CHANGES IN PECTIN STRUCTURE AND LOCALIZATION DURING THE GROWTH OF UNADAPTED AND NACL-ADAPTED TOBACCO CELLS

Tobacco cells adapted to grow in high concentrations of NaCl exhibit a drastically altered growth physiology that results in cells whose ful ly expanded volume is only one-fifth to one-eighth those of unadapted cells. Comparison between NaCl-adapted and unadapted tobacco cells pro vides an opportunity to evaluate current concepts of the structural an d mechanical determinants of cell wall expansion. Both biochemical stu dies of pectic polymers and the ultrastructural localization of pectic epitopes at three specific phases of cell culture, maximal cell divis ion, maximal elongation, and stationary phase are reported here.One-ha lf of the galactosyluronic acid units in wail polymers of NaCl-adapted cells are esterified throughout the culture period, while wall polyme rs of unadapted cells show a rise in esterified polygalacturonic acid from 50 to 80% during elongation and then a decrease to 70% at station ary phase. Methyl esters account for only a proportion of the total es terified 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 p ectic epitopes at different stages of growth and corroborate the chemi cal determinations. Fourier transform infrared (FTIR) microspectroscop y of representative walls of both NaCl-adapted and unadapted cells con firms, at the single cell wall level, that results obtained from chemi cal analysis of bulk samples are applicable to the entire cell populat ion. FTIR microspectroscopy also reveals an increase in wall protein i n the walls of adapted cells. Images obtained by the fast-freeze, deep -etch, rotary-shadowed replica technique show clearly different cell w all architectures in NaCl-adapted compared with unadapted cells; wails of elongating unadapted cells contain long, thin fibres that show a n et orientation with respect to the long axis of the cell, whereas wall s of adapted cells have thicker, flatter bundles of fibres with no cle ar net orientation. Polarized FTIR microspectroscopy indicates that, i n unadapted tobacco cells during elongation, pectin molecules may be o riented within the wall in a similar manner to cellulose. Possible way s in which pectin structure and conformation may affect the behaviour of the cellulose-xyloglucan network are discussed.