Correlation of Apiose Levels and Growth Rates in Duckweeds

Differentiation between chemo- and radiotoxicity of 137Cs and 60Co on Lemna minor

The uptake and effects of stable Cs and Co on L.minor were extensively studied, together with the effects of gamma radiation using a 137Cs or 60Co source. Innovative is that we combined external irradiation (from 137Cs or 60Co sources) with the direct uptake of certain amounts of stable Cs or Co to simulate the impact of the same mass of a radioisotope compared with that of the stable element. Such approach allows to differentiate between chemo- and radiotoxicity of 137Cs or 60Co, permitting to study the 137Cs and 60Co uptake by L. minor without using high concentrations of these elements in solution. Our results indicate that radiotoxicity of both 137Cs and 60Co has a greater importance compared to their chemotoxicity. This was also supported by the independent action and concentration addition concepts. Both concepts resulted in a good prediction of the dose-response curve of the combination exposure. The maximal removal of 137Cs or 60Co per gram dry matter of L. minor was lower compared with the removal of the corresponding stable isotope. The toxicity of 60Co was higher compared to 137Cs based on EC50 values and uptake data. With respect to the effects on photosynthetic pigments, starch and soluble sugars contents, only starch increased in a concentration- and dose-dependent manner.

Duckweed pectic-arabinogalactan-proteins can crosslink through borate diester bonds

Material containing pectin and arabinogalactan-protein (AGP) was released and purified from Spirodela alcohol insoluble residues. Results of carbohydrate analyses and two-dimensional NMR spectroscopy suggest that this material is composed of apiogalacturonan and rhamnogalacturonan-I covalently attached to AGPs. 11B NMR spectroscopy indicated that some of the glycoses in this complex exist as their boric acid monoesters. Borate diesters were formed when the pectic-AGPs were allowed to react at pH above 6.2 with the boron-depleted pectic-AGPs, suggesting that in vitro two pectic-AGP molecules can crosslink to one another through borate. Borate diesters also formed when the pectic-AGPs were incubated with monomeric rhamnogalacturonan-II in the presence of Pb2+ ion at pH 9.2. This data presents evidence of the first wall polymer after rhamnogalacturonan-II to crosslink through borate diesters. We suggest that the formation of these borate-crosslinks may help Spirodela respond to high-pH condition.

The apiosyltransferase celery UGT94AX1 catalyzes the biosynthesis of the flavone glycoside apiin

Apiose is a unique branched-chain pentose found in plant glycosides and a key component of the cell wall polysaccharide pectin and other specialized metabolites. More than 1,200 plant-specialized metabolites contain apiose residues, represented by apiin, a distinctive flavone glycoside found in celery (Apium graveolens) and parsley (Petroselinum crispum) in the family Apiaceae. The physiological functions of apiin remain obscure, partly due to our lack of knowledge on apiosyltransferase during apiin biosynthesis. Here, we identified UGT94AX1 as an A. graveolens apiosyltransferase (AgApiT) responsible for catalyzing the last sugar modification step in apiin biosynthesis. AgApiT showed strict substrate specificity for the sugar donor, UDP-apiose, and moderate specificity for acceptor substrates, thereby producing various apiose-containing flavone glycosides in celery. Homology modeling of AgApiT with UDP-apiose, followed by site-directed mutagenesis experiments, identified unique Ile139, Phe140, and Leu356 residues in AgApiT, which are seemingly crucial for the recognition of UDP-apiose in the sugar donor pocket. Sequence comparison and molecular phylogenetic analysis of celery glycosyltransferases suggested that AgApiT is the sole apiosyltransferase-encoding gene in the celery genome. Identification of this plant apiosyltransferase gene will enhance our understanding of the physioecological functions of apiose and apiose-containing compounds.

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.

