Wall analyses of lophocolea seta cells (bryophyta) before and after elongation

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.

The evolution of the stomatal apparatus: intercellular spaces and sporophyte water relations in bryophytes-two ignored dimensions

Cryo-scanning electron microscopy shows that nascent intercellular spaces (ICSs) in bryophytes are liquid-filled, whereas these are gas-filled from the outset in tracheophytes except in the gametophytes of Lycopodiales. ICSs are absent in moss gametophytes and remain liquid-filled in hornwort gametophytes and in both generations in liverworts. Liquid is replaced by gas following stomatal opening in hornworts and is ubiquitous in moss sporophytes even in astomate taxa. New data on moss water relations and sporophyte weights indicate that the latter are homiohydric while X-ray microanalysis reveals an absence of potassium pumps in the stomatal apparatus. The distribution of ICSs in bryophytes is strongly indicative of very ancient multiple origins. Inherent in this scenario is either the dual or triple evolution of stomata. The absence, in mosses, of any relationship between increases in sporophyte biomass and stomata numbers and absences, suggests that CO2 entry through the stomata, possible only after fluid replacement by gas in the ICSs, makes but a minor contribution to sporophyte nutrition. Save for a single claim of active regulation of aperture dimensions in mosses, all other functional and structural data point to the sporophyte desiccation, leading to spore discharge, as the primeval role of the stomatal apparatus.This article is part of a discussion meeting issue 'The Rhynie cherts: our earliest terrestrial ecosystem revisited'.

Metabolism of L-arabinose in plants

L-Arabinose (L-Ara) is a plant-specific sugar accounting for 5-10 % of cell wall saccharides in Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa). L-Ara occurs in pectic arabinan, rhamnogalacturonan II, arabinoxylan, arabinogalactan-protein (AGP), and extensin in the cell walls, as well as in glycosylated signaling peptides like CLAVATA3 and small glycoconjugates such as quercetin 3-O-arabinoside. This review focuses on recent advances towards understanding the generation of L-Ara and the metabolism of L-Ara-containing molecules in plants.

Physcomitrella patens arabinogalactan proteins contain abundant terminal 3-O-methyl-L: -rhamnosyl residues not found in angiosperms

A biochemical investigation of arabinogalactan proteins (AGPs) in Physcomitrella patens was undertaken with particular emphasis on the glycan chains. Following homogenization and differential centrifugation of moss gametophytes, AGPs were obtained by Yariv phenylglycoside-induced precipitation from the soluble, microsomal membrane, and cell wall fractions. Crossed-electrophoresis indicated that each of these three AGP fractions was a mixture of several AGPs. The soluble AGP fraction was selected for further separation by anion-exchange and gel-permeation chromatography. The latter indicated molecular masses of approximately 100 and 224 kDa for the two major soluble AGP subfractions. The AGPs in both of these subfractions contained the abundant (1,3,6)-linked galactopyranosyl residues, terminal arabinofuranosyl residues, and (1,4)-linked glucuronopyranosyl residues that are typical of many angiosperm AGPs. Unexpectedly, however, the moss AGP glycan chains contained about 15 mol% terminal 3-O-methyl-L: -rhamnosyl residues, which have not been found in angiosperm AGPs. This unusual and relatively nonpolar sugar, also called L: -acofriose, is likely to have considerable effects on the overall polarity of Physcomitrella AGPs. A review of the literature indicates that the capacity to synthesize polymers containing 3-O-methyl-L: -rhamnosyl residues is present in a variety of bacteria, algae and lower land plants but became less common through evolution to the extent that this sugar has been found in only a few species of angiosperms where it occurs as a single residue on steroidal glycosides.

alpha-D-Glucuronosyl-(1-->3)-L-galactose, an unusual disaccharide from polysaccharides of the hornwort Anthoceros caucasicus

Acid hydrolysis of cell wall-rich material from thalli of the hornwort Anthoceros caucasicus yielded substantial amounts of an unusual disaccharide (1). Hydrolysis of 1 yielded only GlcA, Gal and unhydrolysed 1. Compound 1 was identified as alpha-D-GlcpA-(1-->3)-L-Gal by 1H and 13C NMR spectroscopic analysis and by the susceptibility of its monosaccharide units to phosphorylation by enantiomer-specific kinases. Compound 1 was not detected in acid hydrolysates of other land plants including mosses, leafy and thalloid liverworts, lycopodiophytes and euphyllophytes; it was also absent from charophytes. The Anthoceros polysaccharide that yields 1 was partially extractable in cold aqueous buffer (pH 4.7) and Na(2)CO(3), but not in EDTA or NaOH, suggesting that it was not a typical pectin or hemicellulose. The yield of 1 from various polysaccharide fractions correlated with the yields of Xyl, suggesting a previously unreported polymer containing D-GlcA, L-Gal and Xyl. The existence of a unique polysaccharide in an evolutionarily isolated plant (Anthoceros) supports the view that major steps in plant phylogeny were accompanied by significant changes in cell wall composition.

