Changes in the water binding characteristics of the cell walls from transgenic Nicotiana tabacum leaves with enhanced levels of peroxidase activity

Pore size in the cell wall matrix may affect cell wall-water relations, particularly under osmotic stress. Cross linkage of plant cell wall matrix polymers is an important step in the formation of this structure and peroxidases have been proposed to catalyse the cross-linking of phenolic constituents. Transgenic tobacco (Nicotiana tabacum) plants expressing a basic tomato peroxidase gene (TPX2) showed increased apoplastic ferulic acid peroxidase activity in mature leaves. This enhanced activity was not associated with a decreased leaf growth. Differential scanning calorimetry (DSC) of control dried cell walls showed a putative glass transition, after Ca2+ removal, that was absent in the transgenic line. This would indicate that transgenic walls were more rigid. DSC analysis of water-hydrated cell wall preparations distinguished two pools of water, freezable and non-freezable water. The amount of non-freezable water, which corresponds to strongly bound water, was higher in the transgenic line (64 versus 55%). DSC thermograms of the transgenic cell wall were displaced to lower temperatures, and this may be interpreted as the result of a stronger interaction between this freezable water and this wall. Water sorption and desorption isotherms, obtained at relative humidity ranging from 5 to 93%, demonstrated the presence of very strongly bound water in the transgenic cell walls that was absent in controls. Water sorption-desorption hysteresis of the isotherms was evident in the control wall but not in the transgenic line. These changes in cell wall-water interaction seem to be relevant at the organ level because leaf discs of transgenic plants maintained higher relative water content than control discs, at water potentials between -1.05 and-2.31 MPa.