Abstract
The notion that xyloglucans (XG) play a pivotal role in tethering cellulose microfibrils in the primary cell wall of plants can be traced back to the first molecular model of the cell wall proposed in 1973, which was reinforced in the 1990s by the identification of Xyloglucan Endotransglucosylase/Hydrolase (XTH) enzymes that cleave and reconnect xyloglucan crosslinks in the cell wall. However, this tethered network model has been seriously challenged since 2008 by the identification of the Arabidopsis thaliana xyloglucan-deficient mutant (xxt1 xxt2), which exhibits functional cell walls. Thus, the molecular mechanism underlying the physical integration of cellulose microfibrils into the cell wall remains controversial. To resolve this dilemma, we investigated the cell wall regeneration process using mesophyll protoplasts derived from xxt1 xxt2 mutant leaves. Imaging analysis revealed only a slight difference in the structure of cellulose microfibril network between xxt1 xxt2 and wild-type (WT) protoplasts. Additionally, exogenous xyloglucan application did not alter the cellulose deposition patterns or mechanical stability of xxt1 xxt2 mutant protoplasts. These results indicate that xyloglucan is not essential for the initial assembly of the cellulose network, and the cellulose network formed in the absence of xyloglucan provides sufficient tensile strength to the primary cell wall regenerated from protoplasts.
Highlights
Primary cell walls provide plant cells with the flexibility to expand and the mechanical strength to support the cell shape
We investigated the effects of exogenously applied xyloglucan on the construction of cellulose network in the primary cell wall regenerated from xxt1 xxt2 protoplasts
Mesophyll protoplasts isolated from WT and xxt1 xxt2 leaves were incubated in cell wall regeneration medium and stained with Calcofluor, a β-glucan-specific dye
Summary
Primary cell walls provide plant cells with the flexibility to expand and the mechanical strength to support the cell shape. Plants 2020, 9, 629 microfibrils via hydrogen bonds, while other matrix polymers such as pectin were linked to xyloglucan via covalent bonds This original model was supported by studies showing that xyloglucan dynamics is involved in cell wall construction and extension processes [6,7,8,9]. This model was later revised and replaced by the tethered network model, which suggests that a single xyloglucan molecule physically interacts with two or more cellulose microfibrils, functioning as a tether between microfibrils [4,10,11,12]. The tethered network model was reinforced by the identification of a xyloglucan endotransglucosylase/hydrolase, an enzyme that cleaves and reconnects xyloglucan crosslinks [13,14,15]
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