Regulation of polysaccharide breakdown during auxin-induced cell wall loosening

Auxin induces cell elongation by increasing the extensibility of the cell wall. Biochemical modifications of wall constituents lead to such changes in the mechanical properties of the cell wall (wall loosening). The results obtained in the studies using antibodies and lectins as specific probes indicate that the breakdown of xyloglucans in dicotyledons and (1→3), (1→4)-β-glucans in Poaceae is involved in auxin-induced wall loosening. In dicotyledons, xyloglucans are degraded by the direct hydrolysis with an endoglucanase to oligosaccharides and by the two-step reaction via a product with intermediate size. (1→3), (1→4)-β-Glucan breakdown in Poaceae coleoptiles is mediated by the two-step reaction with endo-and exoglucanases. Although auxin inducesde novo synthesis of some hydrolases involved in breakdown of these polysaccharides, the breakdown activity is also regulated by the wall environment such as pH, by the mobility of hydrolases through wall networks, by the interaction of hydrolases with wall polysaccharide complex, and by the presence and the concentrations of different types of regulatory molecules. Recipient of the Botanical Society Award of Young Scientists, 1992.     

Polysaccharide composition of unlignified cell walls of pineapple [Ananas comosus (L.) Merr.] fruit.

The polysaccharides of cell walls isolated from the fleshy, edible part of the fruit of the monocotyledon pineapple [Ananas comosus (L.) Merr.] (family Bromeliaceae) were analyzed chemically. These cell walls were derived mostly from parenchyma cells and were shown histochemically to be unlignified, but they contained ester-linked ferulic acid. The analyses indicated that the noncellulosic polysaccharide composition of the cell walls was intermediate between that of unlignified cell walls of species of the monocotyledon family Poaceae (grasses and cereals) and that of unlignified cell walls of dicotyledons. Glucuronoarabinoxylans were the major non-cellulosic polysaccharides in the pineapple cell walls. Xyloglucans were also present, together with small amounts of pectic polysaccharides and glucomannans (or galactoglucomannans). The large amounts of glucuronoarabinoxylans and small amounts of pectic polysaccharides resemble the noncellulosic polysaccharide composition of the unlignified cell walls of the Poaceae. However, the absence of (1-->3,1-->4)-beta-glucans, the presence of relatively large amounts of xyloglucans, and the possible structure of the xyloglucans resemble the noncellulosic polysaccharide composition of the unlignified cell walls of dicotyledons.     

Perception mechanism of gravistimuli in gravity resistance responses of plants.

Gravity resistance is a response that enables plants to develop against the gravitational force. Hypergravity conditions produced by centrifugation have been used to analyze the mechanisms of gravity resistance responses. Under hypergravity conditions, plants construct short and thick shoots and increase cell wall rigidity for resisting the gravitational force. Hypergravity caused a decrease in the percentage of cells with transverse microtubules, and an increase in that with longitudinal microtubules. Such a prompt reorientation of cortical microtubules is involved in the changes in morphology of shoots by gravity. Hypergravity increased cell wall rigidity by increasing the molecular mass of xyloglucans via suppression of xyloglucan breakdown as well as by the thickening of cell walls. Blocker reagents of mechanoreceptors nullified the above-mentioned changes induced by hypergravity. Gravity resistance responses were brought about normally in mutants deprived of gravitropism. This result indicates that the graviperception mechanism in gravity resistance is independent of that in gravitropism. Gravity resistance responses were brought about independently of the direction of gravistimuli, but the responses disappeared in the presence of blockers of mechanoreceptors. Thus, in gravity responses, plants may perceive the gravitational force independently of the direction of stimuli by mechanoreceptors on the plasma membrane, and may utilize the signal to construct a tough body.     

Physiological functions of plant cell coverings

The cell coverings of plants have two important functions in plant life. Plant cell coverings are deeply involved in the regulation of the life cycle of plants: each stage of the life cycle, such as germination, vegetative growth, reproductive growth, and senescence, is strongly influenced by the nature of the cell coverings. Also, the apoplast, which consists of the cell coverings, is the field where plant cells first encounter the outer environment, and so becomes the major site of plant responses to the environment. In the regulation of each stage of the life cycle and the response to each environmental signal, some specific constituents of the cell coverings, such as xyloglucans in dicotyledons and 1,3,1,4-β-glucans in Gramineae, act as the key component. The physiological functions of plant cell coverings are sustained by the metabolic turnover of these components. The components of the cell coverings are supplied from the symplast, but then they are modified or degraded in the apoplast. Thus, the metabolism of the cell coverings is regulated through the cross-talk between the symplast and the apoplast. The understanding of physiological functions of plant cell coverings will be greatly advanced by the use of genomic approaches. At the same time, we need to introduce nanobiological techniques for clarifying the minute changes in the cell coverings that occur in a small part within each cell. Electronic Publication     

Is water immersion useful for analyzing gravity resistance responses in terrestrial plants?

