Salt Stimulation of Phosphate Uptake in Maize Root Tips Studied by 31P Nuclear Magnetic Resonance

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Abstract
The effects of external salt and Pi on the concentrations of vacuolar Pi, and cytoplasmic Pi, ATP, G-6-P and UDP-glucose in maize root tips were examined using 31P NMR spectroscopy. A more than 2-fold stimulation of Pi uptake from 10 mM KH2PO4 solutions was observed when root tips were exposed to 100 mM NaCl + CaCl2. This stimulation of Pi uptake was associated with an increase in the concentration of cytoplasmic Pi in root tip cells. The molar ratio of cytoplasmic Pi to Pi + ATP + G-6-P + UDP-glucose increased greatly in root tips exposed to salt and Pi. This disturbance in relative concentrations of cytoplasmic phosphatase (which are normally tightly regulated) is apparently responsible for both the greater rate of uptake of Pi by vacuoles by excised maize root tips, and the previously documented stimulation of Pi translocation from root to shoot in whole maize plants exposed to salt and Pi.