A Revised Architecture of Primary Cell Walls Based on Biomechanical Changes Induced by Substrate-Specific Endoglucanases      

Abstract

Xyloglucan is widely believed to function as a tether between cellulose microfibrils in the primary cell wall, limiting cell enlargement by restricting the ability of microfibrils to separate laterally. To test the biomechanical predictions of this “tethered network” model, we assessed the ability of cucumber (Cucumis sativus) hypocotyl walls to undergo creep (long-term, irreversible extension) in response to three family-12 endo-β-1,4-glucanases that can specifically hydrolyze xyloglucan, cellulose, or both. Xyloglucan-specific endoglucanase (XEG from Aspergillus aculeatus) failed to induce cell wall creep, whereas an endoglucanase that hydrolyzes both xyloglucan and cellulose (Cel12A from Hypocrea jecorina) induced a high creep rate. A cellulose-specific endoglucanase (CEG from Aspergillus niger) did not cause cell wall creep, either by itself or in combination with XEG. Tests with additional enzymes, including a family-5 endoglucanase, confirmed the conclusion that to cause creep, endoglucanases must cut both xyloglucan and cellulose. Similar results were obtained with measurements of elastic and plastic compliance. Both XEG and Cel12A hydrolyzed xyloglucan in intact walls, but Cel12A could hydrolyze a minor xyloglucan compartment recalcitrant to XEG digestion. Xyloglucan involvement in these enzyme responses was confirmed by experiments with Arabidopsis (Arabidopsis thaliana) hypocotyls, where Cel12A induced creep in wild-type but not in xyloglucan-deficient (xxt1/xxt2) walls. Our results are incompatible with the common depiction of xyloglucan as a load-bearing tether spanning the 20- to 40-nm spacing between cellulose microfibrils, but they do implicate a minor xyloglucan component in wall mechanics. The structurally important xyloglucan may be located in limited regions of tight contact between microfibrils.

Revision of adsorption models of xyloglucan on microcrystalline cellulose

“…4d, h), which is in accordance with the results obtained from the atomic force microscope for celery (Apium graveolens) cell walls in which the removal of xyloglucan and pectins induced the cellulose microfibrils to self-associate and aggregate (Thimm et al 2002(Thimm et al , 2009). The recent findings suggest that xyloglucan plays the role of the molecular binder and is involved in the adhesion of microfibrils over short distances (Cosgrove and Jarvis 2012;Park and Cosgrove 2012). In the present study, Avicel cellulose was used, which is highly crystalline (53.8 %) and mostly contains the Ib structure (96 %) (Szymańska-Chargot et al 2011).…”

Functional Analysis of Cellulose and Xyloglucan in the Walls of Stomatal Guard Cells of Arabidopsis

“…Given that xxt1 xxt2 and cesa7 irx3-5 guard cells phenocopy each other in having more fibrillar S4B staining patterns, we speculate that cellulose reorganization might be more likely to depend on the proper control of stomatal complex size. It is also possible that the regulation of stomatal apertures and the dynamic changes of cellulose distribution in guard cells involve interactions between cellulose and xyloglucan, which are thought to take place at limited regions and are the targets of expansin action (Park and Cosgrove, 2012b;Cosgrove, 2014). In keeping with this idea, overexpression of the Arabidopsis a-expansin gene AtEXPA1 accelerates stomatal opening, likely by regulating the elastic properties of guard cell walls (Zhang et al, 2011).…”

Group III-AXTHGenes of Arabidopsis Encode Predominant Xyloglucan Endohydrolases That Are Dispensable for Normal Growth

“…This suggests a significant ability to compensate for even drastic effects on xyloglucan composition and quantity. Likewise, recent elegant work from Park and Cosgrove (2012), using ex vivo cucumber (Cucumis sativus) and Arabidopsis cell wall systems, has provided strong evidence that extensive cleavage of bulk matrix xyloglucan chains does not induce wall creep. Rather, these authors suggest that wall extensibility is dominated by comparatively few intimately associated chains of cellulose and xyloglucan, which require mixed-function cellulose/xyloglucanase enzymes to disrupt.…”

“…therein). Indeed, many contemporary models implicate XGs as the main hemicelluloses, which contribute to the strength of the primary wall by cross linking cellulose fibrils (Pauly et al, 1999;Carpita and McCann, 2000;Cosgrove, 2005), although the proposed distance over which this cross linking is thought to occur has recently been called into question (Cosgrove and Jarvis, 2012;Park and Cosgrove, 2012).…”