae, we performed sequential cell wall extractions of four Chara species. The three species Chara globularis, Chara subspinosa and Chara tomentosa revealed comparable cell wall compositions, with pectins, xylans and xyloglucans, whereas Chara aspera stood out with higher amounts of uronic acids in the pectic fractions and lack of reactivity with antibodies binding to xylan- and xyloglucan epitopes. Search for AGPs in the four Chara species and in Nitellopsis obtusa revealed the presence of galactans with pyranosidic galactose in 1,3-, 1,6- and 1,3,6-linkage, which are typical galactan motifs in land plant AGPs. A unique feature of these branched galactans was high portions of 3-O-methylgalactose. Only Nitellopsis contained substantial amounts of arabinose A bioinformatic search for prolyl-4-hydroxylases, involved in the biosynthesis of AGPs, revealed one possible functional sequence in the genome of Chara braunii, but no hydroxyproline could be detected in the four Chara species or in Nitellopsis obtusa. We conclude that AGPs that is typical for land plants are absent, at least in these members of the Charophyceae.

Bricks out of the wall: polysaccharide extramural functions

Plant polysaccharides are components of plant cell walls and/or store energy. However, this oversimplified classification neglects the fact that some cell wall polysaccharides and glycoproteins can localize outside the relatively sharp boundaries of the apoplastic moiety, where they adopt functions not directly related to the cell wall. Such polysaccharide multifunctionality (or 'moonlighting') is overlooked in current research, and in most cases the underlying mechanisms that give rise to unconventional ex muro trafficking, targeting, and functions of polysaccharides and glycoproteins remain elusive. This review highlights major examples of the extramural occurrence of various glycan cell wall components, discusses the possible significance and implications of these phenomena for plant physiology, and lists exciting open questions to be addressed by future research.

The cell biology of charophytes: Exploring the past and models for the future

Charophytes (Streptophyta) represent a diverse assemblage of extant green algae that are the sister lineage to land plants. About 500-600+ million years ago, a charophyte progenitor successfully colonized land and subsequently gave rise to land plants. Charophytes have diverse but relatively simple body plans that make them highly attractive organisms for many areas of biological research. At the cellular level, many charophytes have been used for deciphering cytoskeletal networks and their dynamics, membrane trafficking, extracellular matrix secretion, and cell division mechanisms. Some charophytes live in challenging habitats and have become excellent models for elucidating the cellular and molecular effects of various abiotic stressors on plant cells. Recent sequencing of several charophyte genomes has also opened doors for the dissection of biosynthetic and signaling pathways. While we are only in an infancy stage of elucidating the cell biology of charophytes, the future application of novel analytical methodologies in charophyte studies that include a broader survey of inclusive taxa will enhance our understanding of plant evolution and cell dynamics.

Search for evolutionary roots of land plant arabinogalactan-proteins in charophytes: presence of a rhamnogalactan-protein in Spirogyra pratensis (Zygnematophyceae)

Charophyte green algae (CGA) are assigned to be the closest relatives of land plants and therefore enlighten processes in the colonization of terrestrial habitats. For the transition from water to land, plants needed significant physiological and structural changes, as well as with regard to cell wall composition. Sequential extraction of cell walls of Nitellopsis obtusa (Charophyceae) and Spirogyra pratensis (Zygnematophyceae) offered a comparative overview on cell wall composition of late branching CGA. Because arabinogalactan-proteins (AGPs) are considered common for all land plant cell walls, we were interested in whether these special glycoproteins are present in CGA. Therefore, we investigated both species with regard to characteristic features of AGPs. In the cell wall of Nitellopsis, no hydroxyproline was present and no AGP was precipitable with the β-glucosyl Yariv's reagent (βGlcY). By contrast, βGlcY precipitation of the water-soluble cell wall fraction of Spirogyra yielded a glycoprotein fraction rich in hydroxyproline, indicating the presence of AGPs. Putative AGPs in the cell walls of non-conjugating Spirogyra filaments, especially in the area of transverse walls, were detected by staining with βGlcY. Labelling increased strongly in generative growth stages, especially during zygospore development. Investigations of the fine structure of the glycan part of βGlcY-precipitated molecules revealed that the galactan backbone resembled that of AGPs with 1,3- 1,6- and 1,3,6-linked Galp moieties. Araf was present only in small amounts and the terminating sugars consisted predominantly of pyranosidic terminal and 1,3-linked rhamnose residues. We introduce the term 'rhamnogalactan-protein' for this special AGP-modification present in S. pratensis.

Experimental Manipulation of Pectin Architecture in the Cell Wall of the Unicellular Charophyte, Penium Margaritaceum

Pectins represent one of the main components of the plant primary cell wall. These polymers have critical roles in cell expansion, cell-cell adhesion and response to biotic stress. We present a comprehensive screening of pectin architecture of the unicellular streptophyte, Penium margaritaceum. Penium possesses a distinct cell wall whose outer layer consists of a lattice of pectin-rich fibers and projections. In this study, cells were exposed to a variety of physical, chemical and enzymatic treatments that directly affect the cell wall, especially the pectin lattice. Correlative analyses of pectin lattice perturbation using field emission scanning electron microscopy, confocal laser scanning microscopy, and transmission electron microscopy demonstrate that pectin lattice microarchitecture is both highly sensitive and malleable.

