ORIENTATION OF MACROMOLECULES IN THE WALLS OF ELONGATING CARROT CELLS

When round cells from a carrot cell suspension culture are diluted int o fresh medium without auxin, the cells elongate to almost 50 times th eir original diameter within three days. This process of elongation is accompanied by changes in both the composition and the orientation of cell wall polymers. We have obtained information on the orientation o f wall polymers in elongating cells by two complementary techniques, o ne using microscopy and one spectroscopy. Images obtained by the fast- freeze, deep-etch, rotary-shadowed replica technique show that walls o f round carrot cells have no net orientation of cellulose microfibrils , and that many thin fibres can be seen cross-linking microfibrils. Wa lls of elongated carrot cells, in contrast, show a marked net orientat ion of microfibrils at right angles to the axis of elongation. Fourier Transform Infrared (FTIR) spectra obtained from defined areas of sing le cell walls show that walls of round carrot cells contain more prote in, eaters and phenolics in a given area (10 mu m x 10 mu m) than wall s of elongated carrot cells, that contain proportionally more carbohyd rate. The orientation of particular functional groups, with respect to the direction of elongation of the cell, can be determined by inserti ng a polariser into the path of the infrared beam, before it passes th rough a cell wall sample mounted on the stage of the microscope access ory. In the walls of elongated cells, ester bands, amide bands charact eristic of proteins, and stretching frequencies in the carbohydrate re gion of the spectrum all show a net orientation transverse to the long axis of the cells. In the walls of round carrot cells, however, there is no such net orientation of polymers. Spectra obtained from 25 mu m -thick fresh sections of the etiolated stem of a carrot seedling show that different wall components are polarised in different tissue types . These techniques have therefore enabled us to define differences in both the composition and the architecture of walls of elongating cells at the level of a single cell, and to suggest that polymers not previ ously thought to be ordered, such as pectin and protein, are strictly oriented in some wall types.