Plants microfibril orientation

Plant hormones rarely act alone hormones interact to produce a final effect. According to Gaspar et al., Some responses of plants to auxins may be caused by increased ethylene synthesis in response to auxin treatment. At high ethylene concentrations, microtubule and microfibril orientation are altered, which results in decreased cell elongation and increased cell expansion. The role of ethylene is hard to understand because it effects vary with developmental stage and because low concentrations can promote (or sometimes inhibit) a process, whereas higher levels have the opposite effect [22]. 

Mizuta S. 1985. Evidence for the regulation of the shift in cellulose microfibril orientation in freeze-fractured plasma membrane of Boergesenia forbesii. Plant Cell Physiol 26 53-62. 

Roudier E, Fernandez A., Fujita M., Himmelspach R., Bomer G., Schindelman G., Song S., Baskin T., Dupree P., Wasteneys G, and Benfey P. (2005). COBRA, an Arabidopsis extracellular GPl-anchored protein, specifically controls highly anisotropic expansion through its involvement in cellulose microfibril orientation. The Plant Cell 17 1749-1763. 

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... 

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. 

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. 

Hasezawa, S. and Nozaki, H. 1999. Role of cortical microtubules in the orientation of cellulose microfibril deposition in higher-plant cells. Protoplasma 209, 98-104... 

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. 

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). 

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. 

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