Endomembrane architecture and dynamics during secretion of the extracellular matrix of the unicellular charophyte, Penium margaritaceum

The extracellular matrix (ECM) of many charophytes, the assemblage of green algae that are the sister group to land plants, is complex, produced in large amounts, and has multiple essential functions. An extensive secretory apparatus and endomembrane system are presumably needed to synthesize and secrete the ECM, but structural details of such a system have not been fully characterized. Penium margaritaceum is a valuable unicellular model charophyte for studying secretion dynamics. We report that Penium has a highly organized endomembrane system, consisting of 150-200 non-mobile Golgi bodies that process and package ECM components into different sets of vesicles that traffic to the cortical cytoplasm, where they are transported around the cell by cytoplasmic streaming. At either fixed or transient areas, specific cytoplasmic vesicles fuse with the plasma membrane and secrete their constituents. Extracellular polysaccharide (EPS) production was observed to occur in one location of the Golgi body and sometimes in unique Golgi hybrids. Treatment of cells with brefeldin A caused disruption of the Golgi body, and inhibition of EPS secretion and cell wall expansion. The structure of the endomembrane system in Penium provides mechanistic insights into how extant charophytes generate large quantities of ECM, which in their ancestors facilitated the colonization of land.

Arabinogalactan Proteins and the Extracellular Matrix of Charophytes: A Sticky Business

Charophytes represent the group of green algae whose ancestors invaded land and ultimately gave rise to land plants 450 million years ago. While Zygnematophyceae are believed to be the direct sister lineage to embryophytes, different members of this group (Penium, Spirogyra, Zygnema) and the advanced thallus forming Coleochaete as well as the sarcinoid basal streptophyte Chlorokybus were investigated concerning their vegetative extracellular matrix (ECM) properties. Many taxa exhibit adhesion phenomena that are critical for affixing to a substrate or keeping cells together in a thallus, however, there is a great variety in possible reactions to e.g., wounding. In this study an analysis of adhesion mechanisms revealed that arabinogalactan proteins (AGPs) are most likely key adhesion molecules. Through use of monoclonal antibodies (JIM13) or the Yariv reagent, AGPs were located in cell surface sheaths and cell walls that were parts of the adhesion focal zones on substrates including wound induced rhizoid formation. JIM5, detecting highly methyl-esterfied homoglacturonan and JIM8, an antibody detecting AGP glycan and LM6 detecting arabinans were also tested and a colocalization was found in several examples (e.g., Zygnema) suggesting an interplay between these components. AGPs have been described in this study to perform both, cell to cell adhesion in algae forming thalli and cell to surface adhesion in the filamentous forms. These findings enable a broader evolutionary understanding of the function of AGPs in charophyte green algae.

Homogalacturonan Accumulation in Cell Walls of the Green Alga Zygnema sp. (Charophyta) Increases Desiccation Resistance

Land plants inherited several traits from their green algal ancestors (Zygnematophyceae), including a polysaccharide-rich cell wall, which is a prerequisite for terrestrial survival. A major component of both land plant and Zygnematophyceaen cell walls is the pectin homogalacturonan (HG), and its high water holding capacity may have helped algae to colonize terrestrial habitats, characterized by water scarcity. To test this, HG was removed from the cell walls of Zygnema filaments by pectate lyase (PL), and their effective quantum yield of photosystem II (YII) as a proxy for photosynthetic performance was measured in response to desiccation stress by pulse amplitude modulation (PAM). Old filaments were found to contain more HG and are more resistant against desiccation stress but relatively lose more desiccation resistance after HG removal than young filaments. After rehydration, the photosynthetic performance recovered less efficiently in filaments with a HG content reduced by PL, independently of filament age. Immunolabeling showed that partial or un-methylesterified HG occurs throughout the longitudinal cell walls of both young and old filaments, while no labeling signal occurred when filaments were treated with PL prior labeling. This confirmed that most HG can be removed from the cell walls by PL. The initial labeling pattern was restored after ~3 days. A different form of methylesterified HG was restricted to cell poles and cross cell walls. In conclusion, it was shown that the accumulation of HG in Zygnema filaments increases their resistance against desiccation stress. This trait might have played an important role during the colonization of land by Zygnematophyceae, which founded the evolution of all land plants.

