Plants, higher Cell walls

It has long been recognized that boron is required by higher plants [61, 62], and recent research indicates the involvement of boron in three main aspects of plant physiology cell wall structure, membrane function, and reproduction. In vascular plants, boron in solution moves in the transpiration stream from the roots and accumulates in the stems and leaves. Once in the leaves, the translocation of boron is limited and requires a phloem transport mechanism. The nature of this mechanism was only recently elucidated with the isolation of a number of borate polyol compounds from various plants [63-65]. These include sorbitol-borate ester complexes isolated from the floral nectar of peaches and mannitol-borate ester complexes from the phloem sap of celery. The implication is that the movement of boron in plants depends on borate-polyol ester formation with the particular sugar polyol compounds used as transport molecules in specific plants. 

In fungi, and in higher plants, the cell wall is a dynamic structure, changing during growth and differentiation, or in response to environmental factors. The main polysaccharides found in plant cell-walls are listed in this Section. The biosynthesis of those which are synthesized by way of lipid intermediates (see Table V) will be analyzed later. 

Like mosses and higher plants, liverworts use chlorophyll-a, chlorophyll-b, and carotenoids as photosynthetic pigments and store their food reserves as starch. As in mosses and higher plants, their cell walls are composed of cellulose. 

Polysaccharides are components of almost all living things. They are present in greatest quantity in the higher orders of plants where they constitute approximately three-quarters of the dry weight. The majority of plant polysaccharides are components of the cell walls. In a typical tissue from either an annual or perennial plant, the cell walls consist of three morphologically distinct layers namely, the intercellular cement, or middle lamella, the primary wall, and the secondary wall, as shown 41) in Fig. 2. 

Glucomannans (GM) and galactoglucomannans (GGM), common constituents of plant cell walls, are the major hemicellulosic components of the secondary cell walls of softwoods, whereas in the secondary cell walls of hardwoods they occur in minor amounts. They are suggested to be present together with xylan and fucogalactoxyloglucan in the primary cell walls of higher plants [192]. These polysaccharides were extensively studied in the 1960s [6,193]. 

The primary cell walls of most higher plant species contain XGs of the XXXG type, which bear trisaccharide side chains (8) on the backbone [247]. The seeds of many plants contain XXXG-type XGs, in which about 30% of the xylose units possess a /3-D-Galp residue attached to position 2. Several plant species produce XGs that lack fucose and galactose, and have a-L-Ara/ attached to 0-2 of some of the Xylp side-chains, such as XG isolated from olive fruit [262] and soybean (Glycine maxima) meal [263]. However, a-L-Ara/ residues occur also 2-linked directly to some of the Glcp residues of the backbone [154]. 

Submembranous microtubules are often present in parallel bundles beneath the plasma membrane in the cells of higher plants, particularly during cell wall formation (Hardham and Gimning, 1978). Circular submembranous bundles of microtubules are a feature of bird erythrocytes and mammalian blood platelets, where they maintain the discoid shape of these structures (Dustin, 1980). 

A number of investigators have successfully selected cell lines which have higher tolerance to salinity than the line from which they were selected (see Spiegel-Roy Ben Hayyim, 1985 Rains et al., 1986 for a list of plant species). An evaluation of these selected lines demonstrates a number of differences in ionic status and cell wall regulation, but there are relatively few cases where the salinity tolerance of whole plants that have been regenerated from this material have been determined (see Yeo Flowers, Chapter 12). 

Rhamnogalacturonan 11 (RG-11) is a structurally complex, pectic polysaccharide that is present in the primary cell-walls of higher plants. It is composed of 60 glycosyl residues, and is a very complex molecule indeed. For example, on acid hydrolysis, at least ten different monosaccharides are formed, including the novel aceric acid (30), which is the only branched-... 

Pectins is a general term for a group of natural polymers based on polymerized galacturonic acid partly esterified with methanol. In addition these polymers must be considered as copolymers due to existence of neutral sugar branched zones. [1]. Some uronic acid units may also be esterified on 0-2 or 0-3 position with acetic acid. The pectins occur in the cell wall of higher plants and control at least partly the mechanical properties, the ion exchange properties and the swelling of the cell walls. 

An Hypothesis The Same Six Polysaccharides are Components of the Primary Cell Walls of All Higher Plants... 

We hypothesize that the fundamental processes of cell wall expansion are conserved in all higher plants, that is, growth of the cells of all higher plants requires the synthesis and insertion of the same polysaccharides by the same procedures. If this hypothesis is correct, then all primary cell walls have a common set of stmctural polysaccharides. The commonality of the primary cell wall polysaccharides hypothesis does not require that (i) the common polysaccharides be present in all cell walls in the same proportions, (ii) the polysaccharides be... 

This essay was written in an attempt to explain our overview of primary cell walls and to reach consensus on the nomenclature of primary cell wall polysaccharides. We present evidence supporting the hypothesis that cellulose, xyloglucan, arabinoxylan, homogalacturonan, RG-I, and RG-II are the six polysaccharides common to all primary cell walls of higher plants. In many cells, these six polysaccharides account for all or nearly all of the primary wall polysaccharides. Like the physically interacting proteins that constitute the electron transport machinery of mitochondria, the structures of the six patently ubiquitous polysaccharides of primary cell walls have been conserved during evolution. Indeed, we hypothesize that the common set of six structural polysaccharides of primary cell walls have been structurally... 

Many different glycosyltransferase activities involved in higher plant wall biosynthesis have been identified in cell free membrane fractions, but in only a few cases has glycosyltransferase activity been retained in detergent-solubilized preparations, and in even fewer cases have any purified polypeptides been identified as plant cell wall glycosyltransferases (29,33). 

Such a study has been performed on a model plant system, the Nitella flexilis cell wall [1, 2, 3]. This freshwater alga has giant intemodal cells whose easily isolated cell walls constitute a simplified model of higher plant cell walls it has no lignin and its pectin is not methylesterified. Isolated cell walls are cut in pieces and distributed in different lots over the whole exchange isotherm to reduce variability between experimental points. 

In soil, the chances that any enzyme will retain its activity are very slim indeed, because inactivation can occur by denaturation, microbial degradation, and sorption (61,62), although it is possible that sorption may protect an enzyme from microbial degradation or chemical hydrolysis and retain its activity. The nature of most enzymes, particularly size and charge characteristics, is such that they would have very low mobility in soils, so that if a secreted enzyme is to have any effect, it must operate close to the point of secretion and its substrate must be able to diffuse to the enzyme. Secretory acid phosphatase was found to be produced in response to P-deficiency stress by epidermal cells of the main tap roots of white lupin and in the cell walls and intercellular spaces of lateral roots (63). Such apoplastic phosphatase is safe from soil but can be effective only when presented with soluble organophosphates, which are often present in the soil. solution (64). However, because the phosphatase activity in the rhizo-sphere originates from a number of sources (65), mostly microbial, and is much higher in the rhizosphere than in bulk soil (66), it seems curious that plants would have a need to secrete phosphatase at all. 

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