Changes in cell wall composition and metabolism of tobacco cells upon adaptation to osmotic stress

Adaptation of crop plants to osmotic stress involves changes in their anatomy, physiology, and metabolism. Such changes can be studied more conveniently in suspension-cultured cells. Upon adaptation of tobacco suspension-cultured cells to osmotic stress, mimicked by supplementing the growth medium with either 30% polyethylene glycol or 2.5 NaCl, cell expansion was decreased by 3-5 fold. Rate and absolute degree of cell expansion depends on turgor pressure and chemical and biochemical properties of the wall. The chemical composition and some metabolic aspects of the wall were studied to establish a relationship between the inhibition of cell expansion and alterations in wall structure and metabolism. Classic fractionation of cell walls into pectins, hemicellulose and cellulose followed by determination of chemical and structural characteristics of each fraction showed substantial alterations in cell wall structure upon adaptation to osmotic stress. The proportion of cell wall material, particularly the cellulose, in the total dry weight of cells was decreased dramatically reflecting the shift in the flow of carbon from cell wall synthesis to solute accumulation. The inhibited cell expansion which resulted from adaptation to the stress was accompanied by alterations in pectin structure and organization as well. Walls from adapted cells accumulated loosely bound pectin 3-6 fold over walls of unadapted cells. The degree of branching of the rhamnogalacturonans was increased upon adaptation but the size of the branches was decreased about 3 fold. These changes in wall structure, which may affect the physical and biochemical properties of the wall, were accompanied by an impaired turnover of xyloglucan in intact cells as well as in isolated cell walls. The amounts of xyloglucan and polyuronides released into the growth medium of adapted cells were decreased substantially and walls of adapted cells released xyloglucan only one-third the amount released by walls of unadapted cells. The structural alterations in walls of adapted cells, the accumulation of polyuronides and the impaired turnover of xyloglucan seem to be related to a mechanism controlling cell expansion. (Abstract shortened with permission of author.