Two New Marine Species of Placopus (Vampyrellida, Rhizaria) That Perforate the Theca of Tetraselmis (Chlorodendrales, Viridiplantae)

Vampyrellids (Vampyrellida, Rhizaria) are a major group of predatory amoebae known primarily from freshwater and soil. Environmental sequence data indicate that there is also a considerable diversity of vampyrellids inhabiting marine ecosystems, but their phenotypic traits and ecology remain largely unexplored. We discovered algivorous vampyrellids of the filoflabellate morphotype in coastal habitats in Atlantic Canada, established cultures by single-cell isolation, and characterised three strains using light microscopy, SSU rRNA gene sequencing, feeding experiments and growth experiments at various salinities. These strains exhibit orange, discoid trophozoites with ventral filopodia, moving granules ("membranosomes"), and rolling locomotion, similar to freshwater species previously assigned to Hyalodiscus Hertwig & Lesser, but here moved to Placopus Schulze (due to homonymy with Hyalodiscus Ehrenberg). SSU rRNA gene phylogenies place our strains in two distinct positions within "lineage B3" (here referred to as Placopodidae). Based on these morphological, habitat and molecular data, we describe two new species, Placopus melkoniani sp. nov. and Placopus pusillus sp. nov., both of which feed on chlorophyte flagellates (Tetraselmis, Pyramimonas) and the cryptophyte Chroomonas. They perforate the theca of Tetraselmis to extract the protoplast, and thereby represent the first vampyrellids known to degrade the biochemically exotic cell wall of the Chlorodendrales (Chlorophyta, Viridiplantae).

Biotechnology Potential of Marine Fungi Degrading Plant and Algae Polymeric Substrates

Filamentous fungi possess the metabolic capacity to degrade environment organic matter, much of which is the plant and algae material enriched with the cell wall carbohydrates and polyphenol complexes that frequently can be assimilated by only marine fungi. As the most renewable energy feedstock on the Earth, the plant or algae polymeric substrates induce an expression of microbial extracellular enzymes that catalyze their cleaving up to the component sugars. However, the question of what the marine fungi contributes to the plant and algae material biotransformation processes has yet to be highlighted sufficiently. In this review, we summarized the potential of marine fungi alternatively to terrestrial fungi to produce the biotechnologically valuable extracellular enzymes in response to the plant and macroalgae polymeric substrates as sources of carbon for their bioconversion used for industries and bioremediation.

Cell Wall Structure of Coccoid Green Algae as an Important Trade-Off Between Biotic Interference Mechanisms and Multidimensional Cell Growth

Coccoid green algae can be divided in two groups based on their cell wall structure. One group has a highly chemical resistant cell wall (HR-cell wall) containing algaenan. The other group is more susceptible to chemicals (LR-cell wall – Low resistant cell wall). Algaenan is considered as important molecule to explain cell wall resistance. Interestingly, cell wall types (LR- and HR-cell wall) are not in accordance with the taxonomic classes Chlorophyceae and Trebouxiophyceae, which makes it even more interesting to consider the ecological function. It was already shown that algaenan helps to protect against virus, bacterial and fungal attack, but in this study we show for the first time that green algae with different cell wall properties show different sensitivity against interference competition with the cyanobacterium Microcystis aeruginosa. Based on previous work with co-cultures of M. aeruginosa and two green algae (Acutodesmus obliquus and Oocystis marssonii) differing in their cell wall structure, it was shown that M. aeruginosa could impair only the growth of the green algae if they belong to the LR-cell wall type. In this study it was shown that the sensitivity to biotic interference mechanism shows a more general pattern within coccoid green algae species depending on cell wall structure.

Localisation and substrate specificities of transglycanases in charophyte algae relate to development and morphology

Cell wall-modifying enzymes have been previously investigated in charophyte green algae (CGA) in cultures of uniform age, giving limited insight into their roles. Therefore, we investigated the in situ localisation and specificity of enzymes acting on hemicelluloses in CGA genera of different morphologies and developmental stages. In vivo transglycosylation between xyloglucan and an endogenous donor in filamentous Klebsormidium and Zygnema was observed in longitudinal cell walls of young (1 month) but not old cells (1 year), suggesting that it has a role in cell growth. By contrast, in parenchymatous Chara, transglycanase action occurred in all cell planes. In Klebsormidium and Zygnema, the location of enzyme action mainly occurred in regions where xyloglucans and mannans, and to a lesser extent mixed-linkage β-glucan (MLG), were present, indicating predominantly xyloglucan:xyloglucan endotransglucosylase (XET) activity. Novel transglycosylation activities between xyloglucan and xylan, and xyloglucan and galactomannan were identified in vitro in both genera. Our results show that several cell wall-modifying enzymes are present in CGA, and that differences in morphology and cell age are related to enzyme localisation and specificity. This indicates an evolutionary significance of cell wall modifications, as similar changes are known in their immediate descendants, the land plants. This article has an associated First Person interview with the first author of the paper.