Carbon allocation of Spirodela polyrhiza under boron toxicity

Pectic polysaccharides containing apiose, xylose, and uronic acids are excellent candidates for boron fixation. Duckweeds are the fastest-growing angiosperms that can absorb diverse metals and contaminants from water and have high pectin content in their cell walls. Therefore, these plants can be considered excellent boron (B) accumulators. This work aimed to investigate the relationship between B assimilation capacity with apiose content in the cell wall of Spirodela polyrhiza subjected to different boric acid concentrations. Plants were grown for 7 and 10 days in ½ Schenck-Hildebrandt media supplemented with 0 to 56 mg B.L^-1, the non-structural and structural carbohydrates, and related genes were evaluated. The results showed that B altered the morphology and carbohydrate composition of this species during plant development. The optimum B concentration (1.8 mg B.L^-1) led to the highest relative growth and biomass accumulation, reduced starch, and high pectin and apiose contents, together with increased expression of UDP-apiose/UDP-xylose synthase (AXS) and 1,4-α-galacturonosyltransferase (GAUT). The toxic state (28 and 56 mg B.L^-1) increased the hexose contents in the cell wall with a concomitant reduction of pectins, apiose, and growth. The pectin content of S. polyrhiza was strongly associated with its growth capacity and regulation of B content within the cells, which have AXS as an important regulator. These findings suggest that duckweeds are suitable for B remediation, and their biomass can be used for bioenergy production.

Highlighting the hygroscopic capacities of apiogalacturonans

To meet the needs of dehydrated skin, molecules with a high hygroscopic potential are necessary to hydrate it effectively and durably. In this context, we were interested in pectins, and more precisely in apiogalacturonans (AGA), a singular one that is currently only found in a few species of aquatic plants. As key structures in water regulation of these aquatic plants and thanks to their molecular composition and conformations, we hypothesized that they could have beneficial role for skin hydration. Spirodela polyrhiza is a duckweed known to be naturally rich in AGA. The aim of this study was to investigate the hygroscopic potential of AGA. Firstly, AGA models were built based on structural information obtained from previous experimental studies. Molecular dynamics (MD) simulations were performed, and the hygroscopic potential was predicted in silico by analyzing the frequency of interaction of water molecules with each AGA residue. Quantification of interactions identified the presence of 23 water molecules on average in contact with each residue of AGA. Secondly, the hygroscopic properties were investigated directly in vivo. Indeed, the water capture in the skin was measured in vivo by Raman microspectroscopy thanks to the deuterated water (D₂0) tracking. Investigations revealed that AGA significantly capture and retain more water in the epidermis and deeper than a placebo control. Not only do these original natural molecules interact with water molecules, but they capture and retain them efficiently in the skin.

Making and breaking of boron bridges in the pectic domain rhamnogalacturonan-II at apoplastic pH in vivo and in vitro

Cross-linking of the cell-wall pectin domain rhamnogalacturonan-II (RG-II) via boron bridges between apiose residues is essential for normal plant growth and development, but little is known about its mechanism or reversibility. We characterized the making and breaking of boron bridges in vivo and in vitro at 'apoplastic' pH. RG-II (13-26 μm) was incubated in living Rosa cell cultures and cell-free media with and without 1.2 mm H3 BO3 and cationic chaperones (Ca2+ , Pb2+ , polyhistidine, or arabinogalactan-protein oligopeptides). The cross-linking status of RG-II was monitored electrophoretically. Dimeric RG-II was stable at pH 2.0-7.0 in vivo and in vitro. In-vitro dimerization required a 'catalytic' cation at all pHs tested (1.75-7.0); thus, merely neutralizing the negative charge of RG-II (at pH 1.75) does not enable boron bridging. Pb2+ (20-2500 μm) was highly effective at pH 1.75-4.0, but not 4.75-7.0. Cationic peptides were effective at approximately 1-30 μm; higher concentrations caused less dimerization, probably because two RG-IIs then rarely bonded to the same peptide molecule. Peptides were ineffective at pH 1.75, their pH optimum being 2.5-4.75. d-Apiose (>40 mm) blocked RG-II dimerization in vitro, but did not cleave existing boron bridges. Rosa cells did not take up d-[U-14 C]apiose; therefore, exogenous apiose would block only apoplastic RG-II dimerization in vivo. In conclusion, apoplastic pH neither broke boron bridges nor prevented their formation. Thus boron-starved cells cannot salvage boron from RG-II, and 'acid growth' is not achieved by pH-dependent monomerization of RG-II. Divalent metals and cationic peptides catalyse RG-II dimerization via co-ordinate and ionic bonding respectively (possible and impossible, respectively, at pH 1.75). Exogenous apiose may be useful to distinguish intra- and extra-protoplasmic dimerization.

