OCCURRENCE AND CHARACTERIZATION OF ARABINOGALACTAN-LIKE PROTEINS AND HEMICELLULOSES IN MICRASTERIAS (STREPTOPHYTA)(1)

Chara — a living sister to the land plants with pivotal enzymic toolkit for mannan and xylan remodelling

Land‐plant transglycosylases ‘cut‐and‐paste’ cell‐wall polysaccharides by endo‐transglycosylation (transglycanases) and exo‐transglycosylation (transglycosidases). Such enzymes may remodel the wall, adjusting extensibility and adhesion. Charophytes have cell‐wall polysaccharides that broadly resemble, but appreciably differ from land‐plants'. We investigated whether Chara vulgaris has wall‐restructuring enzymes mirroring those of land‐plants.

Wall enzymes extracted from Chara were assayed in vitro for transglycosylase activities on various donor substrates — β‐(1→4)‐glucan‐based [xyloglucan and mixed‐linkage glucans (MLGs)], β‐(1→4)‐xylans and β‐(1→4)‐mannans — plus related acceptor substrates (tritium‐labelled oligosaccharides, XXXGol, Xyl6‐ol and Man6‐ol), thus 12 donor:acceptor permutations. Also, fluorescent oligosaccharides were incubated in situ with Chara, revealing endogenous enzyme action on endogenous (potentially novel) polysaccharides.

Chara enzymes acted on the glucan‐based polysaccharides with [3H]XXXGol as acceptor substrate, demonstrating ‘glucan:glucan‐type’ transglucanases. Such activities were unexpected because Chara lacks biochemically detectable xyloglucan and MLG. With xylans as donor and [3H]Xyl6‐ol (but not [3H]Man6‐ol) as acceptor, high trans‐β‐xylanase activity was detected. With mannans as donor and either [3H]Man6‐ol or [3H]Xyl6‐ol as acceptor, we detected high levels of both mannan:mannan homo‐trans‐β‐mannanase and mannan:xylan hetero‐trans‐β‐mannanase activity, showing that Chara can not only ‘cut/paste’ these hemicelluloses by homo‐transglycosylation but also hetero‐transglycosylate them, forming mannan→xylan (but not xylan→mannan) hybrid hemicelluloses. In in‐situ assays, Chara walls attached endogenous polysaccharides to exogenous sulphorhodamine‐labelled Man6‐ol, indicating transglycanase (possibly trans‐mannanase) action on endogenous polysaccharides.

In conclusion, cell‐wall transglycosylases, comparable to but different from those of land‐plants, pre‐dated the divergence of the Charophyceae from its sister clade (Coleochaetophyceae/Zygnematophyceae/land‐plants). Thus, the ability to ‘cut/paste’ wall polysaccharides is an evolutionarily ancient streptophytic trait.

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.

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.

The cell wall of hornworts and liverworts: innovations in early land plant evolution?

An important step for plant diversification was the transition from freshwater to terrestrial habitats. The bryophytes and all vascular plants share a common ancestor that was probably the first to adapt to life on land. A polysaccharide-rich cell wall was necessary to cope with newly faced environmental conditions. Therefore, some pre-requisites for terrestrial life have to be shared in the lineages of modern bryophytes and vascular plants. This review focuses on hornwort and liverwort cell walls and aims to provide an overview on shared and divergent polysaccharide features between these two groups of bryophytes and vascular plants. Analytical, immunocytochemical, and bioinformatic data were analysed. The major classes of polysaccharides-cellulose, hemicelluloses, and pectins-seem to be present but have diversified structurally during evolution. Some polysaccharide groups show structural characteristics which separate hornworts from the other bryophytes or are too poorly studied in detail to be able to draw absolute conclusions. Hydroxyproline-rich glycoprotein backbones are found in hornworts and liverworts, and show differences in, for example, the occurrence of glycosylphosphatidylinositol (GPI)-anchored arabinogalactan-proteins, while glycosylation is practically unstudied. Overall, the data are an appeal to researchers in the field to gain more knowledge on cell wall structures in order to understand the changes with regard to bryophyte evolution.

Arabinogalactan Protein-Like Proteins From Ulva lactuca Activate Immune Responses and Plant Resistance in an Oilseed Crop

Natural compounds isolated from macroalgae are promising, ecofriendly, and multifunctional bioinoculants, which have been tested and used in agriculture. Ulvans, for instance, one of the major polysaccharides present in Ulva spp. cell walls, have been tested for their plant growth-promoting properties as well as their ability to activate plant immune defense, on a large variety of crops. Recently, we have characterized for the first time an arabinogalactan protein-like (AGP-like) from Ulva lactuca, which exhibits several features associated to land plant AGPs. In land plant, AGPs were shown to play a role in several plant biological functions, including cell morphogenesis, reproduction, and plant-microbe interactions. Thus, isolated AGP-like proteins may be good candidates for either the plant growth-promoting properties or the activation of plant immune defense. Here, we have isolated an AGP-like enriched fraction from Ulva lactuca and we have evaluated its ability to (i) protect oilseed rape (Brassica napus) cotyledons against Leptosphaeria maculans, and (ii) its ability to activate immune responses. Preventive application of the Ulva AGP-like enriched fraction on oilseed rape, followed by cotyledon inoculation with the fungal hemibiotroph L. maculans, resulted in a major reduction of infection propagation. The noticed reduction correlated with an accumulation of H₂O₂ in treated cotyledons and with the activation of SA and ET signaling pathways in oilseed rape cotyledons. In parallel, an ulvan was also isolated from Ulva lactuca. Preventive application of ulvan also enhanced plant resistance against L. maculans. Surprisingly, reduction of infection severity was only observed at high concentration of ulvan. Here, no such significant changes in gene expression and H₂O₂ production were observed. Together, this study indicates that U. lactuca AGP-like glycoproteins exhibit promising elicitor activity and that plant eliciting properties of Ulva extract, might result not only from an ulvan-originated eliciting activities, but also AGP-like originated.

