Effect of Aluminium and cAMP on Acid Phosphatase from the Apoplast of Barley and Maize Root Cells

Acid phosphatase activity inhibited by 1 mM sodium molybdate was detected at the surface of barley seedling roots and in the cell wall fraction isolated from barley and maize seedling roots. This enzyme hydrolyzed NPP, GP, and PP_i at low pH (4.0 and below). NPP hydrolysis was stimulated by magnesium (but not calcium or manganese) ions, while PP_i hydrolysis was independent of the presence of bivalent ions. The activity of phosphatase localized in the cell walls of the both crops increased in the presence of 100 M AlCl₃ or CuCl₂. Stimulation of NPP hydrolysis by micromolar concentrations of aluminium and copper as well as by millimolar concentrations of magnesium decreased in the presence of 25 M cAMP. This agrees with the previous data on the enzyme localized at the outer side of the properly oriented vesicles in the microsomal fraction of plasmalemma. The role of the root extracellular acid phosphatase loosely associated with various apoplast structures in plant adaptation to toxic effect of aluminium in the acidic soils as well as possible control of this process by cAMP secretion to the apoplast are discussed.     

The Effect of cAMP on Phosphohydrolase Activity of the Barley Root Cells

We studied the effects of cAMP on phosphohydrolase activity in the microsomal fraction and its 0.5 M KCl extract. In both preparations, cAMP inhibited the activity of acid phosphatases. In the microsomal fraction, a 50% inhibition of acid cation-dependent (Mg²⁺, Cu²⁺) phosphatase was induced at pH 6.0 by 10 M cAMP; whereas, for cation-independent phosphatase, this effect was exerted only by 50 M cAMP. In the KCl extract, the sensitivity of both cation-dependent and cation-independent acid phosphatases to cAMP was higher than in the microsomal fraction. Cyclic AMP did not reduce the activity of nucleosidases. Selectivity in the cAMP action on acid phosphatases is assumed to be one of the mechanisms controlling the biochemical activity outside the barley root cells.     

Biochemical Adaptation of Barley Root Cells to Toxic Substances. II. Effect of Heavy Metals on Phosphohydrolase Activity

We studied the effect of heavy metals such as Cu, Zn, Cd, and Pb on the activity of membrane-bound and soluble phosphohydrolases of barley root cell apoplast. The effect of heavy metals on the activity of acid phosphatase and acid nucleotidases proved to depend on the presence of calcium and magnesium ions in the incubation buffer, the condition and form of the enzymes, as well as the dose and moment of the addition of heavy metals. We propose changing the activity of acid phosphohydrolases in barley root cell apoplast as a possible mechanism of barley adaptation to the toxic effect of heavy metals.