The Cell Wall of Seagrasses: Fascinating, Peculiar and a Blank Canvas for Future Research

Seasonal Variation in Cell Wall Composition and Carbohydrate Metabolism in the Seagrass Posidonia oceanica Growing at Different Depths

Posidonia oceanica is a common seagrass in the Mediterranean Sea that is able to sequester large amounts of carbon. The carbon assimilated during photosynthesis can be partitioned into non-structural sugars and cell-wall polymers. In this study, we investigated the distribution of carbon in starch, soluble carbohydrates and cell-wall polymers in leaves and rhizomes of P. oceanica. Analyses were performed during summer and winter in meadows located south of the Frioul archipelago near Marseille, France. The leaves and rhizomes were isolated from plants collected in shallow (2 m) and deep water (26 m). Our results showed that P. oceanica stores more carbon as starch, sucrose and cellulose in summer and that this is more pronounced in rhizomes from deep-water plants. In winter, the reduction in photoassimilates was correlated with a lower cellulose content, compensated with a greater lignin content, except in rhizomes from deep-water plants. The syringyl-to-guaiacyl (S/G) ratio in the lignin was higher in leaves than in rhizomes and decreased in rhizomes in winter, indicating a change in the distribution or structure of the lignin. These combined data show that deep-water plants store more carbon during summer, while in winter the shallow- and deep-water plants displayed a different cell wall composition reflecting their environment.

A review of metal contamination in seagrasses with an emphasis on metal kinetics and detoxification

Seagrasses are important foundation species in coastal ecosystems, and they provide food and habitat that supports high biodiversity. However, seagrasses are increasingly subjected to anthropogenic disturbances such as metal pollution, which has been implicated as a significant factor driving seagrass losses. There have been several reviews synthesizing the metal concentrations in seagrasses and evaluating their utility as biomonitors for metal pollution in the coastal environment at the local scale. However, the interpretation of metal data in seagrass biomonitors requires a more mechanistic understanding of the processes governing metal bioaccumulation and detoxification. In this review, the progress and trends in metal studies in seagrasses between 1973 and 2022 were analyzed to identify frontier topics in this field. In addition, we tried to (1) analyze and assess the current status of metal contamination in seagrasses on a global scale by incorporating more metal data from tropical and Indo-Pacific seagrasses, (2) summarize the geochemical and biological factors governing metal uptake and loss in seagrasses, and (3) provide an up-to-date understanding of metals' effects on seagrasses and their physiological responses to metal challenges. This review improves our understanding of the highly variable metal concentrations observed in the field.

The Growth Oscillator and Plant Stomata: An Open and Shut Case

Since Darwin's "Power of Movement in Plants" the precise mechanism of oscillatory plant growth remains elusive. Hence the search continues for the hypothetical growth oscillator that regulates a huge range of growth phenomena ranging from circumnutation to pollen tube tip growth and stomatal movements. Oscillators are essentially simple devices with few components. A universal growth oscillator with only four major components became apparent recently with the discovery of a missing component, notably arabinogalactan glycoproteins (AGPs) that store dynamic Ca²⁺ at the cell surface. Demonstrably, auxin-activated proton pumps, AGPs, Ca²⁺ channels, and auxin efflux "PIN" proteins, embedded in the plasma membrane, combine to generate cytosolic Ca²⁺ oscillations that ultimately regulate oscillatory growth: Hechtian adhesion of the plasma membrane to the cell wall and auxin-activated proton pumps trigger the release of dynamic Ca²⁺ stored in periplasmic AGP monolayers. These four major components represent a molecular PINball machine a strong visual metaphor that also recognises auxin efflux "PIN" proteins as an essential component. Proton "pinballs" dissociate Ca²⁺ ions bound by paired glucuronic acid residues of AGP glycomodules, hence reassessing the role of proton pumps. It shifts the prevalent paradigm away from the recalcitrant "acid growth" theory that proposes direct action on cell wall properties, with an alternative explanation that connects proton pumps to Ca²⁺ signalling with dynamic Ca²⁺ storage by AGPs, auxin transport by auxin-efflux PIN proteins and Ca²⁺ channels. The extensive Ca²⁺ signalling literature of plants ignores arabinogalactan proteins (AGPs). Such scepticism leads us to reconsider the validity of the universal growth oscillator proposed here with some exceptions that involve marine plants and perhaps the most complex stress test, stomatal regulation.

