Advances in determination of polymer structure and in preservation of structure for electron microscopy provide the best view to date of how polysaccharides and structural proteins are organized into plant cell walls. The walls that form and partition dividing cells are modified chemically and structurally from the walls expanding to provide a cell with its functional form. In grasses, the chemical structure of the wall differs from that of all other flowering plant species that have been examined. Nevertheless, both types of wall must conform to the same physical laws. Cell expansion occurs via strictly regulated reorientation of each of the wall's components that first permits the wall to stretch in specific directions and then lock into final shape. This review integrates information on the chemical structure of individual polymers with data obtained from new techniques used to probe the arrangement of the polymers within the walls of individual cells. We provide structural models of two distinct types of walls in flowering plants consistent with the physical properties of the wall and its components.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-313X.1993.tb00007.x

Structural models of primary cell walls in flowering plants: consistency of molecular structure with the physical properties of the walls during growth

This review deals with recent advances in the structural characterization of pectins and the gels which they form, in relation to auxin-induced extension growth, the ripening of fruit, and cellular recognition. Pectins are block polysaccharides. Heavily branched, largely methyl-esterified blocks alternate with unbranched blocks of varying degrees of esterification. The unbranched, non-esterified blocks can aggregate through calcium binding to form the junction zones that hold a gel together. The aggregates are of two, or possibly four, chains at low calcium levels, and larger with excess calcium. The fall in wall pH during auxin-induced growth activates glycanase enzymes. These may attack some components of the pectic fraction, as well as xyloglucans. Pectin-bound calcium ions may be displaced but this probably has little effect on gel strength. Pectins may be cross-linked by diferulate esters when growth stops. The softening of ripe fruit is due to loss of cohesion in the pectin gel. In apples this results from replacement of the pectins by more esterified forms. In many other fruits it results from depolymerization by polygalacturonases, assisted by pectinesterases, so that the remaining segments are too short for effective calcium binding. Pectins have a further role in the recognition reactions between plant cells and some of their bacterial and fungal pathogens.

Structure and properties of pectin gels in plant cell walls

Publisher Summary Pollen germination and pollen-tube growth events when viewed either in culture or in the style are considered as biochemically, physiologically, and structurally complex. The various processes involved are each independent in some measure, but still all are interactive—namely, the successful attainment of the delivery of two male gametes into the embryo sac, requires a high degree of coordination within the male gametophyte and at the same time a remarkable sensitivity to a considerable range of influences both in the physical environment and in the pistil. Much of the activity of the vegetative cell has the air of being preprogrammed; and this must be so in those species where there is little or no gene transcription in the vegetative nucleus. The capacity to function successfully in a complex and varying environment betokens a high degree of self-regulation. Some of the feedback mechanisms likely to contribute to stability are reviewed. They are analogous with the homeorhetic systems postulated in other instances of biological development. However, two features make the angiosperm male gametophyte almost unique among higher organisms, namely the versatility expressed by a single haploid cell, and the degree of coadaptation it shows with its diploid host.

Pollen germination and pollen-tube growth

Methanol has been recognised as an important constituent of the background atmosphere, but little is known about its overall cycle in the biosphere/atmosphere system A model is proposed for the production and emission to the atmosphere of methanol by flowering plants based on plant structure and metabolic properties, particularly the demethylation of pectin in the primary cell walls This model provides a framework to extend seven sets of measurements of methanol emission rates to the global terrestrial biosphere A global rate of release of methanol from plants to the atmosphere of 100 Tg y−1 is calculated A separate model of the global cycle of methanol is constructed involving emissions from plant growth and decay, atmospheric and oceanic chemical production, biomass burning and industrial production Removal processes occur through hydroxyl radical attack in the atmosphere, in clouds and oceans, and wet and dry deposition The model successfully reproduces the methanol concentrations in the continental boundary-layer and the free atmosphere, including the inter-hemispheric gradient in the free atmosphere The model demonstrates a new concept in global biogeochemistry, the coupling of plant cell growth with the global atmospheric concentration of methanol The model indicates that the ocean provides a storage reservoir capable of holding at least 66 times more methanol than the atmosphere The ocean surface layer reservoir essentially buffers the atmospheric concentration of methanol, providing a physically based smoothing mechanism with a time constant of the order of one year

The Production of Methanol by Flowering Plants and the Global Cycle of Methanol

Infrared and Raman spectra of sequentially extracted primary cell walls and their pectic polymers were obtained from five angiosperm plants. Fourier-transform Raman spectrometry was shown to be a powerful tool for the investigation of primary cell-wall architecture at a molecular level, providing complementary information to that obtained by Fourier-transform infrared microspectroscopy. The use of an extraction procedure using imidazole instead of cyclohexane trans-1,2-N,N,N[prime],N[prime]-diaminotetraacetate allows the extension of the infrared spectral window for data interpretation from 1300 to 800 cm-1, to 2000 to 800 cm-1, and allows us to obtain Raman spectra from extracted cell-wall material. Wall constituents such as pectins, proteins, aromatic phenolics, cellulose, and hemicellulose have characteristic spectral features that can be used to identify and/or fingerprint these polymers without, in most cases, the need for any physical separation. The Gramineae (rice [Oryza sativa], polypogon [Polypogon fugax steud], and sweet corn [Zea mays]) are spectroscopically very different from the nongraminaceous monocotyledon (onion [Allium cepa]) and the dicotyledon (carrot [Daucus carota]); this reflects differences in chemical composition and cross-linking of the walls. The possibility of a taxonomic classification of plant cell walls based on infrared and Raman spectroscopies and the use of spectral fingerprinting for authentication and detection of adulteration of products rich in cell-wall materials are discussed.

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Norihiko Nakajima

Papers

Differential effect of Calcium and Magnesium on Mechanical Properties of Pea Stem Cell Walls

Oriented structure of pectic polysaccharides in pea epidermal cell walls.