Dynamics of the sucrose metabolism and related gene expression in tomato fruits under water deficit

The impact of water deficit on sucrose metabolism in sink organs like the fruit remains poorly known despite the need to improve fruit crops resilience to drought in the face of climate change. The present study investigated the effects of water deficit on sucrose metabolism and related gene expression in tomato fruits, aiming to identify candidate genes for improving fruit quality upon low water availability. Tomato plants were subjected to irrigated control and water deficit (-60% water supply compared to control) treatments, which were applied from the first fruit set to first fruit maturity stages. The results have shown that water deficit significantly reduced fruit dry biomass and number, among other plant physiological and growth variables, but substantially increased the total soluble solids content. The determination of soluble sugars on the basis of fruit dry weight revealed an active accumulation of sucrose and concomitant reduction in glucose and fructose levels in response to water deficit. The complete repertoire of genes encoding sucrose synthase (SUSY1-7), sucrose-phosphate synthase (SPS1-4), and cytosolic (CIN1-8), vacuolar (VIN1-2) and cell wall invertases (WIN1-4) was identified and characterized, of which SlSUSY4, SlSPS1, SlCIN3, SlVIN2, and SlCWIN2 were shown to be positively regulated by water deficit. Collectively, these results show that water deficit regulates positively the expression of certain genes from different gene families related to sucrose metabolism in fruits, favoring the active accumulation of sucrose in this organ under water-limiting conditions.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-023-01288-7.

The Effect of Sugarcane Straw Aging in the Field on Cell Wall Composition

Cellulosic ethanol is an alternative for increasing the amount of bioethanol production in the world. In Brazil, sugarcane leads the bioethanol production, and to improve its yield, besides bagasse, sugarcane straw is a possible feedstock. However, the process that leads to cell wall disassembly under field conditions is unknown, and understanding how this happens can improve sugarcane biorefinery and soil quality. In the present work, we aimed at studying how sugarcane straw is degraded in the field after 3, 6, 9, and 12 months. Non-structural and structural carbohydrates, lignin content, ash, and cellulose crystallinity were analyzed. The cell wall composition was determined by cell wall fractionation and determination of monosaccharide composition. Non-structural carbohydrates degraded quickly during the first 3 months in the field. Pectins and lignin remained in the plant waste for up to 12 months, while the hemicelluloses and cellulose decreased 7.4 and 12.4%, respectively. Changes in monosaccharide compositions indicated solubilization of arabinoxylan (xylose and arabinose) and β-glucans (β-1,3 1,4 glucan; after 3 months) followed by degradation of cellulose (after 6 months). Despite cellulose reduction, the xylose:glucose ratio increased, suggesting that glucose is consumed faster than xylose. The degradation and solubilization of the cell wall polysaccharides concomitantly increased the level of compounds related to recalcitrance, which led to a reduction in saccharification and an increase in minerals and ash contents. Cellulose crystallinity changed little, with evidence of silica at the latter stages, indicating mineralization of the material. Our data suggest that for better soil mineralization, sugarcane straw must stay in the field for over 1 year. Alternatively, for bioenergy purposes, straw should be used in less than 3 months.

Duckweeds: their utilization, metabolites and cultivation

Duckweeds are floating plants of the family Lemnaceae, comprising 5 genera and 36 species. They typically live in ponds or lakes and are found worldwide, except the polar regions. There are two duckweed subfamilies-namely Lemnoidea and Wolffioideae, with 15 and 21 species, respectively. Additionally, they have characteristic reproduction methods. Several metabolites have also been reported in various duckweeds. Duckweeds have a wide range of adaptive capabilities and are particularly suitable for experiments requiring high productivity because of their speedy growth and reproduction rates. Duckweeds have been studied for their use as food/feed resources and pharmaceuticals, as well as for phytoremediation and industrial applications. Because there are numerous duckweed species, culture conditions should be optimized for industrial applications. Here, we review and summarize studies on duckweed species and their utilization, metabolites, and cultivation methods to support the extended application of duckweeds in future.

