Estimation of cytoplasmic and vacuolar pH in higher plant cells by ³¹P NMR

Abstract

It has recently been emphasised that the intracellular pH of plant cells must be actively regulated to offset acid–base imbalances that occur during metabolism and solute uptake (for review see ref. 1). Changes in cytoplasmic pH might be important (1) in controlling various aspects of metabolism², including enzyme activities responsible for malate synthesis during cation accumulation^1,2, (2) in polar transport of the plant hormone, auxin³, and (3) in intracellular responses to substances such as auxin and the phytotoxin fusicoccin, which cause proton extrusion^1,4,5. Almost no direct measurements of cytoplasmic pH have been made on the small cells typical of higher plants¹. The task is hampered by the presence of a generally acidic vacuole which occupies much of the cell volume. Use of the indirect method of ¹⁴C-5, 5-dimethyloxazolidine-2, 4-dione (DMO) distribution is subject to considerable uncertainties because of corrections needed for vacuolar pH and volume, and DMO metabolism^6,7. Here we report the use of ³¹P nuclear magnetic resonance (NMR) to estimate the pH of vacuole and cytoplasm in maize root tips.