Versatility of the green microalga cell vacuole function as revealed by analytical transmission electron microscopy

Vacuole is a multifunctional compartment central to a large number of functions (storage, catabolism, maintenance of the cell homeostasis) in oxygenic phototrophs including microalgae. Still, microalgal cell vacuole is much less studied than that of higher plants although knowledge of the vacuolar structure and function is essential for understanding physiology of nutrition and stress tolerance of microalgae. Here, we combined the advanced analytical and conventional transmission electron microscopy methods to obtain semi-quantitative, spatially resolved at the subcellular level information on elemental composition of the cell vacuoles in several free-living and symbiotic chlorophytes. We obtained a detailed record of the changes in cell and vacuolar ultrastructure in response to environmental stimuli under diverse conditions. We suggested that the vacuolar inclusions could be divided into responsible for storage of phosphorus (mainly in form of polyphosphate) and those accommodating non-protein nitrogen (presumably polyamine) reserves, respectively.The ultrastructural findings, together with the data on elemental composition of different cell compartments, allowed us to speculate on the role of the vacuolar membrane in the biosynthesis and sequestration of polyphosphate. We also describe the ultrastructural evidence of possible involvement of the tonoplast in the membrane lipid turnover and exchange of energy and metabolites between chloroplasts and mitochondria. These processes might play a significant role in acclimation in different stresses including nitrogen starvation and extremely high level of CO₂ and might also be of importance for microalgal biotechnology. Advantages and limitations of application of analytical electron microscopy to biosamples such as microalgal cells are discussed.

Spatio-temporal diversification of the cell wall matrix materials in the developing stomatal complexes of Zea mays

The matrix cell wall materials, in developing Zea mays stomatal complexes are asymmetrically distributed, a phenomenon appearing related to the local cell wall expansion and deformation, the establishment of cell polarity, and determination of the cell division plane. In cells of developing Zea mays stomatal complexes, definite cell wall regions expand determinately and become locally deformed. This differential cell wall behavior is obvious in the guard cell mother cells (GMCs) and the subsidiary cell mother cells (SMCs) that locally protrude towards the adjacent GMCs. The latter, emitting a morphogenetic stimulus, induce polarization/asymmetrical division in SMCs. Examination of immunolabeled specimens revealed that homogalacturonans (HGAs) with a high degree of de-esterification (2F4- and JIM5-HGA epitopes) and arabinogalactan proteins are selectively distributed in the extending and deformed cell wall regions, while their margins are enriched with rhamnogalacturonans (RGAs) containing highly branched arabinans (LM6-RGA epitope). In SMCs, the local cell wall matrix differentiation constitutes the first structural event, indicating the establishment of cell polarity. Moreover, in the premitotic GMCs and SMCs, non-esterified HGAs (2F4-HGA epitope) are preferentially localized in the cell wall areas outlining the cytoplasm where the preprophase band is formed. In these areas, the forthcoming cell plate fuses with the parent cell walls. These data suggest that the described heterogeneity in matrix cell wall materials is probably involved in: (a) local cell wall expansion and deformation, (b) the transduction of the inductive GMC stimulus, and

Cell wall matrix polysaccharide distribution and cortical microtubule organization: two factors controlling mesophyll cell morphogenesis in land plants

BACKGROUND AND AIMS

This work investigates the involvement of local differentiation of cell wall matrix polysaccharides and the role of microtubules in the morphogenesis of mesophyll cells (MCs) of three types (lobed, branched and palisade) in the dicotyledon Vigna sinensis and the fern Asplenium nidus.

METHODS

Homogalacturonan (HGA) epitopes recognized by the 2F4, JIM5 and JIM7 antibodies and callose were immunolocalized in hand-made leaf sections. Callose was also stained with aniline blue. We studied microtubule organization by tubulin immunofluorescence and transmission electron microscopy.

RESULTS

In both plants, the matrix cell wall polysaccharide distribution underwent definite changes during MC differentiation. Callose constantly defined the sites of MC contacts. The 2F4 HGA epitope in V. sinensis first appeared in MC contacts but gradually moved towards the cell wall regions facing the intercellular spaces, while in A. nidus it was initially localized at the cell walls delimiting the intercellular spaces, but finally shifted to MC contacts. In V. sinensis, the JIM5 and JIM7 HGA epitopes initially marked the cell walls delimiting the intercellular spaces and gradually shifted in MC contacts, while in A. nidus they constantly enriched MC contacts. In all MC types examined, the cortical microtubules played a crucial role in their morphogenesis. In particular, in palisade MCs, cortical microtubule helices, by controlling cellulose microfibril orientation, forced these MCs to acquire a truncated cone-like shape. Unexpectedly in V. sinensis, the differentiation of colchicine-affected MCs deviated completely, since they developed a cell wall ingrowth labyrinth, becoming transfer-like cells.

CONCLUSIONS

The results of this work and previous studies on Zea mays (Giannoutsou et al., Annals of Botany 2013; 112: : 1067-1081) revealed highly controlled local cell wall matrix differentiation in MCs of species belonging to different plant groups. This, in coordination with microtubule-dependent cellulose microfibril alignment, spatially controlled cell wall expansion, allowing MCs to acquire their particular shape.