Primary cell wall composition of bryophytes and charophytes

Major differences in primary cell wall (PCW) components between non-vascular plant taxa are reported. (1) Xyloglucan: driselase digestion yielded isoprimeverose (the diagnostic repeat unit of xyloglucan) from PCW-rich material of Anthoceros (a hornwort), mosses and both leafy and thalloid liverworts, as well as numerous vascular plants, showing xyloglucan to be a PCW component in all land plants tested. In contrast, charophycean green algae (Klebsormidium flaccidium, Coleochaete scutata and Chara corallina), thought to be closely related to land plants, did not contain xyloglucan. They did not yield isoprimeverose; additionally, charophyte material was not digestible with xyloglucan-specific endoglucanase or cellulase to give xyloglucan-derived oligosaccharides. (2) Uronic acids: acid hydrolysis of PCW-rich material from the charophytes, the hornwort, thalloid and leafy liverworts and a basal moss yielded higher concentrations of glucuronic acid than that from the remaining land plants including the less basal mosses and all vascular plants tested. Polysaccharides of the hornwort Anthoceros contained an unusual repeat-unit, glucuronic acid-alpha(1-->3)-galactose, not found in appreciable amounts in any other plants tested. Galacturonic acid was consistently the most abundant PCW uronic acid, but was present in higher concentrations in acid hydrolysates of bryophytes and charophytes than in those of any of the vascular plants. Mannuronic acid was not detected in any of the species surveyed. (3) Mannose: acid hydrolysis of charophyte and bryophyte PCW-rich material also yielded appreciably higher concentrations of mannose than are found in vascular plant PCWs. (4) Mixed-linkage glucan (MLG) was absent from all algae and bryophytes tested; however, upon digestion with licheninase, PCW-rich material from the alga Ulva lactuca and the leafy liverwort Lophocolea bidentata yielded penta- to decasaccharides, indicating the presence of MLG-related polysaccharides. Our results show that major evolutionary events are often associated with changes in PCW composition. In particular, the acquisition of xyloglucan may have been a pre-adaptive advantage that allowed colonization of land.

3-O-methyl-D-galactose residues in lycophyte primary cell walls

Acid hydrolysis of cell wall-rich material from young leaves of the lycophyte Selaginella apoda (L.) Spring yielded substantial amounts of 3-O-methyl-D-galactose (1) in addition to the usual major monosaccharides (glucose, galactose, arabinose, xylose and galacturonic acid). The yield of 1 approximately equalled that of galacturonic acid. Compound 1 was identified as 3-O-methylgalactose by its 1H and 13C NMR spectra, and shown to be the D-enantiomer by its susceptibility to D-galactose oxidase. Compound 1 was detected in acid hydrolysates of the alcohol-insoluble residues from young leaves of all lycophytes tested, both homosporous (Lycopodium, Huperzia and Diphasiastrum) and heterosporous (Selaginella). It was not detectable in the charophyte green algae Coleochaete scutata, Chara coralina or Klebsormidium flaccidum, any bryophytes [a hornwort (Anthoceros), four liverworts and three mosses], or any euphyllophytes [a psilopsid (Psilotum), a horsetail (Equisetum), eusporangiate and leptosporangiate ferns, the gymnosperm Gnetum, and diverse angiosperms]. A high content of 1 is thus an autapomorphy of the lycophytes.

Solute production and net wall synthesis in the growing and non-growing cells of gravistimulated sunflower hypocotyls

Solute generation and cell wall synthesis were examined in sunflower hypocotyl peripheral layers, the growth rate of which had been altered by gravistimulation. Measurements of both the concentrations of the major solutes and the osmotic potential showed that although upper cells stopped growing, the solute levels in these cells continued to increase at rates comparable to those in lower cells. This indicated that altered growth rates, generated during gravicurvature, are not based on solute generation but must result from cell wall changes. Gravimetric and precursor incorporation studies showed that net wall synthesis continued in upper cells despite their lack of growth. An ultrastructural study of the epidermal cells on the uppermost (non-elongating) and lowermost (elongating) surfaces of horizontal cucumber hypocotyls showed that the relative amounts of the various membrane fractions were similar in upper and lower cells despite their very different growth rates.