Water immersion has been used as a simulator of microgravity for analyzing gravity responses in semiaquatic plants such as rice. To examine whether or not water immersion for a short experimental period is a useful microgravity simulator even in terrestrial plants, we analyzed effects of water immersion on the cell wall rigidity and metabolisms of its constituents in azuki bean epicotyls. The cell wall rigidity of epicotyls grown underwater was significantly lower than that in the control. Water immersion also caused a decrease in molecular mass of xyloglucans as well as the thinning of the cell wall. Such changes in the mechanical and chemical properties of the cell wall underwater were similar to those observed in microgravity conditions in space. These results suggest that water immersion for a short period is a useful system for analyzing gravity resistance responses even in terrestrial plants.     

Inhibition of auxin-induced elongation and xyloglucan breakdown in azuki bean epicotyl segments by fucose-binding lectins

Auxin-induced elongation of epicotyl segments of azuki bean ( Vigna angularis Ohwi and Ohashi cv. Takara) was suppressed by fucose-binding lectins from Tetragonolobus purpureus Moench and Ulex europaeus L. These lectins also inhibited auxin-induced cell wall loosening (decrease in the minimum stress-relaxation time of the cell walls) of segments. Auxin caused a decrease in molecular mass of xyloglucans extracted with 24% KOH from the cell walls. The lectins inhibited auxin-induced changes in molecular mass of the xyloglucans. The autolytic release of xylose-containing products from the pectinase-treated cell walls was also suppressed by the lectins. Fucose-binding lectins pretreated with fucose exhibited little or no inhibitory effect on auxin-induced elongation, cell wall loosning, or breakdown of xyloglucans. These results support the view that the breakdown of xyloglucans is involved in the cell wall loosening responsible for auxin-induced elongation in dicotyledons.     

Development of the anti-gravitational system in land plants and its implication for the interaction between plants and other organisms.

After they first went ashore during the Silurian epoch, plants have developed the anti-gravitational system to survive under terrestrial environment with the strong gravitational force. The cell wall acts as a principal component of the anti-gravitational system in plants, probably with the aid of links to the plasma membrane and the cytoskeleton. The cell wall has well developed in land plants and often represents more than 90% of the dry weight of the plant. The development of the cell wall has greatly influenced the interaction between plants and other organisms, such as feeding, sheltering, invasion, and symbiosis, and has been involved in the regulation of the global environment throughout the evolution.     

Stimulation of elongation growth and xyloglucan breakdown in Arabidopsis hypocotyls under microgravity conditions in space

Seedlings of Arabidopsis thaliana (L.) Heynh. (ecotype Columbia and an ethylene-resistant mutant etr1-1) were cultivated for 68.5, 91.5 and 136 h on board during the Space Shuttle STS-95 mission, and changes in the elongation growth and the cell wall properties of hypocotyls were analyzed. Elongation growth of dark-grown hypocotyls of both Columbia and etr1-1 was stimulated under microgravity conditions in space. There were no clear differences in the degree of growth stimulation between Columbia and etr1-1, indicating that the ethylene level was not abnormally high in the cultural environment of this space experiment. Microgravity also increased the mechanical extensibility of cell walls in both cultivars, and such an increase was attributed to the increase in the apparent irreversible extensibility. The levels of cell wall polysaccharides per unit length of hypocotyls decreased in space. Microgravity also reduced the weight-average molecular mass of xyloglucans in the hemicellulose-II fraction. Also, the activity of xyloglucan-degrading enzymes extracted from hypocotyl cell walls increased under microgravity conditions. These results suggest that microgravity reduces the molecular mass of xyloglucans by increasing xyloglucan-degrading activity. Modifications of xyloglucan metabolism as well as the thickness of cell wall polysaccharides seem to be involved in an increase in the cell wall extensibility, leading to growth stimulation of Arabidopsis hypocotyls in space.     