Charophytes: Evolutionary Giants and Emerging Model Organisms

Charophytes are the group of green algae whose ancestral lineage gave rise to land plants in what resulted in a profoundly transformative event in the natural history of the planet. Extant charophytes exhibit many features that are similar to those found in land plants and their relatively simple phenotypes make them efficacious organisms for the study of many fundamental biological phenomena. Several taxa including Micrasterias, Penium, Chara, and Coleochaete are valuable model organisms for the study of cell biology, development, physiology and ecology of plants. New and rapidly expanding molecular studies are increasing the use of charophytes that in turn, will dramatically enhance our understanding of the evolution of plants and the adaptations that allowed for survival on land. The Frontiers in Plant Science series on "Charophytes" provides an assortment of new research reports and reviews on charophytes and their emerging significance as model plants.

Abiotic Stress Tolerance of Charophyte Green Algae: New Challenges for Omics Techniques

Charophyte green algae are a paraphyletic group of freshwater and terrestrial green algae, comprising the classes of Chlorokybophyceae, Coleochaetophyceae, Klebsormidiophyceae, Zygnematophyceae, Mesostigmatophyceae, and Charo- phyceae. Zygnematophyceae (Conjugating green algae) are considered to be closest algal relatives to land plants (Embryophyta). Therefore, they are ideal model organisms for studying stress tolerance mechanisms connected with transition to land, one of the most important events in plant evolution and the Earth's history. In Zygnematophyceae, but also in Coleochaetophyceae, Chlorokybophyceae, and Klebsormidiophyceae terrestrial members are found which are frequently exposed to naturally occurring abiotic stress scenarios like desiccation, freezing and high photosynthetic active (PAR) as well as ultraviolet (UV) irradiation. Here, we summarize current knowledge about various stress tolerance mechanisms including insight provided by pioneer transcriptomic and proteomic studies. While formation of dormant spores is a typical strategy of freshwater classes, true terrestrial groups are stress tolerant in vegetative state. Aggregation of cells, flexible cell walls, mucilage production and accumulation of osmotically active compounds are the most common desiccation tolerance strategies. In addition, high photophysiological plasticity and accumulation of UV-screening compounds are important protective mechanisms in conditions with high irradiation. Now a shift from classical chemical analysis to next-generation genome sequencing, gene reconstruction and annotation, genome-scale molecular analysis using omics technologies followed by computer-assisted analysis will give new insights in a systems biology approach. For example, changes in transcriptome and role of phytohormone signaling in Klebsormidium during desiccation were recently described. Application of these modern approaches will deeply enhance our understanding of stress reactions in an unbiased non-targeted view in an evolutionary context.

Localization and Quantification of Callose in the Streptophyte Green Algae Zygnema and Klebsormidium: Correlation with Desiccation Tolerance

Freshwater green algae started to colonize terrestrial habitats about 460 million years ago, giving rise to the evolution of land plants. Today, several streptophyte green algae occur in aero-terrestrial habitats with unpredictable fluctuations in water availability, serving as ideal models for investigating desiccation tolerance. We tested the hypothesis that callose, a β-d-1,3-glucan, is incorporated specifically in strained areas of the cell wall due to cellular water loss, implicating a contribution to desiccation tolerance. In the early diverging genus Klebsormidium, callose was drastically increased already after 30 min of desiccation stress. Localization studies demonstrated an increase in callose in the undulating cross cell walls during cellular water loss, allowing a regulated shrinkage and expansion after rehydration. This correlates with a high desiccation tolerance demonstrated by a full recovery of the photosynthetic yield visualized at the subcellular level by Imaging-PAM. Furthermore, abundant callose in terminal cell walls might facilitate cell detachment to release dispersal units. In contrast, in the late diverging Zygnema, the callose content did not change upon desiccation for up to 3.5 h and was primarily localized in the corners between individual cells and at terminal cells. While these callose deposits still imply reduction of mechanical damage, the photosynthetic yield did not recover fully in the investigated young cultures of Zygnema upon rehydration. The abundance and specific localization of callose correlates with the higher desiccation tolerance in Klebsormidium when compared with Zygnema.

In Silico Analysis of the Metabolic Potential and Niche Specialization of Candidate Phylum "Latescibacteria" (WS3)

