Effect of Phosphorus Deficiency on Phosphatase Activity of Cell Walls from Roots of Subterranean Clover

Abstract

Clover (Trifolium subterraneum L. cv. Mt. Barker) was grown in solution culture with adequate (+P) or no phosphate (−P). Cell walls were extracted from roots in such a way that they were uncontaminated by other cellular materials. Phosphatase activity was assayed using p-nitro-phenylphosphate (NPP). Phosphatase bound to cell walls had a pH optimum between 5.0 and 6.0, irrespective of the P supply to the plants. Activity of phosphatase bound to cell walls increased with electrolyte concentration of the assay medium at pH 6.5 but not at pH 5.5. This increase in activity was probably due to a higher degree of ionization of the cell wall at pH 6.5 than at pH 5.5, and to effects of high ionic strength in decreasing the mutual repulsion of negatively charged NPP from negative charges on the cell walls. Cell wall-bound phosphatase did not exhibit Michaelis-Menten kinetics: the concentration of NPP at which activity was half the maximum rate (S_0.5) was 0.7 mM for cell walls extracted from roots of both +P and −P plants. Up to 30% of the phosphatase activity bound to cell walls could be removed using buffer solutions of high pH and high ionic strength which contained Triton X100. Both soluble and cell wall-bound phosphatase(s) of roots increased in activity with P deficiency. The phosphatase activity of cell walls increased 1.5 fold as the P concentration in the roots fell from 0.4–0.2% dry weight. Experiments with sterile roots of clover showed that increases in cell wall-bound phosphatase activity associated with P deficiency were not due to microbial contamination. It is argued that phosphatase(s) in cell walls of roots could make a substantial contribution to the P nutrition of clover in soils deficient in inorganic phosphate by hydrolysing organic phosphate compounds in the soil.