Plant cell-walls microfibril orientation

As already noted (see p. 327), some experimental evidence has been educed for parallel orientation of the microfibrils in crossed layers under the influence of 0-(carboxymethyl)cellulose. It has been proposed that microfibrils in the plant cell-walls are oriented in the same way under the influence of charged polysaccharides (such as pectins) found in the middle lamella and the primary wall (see p. 348). 

Emons A.M.C., Derksen X, and Sassen M.M.A. 1992. Do microtubules orient plant cell wall microfibrils Physiol Plant 84 486-493. 

FIGURE 20-29 Cellulose structure. The plant cell wall is made up in part of cellulose molecules arranged side by side to form paracrys-talline arrays—cellulose microfibrils. Many microfibrils combine to form a cellulose fiber, seen in the scanning electron microscope as a structure 5 to 12 nm in diameter, laid down on the cell surface in several layers distinguishable by the different orientations of their fibers. 

The general discussion and controversy as to whether the microfibrils are formed by apposition or deposition of cellulosic materials also applies to the plant cell-wall but, here, the question assumes much greater significance, especially with respect to the architecture of the cell wall and the precise orientation of the microfibrils within its layers and lamellae. How these structures are formed and to what extent the processes involved are carried out and controlled by the living cell, or by inanimate, physical forces, pose major questions that have been extensively investigated and discussed. Various theories for the passive and active orientation of the microfibrils in growing-plant cell-walls have been reviewed in several botanical ar-ticles with excellent discussions, and will only briefly be mentioned here as background. 

The filaments of all plant fibers consist of several cells. These cells form crystalline microfibrils (cellulose), which are connected together into a complete layer by amorphous lignin and hemi-cellulose. Multiple layers stick together to form multiple layer composites, filaments. A single cell is subdivided into several concentric layers, one primary and three secondary layers. Figure 5 shows a jute cell. The cell walls differ in their composition and in the orientation of the cellulose microfibrils whereby the characteristic values change from one natural fiber to another. 

The cell walls differ among themselves in their composition and orientation of the cellulose microfibrils. In most plant fibres, these microfibrils are oriented at an angle to the normal axis called the microfibrillar angle (Fig. 19.2). The characteristic value for this structural parameter varies from one plant fibre to another. 

Zhong R., Burk D.H., Morrison III, W.H., and Ye Z.-H. 2002. A kinesin-like protein essential for oriented deposition of cellulose microfibrils and cell wall strength. Plant Cell 14 3101-3117. 

Plants utilize their ability to control the local composition and the microfibril orientation within the cell walls to accommodate differential strains within their composite architecture. This is evidenced for example by changes in the microfibril orientation in trees depending on the specific function of the tissue,as well as in the graded change in elastic modulus... 

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