The “Latescibacteria” (formerly WS3), member of the Fibrobacteres–Chlorobi–Bacteroidetes (FCB) superphylum, represents a ubiquitous candidate phylum found in terrestrial, aquatic, and marine ecosystems. Recently, single-cell amplified genomes (SAGs) representing the “Latescibacteria” were obtained from the anoxic monimolimnion layers of Sakinaw Lake (British Columbia, Canada), and anoxic sediments of a coastal lagoon (Etoliko lagoon, Western Greece). Here, we present a detailed in-silico analysis of the four SAGs to gain some insights on their metabolic potential and apparent ecological roles. Metabolic reconstruction suggests an anaerobic fermentative mode of metabolism, as well as the capability to degrade multiple polysaccharides and glycoproteins that represent integral components of green (Charophyta and Chlorophyta) and brown (Phaeophycaea) algae cell walls (pectin, alginate, ulvan, fucan, hydroxyproline-rich glycoproteins), storage molecules (starch and trehalose), and extracellular polymeric substances (EPSs). The analyzed SAGs also encode dedicated transporters for the uptake of produced sugars and amino acids/oligopeptides, as well as an extensive machinery for the catabolism of all transported sugars, including the production of a bacterial microcompartment (BMC) to sequester propionaldehyde, a toxic intermediate produced during fucose and rhamnose metabolism. Finally, genes for the formation of gas vesicles, flagella, type IV pili, and oxidative stress response were found, features that could aid in cellular association with algal detritus. Collectively, these results indicate that the analyzed “Latescibacteria” mediate the turnover of multiple complex organic polymers of algal origin that reach deeper anoxic/microoxic habitats in lakes and lagoons. The implications of such process on our understanding of niche specialization in microbial communities mediating organic carbon turnover in stratified water bodies are discussed.

Penium margaritaceum as a model organism for cell wall analysis of expanding plant cells

The growth of a plant cell encompasses a complex set of subcellular components interacting in a highly coordinated fashion. Ultimately, these activities create specific cell wall structural domains that regulate the prime force of expansion, internally generated turgor pressure. The precise organization of the polymeric networks of the cell wall around the protoplast also contributes to the direction of growth, the shape of the cell, and the proper positioning of the cell in a tissue. In essence, plant cell expansion represents the foundation of development. Most studies of plant cell expansion have focused primarily upon late divergent multicellular land plants and specialized cell types (e.g., pollen tubes, root hairs). Here, we describe a unicellular green alga, Penium margaritaceum (Penium), which can serve as a valuable model organism for understanding cell expansion and the underlying mechanics of the cell wall in a single plant cell.

Multicellularity in green algae: upsizing in a walled complex

Modern green algae constitute a large and diverse taxonomic assemblage that encompasses many multicellular phenotypes including colonial, filamentous, and parenchymatous forms. In all multicellular green algae, each cell is surrounded by an extracellular matrix (ECM), most often in the form of a cell wall. Volvocalean taxa like Volvox have an elaborate, gel-like, hydroxyproline rich glycoprotein covering that contains the cells of the colony. In "ulvophytes," uronic acid-rich and sulfated polysaccharides are the likely adhesion agents that maintain the multicellular habit. Charophytes also produce polysaccharide-rich cell walls and in late divergent taxa, pectin plays a critical role in cell adhesion in the multicellular complex. Cell walls are products of coordinated interaction of membrane trafficking, cytoskeletal dynamics and the cell's signal transduction machinery responding both to precise internal clocks and external environmental cues. Most often, these activities must be synchronized with the secretion, deposition and remodeling of the polymers of the ECM. Rapid advances in molecular genetics, cell biology and cell wall biochemistry of green algae will soon provide new insights into the evolution and subcellular processes leading to multicellularity.

Evidence for land plant cell wall biosynthetic mechanisms in charophyte green algae

BACKGROUND AND AIMS

The charophyte green algae (CGA) are thought to be the closest living relatives to the land plants, and ancestral CGA were unique in giving rise to the land plant lineage. The cell wall has been suggested to be a defining structure that enabled the green algal ancestor to colonize land. These cell walls provide support and protection, are a source of signalling molecules, and provide developmental cues for cell differentiation and elongation. The cell wall of land plants is a highly complex fibre composite, characterized by cellulose cross-linked by non-cellulosic polysaccharides, such as xyloglucan, embedded in a matrix of pectic polysaccharides. How the land plant cell wall evolved is currently unknown: early-divergent chlorophyte and prasinophyte algae genomes contain a low number of glycosyl transferases (GTs), while land plants contain hundreds. The number of GTs in CGA is currently unknown, as no genomes are available, so this study sought to give insight into the evolution of the biosynthetic machinery of CGA through an analysis of available transcriptomes.

METHODS

Available CGA transcriptomes were mined for cell wall biosynthesis GTs and compared with GTs characterized in land plants. In addition, gene cloning was employed in two cases to answer important evolutionary questions.

KEY RESULTS

Genetic evidence was obtained indicating that many of the most important core cell wall polysaccharides have their evolutionary origins in the CGA, including cellulose, mannan, xyloglucan, xylan and pectin, as well as arabino-galactan protein. Moreover, two putative cellulose synthase-like D family genes (CSLDs) from the CGA species Coleochaete orbicularis and a fragment of a putative CSLA/K-like sequence from a CGA Spirogyra species were cloned, providing the first evidence that all the cellulose synthase/-like genes present in early-divergent land plants were already present in CGA.