Over 100-Year Preservation and Temporal Fluctuations of Cell Wall Polysaccharides in Marine Sediments

Polysaccharides constitute an important carbon pool in marine systems, but much is still unknown about the fate and degradation of these compounds. They are derived partly from production in situ, and in coastal areas, they are partly terrestrially derived, originating from freshwater runoff from land. The aim of this study was to test the applicability of high-throughput polysaccharide profiling for plant and algal cell-wall compounds in dated sediment cores from a coastal marine environment, to examine the preservation of cell-wall polysaccharides and explore their potential as proxies for temporal environmental changes. Preserved compounds and remains of organisms are routinely used as paleoenvironmental proxies as the amount and composition of different compounds that can provide insight into past environmental conditions, and novel means for reporting environmental changes are highly sought.

From Cancer Therapy to Winemaking: The Molecular Structure and Applications of β-Glucans and β-1, 3-Glucanases

β-glucans are a diverse group of polysaccharides composed of β-1,3 or β-(1,3-1,4) linked glucose monomers. They are mainly synthesized by fungi, plants, seaweed and bacteria, where they carry out structural, protective and energy storage roles. Because of their unique physicochemical properties, they have important applications in several industrial, biomedical and biotechnological processes. β-glucans are also major bioactive molecules with marked immunomodulatory and metabolic properties. As such, they have been the focus of many studies attesting to their ability to, among other roles, fight cancer, reduce the risk of cardiovascular diseases and control diabetes. The physicochemical and functional profiles of β-glucans are deeply influenced by their molecular structure. This structure governs β-glucan interaction with multiple β-glucan binding proteins, triggering myriad biological responses. It is then imperative to understand the structural properties of β-glucans to fully reveal their biological roles and potential applications. The deconstruction of β-glucans is a result of β-glucanase activity. In addition to being invaluable tools for the study of β-glucans, these enzymes have applications in numerous biotechnological and industrial processes, both alone and in conjunction with their natural substrates. Here, we review potential applications for β-glucans and β-glucanases, and explore how their functionalities are dictated by their structure.

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.

Raman imaging of Micrasterias: new insights into shape formation

The algae Micrasterias with its star-shaped cell pattern is a perfect unicellular model system to study morphogenesis. How the indentations are formed in the primary cell wall at exactly defined areas puzzled scientists for decades, and they searched for chemical differences in the primary wall of the extending tips compared to the resting indents. We now tackled the question by Raman imaging and scanned in situ Micrasterias cells at different stages of development. Thousands of Raman spectra were acquired from the mother cell and the developing semicell to calculate chemical images based on an algorithm finding the most different Raman spectra. Each of those spectra had characteristic Raman bands, which were assigned to molecular vibrations of BaSO4, proteins, lipids, starch, and plant cell wall carbohydrates. Visualizing the cell wall carbohydrates revealed a cell wall thickening at the indentations of the primary cell wall of the growing semicell and uniplanar orientation of the cellulose microfibrils to the cell surface in the secondary cell wall. Crystalline cellulose dominated in the secondary cell wall spectra, while in the primary cell wall spectra, also xyloglucan and pectin were reflected. Spectral differences between the indent and tip region of the primary cell wall were scarce, but a spectral mixing approach pointed to more cellulose fibrils deposited in the indent region. Therefore, we suggest that cell wall thickening together with a denser network of cellulose microfibrils stiffens the cell wall at the indent and induces different cell wall extensibility to shape the lobes.

Precursor biosynthesis regulation of lignin, suberin and cutin

The extracellular matrix of plants can contain the hydrophobic biopolymers lignin, suberin and/or cutin, which provide mechanical strength and limit water loss and pathogen invasion. Due to their remarkable chemical resistance, these polymers have a high potential in various biotechnological applications and can replace petrol-based resources, for example, in the packing industry. However, despite the importance of these polymers, the regulation of their precursor biosynthesis is far from being fully understood. This is particularly true for suberin and cutin, which hinders efforts to engineer their formation in plants and produce customised biopolymers. This review brings attention to knowledge gaps in the current research and highlights some of the most recent findings on transcription factors that regulate lignin, suberin and cutin precursor biosynthesis. Finally, we also briefly discuss how some of the remaining knowledge gaps can be closed.

Hemicellulose-remodelling transglycanase activities from charophytes: towards the evolution of the land-plant cell wall

Transglycanases remodel cell-wall polymers, having a critical impact on many physiological processes. Unlike xyloglucan endotransglucosylase (XET) activity, widely studied in land plants, very little is known about charophyte wall-modifying enzymes - information that would promote our understanding of the 'primordial' wall, revealing how the wall matrix is remodelled in the closest living algal relatives of land plants, and what changed during terrestrialisation. We conducted various in-vitro assays for wall-remodelling transglycosylases, monitoring either (a) polysaccharide-to-[³ H]oligosaccharide transglycosylation or (b) non-radioactive oligosaccharide-to-oligosaccharide transglycosylation. We screened a wide collection of enzyme extracts from charophytes (and early-diverging land plants for comparison) and discovered several homo- and hetero-transglycanase activities. In contrast to most land plants, charophytes possess high trans-β-1,4-mannanase activity, suggesting that land plants' algal ancestors prioritised mannan remodelling. Trans-β-1,4-xylanase activity was also found, most abundantly in Chara, Nitella and Klebsormidium. Exo-acting transglycosidase activities (trans-β-1,4-xylosidase and trans-β-1,4-mannosidase) were also detected. In addition, charophytes exhibited homo- and hetero-trans-β-glucanase activities (XET, mixed-linkage glucan [MLG]:xyloglucan endotransglucosylase and cellulose:xyloglucan endotransglucosylase) despite the paucity or lack of land-plant-like xyloglucan and MLG as potential donor substrates in their cell walls. However, trans-α-xylosidase activity (which remodels xyloglucan in angiosperms) was absent in charophytes and early-diverging land plants. Transglycanase action was also found in situ, acting on endogenous algal polysaccharides as donor substrates and fluorescent xyloglucan oligosaccharides as acceptor substrates. We conclude that trans-β-mannanase and trans-β-xylanase activities are present and thus may play key roles in charophyte walls (most of which possess little or no xyloglucan and MLG, but often contain abundant β-mannans and β-xylans), comparable to the roles of XET in xyloglucan-rich land plants.