Comparison of macrofaunal assemblages in temperate seagrass meadows and neighboring seabeds along the southeastern coast of Shandong Peninsula, China

Comparative study of macrofaunal assemblages in seagrass meadows and neighboring seabeds along the southeastern coast of Shandong Peninsula, China were performed. A total of 136 species were identified, including polychaetes (49 species), crustaceans (28), molluscs (58), and echinoderms (1). Species numbers of macrofauna in seagrass meadows and the neighboring seabeds were 52 and 65, respectively, whereas those in autumn were 90 and 56, respectively. Average macrofaunal abundances in spring in seagrass and neighboring seabeds were 2388.9 and 2516.7 ind./m², respectively, whereas those in autumn were 11,689.0 and 1733.3 ind./m², respectively. Ranges of species richness index, evenness index, and Shannon-Wiener index in seagrass meadows and the neighboring seabeds were 1.3-2.7, 0.7-0.9, 2.8-3.8, and 1.04-2.4, 0.5-0.9, 1.6-3.4 during spring, whereas those in autumn were 0.1-4.2, 0.3-0.8, 0.8-3.6 and 1.4-3.5, 0.5-0.9, 2.2-4.5. Bottom water temperature, salinity, sediment chlorophyll a concentration, and water content were the most important environmental factors influencing macrofaunal assemblages.

Pectin from Three Vietnamese Seagrasses: Isolation, Characterization, and Antioxidant Activity

This study focused on the isolation and structural characterization of pectin from three distinct species of Vietnamese seagrass including Enhalus acoroides, Thalassia hemprichii, and Halophila ovalis. The pectin yield obtained from Enhalus acoroides was the highest, corresponding to 24.15%, followed by those from Thalassia hemprichii (20.04%) and Halophila ovalis (19.14%). The physicochemical properties of pectin including total carbohydrate content, anhydrouronic acid (AUA) content, equivalent weight (EW), methoxyl content (MeO), and degree of esterification (DE) were determined using various analysis techniques. The pectin obtained from all three species were found to be low-methyl-esterified pectin, with the MeO content and DE for E. acoroides, T. hemprichii, and H. ovalis being 6.15% and 27.18%, 3.26% and 43.31%, and 4.65% and 33.25%, respectively. The average molecular weight (MW) of pectin was analyzed by size-exclusion chromatography. Pectin from T. hemprichii had the highest MW of 173.01 kDa, followed by pectin from E. acoroides, with a MW of 127.32 kDa, and that from H. ovalis, with a MW of 56.06 kDa. Furthermore, the pectins from all three seagrass species exhibited high antioxidant activity and might be promising as antioxidants.

Structures of the xyloglucans in the monocotyledon family Araceae (aroids)

The xyloglucans of all aquatic Araceae species examined had unusual structures compared with those of other non-commelinid monocotyledon families previously examined. The aquatic Araceae species Lemna minor was earlier shown to have xyloglucans with a different structure from the fucogalactoxyloglucans of other non-commelinid monocotyledons. We investigated 26 Araceae species (including L. minor), from five of the seven subfamilies. All seven aquatic species examined had xyloglucans that were unusual in having one or two of three features: < 77% XXXG core motif [L. minor (Lemnoideae) and Orontium aquaticum (Orontioideae)]; no fucosylation [L. minor (Lemnoideae), Cryptocoryne aponogetonifolia, and Lagenandra ovata (Aroideae, Rheophytes clade)]; and > 14% oligosaccharide units with S or D side chains [Spirodela polyrhiza and Landoltia punctata (Lemnoideae) and Pistia stratiotes (Aroideae, Dracunculus clade)]. Orontioideae and Lemnoideae are the two most basal subfamilies, with all species being aquatic, and Aroideae is the most derived. Two terrestrial species [Dieffenbachia seguine and Spathicarpa hastifolia (Aroideae, Zantedeschia clade)] also had xyloglucans without fucose indicating this feature was not unique to aquatic species.