CONCLUSIONS

The results provide new insights into the evolution of cell walls and support the notion that the CGA were pre-adapted to life on land by virtue of the their cell wall biosynthetic capacity. These findings are highly significant for understanding plant cell wall evolution as they imply that some features of land plant cell walls evolved prior to the transition to land, rather than having evolved as a result of selection pressures inherent in this transition.

Ultrastructure of the Algivorous Amoeboflagellate Viridiraptor invadens (Glissomonadida, Cercozoa)

The family Viridiraptoridae represents a morphologically and ecologically distinct lineage of glissomonad flagellates (Cercozoa, Rhizaria). It currently comprises two highly specialised, algivorous genera inhabiting freshwater ecosystems, Orciraptor and Viridiraptor, for which ultrastructural data were lacking. In this study, the ultrastructure of Viridiraptor invadensHess et Melkonian, the sole described species of the viridiraptorid type genus, has been studied by transmission and scanning electron microscopy. In particular the flagellar transitional region and basal apparatus, both reconstructed from serial sections, revealed ultrastructural traits, that agree with the phylogenetic placement of the viridiraptorids within Glissomonadida: The transitional region contains a distal plate/collar complex and the basal apparatus comprises two ventral posterior roots and an anterior root, all known from other glissomonads. However, two additional small microtubular roots, two conspicuous rhizoplasts and probasal bodies present during interphase represent novel characteristics. Furthermore, an acorn/V-shaped filament system was discovered at the proximal end of the flagellar transitional region and used to establish a basal body triplet numbering system for flagellate cells of the Rhizaria. Finally, ultrastructural data on perforated algal cell walls suggest that the previously described reticulocyst of Viridiraptor represents an extrusome-derived, mesh-like coat supporting the invasion/feeding process.

Pectin metabolism and assembly in the cell wall of the charophyte green alga Penium margaritaceum

The pectin polymer homogalacturonan (HG) is a major component of land plant cell walls and is especially abundant in the middle lamella. Current models suggest that HG is deposited into the wall as a highly methylesterified polymer, demethylesterified by pectin methylesterase enzymes and cross-linked by calcium ions to form a gel. However, this idea is based largely on indirect evidence and in vitro studies. We took advantage of the wall architecture of the unicellular alga Penium margaritaceum, which forms an elaborate calcium cross-linked HG-rich lattice on its cell surface, to test this model and other aspects of pectin dynamics. Studies of live cells and microscopic imaging of wall domains confirmed that the degree of methylesterification and sufficient levels of calcium are critical for lattice formation in vivo. Pectinase treatments of live cells and immunological studies suggested the presence of another class of pectin polymer, rhamnogalacturonan I, and indicated its colocalization and structural association with HG. Carbohydrate microarray analysis of the walls of P. margaritaceum, Physcomitrella patens, and Arabidopsis (Arabidopsis thaliana) further suggested the conservation of pectin organization and interpolymer associations in the walls of green plants. The individual constituent HG polymers also have a similar size and branched structure to those of embryophytes. The HG-rich lattice of P. margaritaceum, a member of the charophyte green algae, the immediate ancestors of land plants, was shown to be important for cell adhesion. Therefore, the calcium-HG gel at the cell surface may represent an early evolutionary innovation that paved the way for an adhesive middle lamella in multicellular land plants.

Early local differentiation of the cell wall matrix defines the contact sites in lobed mesophyll cells of Zea mays

BACKGROUND AND AIMS

The morphogenesis of lobed mesophyll cells (MCs) is highly controlled and coupled with intercellular space formation. Cortical microtubule rings define the number and the position of MC isthmi. This work investigated early events of MC morphogenesis, especially the mechanism defining the position of contacts between MCs. The distributions of plasmodesmata, the hemicelluloses callose and (1 → 3,1 → 4)-β-d-glucans (MLGs) and the pectin epitopes recognized by the 2F4, JIM5, JIM7 and LM6 antibodies were studied in the cell walls of Zea mays MCs.

METHODS

Matrix cell wall polysaccharides were immunolocalized in hand-made sections and in sections of material embedded in LR White resin. Callose was also localized using aniline blue in hand-made sections. Plasmodesmata distribution was examined by transmission electron microscopy.

RESULTS

Before reorganization of the dispersed cortical microtubules into microtubule rings, particular bands of the longitudinal MC walls, where the MC contacts will form, locally differentiate by selective (1) deposition of callose and the pectin epitopes recognized by the 2F4, LM6, JIM5 and JIM7 antibodies, (2) degradation of MLGs and (3) formation of secondary plasmodesmata clusterings. This cell wall matrix differentiation persists in cell contacts of mature MCs. Simultaneously, the wall bands between those of future cell contacts differentiate with (1) deposition of local cell wall thickenings including cellulose microfibrils, (2) preferential presence of MLGs, (3) absence of callose and (4) transient presence of the pectins identified by the JIM5 and JIM7 antibodies. The wall areas between cell contacts expand determinately to form the cell isthmi and the cell lobes.