Arabinogalactan-like Glycoproteins from Ulva lactuca (Chlorophyta) Show Unique Features Compared to Land Plants AGPs

Arabinogalactan proteins (AGPs) encompass a diverse group of plant cell wall proteoglycans, which play an essential role in plant development, signaling, plant-microbe interactions, and many others. Although they are widely distributed throughout the plant kingdom and extensively studied, they remain largely unexplored in the lower plants, especially in seaweeds. Ulva species have high economic potential since various applications were previously described including bioremediation, biofuel production, and as a source of bioactive compounds. This article presents the first experimental confirmation of AGP-like glycoproteins in Ulva species and provides a simple extraction protocol of Ulva lactuca AGP-like glycoproteins, their partial characterization and unique comparison to scarcely described Solanum lycopersicum AGPs. The reactivity with primary anti-AGP antibodies as well as Yariv reagent showed a great variety between Ulva lactuca and Solanum lycopersicum AGP-like glycoproteins. While the amino acid analysis of the AGP-like glycoproteins purified by the β-d-glucosyl Yariv reagent showed a similarity between algal and land plant AGP-like glycoproteins, neutral saccharide analysis revealed unique glycosylation of the Ulva lactuca AGP-like glycoproteins. Surprisingly, arabinose and galactose were not the most prevalent monosaccharides and the most outstanding was the presence of 3-O-methyl-hexose, which has never been described in the AGPs. The exceptional structure of the Ulva lactuca AGP-like glycoproteins implies a specialized adaptation to the marine environment and might bring new insight into the evolution of the plant cell wall.

Structures, Biosynthesis, and Physiological Functions of (1,3;1,4)-β-D-Glucans

(1,3;1,4)-β-d-Glucans, also named as mixed-linkage glucans, are unbranched non-cellulosic polysaccharides containing both (1,3)- and (1,4)-β-linkages. The linkage ratio varies depending upon species origin and has a significant impact on the physicochemical properties of the (1,3;1,4)-β-d-glucans. (1,3;1,4)-β-d-Glucans were thought to be unique in the grasses family (Poaceae); however, evidence has shown that (1,3;1,4)-β-d-glucans are also synthesized in other taxa, including horsetail fern Equisetum, algae, lichens, and fungi, and more recently, bacteria. The enzyme involved in (1,3;1,4)-β-d-glucan biosynthesis has been well studied in grasses and cereal. However, how this enzyme is able to assemble the two different linkages remains a matter of debate. Additionally, the presence of (1,3;1,4)-β-d-glucan across the species evolutionarily distant from Poaceae but absence in some evolutionarily closely related species suggest that the synthesis is either highly conserved or has arisen twice as a result of convergent evolution. Here, we compare the structure of (1,3;1,4)-β-d-glucans present across various taxonomic groups and provide up-to-date information on how (1,3;1,4)-β-d-glucans are synthesized and their functions.

Localization, Extraction, and Quantification of Plant and Algal Arabinogalactan Proteins

Arabinogalactan proteins are a diverse group of cell wall-associated proteoglycans. While structural and molecular genetic analyses have contributed to the emerging improved understanding of the wide-range of biological processes in which AGPs are implicated; the ability to detect, localize, and quantify them is fundamentally important. This chapter describes three methods: histological staining, radial gel diffusion, and colorimetric quantification, each of which utilize the ability of Yariv reagent to bind to AGPs.

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.

N- and O-Glycosylation Pathways in the Microalgae Polyphyletic Group

The term microalga refers to various unicellular and photosynthetic organisms representing a polyphyletic group. It gathers numerous species, which can be found in cyanobacteria (i.e., Arthrospira) as well as in distinct eukaryotic groups, such as Chlorophytes (i.e., Chlamydomonas or Chlorella) and Heterokonts (i.e., diatoms). This phylogenetic diversity results in an extraordinary variety of metabolic pathways, offering large possibilities for the production of natural compounds like pigments or lipids that can explain the ever-growing interest of industrials for these organisms since the middle of the last century. More recently, several species have received particular attention as biofactories for the production of recombinant proteins. Indeed, microalgae are easy to grow, safe and cheap making them attractive alternatives as heterologous expression systems. In this last scope of applications, the glycosylation capacity of these organisms must be considered as this post-translational modification of proteins impacts their structural and biological features. Although these mechanisms are well known in various Eukaryotes like mammals, plants or insects, only a few studies have been undertaken for the investigation of the protein glycosylation in microalgae. Recently, significant progresses have been made especially regarding protein N-glycosylation, while O-glycosylation remain poorly known. This review aims at summarizing the recent data in order to assess the state-of-the art knowledge in glycosylation processing in microalgae.

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.

The laterally acquired GH5 ZgEngAGH5_4 from the marine bacterium Zobellia galactanivorans is dedicated to hemicellulose hydrolysis

Cell walls of marine macroalgae are composed of diverse polysaccharides that provide abundant carbon sources for marine heterotrophic bacteria. Among them, Zobellia galactanivorans is considered as a model for studying algae-bacteria interactions. The degradation of typical algal polysaccharides, such as agars or alginate, has been intensively studied in this model bacterium, but the catabolism of plant-like polysaccharides is essentially uncharacterized. Here, we identify a polysaccharide utilization locus in the genome of Z. galactanivorans, induced by laminarin (β-1,3-glucans), and containing a putative GH5 subfamily 4 (GH5_4) enzyme, currently annotated as a endoglucanase (ZgEngA_{GH5_4}). A phylogenetic analysis indicates that ZgEngA_{GH5_4} was laterally acquired from an ancestral Actinobacteria We performed the biochemical and structural characterization of ZgEngA_{GH5_4} and demonstrated that this GH5 is, in fact, an endo-β-glucanase, most active on mixed-linked glucan (MLG). Although ZgEngA_{GH5_4} and GH16 lichenases both hydrolyze MLG, these two types of enzymes release different series of oligosaccharides. Structural analyses of ZgEngA_{GH5_4} reveal that all the amino acid residues involved in the catalytic triad and in the negative glucose-binding subsites are conserved, when compared with the closest relative, the cellulase EngD from Clostridium cellulovorans, and some other GH5s. In contrast, the positive glucose-binding subsites of ZgEngA_{GH5_4} are different and this could explain the preference for MLG, with respect to cellulose or laminarin. Molecular dynamics computer simulations using different hexaoses reveal that the specificity for MLG occurs through the +1 and +2 subsites of the binding pocket that display the most important differences when compared with the structures of other GH5_4 enzymes.

