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Background: The polyphyletic group of seagrasses shows an evolutionary history from early monocotyledonous land plants to the marine environment. Seagrasses form important coastal ecosystems worldwide and large amounts of seagrass detritus washed on beaches might also be valuable bioeconomical resources. Despite this importance and potential, little is known about adaptation of these angiosperms to the marine environment and their cell walls., Results: We investigated polysaccharide composition of nine seagrass species from the Mediterranean, Red Sea and eastern Indian Ocean. Sequential extraction revealed a similar seagrass cell wall polysaccharide composition to terrestrial angiosperms: arabinogalactans, pectins and different hemicelluloses, especially xylans and/or xyloglucans. However, the pectic fractions were characterized by the monosaccharide apiose, suggesting unusual apiogalacturonans are a common feature of seagrass cell walls. Detailed analyses of four representative species identified differences between organs and species in their constituent monosaccharide composition and lignin content and structure. Rhizomes were richer in glucosyl units compared to leaves and roots. Enhalus had high apiosyl and arabinosyl abundance, while two Australian species of Amphibolis and Posidonia, were characterized by high amounts of xylosyl residues. Interestingly, the latter two species contained appreciable amounts of lignin, especially in roots and rhizomes whereas Zostera and Enhalus were lignin-free. Lignin structure in Amphibolis was characterized by a higher syringyl content compared to that of Posidonia., Conclusions: Our investigations give a first comprehensive overview on cell wall composition across seagrass families, which will help understanding adaptation to a marine environment in the evolutionary context and evaluating the potential of seagrass in biorefinery incentives., (© 2022. The Author(s).)

Seagrasses evolved from monocotyledonous land plants that returned to the marine habitat. This transition was accomplished by substantial changes in cell wall composition, revealing habitat-driven adaption to the new environment. Whether arabinogalactan-proteins (AGPs), important signalling molecules of land plants, are present in seagrass cell walls is of evolutionary and plant development interest. AGPs of Zostera marina L. were isolated and structurally characterised by analytical and bioinformatics methods as well as by ELISA with different anti-AGP antibodies. Calcium-binding capacity of AGPs was studied by isothermal titration calorimetry (ITC) and microscopy. Bioinformatic searches of the Z. marina proteome identified 9 classical AGPs and a large number of chimeric AGPs. The glycan structures exhibit unique features, including a high degree of branching and an unusually high content of terminating 4-O-methyl-glucuronic acid (4-OMe GlcA) residues. Although the common backbone structure of land plant AGPs is conserved in Z. marina, the terminating residues are distinct with high amounts of uronic acids. These differences likely result from the glycan-active enzymes (glycosyltransferases and methyltransferases) and are essential for calcium-binding properties. The role of this polyanionic surface is discussed with regard to adaption to the marine environment.

The increasing evidence for the physiological significance of glycan-protein (lectin) interactions prompts considerations for respective bioactivity of plant polysaccharides. Arabinogalactan from larch, a polysaccharide with a β1,3-linked galactose core and branches at the 6'-hydroxyl, was thus tested, together with two processed forms treated either with oxalic or trifluoroacetic acid. Hydrolysis by acid reduced the arabinose contents without backbone degradation. The three preparations were tested as an inhibitor of lectin binding in solid-phase and cell-based assays, using the toxin from Viscum album and a panel of seven human lectins (six galectins and a C-type lectin). Increasing potency correlating with the molecular contents of galactose was seen for the plant toxin. In general, relatively weak or no inhibitory capacity was detected for the three preparations, when binding of the human galectins and avian orthologues used as controls was measured. Acid-treated polysaccharides also weakly interfered with binding of the galactose-specific C-type lectin of human macrophages. Larch arabinogalactan, tested as a model, will thus most likely not impair (ga)lectin functionality physiologically., (Georg Thieme Verlag KG Stuttgart · New York.)

Mucilage is thought to play a role in salinity tolerance in certain halophytic species by regulating water ascent and ion transport. The localization and composition of mucilage in the halophyte Kosteletzkya virginica was therefore investigated. Plants were grown in a hydroponic system in the presence or absence of 100mM NaCl and regularly harvested for growth parameter assessment and mucilage analysis with the gas liquid chromatography method. NaCl treatment stimulated shoot growth and biomass accumulation, had little effect on shoot and root water content, and reduced leaf water potential (Psi(w)), osmotic potential (Psi(s)) as well as stomatal conductance (g(s)). Mucilage increased in shoot, stems and roots in response to salt stress. Furthermore, changes were also observed in neutral monosaccharide components. Levels of rhamnose and uronic acid increased with salinity. Staining with a 0.5% alcian blue solution revealed the presence of mucopolyssacharides in xylem vessels and salt-induced mucilaginous precipitates on the leaf abaxial surface. Determination of ion concentrations showed that a significant increase of Na(+) and a decrease of K(+) and Ca(2+) simultaneously occurred in tissues and in mucilage under salt stress. Considering the high proportion of rhamnose and uronic acid in stem mucilage, we suggest that the pectic polysaccharide could be involved in Na(+) fixation, though only a minor fraction of accumulated sodium appeared to be firmly bound to mucilage., (Copyright 2009 Elsevier GmbH. All rights reserved.)