Flavonoids from duckweeds: potential applications in the human diet

Duckweeds are the smallest free-floating flowering aquatic plants. Their biotechnological applications include their use as food, bioenergy, and environmental sustainability, as they can help clean polluted water. The high growth capacity and their chemical properties make them suitable for human health applications. Here we evaluated the ethanolic extracts from five species of duckweeds by HPLC-DAD/MS-MS for chemical characterization. Sixteen compounds were identified and quantified, in which three were chlorogenic acid derivatives and eleven apigenin and luteolin derivatives. We describe for the first time the presence in duckweeds of 5-O-(E)-caffeoylquinic acid (1), 3-O-(E)-coumaroylquinic acid (2), luteolin-7-O-glucoside-C-glucoside (3), 4-O-(E)-coumaroylquinic acid (4), luteolin-6-C-glucoside-8-C-rhamnoside (5), and luteolin-8-C-glucoside-6-C-rhamnoside (6). The flavonoids diversity showed a significant content of luteolin and its derivatives, except for Landoltia punctata that had significant apigenin content. Flavones identified in duckweeds were mostly C-glycosides, which can benefit human diets, and its abundance seems to be related to the higher antioxidant and anticancer capacities of Wolffiella caudata, Wolffia borealis, and Landoltia punctata. Our findings reinforce the idea that duckweeds could be valuable additives to the human diet, and their potential should be further explored.

Aquatic Plants, Landoltia punctata, and Azolla filiculoides as Bio-Converters of Wastewater to Biofuel

The aquatic plants, Azolla filiculoides, and Landoltia punctate, were used as complementing phytoremediators of wastewater containing high levels of phosphate, which simulates the effluents from textile, dyeing, and laundry detergent industries. Their complementarities are based on differences in capacities to uptake nitrogen and phosphate components from wastewater. Sequential treatment by L. punctata followed by A. filiculoides led to complete removal of NH4, NO3, and up to 93% reduction of PO4. In experiments where L. punctata treatment was followed by fresh L. punctata, PO4 concentration was reduced by 65%. The toxicity of wastewater assessed by shrimps, Paratya australiensis, showed a four-fold reduction of their mortality (LC50 value) after treatment. Collected dry biomass was used as an alternative carbon source for heterotrophic marine protists, thraustochytrids, which produced up to 35% dry weight of lipids rich in palmitic acid (50% of total fatty acids), the key fatty acid for biodiesel production. The fermentation of treated L. punctata biomass by Enterobacter cloacae yielded up to 2.14 mol H2/mole of reduced sugar, which is comparable with leading terrestrial feedstocks. A. filiculoides and L. punctata can be used as a new generation of feedstock, which can treat different types of wastewater and represent renewable and sustainable feedstock for bioenergy production.

Linkage structure of cell-wall polysaccharides from three duckweed species

Rapid growth and easily digestible walls that are naturally low in lignin make the aquatic plant family Lemnaceae, or duckweed, a promising feedstock for biofuel production. Monosaccharide and linkage analysis of cell walls from three species of duckweed: Spirodela polyrhiza, Lemna gibba, and Wolffia australiana showed that apiogalacturonans and/or xylogalacturonans, and smaller amounts of rhamnogalacturonan I, constituted 57%, 51% and 48% of their respective wall mass. Hemicellulosic xylan, xyloglucan, and glucomannan made up lesser amounts wall mass across the three species. Apiose residues were either non-reducing terminal or 3'-linked, but their ratios varied substantially from nearly 1:1 for S. polyrhiza and 2:1 for L. gibba, to 10:1 for W. australiana. These findings will help guide future research to design efficient strategies for disassembly of duckweed cell walls into sugars and uronic acids for conversion of duckweed biomass into usable fuel, and to facilitate extraction of other bioproducts from its polysaccharides.