The Secretome and N-Glycosylation Profiles of the Charophycean Green Alga, Penium margaritaceum, Resemble Those of Embryophytes

The secretome can be defined as the population of proteins that are secreted into the extracellular environment. Many proteins that are secreted by eukaryotes are N-glycosylated. However, there are striking differences in the diversity and conservation of N-glycosylation patterns between taxa. For example, the secretome and N-glycosylation structures differ between land plants and chlorophyte green algae, but it is not clear when this divergence took place during plant evolution. A potentially valuable system to study this issue is provided by the charophycean green algae (CGA), which is the immediate ancestors of land plants. In this study, we used lectin affinity chromatography (LAC) coupled with mass spectrometry to characterize the secretome including secreted N-glycoproteins of Penium margaritaceum, which is a member of the CGA. The identified secreted proteins and N-glycans were compared to those known from the chlorophyte green alga Chlamydomonas reinhardtii and the model land plant, Arabidopsis thaliana, to establish their evolutionary context. Our approach allowed the identification of cell wall proteins and proteins modified with N-glycans that are identical to those of embryophytes, which suggests that the P. margaritaceum secretome is more closely related to those of land plants than to those of chlorophytes. The results of this study support the hypothesis that many of the proteins associated with plant cell wall modification as well as other extracellular processes evolved prior to the colonization of terrestrial habitats.

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.

Insoluble (1 → 3), (1 → 4)-β-D-glucan is a component of cell walls in brown algae (Phaeophyceae) and is masked by alginates in tissues

Brown algae are photosynthetic multicellular marine organisms. They belong to the phylum of Stramenopiles, which are not closely related to land plants and green algae. Brown algae share common evolutionary features with other photosynthetic and multicellular organisms, including a carbohydrate-rich cell-wall. Brown algal cell walls are composed predominantly of the polyanionic polysaccharides alginates and fucose-containing sulfated polysaccharides. These polymers are prevalent over neutral and crystalline components, which are believed to be mostly, if not exclusively, cellulose. In an attempt to better understand brown algal cell walls, we performed an extensive glycan array analysis of a wide range of brown algal species. Here we provide the first demonstration that mixed-linkage (1 → 3), (1 → 4)-β-D-glucan (MLG) is common in brown algal cell walls. Ultra-Performance Liquid Chromatography analyses indicate that MLG in brown algae solely consists of trisaccharide units of contiguous (1 → 4)-β-linked glucose residues joined by (1 → 3)-β-linkages. This regular conformation may allow long stretches of the molecule to align and to form well-structured microfibrils. At the tissue level, immunofluorescence studies indicate that MLG epitopes in brown algae are unmasked by a pre-treatment with alginate lyases to remove alginates. These findings are further discussed in terms of the origin and evolution of MLG in the Stramenopile lineage.

Micrasterias as a Model System in Plant Cell Biology

The unicellular freshwater alga Micrasterias denticulata is an exceptional organism due to its complex star-shaped, highly symmetric morphology and has thus attracted the interest of researchers for many decades. As a member of the Streptophyta, Micrasterias is not only genetically closely related to higher land plants but shares common features with them in many physiological and cell biological aspects. These facts, together with its considerable cell size of about 200 μm, its modest cultivation conditions and the uncomplicated accessibility particularly to any microscopic techniques, make Micrasterias a very well suited cell biological plant model system. The review focuses particularly on cell wall formation and composition, dictyosomal structure and function, cytoskeleton control of growth and morphogenesis as well as on ionic regulation and signal transduction. It has been also shown in the recent years that Micrasterias is a highly sensitive indicator for environmental stress impact such as heavy metals, high salinity, oxidative stress or starvation. Stress induced organelle degradation, autophagy, adaption and detoxification mechanisms have moved in the center of interest and have been investigated with modern microscopic techniques such as 3-D- and analytical electron microscopy as well as with biochemical, physiological and molecular approaches. This review is intended to summarize and discuss the most important results obtained in Micrasterias in the last 20 years and to compare the results to similar processes in higher plant cells.

Immuno and Affinity Cytochemical Analysis of Cell Wall Composition in the Moss Physcomitrella patens

In contrast to homeohydric vascular plants, mosses employ a poikilohydric strategy for surviving in the dry aerial environment. A detailed understanding of the structure, composition, and development of moss cell walls can contribute to our understanding of not only the evolution of overall cell wall complexity, but also the differences that have evolved in response to selection for different survival strategies. The model moss species Physcomitrella patens has a predominantly haploid lifecycle consisting of protonemal filaments that regenerate from protoplasts and enlarge by tip growth, and leafy gametophores composed of cells that enlarge by diffuse growth and differentiate into several different types. Advantages for genetic studies include methods for efficient targeted gene modification and extensive genomic resources. Immuno and affinity cytochemical labeling were used to examine the distribution of polysaccharides and proteins in regenerated protoplasts, protonemal filaments, rhizoids, and sectioned gametophores of P. patens. The cell wall composition of regenerated protoplasts was also characterized by flow cytometry. Crystalline cellulose was abundant in the cell walls of regenerating protoplasts and protonemal cells that developed on media of high osmolarity, whereas homogalactuonan was detected in the walls of protonemal cells that developed on low osmolarity media and not in regenerating protoplasts. Mannan was the major hemicellulose detected in all tissues tested. Arabinogalactan proteins were detected in different cell types by different probes, consistent with structural heterogneity. The results reveal developmental and cell type specific differences in cell wall composition and provide a basis for analyzing cell wall phenotypes in knockout mutants.