This investigation shows structural features of two macromolecules from roots of Echinacea pallida (Nutt.) Nutt: an arabinogalactan-protein (AGP) and an arabinan. The arabinogalactan-protein was precipitated with beta-glucosyl Yariv reagent from a high molecular weight fraction. Investigations of the neutral sugar composition revealed Gal (52.1% w/w) and Ara (38.2% w/w) in a ratio of 1.4:1, accompanied by Glc (6.9% w/w) and Rha (2.8% w/w). The content of uronic acids was 6.2%. Mild acid hydrolysis detects Ara and Glc being located at the periphery of the molecule. Linkage analyses and NMR spectroscopy revealed a backbone of the polysaccharide mainly consisting of 3-linked and 3,6-linked Galp-residues. Side chains are composed of 3,6-linked or 6-linked Galp terminating in 5-linked Araf, terminal Araf, Glcp and GlcAp. The protein part (3.9% w/w) of the AGP is rich in Hyp, Ser, Ala, Thr, Glu, Asp and Gly. The amount of Hyp was determined by a colorimetric method and found to be (0.65% (w/w) of the AGP, which is in good agreement with the result obtained by amino acid hydrolysis (0.67% w/w). The arabinan was isolated from the supernatant of the Yariv precipitation on the basis of solubility in EtOH (80%). It mainly consists of Ara (85.8%). Linkage analyses and NMR spectroscopy indicate a highly branched molecule, consisting of 3,5-linked, 5-linked and terminal Araf-residues in equal amounts.

SUMMARY: 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. [ABSTRACT FROM AUTHOR]

Galectins represent β-galactoside-binding proteins with numerous functions. Due to their role in tumor progression, human galectins-1, -3 and -7 (Gal-1, -3 and -7) are potential targets for cancer therapy. As plant derived glycans might act as galectin inhibitors, we prepared galactans by partial degradation of plant arabinogalactan-proteins. Besides commercially purchased galectins, we produced Gal-1 and -7 in a cell free system and tested binding capacities of the galectins to the galactans by biolayer-interferometry. Results for commercial and cell-free expressed galectins were comparable confirming functionality of the cell-free produced galectins. Our results revealed that galactans from Echinacea purpurea bind to Gal-1 and -7 with KD values of 1–2 µM and to Gal-3 slightly stronger with KD values between 0.36 and 0.70 µM depending on the sensor type. Galactans from the seagrass Zostera marina with higher branching of the galactan and higher content of uronic acids showed stronger binding to Gal-3 (0.08–0.28 µM) compared to galactan from Echinacea. The results contribute to knowledge on interactions between plant polysaccharides and galectins. Arabinogalactan-proteins have been identified as a new source for production of galactans with possible capability to act as galectin inhibitors. [ABSTRACT FROM AUTHOR]

Arabinogalactan (AG) isolated from dust of a traditional farm prevents disease in murine models of allergy. However, it is unclear whether this polysaccharide has immune regulatory properties in humans. The aim of this study w as to test the influence of AG on the immune-stimulating properties of human dendritic cells (DCs). Moreover, we sought to identify the receptor to which AG binds. AG was produced from plant callus tissue under sterile conditions to avoid the influence of pathogen-associated molecular patterns in subsequent experiments. The influence of AG on the human immune system was investigated by analyzing its impact on monocyte-derived DCs. To analyze whether the T cell stimulatory capacity of AG-stimulated DCs is altered, an MLR with naive Th cells was performed. We revealed that AG reduced T cell proliferation in a human MLR. In the search for a molecular mechanism, we found that AG binds to the immune modulatory receptors DC-specific ICAM-3-grabbing non integrin (DC-SIGN) and macrophage mannose receptor 1 (MMR-1). Stimulation of these receptors with AG simultaneously with TLR4 stimulation with LPS increased the expression of the E3 ubiquitin-protein ligase tripartite motif-containing protein 21 and decreased the phosphorylation of NF-κB p65 in DCs. This led to a reduced activation profile with reduced costimulatory molecules and proinflammatory cytokine production. Blocking of MMR-1 or DC-SIGN with neutralizing Abs partially inhibits this effect. We conclude that AG dampens the activation of human DCs by LPS via binding to DC-SIGN and MMR-1, leading to attenuated TLR signaling. This results in a reduced T cell activation capacity of DCs. [ABSTRACT FROM AUTHOR]