Direct visualization of cross-links in the primary plant cell wall

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Abstract
We have investigated the structure of the onion primary cell wall at high resolution, using shadowed replicas of rapidly frozen deep-etched specimens. We have sequentially extracted polymers from the wall and have visualized both these and the remaining structures at each extraction step. By viewing the structures in as near their native state as possible, an accurate three-dimensional picture of wall construction has been assembled, facilitated by viewing stereo pairs of micrographs. Our observations show that the physical links between cellulose microfibrils that we observe in the intact wall are generally shorter (20–40 nm) than the isolated molecules we extract (30->700nm), suggesting that lateral interactions must occur between linking polymers and cellulose in muro. These cross-links are hemicellulosic and we believe them to be xyloglucans: their removal allows increased lateral association of microfibrils. Na2CO3-extractable pectic fractions form a separate coextensive network, the removal of which does not affect basic cellulose/ hemicellulose architecture. Preliminary evidence for a lamellate model of wall construction has been obtained. In addition, we propose a positive role for hemicellulose in maintaining the ordered spacing of cellulose micronbrils, perhaps regulating wall porosity and strength. The basic wall parameters that we derive impose constraints on possible cell wall models.