Changes in Wall Polysaccharides of Squash (Cucurbita maxima Duch.) Hypocotyls under Water Stress Condition: II. Composition of Pectic and Hemicellulosic Polysaccharides

Hypocotyl growth of dark-grown squash (Cucurbita maxima Duch.)seedlings was greatly reduced by the addition of polyethyleneglycol (60 mM) to the hydroponic solution through inhibitionof cell elongation. Measurement of the mechanical propertiesof the cell walls revealed that the cell wall of stressed hypocotylswas loosened as much as that of the unstressed hypocotyls, suggestingthat the stressed hypocotyl could not elongate even though thecell wall loosened. Galactose and arabinose in the pectic fraction,which are probably attached to high mol wt rhamnogalacturonans,increased under stressed as well as under unstressed condition.Other polysaccharides including pectic low mol wt galacturonans,hemicellulosic xyloglucans, galactoglucomannans, arabinans,and glucuronoarabinoxylans increased more under unstressed condition.The mol wt of xyloglucans in the hemicellulosic fraction increasedunder unstressed but not under stressed condition. These results suggest that changes in wall structure, such asincreases in high mol wt rhamnogaracturonans rich in arabinoseand galactose residues, and the suppression of polymerizationof xyloglucans are involved in the process of cell wall loosening. (Received December 15, 1986; Accepted June 8, 1987)     

Cell wall carbohydrates as signals in plants

Plant and fungal cells are surrounded by a cell wall rich in diverse polysaccharides and proteins. It has become apparent in recent years that the carbohydrates in the cell wall function not only to maintain cell shape and integrity, but also may serve as signals in plants. This review summarizes the evidence that biologically-active oligosaccharides (oligosaccharins) released from plant or microbial cell walls can serve as signals to regulate plant defense and plant growth and development. The oligosaccharins discussed include the fungal-derived hepta-β-glucoside and the plant cell wall-derived oligogalacturonides and xyloglucans. Possible mechanisms by which oligosaccharins may exert their effects on plant cells are discussed.     

Endo-xyloglucan transferase, a novel class of glycosyltransferase that catalyzes transfer of a segment of xyloglucan molecule to another xyloglucan molecule.

Xyloglucans are the major component of plant cell walls and bind tightly to the surface of individual cellulose microfibrils, thereby cross-linking them into a complex polysaccharide network of the cell wall. The cleavage and reconnection of xyloglucan cross-links are considered to play the leading role during chemical processes essential for wall expansion and, therefore, cell growth and differentiation. Although it is hypothesized that some transglycosylation is involved in these chemical processes, the enzyme responsible for the reaction was not identified. We have now purified a novel class of endo-type glycosyltransferase to apparent homogeneity from the extracellular space or the cell wall of the epicotyls of Vigna angularis, a bean plant. The enzyme is a glycoprotein with a molecular mass of about 33 kDa. The enzyme catalyzes both 1) endo-type splitting of a xyloglucan molecule and 2) linking of a newly generated reducing end of the xyloglucan to the nonreducing end of another xyloglucan molecule, thereby mediating the transfer of a large segment of the xyloglucan to another xyloglucan molecule. The transferase exhibits no glycosidase or glycanase activity. Substrate specificity of the enzyme was investigated using several polysaccharides with different glycosidic linkages as donor substrates and pyridylamino oligosaccharides as acceptor substrates, in which the reducing end of the carbohydrate was tagged with a fluorescent group. The enzyme required a basic xyloglucan structure, i.e. a beta-(1-->4)-glucosyl backbone with xylosyl side chains, for both acceptor and donor activity. Galactosyl or fucosyl side chains on the main chain were not required for the acceptor activity. The enzyme exhibited higher reaction rates when xyloglucans with higher M(r) were used as donor substrates. Xyloglucans smaller than 10 kDa were no longer the donor substrate. On the other hand, pyridylamino heptasaccharide acted as a good acceptor as did xyloglucan polymers. Based on these results we propose to designate this novel enzyme a xyloglucan: xyloglucano-transferase, to be abbreviated endo-xyloglucan transferase (EXT) or xyloglucan recombinase. This enzyme is the first enzyme identified that mediates the transfer of a high M(r) segment between polysaccharide molecules to generate chimeric polymers. We conclude that endo-xyloglucan transferase functions as a reconnecting enzyme for xyloglucans and is involved in the interweaving or reconstruction of cell wall matrix, which is responsible for chemical creepage that leads to morphological changes in the cell wall.     

Use of an alpha-D-glucosidase for the specific cytochemical identification of lateral alpha-D-xylosyl end groups in plant xyloglucans

alpha-Linked D-xylosyl side chains represent the typical feature common to all xyloglucans not shared by other cell wall polysaccharides. Since no easily available alpha-D-xyloxidase is known, advantage was taken of the conformational and configurational homologies between alpha-D-xylopyranose and alpha-D-glucopyranose to make an alpha-D-glucosidase-gold complex which was able to recognize alpha-D-xylosyl terminal residues of xyloglucans. This marker was used together with alpha-L-fucosidase gold complex for the double labeling on two different structural features of the same macromolecule in plant primary cell wall.     