Bioinformatic Identification and Analysis of Extensins in the Plant Kingdom

Extensins (EXTs) are a family of plant cell wall hydroxyproline-rich glycoproteins (HRGPs) that are implicated to play important roles in plant growth, development, and defense. Structurally, EXTs are characterized by the repeated occurrence of serine (Ser) followed by three to five prolines (Pro) residues, which are hydroxylated as hydroxyproline (Hyp) and glycosylated. Some EXTs have Tyrosine (Tyr)-X-Tyr (where X can be any amino acid) motifs that are responsible for intramolecular or intermolecular cross-linkings. EXTs can be divided into several classes: classical EXTs, short EXTs, leucine-rich repeat extensins (LRXs), proline-rich extensin-like receptor kinases (PERKs), formin-homolog EXTs (FH EXTs), chimeric EXTs, and long chimeric EXTs. To guide future research on the EXTs and understand evolutionary history of EXTs in the plant kingdom, a bioinformatics study was conducted to identify and classify EXTs from 16 fully sequenced plant genomes, including Ostreococcus lucimarinus, Chlamydomonas reinhardtii, Volvox carteri, Klebsormidium flaccidum, Physcomitrella patens, Selaginella moellendorffii, Pinus taeda, Picea abies, Brachypodium distachyon, Zea mays, Oryza sativa, Glycine max, Medicago truncatula, Brassica rapa, Solanum lycopersicum, and Solanum tuberosum, to supplement data previously obtained from Arabidopsis thaliana and Populus trichocarpa. A total of 758 EXTs were newly identified, including 87 classical EXTs, 97 short EXTs, 61 LRXs, 75 PERKs, 54 FH EXTs, 38 long chimeric EXTs, and 346 other chimeric EXTs. Several notable findings were made: (1) classical EXTs were likely derived after the terrestrialization of plants; (2) LRXs, PERKs, and FHs were derived earlier than classical EXTs; (3) monocots have few classical EXTs; (4) Eudicots have the greatest number of classical EXTs and Tyr-X-Tyr cross-linking motifs are predominantly in classical EXTs; (5) green algae have no classical EXTs but have a number of long chimeric EXTs that are absent in embryophytes. Furthermore, phylogenetic analysis was conducted of LRXs, PERKs and FH EXTs, which shed light on the evolution of three EXT classes.

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.

Structural stress responses and degradation of dictyosomes in algae analysed by TEM and FIB-SEM tomography

Stress-induced physiological deficiencies in cells are reflected in structural, morphological and functional reactions of organelles. Although numerous investigations have focused on chloroplasts and mitochondria as main targets of different stressors in plant cells, there is insufficient information on the plant Golgi apparatus as stress sensor. By using the advantages of field emission scanning electron microscopy tomography in combination with classical ultrathin sectioning and transmission electron microscopic analyses, we provide structural evidence for common stress responses of the large and highly stable dictyosomes in the algal model system Micrasterias. Stress is induced by different metals such as manganese and lead, by starvation in 9 weeks of darkness or by inhibiting photosynthesis or glycolysis and by disturbing ionic homeostasis via KCl. For the first time a stress-induced degradation pathway of dictyosomes is described that does not follow "classical" autophagy but occurs by disintegration of cisternae into single membrane balls that seem to be finally absorbed by the endoplasmic reticulum (ER). Comparison of the morphological features that accompany dictyosomal degradation in Micrasterias to similar reactions observed during the same stress application in Nitella indicates an ubiquitous degradation process at least in algae. As the algae investigated belong to the closest relatives of higher land plants these results may also be relevant for understanding dictyosomal stress and degradation responses in the latter phylogenetic group. In addition, this study shows that two-dimensional transmission electron microscopy is insufficient for elucidating complex processes such as organelle degradation, and that information from three-dimensional reconstructions as provided by field emission scanning electron microscopy tomography is absolutely required for a comprehensive understanding of the phenomenon.

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.

Subcellular Sequestration and Impact of Heavy Metals on the Ultrastructure and Physiology of the Multicellular Freshwater Alga Desmidium swartzii

Due to modern life with increasing traffic, industrial production and agricultural practices, high amounts of heavy metals enter ecosystems and pollute soil and water. As a result, metals can be accumulated in plants and particularly in algae inhabiting peat bogs of low pH and high air humidity. In the present study, we investigated the impact and intracellular targets of aluminum, copper, cadmium, chromium VI and zinc on the filamentous green alga Desmidium swartzii, which is an important biomass producer in acid peat bogs. By means of transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) it is shown that all metals examined are taken up into Desmidium readily, where they are sequestered in cell walls and/or intracellular compartments. They cause effects on cell ultrastructure to different degrees and additionally disturb photosynthetic activity and biomass production. Our study shows a clear correlation between toxicity of a metal and the ability of the algae to compartmentalize it intracellularly. Cadmium and chromium, which are not compartmentalized, exert the most toxic effects. In addition, this study shows that the filamentous alga Desmidium reacts more sensitively to aluminum and zinc when compared to its unicellular relative Micrasterias, indicating a severe threat to the ecosystem.

Determining the subcellular location of synthesis and assembly of the cell wall polysaccharide (1,3; 1,4)-β-D-glucan in grasses

The current dogma for cell wall polysaccharide biosynthesis is that cellulose (and callose) is synthesized at the plasma membrane (PM), whereas matrix phase polysaccharides are assembled in the Golgi apparatus. We provide evidence that (1,3;1,4)-β-D-glucan (mixed-linkage glucan [MLG]) does not conform to this paradigm. We show in various grass (Poaceae) species that MLG-specific antibody labeling is present in the wall but absent over Golgi, suggesting it is assembled at the PM. Antibodies to the MLG synthases, cellulose synthase-like F6 (CSLF6) and CSLH1, located CSLF6 to the endoplasmic reticulum, Golgi, secretory vesicles, and the PM and CSLH1 to the same locations apart from the PM. This pattern was recreated upon expression of VENUS-tagged barley (Hordeum vulgare) CSLF6 and CSLH1 in Nicotiana benthamiana leaves and, consistent with our biochemical analyses of native grass tissues, shown to be catalytically active with CSLF6 and CSLH1 in PM-enriched and PM-depleted membrane fractions, respectively. These data support a PM location for the synthesis of MLG by CSLF6, the predominant enzymatically active isoform. A model is proposed to guide future experimental approaches to dissect the molecular mechanism(s) of MLG assembly.

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.

Future directions for the development of Chlamydomonas-based vaccines

Besides serving as a valuable model in biological sciences, Chamydomonas reinhardtii has been used during the last decade in the biotechnology arena to establish models for the low cost production of vaccines. Antigens from various pathogens including Plasmodium falciparum, foot and mouth disease virus, Staphylococcus aureus, classical swine fever virus (CSFV) as well as some auto-antigens, have been produced in C. reinhardtii. Although some of them have been functionally characterized with promising results, this review identifies future directions for the advancement in the exploitation of this robust and safe vaccine production platform. The present analysis reflects that important immunological implications exist for this system and remain unexplored, including the possible adjuvant effects of algae biomolecules, the effect of bioencapsulation on immunogenicity and the possible development of whole-cell vaccines as an approach to trigger cytotoxic immune responses. Recently described molecular strategies that aim to optimize the expression of nuclear-encoded target antigens are also discussed.