CONCLUSIONS

The morphogenesis of lobed MCs is characterized by the early patterned differentiation of two distinct cell wall subdomains, defining the sites of the future MC contacts and of the future MC isthmi respectively. This patterned cell wall differentiation precedes cortical microtubule reorganization and may define microtubule ring disposition.

Polar Expansion Dynamics in the Plant Kingdom: A Diverse and Multifunctional Journey on the Path to Pollen Tubes

Polar expansion is a widespread phenomenon in plants spanning all taxonomic groups from the Charophycean Green Algae to pollen tubes in Angiosperms and Gymnosperms. Current data strongly suggests that many common features are shared amongst cells displaying polar growth mechanics including changes to the structural features of localized regions of the cell wall, mobilization of targeted secretion mechanisms, employment of the actin cytoskeleton for directing secretion and in many cases, endocytosis and coordinated interaction of multiple signal transduction mechanisms prompted by external biotic and abiotic cues. The products of polar expansion perform diverse functions including delivery of male gametes to the egg, absorption, anchorage, adhesion and photo-absorption efficacy. A comparative analysis of polar expansion dynamics is provided with special emphasis on those found in early divergent plants.

Evolving views of pectin biosynthesis

Recent progress in the identification and characterization of pectin biosynthetic proteins and the discovery of pectin domain-containing proteoglycans are changing our view of how pectin, the most complex family of plant cell wall polysaccharides, is synthesized. The functional confirmation of four types of pectin biosynthetic glycosyltransferases, the identification of multiple putative pectin glycosyl- and methyltransferases, and the characteristics of the GAUT1:GAUT7 homogalacturonan biosynthetic complex with its novel mechanism for retaining catalytic subunits in the Golgi apparatus and its 12 putative interacting proteins are beginning to provide a framework for the pectin biosynthetic process. We propose two partially overlapping hypothetical and testable models for pectin synthesis: the consecutive glycosyltransferase model and the domain synthesis model.

The Cell Walls of Green Algae: A Journey through Evolution and Diversity

The green algae represent a large group of morphologically diverse photosynthetic eukaryotes that occupy virtually every photic habitat on the planet. The extracellular coverings of green algae including cell walls are also diverse. A recent surge of research in green algal cell walls fueled by new emerging technologies has revealed new and critical insight concerning these coverings. For example, the late divergent taxa of the Charophycean green algae possess cell walls containing assemblages of polymers with notable similarity to the cellulose, pectins, hemicelluloses, arabinogalactan proteins (AGPs), extensin, and lignin present in embryophyte walls. Ulvophycean seaweeds have cell wall components whose most abundant fibrillar constituents may change from cellulose to β-mannans to β-xylans and during different life cycle phases. Likewise, these algae produce complex sulfated polysaccharides, AGPs, and extensin. Chlorophycean green algae produce a wide array of walls ranging from cellulose-pectin complexes to ones made of hydroxyproline-rich glycoproteins. Larger and more detailed surveys of the green algal taxa including incorporation of emerging genomic and transcriptomic data are required in order to more fully resolve evolutionary trends within the green algae and in relationship with higher plants as well as potential applications of wall components in the food and pharmaceutical industries.

H2O2 localization in the green alga Micrasterias after salt and osmotic stress by TEM-coupled electron energy loss spectroscopy

Reactive oxygen species (ROS), including hydrogen peroxide (H(2)O(2)), are constantly generated as by-products of normal metabolic cellular pathways and can be overproduced in response to stress. In this study, we investigated ROS production and localization of H(2)O(2) after salt (200 mM KCl) and osmotic (iso-osmotic sorbitol concentration) stress in the unicellular green alga Micrasterias. By means of the dye H(2)DCFDA and confocal laser scanning microscopy, most ROS production could be detected in KCl-treated cells when compared to sorbitol-exposed cells and controls. For ultrastructural detection of H(2)O(2), CeCl(3), which reacts with H(2)O(2) and produces cerium perhydroxide deposits, has been used. Cerium was identified by transmission electron microscopy (TEM)-coupled electron energy loss spectroscopy (EELS) in organelles of KCl- and sorbitol-treated cells and in controls. Statistical measurements of the presence of the cerium M(4,5) edge were performed in mitochondria, chloroplasts, cell walls, and cytoplasmic sites of five individual cells after each treatment. The most pronounced increase in H(2)O(2) production was found in chloroplasts of KCl- and sorbitol-treated cells. This shows that the chloroplast reveals the strongest response in H(2)O(2) production after stress induction in Micrasterias. Significant elevation of H(2)O(2) production also occurred in mitochondria and cytoplasm, whereas H(2)O(2) levels remained unchanged or even slightly decreased in cell walls of treated cells. Additionally, TEM micrographs and EELS analyses provided indirect evidence for an increased H(2)O(2) production at the plasma membrane of KCl-treated cells, indicating an involvement of the plasma membrane NADPH oxidase in H(2)O(2) generation.