Differential Effect of Auxin on Molecular Weight Distributions of Xyloglucans in Cell Walls of Outer and Inner Tissues from Segments of Dark Grown Squash (Cucurbita maxima Duch.) Hypocotyls

Effects of indole-3-acetic acid (IAA) on the mechanical properties of cell walls and structures of cell wall polysaccharides in outer and inner tissues of segments of dark grown squash (Cucurbita maxima Duch.) hypocotyls were investigated. IAA induced the elongation of unpeeled, intact segments, but had no effect on the elongation of peeled segments. IAA induced the cell wall loosening in outer tissues as studied by the stress-relaxation analysis but not in inner tissues. IAA-induced changes in the net sugar content of cell wall fractions in outer and inner tissues were very small. Extracted hemicellulosic xyloglucans derived from outer tissues had a molecular weight about two times as large as in inner tissues, and the molecular weight of xyloglucans in both outer and inner tissues decreased during incubation. IAA substantially accelerated the depolymerization of xyloglucans in outer tissues, while it prevented that in inner tissues. These results suggest that IAA-induced growth in intact segments is due to the cell wall loosening in outer tissues, and that IAA-accelerated depolymerization of hemicellulosic xyloglucans in outer tissues is involved in the cell wall loosening processes.     

Functional Diversity of Xyloglucan-Related Proteins and its Implications in the Cell Wall Dynamics in Plants

Abstract: The plant cell wall is a dynamic apparatus responsible for both morphogenesis and responsiveness to environmental conditions. In the cell wall of most seed plants, cellulose microfibrils are cross-linked by xyloglucans to form a cellulose/xyloglucan framework, which functions as the mechanical underpinning of the cell wall. Endoxyloglucan transferases are a class of enzymes that play a central role in construction and modification of the plant cell wall. These enzymes are encoded by a large multi-gene family termed xyloglucan-related proteins (XRPs). More than 24 members of the XRP family have so far been identified in Arabidopsis thaliana. Each member of this family functions as either a hydrolase or a transferase acting on xyloglucans. The primary structures of proteins and gene-expression profiles have strongly suggested their potentially divergent roles in plant morphogenesis: different members of this family are expressed in different types of tissues at distinct developmental stages and respond differentially to individual hormones as well as environmental stimuli. These facts imply that each member of this gene family is individually committed to a specific process that proceeds in a specific tissue at a specific stage of development. Probably the generation and maintenance of the cell walls in a whole organ, and thus in the whole plant, is achieved by the ensemble of individual members of the XRP family.     

Action of xyloglucan hydrolase within the native cell wall architecture and its effect on cell wall extensibility in azuki bean epicotyls

Xyloglucan hydrolase (XGH) has recently been purified from the cell wall of azuki bean (Vigna angularis Ohwi et Ohashi) epicotyls as a new type of xyloglucan-degrading enzyme [Tabuchi et al. (2001) Plant Cell Physiol. 42: 154]. In the present study, the effects of XGH on the mechanical properties of the cell wall and on the level and the molecular size of xyloglucans within the native wall architecture were examined in azuki bean epicotyls. When the epidermal tissue strips from the growing regions of azuki bean epicotyls were incubated with XGH, the mechanical extensibility of the cell wall dramatically increased. XGH exogenously applied to cell wall materials (homogenates) or epidermal tissue strips decreased the amount of xyloglucans via the solubilization of the polysaccharides. Also, XGH substantially decreased the molecular mass of xyloglucans in both materials. These results indicate that XGH is capable of hydrolyzing xyloglucans within the native cell wall architecture and thereby increasing the cell wall extensibility in azuki bean epicotyls.     

Plant glycoside hydrolases involved in cell wall polysaccharide degradation.

The cell wall plays a key role in controlling the size and shape of the plant cell during plant development and in the interactions of the plant with its environment. The cell wall structure is complex and contains various components such as polysaccharides, lignin and proteins whose composition and concentration change during plant development and growth. Many studies have revealed changes in cell walls which occur during cell division, expansion, and differentiation and in response to environmental stresses; i.e. pathogens or mechanical stress. Although many proteins and enzymes are necessary for the control of cell wall organization, little information is available concerning them. An important advance was made recently concerning cell wall organization as plant enzymes that belong to the superfamily of glycoside hydrolases and transglycosidases were identified and characterized; these enzymes are involved in the degradation of cell wall polysaccharides. Glycoside hydrolases have been characterized using molecular, genetic and biochemical approaches. Many genes encoding these enzymes have been identified and functional analysis of some of them has been performed. This review summarizes our current knowledge about plant glycoside hydrolases that participate in the degradation and reorganisation of cell wall polysaccharides in plants focussing particularly on those from Arabidopsis thaliana.     