An acetate-hydroxide gradient for the quantitation of the neutral sugar and uronic acid profile of pectins by HPAEC-PAD without postcolumn pH adjustment

An HPAEC-PAD method was developed and validated to quantitate seven neutral sugars and two uronic acids of hydrolyzed pectic polysaccharides without postcolumn pH adjustment. Due to a short gradient phase minimizing the ion concentrations after equilibrating the CarboPac PA20 column with sodium acetate and hydroxide, subsequent isocratic separation of the neutral sugars was characterized by almost baseline resolution of rhamnose and arabinose (1.45 ± 0.15) and xylose and mannose (1.21 ± 0.02) at their maximal concentrations. Linearity was shown (R² = 0.9975-0.9998) for the relevant ranges (0.28-30.3 μmol L⁻¹); galacturonic acid, 1.7-128 μmol L⁻¹) above the limits of detection (30-81 nmol L⁻¹; galacturonic acid, 179 nmol L⁻¹) and ∼3.8 times higher limits of quantification. Conformity of the findings for four pectins after methanolysis plus hydrolysis in trifluoroacetic acid with those of reference procedures (total uronic acids, 95-102%; total neutral sugars, 97-105%) proved the accuracy.

Evolution and development of cell walls in cereal grains

The composition of cell walls in cereal grains and other grass species differs markedly from walls in seeds of other plants. In the maternal tissues that surround the embryo and endosperm of the grain, walls contain higher levels of cellulose and in many cases are heavily lignified. This may be contrasted with walls of the endosperm, where the amount of cellulose is relatively low, and the walls are generally not lignified. The low cellulose and lignin contents are possible because the walls of the endosperm perform no load-bearing function in the mature grain and indeed the low levels of these relatively intractable wall components are necessary because they allow rapid degradation of the walls following germination of the grain. The major non-cellulosic components of endosperm walls are usually heteroxylans and (1,3;1,4)-β-glucans, with lower levels of xyloglucans, glucomannans, and pectic polysaccharides. Pectic polysaccharides and xyloglucans are the major non-cellulosic wall constituents in most dicot species, in which (1,3;1,4)-β-glucans are usually absent and heteroxylans are found at relatively low levels. Thus, the "core" non-cellulosic wall polysaccharides in grain of the cereals and other grasses are the heteroxylans and, more specifically, arabinoxylans. The (1,3;1,4)-β-glucans appear in the endosperm of some grass species but are essentially absent from others; they may constitute from zero to more than 45% of the cell walls of the endosperm, depending on the species. It is clear that in some cases these (1,3;1,4)-β-glucans function as a major store of metabolizable glucose in the grain. Cereal grains and their constituent cell wall polysaccharides are centrally important as a source of dietary fiber in human societies and breeders have started to select for high levels of non-cellulosic wall polysaccharides in grain. To meet end-user requirements, it is important that we understand cell wall biology in the grain both during development and following germination.

3-D analysis of dictyosomes and multivesicular bodies in the green alga Micrasterias denticulata by FIB/SEM tomography

In the present study we employ FIB/SEM tomography for analyzing 3-D architecture of dictyosomes and formation of multivesicular bodies (MVB) in high pressure frozen and cryo-substituted interphase cells of the green algal model system Micrasterias denticulata. The ability of FIB/SEM of milling very thin ‘slices’ (5–10 nm), viewing the block face and of capturing cytoplasmic volumes of several hundred μm³ provides new insight into the close spatial connection of the ER–Golgi machinery in an algal cell particularly in z-direction, complementary to informations obtained by TEM serial sectioning or electron tomography.

Our FIB/SEM series and 3-D reconstructions show that interphase dictyosomes of Micrasterias are not only closely associated to an ER system at their cis-side which is common in various plant cells, but are surrounded by a huge “trans-ER” sheath leading to an almost complete enwrapping of dictyosomes by the ER. This is particularly interesting as the presence of a trans-dictyosomal ER system is well known from mammalian secretory cells but not from cells of higher plants to which the alga Micrasterias is closely related. In contrast to findings in plant storage tissue indicating that MVBs originate from the trans-Golgi network or its derivatives our investigations show that MVBs in Micrasterias are in direct spatial contact with both, trans-Golgi cisternae and the trans-ER sheath which provides evidence that both endomembrane compartments are involved in their formation.

Walkabout on the long branches of plant evolution

The closest living relatives of land plants, the Charophytes, and early diverging land plant lineages, the bryophytes, reside in a phylogenetic grade. Recent analyses have resolved relationships and demonstrated that some components of the land plant developmental genetic toolkit have their origin in algal ancestors. Phylogenetic grades of taxa imply long independent evolutionary histories, with extant species diversity potentially relictual and highly derived morphologically, making reconstruction of ancestral states problematic. Incorporating data on the genetic bases of character states may be phylogenetically informative in elucidating ancestral states in cases where morphology is highly divergent.

Immunolocalization of arabinogalactan proteins (AGPs) in reproductive structures of an early-divergent angiosperm, Trithuria (Hydatellaceae)

BACKGROUND AND AIMS

Trithuria is the sole genus of Hydatellaceae, a family of the early-divergent angiosperm lineage Nymphaeales (water-lilies). In this study different arabinogalactan protein (AGP) epitopes in T. submersa were evaluated in order to understand the diversity of these proteins and their functions in flowering plants.

METHODS

Immunolabelling of different AGPs and pectin epitopes in reproductive structures of T. submersa at the stage of early seed development was achieved by immunofluorescence of specific antibodies.

KEY RESULTS

AGPs in Trithuria pistil tissues could be important as structural proteins and also as possible signalling molecules. Intense labelling was obtained with anti-AGP antibodies both in the anthers and in the intine wall, the latter associated with pollen tube emergence.

