Tobacco cells adapted to grow in high concentrations of NaCl exhibit a drastically altered growth physiology that results in cells whose fully expanded volume is only one-fifth to one-eighth those of unadapted cells. Comparison between NaCl-adapted and unadapted tobacco cells provides an opportunity to evaluate current concepts of the structural and mechanical determinants of cell wall expansion. Both biochemical studies of pectic polymers and the ultrastructural localization of pectic epitopes at three specific phases of cell culture, maximal cell division, maximal elongation, and stationary phase are reported here.
One-half of the galactosyluronic acid units in wall polymers of NaCl-adapted cells are esterified throughout the culture period, while wall polymers of unadapted cells show a rise in esterified polygalacturonic acid from 50 to 80% during elongation and then a decrease to 70% at stationary phase. Methyl esters account for only a proportion of the total esterified polygalacturonic acid at any stage in both unadapted and NaCl-adapted cell walls. Using monoclonal antibodies, we show differences in the localization of relatively methyl-esterified and unesterified pectic epitopes at different stages of growth and corroborate the chemical determinations. Fourier transform infrared (FTIR) microspectroscopy of representative walls of both NaCl-adapted and unadapted cells confirms, at the single cell wall level, that results obtained from chemical analysis of bulk samples are applicable to the entire cell population.
FTIR microspectroscopy also reveals an increase in wall protein in the walls of adapted cells. Images obtained by the fast-freeze, deep-etch, rotary-shadowed replica technique show clearly different cell wall architectures in NaCl-adapted compared with unadapted cells; walls of elongating unadapted cells contain long, thin fibres that show a net orientation with respect to the long axis of the cell, whereas walls of adapted cells have thicker, flatter bundles of fibres with no clear net orientation. Polarized FTIR microspectroscopy indicates that, in unadapted tobacco cells during elongation, pectin molecules may be oriented within the wall in a similar manner to cellulose. Possible ways in which pectin structure and conformation may affect the behaviour of the cellulose-xyloglucan network are discussed.