Hypergravity increases the molecular mass of xyloglucans by decreasing xyloglucan-degrading activity in azuki bean epicotyls.

Elongation growth of dark-grown azuki bean (Vigna angularis Ohwi et Ohashi cv. Takara) epicotyls was suppressed by hypergravity at 30 x g and above. Acceleration at 300 x g significantly decreased the mechanical extensibility of cell walls. The amounts of cell wall polysaccharides (pectin, hemicellulose-II and cellulose) per unit length of epicotyls increased under the hypergravity condition. Hypergravity also increased the amounts and the weight-average molecular mass of xyloglucans in the hemicellulose-II fraction, while decreasing the activity of xyloglucan-degrading enzymes extracted from epicotyl cell walls. These results suggest that hypergravity increases the amounts and the molecular mass of xyloglucans by decreasing xyloglucan-degrading activity. Modification of xyloglucan metabolism as well as the thickening of cell walls under hypergravity conditions seems to be involved in making the cell wall mechanically rigid, thereby inhibiting elongation growth of azuki bean epicotyls.     

Immunohistochemical investigation of the necrotrophic phase of the fungus Colletotrichum gloeosporioides in the biocontrol of hemp sesbania (Sesbania exaltata; Papilionaceae)

? Premise of the study: Fungal plant pathogens exert much of their effect on plant cells through alterations in the host cell walls. However, obtaining biochemical proof for this change is difficult because of the relatively small number of cells that are affected by the pathogen relative to the bulk of host tissue. In this study, we examined the differences in host wall composition between infected and uninfected areas of seedlings of the weed hemp sesbania (Sesbania exaltata) that were treated with the biocontrol agent Colletotrichum gloeosporioides. ? Methods: To determine the changes in cell wall composition, we used semi-thin sections and a battery of antibody probes that recognize components of the cell wall and immunogold-silver cytochemistry to visualize the probes. ? Key results: A loss of specific plant cell wall polysaccharides in the region surrounding the primary fungal infection and the creation of a defensive layer by the plant to limit the fungal invasion were the two most obvious changes noted in this study. At the invasion site, there was significant loss of rhamnogalacturon-1 (RGI) and esterified and de-esterified homogalacturonan (HG)-reactive epitopes from the cell walls. In contrast, boundary tissue between the vascular tissue and the fungal lesion reacted more strongly with antibodies that recognize arabinogalactan proteins (AGPs) and xyloglucans than in unaffected areas. ? Conclusions: These data strongly indicate a role of pectinases in the invasion of the biocontrol agent and the importance of extensins, AGPs, and xyloglucans as defense by the host.     

Adsorption of a hydrophobic mutagen to five contrasting dietary fiber preparations.

The ability of five plant cell wall (dietary fiber) preparations with contrasting compositions to adsorb in vitro the hydrophobic, environmental mutagen, 1,8-dinitropyrene (DNP), was investigated. Many of the fruits and vegetables in Western diets are from dicotyledonous (broad leaved) plants and the dietary fiber from these consists mainly of unlignified cell walls. A representative of this wall type, prepared from immature cabbage leaves, showed little ability to adsorb DNP. Two other cell-wall preparations, representing lignified walls of dicotyledons and unlignified walls of vegetative parts of grasses and cereals (monocotyledons belonging to the family Poaceae), adsorbed DNP much more effectively. However, two further preparations, representing suberized walls of cork cells and lignified walls of vegetative parts of grasses and cereals, were the most effective in adsorbing DNP. Extrapolation of these data to the in vivo situation would indicate that increased consumption of the vegetative parts of grasses or cereals and plant material containing cork cells, for example potato skins, could be effective in removing hydrophobic mutagens from potential contact with colonic mucosal cells.     

Cell wall integrity: Targeted post-synthetic modifications to reveal its role in plant growth and defense against pathogens

The plant cell wall, a dynamic network of polysaccharides and glycoproteins of significant compositional and structural complexity, functions in plant growth, development and stress responses. In recent years, the existence of plant cell wall integrity (CWI) maintenance mechanisms has been demonstrated, but little is known about the signaling pathways involved, or their components. Examination of key mutants has shed light on the relationships between cell wall remodeling and plant cell responses, indicating a central role for the regulatory network that monitors and controls cell wall performance and integrity. In this review, we present a short overview of cell wall composition and discuss post-synthetic cell wall modification as a valuable approach for studying CWI perception and signaling pathways.