CONCLUSIONS

AGPs could play a significant role in Trithuria reproduction, due to their specific presence in the pollen tube pathway. The results agree with labellings obtained for Arabidopsis and confirms the importance of AGPs in angiosperm reproductive structures as essential structural components and probably important signalling molecules.

The quest for four-dimensional imaging in plant cell biology: it's just a matter of time

BACKGROUND

Analysis of plant cell dynamics over time, or four-dimensional imaging (4-DI), represents a major goal of plant science. The ability to resolve structures in the third dimension within the cell or tissue during developmental events or in response to environmental or experimental stresses (i.e. 4-DI) is critical to our understanding of gene expression, post-expression modulations of macromolecules and sub-cellular system interactions.

SCOPE

Microscopy-based technologies have been profoundly integral to this type of investigation, and new and refined microscopy technologies now allow for the visualization of cell dynamics with unprecedented resolution, contrast and experimental versatility. However, certain realities of light and electron microscopy, choice of specimen and specimen preparation techniques limit the scope of readily attaining 4-DI. Today, the plant microscopist must use a combinatorial strategy whereby multiple microscopy-based investigations are used. Modern fluorescence, confocal laser scanning, transmission electron and scanning electron microscopy provide effective conduits for synthesizing data detailing live cell dynamics and highly resolved snapshots of specific cell structures that will ultimately lead to 4-DI. This review provides a synopsis of such technologies available.

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.

Evolution of mixed-linkage (1 -> 3, 1 -> 4)-β-D-glucan (MLG) and xyloglucan in Equisetum (horsetails) and other monilophytes

BACKGROUND AND AIMS

Horsetails (Equisetopsida) diverged from other extant eusporangiate monilophytes in the Upper Palaeozoic. They are the only monilophytes known to contain the hemicellulose mixed-linkage (1 → 3, 1 → 4)-β-d-glucan (MLG), whereas all land plants possess xyloglucan. It has been reported that changes in cell-wall chemistry often accompanied major evolutionary steps. We explored changes in hemicelluloses occurring during Equisetum evolution.

METHODS

Hemicellulose from numerous monilophytes was treated with lichenase and xyloglucan endoglucanase. Lichenase digests MLG to di-, tri- and tetrasaccharide repeat-units, resolvable by thin-layer chromatography.

KEY RESULTS

Among monilophytes, MLG was confined to horsetails. Our analyses support a basal trichotomy of extant horsetails: MLG was more abundant in subgenus Equisetum than in subgenus Hippochaete, and uniquely the sister group E. bogotense yielded almost solely the tetrasaccharide repeat-unit (G4G4G3G). Other species also gave the disaccharide, whereas the trisaccharide was consistently very scarce. Tetrasaccharide : disaccharide ratios varied interspecifically, but with no consistent difference between subgenera. Xyloglucan was scarce in Psilotum and subgenus Equisetum, but abundant in subgenus Hippochaete and in the eusporangiate ferns Marattia and Angiopteris; leptosporangiate ferns varied widely. All monilophytes shared a core pattern of xyloglucan repeat-units, major XEG products co-chromatographing on thin-layer chromatography with non-fucosylated hepta-, octa- and nonasaccharides and fucose-containing nona- and decasaccharides.

CONCLUSIONS

G4G4G3G is the ancestral repeat-unit of horsetail MLG. Horsetail evolution was accompanied by quantitative and qualitative modification of MLG; variation within subgenus Hippochaete suggests that the structure and biosynthesis of MLG is evolutionarily plastic. Xyloglucan quantity correlates negatively with abundance of other hemicelluloses; but qualitatively, all monilophyte xyloglucans conform to a core pattern of repeat-unit sizes.

Intracellular chromium localization and cell physiological response in the unicellular alga Micrasterias

Various contaminants like metals and heavy metals are constantly released into the environment by anthropogenic activities. The heavy metal chromium has a wide industrial use and exists in two stable oxidation states: trivalent and hexavalent. Chromium can cause harm to cell metabolism and development, when it is taken up by plants instead of necessary micronutrients such as for example iron. The uptake of Cr VI into plant cells has been reported to be an active process via carriers of essential anions, while the cation Cr III seems to be taken up inactively. Micrasterias denticulata, an unicellular green alga of the family Desmidiaceae is a well-studied cell biological model organism. Cr III and VI had inhibiting effects on its cell development, while cell division rates were only impaired by Cr VI. Transmission electron microscopy (TEM) revealed ultrastructural changes such as increased vacuolization, condensed cytoplasm and dark precipitations in the cell wall after 3 weeks of Cr VI treatment. Electron energy loss spectroscopy (EELS) and electron spectroscopic imaging (ESI) were applied to measure intracellular chromium distribution. Chromium was only detected after 3 weeks of 10 μM Cr VI treatment in electron dense precipitations found in bag-like structures along the inner side of the cell walls together with iron and elevated levels of oxygen, pointing toward an accumulation respectively extrusion of chromium in form of an iron-oxygen compound. Atomic emission spectroscopy (EMS) revealed that Micrasterias cells are able to accumulate considerable amounts of chromium and iron. During chromium treatment the Cr:Fe ratio shifted in favor of chromium, which implied that chromium may be taken up instead of iron. Significant and rapid increase of ROS production within the first 5 min of treatment confirms an active Cr VI uptake. SOD and CAT activity after Cr VI treatment did not show a response, while the glutathione pool determined by immuno-TEM decreased significantly in chromium treated cells, showing that glutathione is playing a major role in intracellular ROS and chromium detoxification.

The charophycean green algae provide insights into the early origins of plant cell walls

Numerous evolutionary innovations were required to enable freshwater green algae to colonize terrestrial habitats and thereby initiate the evolution of land plants (embryophytes). These adaptations probably included changes in cell-wall composition and architecture that were to become essential for embryophyte development and radiation. However, it is not known to what extent the polymers that are characteristic of embryophyte cell walls, including pectins, hemicelluloses, glycoproteins and lignin, evolved in response to the demands of the terrestrial environment or whether they pre-existed in their algal ancestors. Here we show that members of the advanced charophycean green algae (CGA), including the Charales, Coleochaetales and Zygnematales, but not basal CGA (Klebsormidiales and Chlorokybales), have cell walls that are comparable in several respects to the primary walls of embryophytes. Moreover, we provide both chemical and immunocytochemical evidence that selected Coleochaete species have cell walls that contain small amounts of lignin or lignin-like polymers derived from radical coupling of hydroxycinnamyl alcohols. Thus, the ability to synthesize many of the components that characterize extant embryophyte walls evolved during divergence within CGA. Our study provides new insight into the evolutionary window during which the structurally complex walls of embryophytes originated, and the significance of the advanced CGA during these events.

