Suppression and acceleration of cell elongation by integration of xyloglucans in pea stem segments

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Abstract

Xyloglucan is a key polymer in the walls of growing plant cells. Using split pea stem segments and stem segments from which the epidermis had been peeled off, we demonstrate that the integration of xyloglucan mediated by the action of wall-bound xyloglucan endotransglycosylase suppressed cell elongation, whereas that of its fragment oligosaccharide accelerated it. Whole xyloglucan was incorporated into the cell wall and induced the rearrangement of cortical microtubules from transverse to longitudinal; in contrast, the oligosaccharide solubilized xyloglucan from the cell wall and maintained the microtubules in a transverse orientation. This paper proposes that xyloglucan metabolism controls the elongation of plant cells.

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Xyloglucan endotransglycosylase, a new wall-loosening enzyme activity from plants.

S C Fry, R C Smith, K F Renwick … (1992)

1. Cell-free extracts of all plants tested contained a novel enzyme activity (xyloglucan endotransglycosylase, XET) able to transfer a high-Mr portion from a donor xyloglucan to a suitable acceptor such as a xyloglucan-derived nonasaccharide (Glc4Xyl3GalFuc; XG9). 2. A simple assay for the enzyme, using [3H]XG9 and based on the ability of the [3H]polysaccharide product to bind to filter paper, is described. 3. The enzyme was highly specific for xyloglucan as the glycosyl donor, and showed negligible transglycosylation of other polysaccharides, including CM-cellulose. 4. The Km for XG9 was 50 microM; certain other 3H-labelled xyloglucan oligosaccharides also acted as acceptors, and certain non-radioactive xyloglucan oligosaccharides competed with [3H]XG9 as acceptor; the minimum acceptor structure was deduced to be: [formula: see text] 5. The pH optimum was approx. 5.5 and the enzyme was less than half as active at pH 7.0. The enzyme was slightly activated by Ca2+, Mg2+, Mn2+, spermidine, ascorbate and 2-mercaptoethanol, and inhibited by Ag+, Hg2+, Zn2+ and La3+. 6. XET activity was essentially completely extracted by aqueous solutions of low ionic strength; Triton X-100, Ca2+, La3+, and Li+ did not enhance extraction. Negligible activity was left in the unextractable (cell-wall-rich) residue. 7. The enzyme differed from the major cellulases (EC 3.2.1.4) of pea in: (a) susceptibility to inhibition by cello-oligosaccharides, (b) polysaccharide substrate specificity, (c) inducibility by auxin, (d) requirement for salt in the extraction buffer and (e) activation by 2-mercaptoethanol. XET is therefore concluded to be a new enzyme activity (xyloglucan: xyloglucan xyloglucanotransferase; EC 2.4.1.-). 8. XET was detected in extracts of the growing portions of dicotyledons, monocotyledons (graminaceous and liliaceous) and bryophytes. 9. The activity was positively correlated with growth rate in different zones of the pea stem. 10. We propose that XET is responsible for cutting and rejoining intermicrofibrillar xyloglucan chains and that it thus causes the wall-loosening required for plant cell expansion.

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Cooperative disassembly of the cellulose-xyloglucan network of plant cell walls: parallels between cell expansion and fruit ripening.

Jocelyn Rose, Alan Bennett (1999)

Modification of the plant primary cell wall is required for both cell expansion and for developmental events, such as fruit softening, where cell size remains static but where wall loosening is an important feature. Recent studies suggest that the cellulose-xyloglucan network is targeted by similar enzymatic activities in both expanding cells and ripening fruit but that unique isoforms are expressed in each process. Disassembly of this structural network probably involves the concerted and synergistic action of suites of these enzyme families, where one family of cell wall modifying proteins might mediate the activity of another, providing the basis for orchestrating ordered cell wall restructuring and turnover.

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Adaptation of roots to low water potentials by changes in cell wall extensibility and cell wall proteins.

Benjamin Cosgrove, Y. Wu (2000)

It is common for the root/shoot ratio of plants to increase when water availability is limiting. This ratio increases because roots are less sensitive than shoots to growth inhibition by low water potentials. The physiological and molecular mechanisms that assist root growth under drought conditions are reviewed, with a focus on changes in cell walls. Maize seedlings adapt to low water potential by making the walls in the apical part of the root more extensible. In part, this is accomplished by increases in expansin activity and in part by other, more complex changes in the wall. The role of xyloglucan endotransglycosylase, peroxidase and other wall enzymes in root adaptation to low water potential is evaluated and some of the complications in the field of study are listed.