In vitro salt tolerance of cell wall enzymes from halophytes and glycophytes

The halophyte Suaeda maritima grows optimally in high concentrations (40-60% seawater) of salt, In these conditions the concentration of salt in the apoplast of the leaves is at least 500 mM, a concentration which severely inhibits the activity of cytoplasmic enzymes of both glycophytes and halophytes, The in vitro salt tolerance of a number of cell wall enzymes was assayed in the presence of a range of concentrations of NaCl. There was no significant inhibition of the activity of galactosidase, glucosidase, peroxidase or xyloglucan endo-transglycosylase extracted from Suaeda maritima by in vitro concentrations of NaCl up to at least 1 M. in vitro salt tolerance of cell wall enzymes was not restricted to the halophyte, similar enzymes from the non-halophilic relative Kochia tricophylla, and from the glycophytes Vigna radiata and Cicer arietinum, were inhibited little, or not at all, by the same concentrations of salt. Pectin esterase was somewhat less tolerant, but activity at 500 mM NaCl was still greater than at 0 mM NaCl in both Suaeda and Vigna. It is concluded that these enzymes of the cell wall compartment are much more salt-tolerant than cytoplasmic enzymes of higher plants. The results are discussed in relation to conditions thought to pertain in the apoplast.