Transcriptional analysis of cell growth and morphogenesis in the unicellular green alga Micrasterias (Streptophyta), with emphasis on the role of expansin

BACKGROUND

Streptophyte green algae share several characteristics of cell growth and cell wall formation with their relatives, the embryophytic land plants. The multilobed cell wall of Micrasterias denticulata that rebuilds symmetrically after cell division and consists of pectin and cellulose, makes this unicellular streptophyte alga an interesting model system to study the molecular controls on cell shape and cell wall formation in green plants.

RESULTS

Genome-wide transcript expression profiling of synchronously growing cells identified 107 genes of which the expression correlated with the growth phase. Four transcripts showed high similarity to expansins that had not been examined previously in green algae. Phylogenetic analysis suggests that these genes are most closely related to the plant EXPANSIN A family, although their domain organization is very divergent. A GFP-tagged version of the expansin-resembling protein MdEXP2 localized to the cell wall and in Golgi-derived vesicles. Overexpression phenotypes ranged from lobe elongation to loss of growth polarity and planarity. These results indicate that MdEXP2 can alter the cell wall structure and, thus, might have a function related to that of land plant expansins during cell morphogenesis.

CONCLUSIONS

Our study demonstrates the potential of M. denticulata as a unicellular model system, in which cell growth mechanisms have been discovered similar to those in land plants. Additionally, evidence is provided that the evolutionary origins of many cell wall components and regulatory genes in embryophytes precede the colonization of land.

Leaf and root pectin methylesterase activity and 13C/12C stable isotopic ratio measurements of methanol emissions give insight into methanol production in Lycopersicon esculentum

Plant production of methanol (MeOH) is a poorly understood aspect of metabolism, and understanding MeOH production in plants is crucial for modeling MeOH emissions. Here, we have examined the source of MeOH emissions from mature and immature leaves and whether pectin methylesterase (PME) activity is a good predictor of MeOH emission. We also investigated the significance of below-ground MeOH production for mature leaf emissions. We present measurements of MeOH emission, PME activity, and MeOH concentration in mature and immature tissues of tomato (Lycopersicon esculentum). We also present stable carbon isotopic signatures of MeOH emission and the pectin methoxyl pool. Our results suggest that below-ground MeOH production was not the dominant contributor to daytime MeOH emissions from mature and immature leaves. Stable carbon isotopic signatures of mature and immature leaf MeOH were similar, suggesting that they were derived from the same pathway. Foliar PME activity was related to MeOH flux, but unexplained variance suggested PME activity could not predict emissions. The data show that MeOH production and emission are complex and cannot be predicted using PME activity alone. We hypothesize that substrate limitation of MeOH synthesis and MeOH catabolism may be important regulators of MeOH emission.

INTRACELLULAR METAL COMPARTMENTALIZATION IN THE GREEN ALGAL MODEL SYSTEM MICRASTERIAS DENTICULATA (STREPTOPHYTA) MEASURED BY TRANSMISSION ELECTRON MICROSCOPY-COUPLED ELECTRON ENERGY LOSS SPECTROSCOPY1

Entry of metals in form of aerosols into areas of high air humidity such as peat bogs represents a serious danger for inhabiting organisms such as the unicellular desmid Micrasterias denticulata Bréb. ex Ralfs (Desmidiaceae, Zynematophyceae, Streptophyta). To understand cellular detoxification and tolerance mechanisms, detailed intracellular localization of metal pollutants is required. This study localizes the metals aluminum (Al), zinc (Zn), copper (Cu), and cadmium (Cd) in the green algal model system Micrasterias after experimental exposure to sulfate solutions by highly sensitive TEM-coupled electron energy loss spectroscopy (EELS). Concentrations of the metals shown to induce inhibiting effects on cell development and cytomorphogenesis were chosen for these experiments. Long-term exposure to these metal concentrations led to a pronounced impact on cell physiology expressed by a general decrease in apparent photosynthesis. After long-term treatment, Zn, Al, and Cu were detected in the cell walls by EELS. Zn was additionally found in vacuoles and mucilage vesicles, and Cu in starch grains and also in mucilage vesicles. Elevated amounts of oxygen in areas where Zn, Al, and Cu were localized suggest sequestration of these metals as oxides. The study demonstrated that Micrasterias can cope differently with metal pollutants. In low doses and during a limited time period, the cells were able to compartmentalize Cu the best, followed by Zn and Al. Cu and Zn were taken up into intracellular compartments, whereas Al was only bound to the cell wall. Cd was not compartmentalized at all, which explains its strongest impact on growth, cell division rate, and photosynthesis in Micrasterias.

Extraction and detection of arabinogalactan proteins

Arabinogalactan proteins are a diverse group of plant cell wall-associated proteoglycans. While structural and molecular genetic analyses have contributed to the emerging improved understanding of the wide-range of biological processes in which AGPs are implicated; the ability to detect, localise, and quantify them is fundamentally important. This chapter describes two commonly used methods, histological staining and radial gel diffusion, both of which utilise the ability of Yariv reagent to bind to AGPs.

Evolution and diversity of plant cell walls: from algae to flowering plants

All photosynthetic multicellular Eukaryotes, including land plants and algae, have cells that are surrounded by a dynamic, complex, carbohydrate-rich cell wall. The cell wall exerts considerable biological and biomechanical control over individual cells and organisms, thus playing a key role in their environmental interactions. This has resulted in compositional variation that is dependent on developmental stage, cell type, and season. Further variation is evident that has a phylogenetic basis. Plants and algae have a complex phylogenetic history, including acquisition of genes responsible for carbohydrate synthesis and modification through a series of primary (leading to red algae, green algae, and land plants) and secondary (generating brown algae, diatoms, and dinoflagellates) endosymbiotic events. Therefore, organisms that have the shared features of photosynthesis and possession of a cell wall do not form a monophyletic group. Yet they contain some common wall components that can be explained increasingly by genetic and biochemical evidence.