Effect of Xyloglucan Oligosaccharides on Growth, Viscoelastic Properties, and Long-Term Extension of Pea Shoots

Proc. Natl. Acad. Sci. U.S.A.

Furuta

Awano

et al. 2002

184

141

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.X yloglucan, which occurs widely in the primary walls of higher plants, possesses a 1,4-␤-glucan backbone with 1,6-␣-xylosyl residues along the backbone. Because the 1,4-␤-glucan backbone can bind specifically to cellulose microfibrils by hydrogen bonds (1), the xyloglucan probably contributes to the rigidity of the cell wall by cross-linking adjacent microfibrils (2). In fact, microfibrils seem to be coated with xyloglucan, which is located both on and between microfibrils throughout cell elongation (3). Masking xyloglucans in cell walls should prevent xyloglucan metabolism; in agreement with this prediction, an antibody specific to xyloglucan prevented an auxin-induced decrease in molecular size of xyloglucan and inhibited indole-3-acetic acid (IAA)-induced cell elongation in azuki hypocotyl segments (4). Xyloglucan endotransglycosylase (XET) has been proposed to participate in the dynamic changes of xyloglucan cross-linking (5, 6), but there is little direct evidence for this hypothesis. The question at issue concerns the structural function of xyloglucan, namely whether xyloglucan contributes to the extensibility of the wall by cross-linking adjacent microfibrils.Fucose-containing xyloglucan oligosaccharides have been shown to inhibit auxin-induced elongation of pea stems (7,8). Their inhibitory activity is approximately maximal at a concentration of 10 Ϫ8 to 10 Ϫ9 M. In the absence of auxin, a high concentration (Ͼ10 Ϫ8 M) of xyloglucan oligosaccharide did not inhibit but slightly promoted cell elongation in pea stem segments (9). Thus, the oligosaccharides may provide either negative or positive feedback control during cell elongation. However, the positive feedback reaction has not been reproducibly observed in pea stems (T.T. and T.H., unpublished results) and also was not observed in maize primary roots (10). In the present communication, we examine whether xyloglucan oligosaccharides control the elongation growth of plant cells.In cylindrical plant organs such as roots or stems, cells expand anisotropically, with the direction of most rapid growth parallel to the long axis of the organ, leading to elongation of the organ. It has been known for many years that the direction of elongation is perpendicular to the direction of net orientation of cellulose microfibrils (11). Therefore, paral...

Section: Discussionmentioning

confidence: 57%

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

Proc. Natl. Acad. Sci. U.S.A.

Furuta

Awano

et al. 2002

184

141

“…2000) and root hairs (Vissenberg et al 2003). Furthermore, exogenous xyloglucan oligosaccharides, which would be expected to compete with cell wall xyloglucan (polysaccharide) as the acceptor substrate for endogenous XTHs, promote cell expansion in living pea stem segments (McDougall and Fry 1990;Cutillas-Iturralde and Lorences 1997;Takeda et al 2002). Direct evidence for the re-structuring of existing wall material by XTH-catalysed interpolymeric transglycosylation was provided by 13 C/ 3 H dual-labelling experiments (Thompson and Fry 2001).…”

Section: Introductionmentioning

confidence: 97%

Control of xyloglucan endotransglucosylase activity by salts and anionic polymers

Fry

2004

Planta

Crude extracts of cauliflower florets had high xyloglucan endotransglucosylase (XET) activity, but this was largely lost after partial purification and de-salting. Activity was restored (promoted up to 40-fold) by any of a wide variety of inorganic and organic salts. Optimum concentrations for Na+, K+ and NH4+ salts were typically approximately 300 mM. The chlorides of Ca2+, Mg2+, Al3+ and La3+ were optimally active at lower concentrations (e.g. 0.1 mM LaCl3), but became inhibitory at higher concentrations (e.g. 5 mM LaCl3). Some anionic polysaccharides at 0.04-0.2% w/v (e.g. gum arabic, pectin and hypochlorite-oxidised xyloglucan) promoted the XET activity of de-salted enzyme, especially if a sub-optimal concentration of NaCl was also present; others (e.g. homogalacturonan, 4- O-methyl-glucuronoxylan and alginate) were inhibitory. Similar ionic effects were noted on the XET activity of the Arabidopsis protein XTH24 (heterologously expressed by insect cells); in this case carboxymethylcellulose was also stimulatory. To look for endogenous modulators of XET activity, we prepared a cold-water extract of cauliflower florets; after boiling and centrifugation, the supernatant [boiled cauliflower preparation (BCP)] promoted the XET activity of de-salted cauliflower enzyme and of XTH24. About half the activator present in BCP was an ethanol-precipitable, anionic polymer of apparent Mr <5,000. After acid hydrolysis the polymer yielded much arabinose and galactose, and small amounts of galacturonic and glucuronic acids amino acids were also present. The polymer may thus contain arabinogalactan-proteins. We suggest that acidic polymers and/or other apoplastic ions are naturally occurring regulators of XET action in vivo, and may thus control cell wall assembly, loosening, and growth.

“…In living pea stem segments, exogenous XGOs can promote cell expansion, probably owing to their ability to act as competing acceptor substrates in place of endogenous xyloglucan (McDougall and Fry 1990;Cutillas-Iturralde and Lorences 1997;Takeda et al 2002): wall xyloglucans are cleaved without subsequent restoration of inter-microfibrillar tethers, and therefore the wall is loosened more lastingly than in the absence of XGOs. Xyloglucan-to-xyloglucan transglycosylation, with donor and acceptor both pre-existing in the cell wall (i.e., 're-structuring transglycosylation'), has been demonstrated by 13 C density labelling experiments in living Rosa cells (Thompson and Fry 2001).…”

Section: Introductionmentioning

confidence: 99%

Anionic derivatives of xyloglucan function as acceptor but not donor substrates for xyloglucan endotransglucosylase activity

Miller

Fry

2007

Planta

Tamarind xyloglucan was oxidised by reaction with sodium hypochlorite in the presence of 2,2,6,6-tetramethyl-1-piperidinyloxy free radical (TEMPO). Galactose residues and non-xylosylated glucose residues were thus converted into galacturonic and glucuronic acid residues, respectively, producing an anionic polysaccharide. Acid hydrolysis of oxidised xyloglucan yielded two aldobiouronic acids, deduced to be beta-D: -GalpA-(1-->2)-D-Xyl and beta-D: -GlcpA-(1-->4)-D-Glc. Anionic xyloglucan had a decreased ability to hydrogen-bond to cellulose and to complex with iodine. It was almost totally resistant to digestion by cellulase [endo-(1-->4)-beta-glucanase] and did not serve as a donor substrate for xyloglucan endotransglucosylase (XET) activity. Like several other anionic polysaccharides, it promoted XET activity when unmodified (non-ionic) xyloglucan was used as donor substrate. Anionic xyloglucan may mimic polyanions whose presence in the plant cell wall promotes the action of endogenous XTH proteins. NaOCl with TEMPO oxidised the heptasaccharide, XXXG, to form XXX-glucarate, which did serve as an acceptor substrate although at a rate approximately fourfold less than XXXG itself. Anionic derivatives of xyloglucan, acting as acceptor but not donor substrates, may be valuable tools for exploring the biological roles of XTHs in the integration versus the re-structuring of xyloglucan in the plant cell wall.