Microbial communities on eelgrass (Zostera marina) thriving in Tokyo Bay and the possible source of leaf-attached microbes

Zostera marina (eelgrass) is classified as one of the marine angiosperms and is widely distributed throughout much of the Northern Hemisphere. The present study investigated the microbial community structure and diversity of Z. marina growing in Futtsu bathing water, Chiba prefecture, Japan. The purpose of this study was to provide new insight into the colonization of eelgrass leaves by microbial communities based on leaf age and to compare these communities to the root-rhizome of Z. marina, and the surrounding microenvironments (suspended particles, seawater, and sediment). The microbial composition of each sample was analyzed using 16S ribosomal gene amplicon sequencing. Each sample type was found to have a unique microbial community structure. Leaf-attached microbes changed in their composition depending on the relative age of the eelgrass leaf. Special attention was given to a potential microbial source of leaf-attached microbes. Microbial communities of marine particles looked more like those of eelgrass leaves than those of water samples. This finding suggests that leaf-attached microbes were derived from suspended particles, which could allow them to go back and forth between eelgrass leaves and the water column.

Early evidence of the impacts of microplastic and nanoplastic pollution on the growth and physiology of the seagrass Cymodocea nodosa

Microplastics (MPs) and nanoplastics (NPs) are ubiquitous in natural habitats and the risks their presence poses to marine environments and organisms are of increasing concern. There is evidence that seagrass meadows are particularly prone to accumulate plastic debris, including polystyrene particles, but the impacts of this pollutant on seagrass performance are currently unknown. This is a relevant knowledge gap as seagrasses provide multiple ecosystem services and are declining globally due to anthropogenic impact and climate-change-related stressors. Here, we explored the potential effects of a 12 day-exposure of seagrasses to one concentration (68 μg/L) of polystyrene MPs and NPs on the growth, oxidative status, and photosynthetic efficiency of plants using the foundation species Cymodocea nodosa as a model. Among plant organs, adventitious roots were particularly affected by MPs and NPs showing complete degeneration. The number of leaves per shoot was lower in MPs- and NPs-treated plants compared to control plants, and leaf loss exceeded new leaf production in MPs-treated plants. MPs also reduced photochemical efficiency and increased pigment content compared to control plants. Shoots of NPs-treated plants showed a greater oxidative damage and phenol content than those of control plants and MPs-treated plants. Biochemical data about oxidative stress markers were consistent with histochemical results. The effects of MPs on C. nodosa could be related to their adhesion to plant surface while those of NPs to entering tissues. Our study provides the first experimental evidence of the potential harmful effects of MPs/NPs on seagrass development. It also suggests that the exposure of seagrasses to MPs/NPs in natural environments could have negative consequences on the functioning of seagrass ecosystems. This stresses the importance of implementing cleaning programs to remove all plastics already present in marine habitats as well as of undertaking specific actions to prevent the introduction of these pollutants within seagrass meadows.

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.

Profiling the cell walls of seagrasses from A (Amphibolis) to Z (Zostera)

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.

"Neptune Balls" Polysaccharides: Disentangling the Wiry Seagrass Detritus

Each year, high amounts of dead seagrass material are washed ashore at beaches world-wide. In the Mediterranean region, the seagrass Posidonia oceanica is responsible for huge agglomerates of ball-like seagrass litter. As these are often removed due to touristic reasons, a reuse method would be a step towards a more ecologically oriented society. In this study, the main polysaccharide components were analyzed, in order to propose possible usage options. To do this, different aqueous fractions were extracted, analyzed by classical carbohydrate analysis methods (GC-FID/MS, colorimetric assay and elemental analysis), and purified by ion-exchange chromatography, as well as selective precipitation with a detecting agent for highly glycosylated glycoproteins. The obtained purified fractions were analyzed in detail and a linkage-type analysis of the most promising extract was conducted via permethylation. Only low amounts of glycoproteins, as well as medium amounts of the characteristic apiogalacturonan were likely to be present, while xylan seemed to be the most abundant polysaccharide in most fractions. A partial structural proposal showed general accordance with land plant xylans, presenting reuse options in the field of biofuel and bioplastic generation.