PHOSPHATE-UPTAKE BY EXCISED MAIZE ROOT-TIPS STUDIED BY INVIVO P-31 NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY

The extent of phosphate uptake measured by the relative changes in cytoplasmic Pi, vacuolar Pi, ATP, glucose-6-phosphate, and UDPG was determined using in vivo 31P nuclear magnetic resonance spectroscopy. Maize (Zea mays) root tips were perfused with a solution containing 0.5 or 1.0 millimolar phosphate at pH ∼6.5 under different conditions. In the aerated state, phosphate uptake resulted in a significant increase (>80%) in vacuolar Pi, but cytoplasmic Pi only transiently increased by 10%. Under N2, the cytoplasmic Pi increased ∼150% which could be attributed to a large extent to the breakdown of ATP, sugar phosphates and UDPG. Vacuolar Pi increased but only to the extent of ∼10% of that seen under aerobic conditions. 2-deoxyglucose pretreatment was utilized to decrease the level of cytoplasmic Pi. When pretreated with the 2-deoxyglucose, the excised maize roots absorbed phosphate from the perfusate with a significant increase in the cytoplasmic Pi. The increase could only be traced to external phosphate since the concentrations of other phosphorus containing species remained constant during the uptake period. With 2-deoxyglucose pretreatment, phosphate uptake under anaerobic conditions was substantially inhibited with only the vacuolar phosphate showing a slight increase. When roots were treated with carbonyl cyanide m-chlorophenyl hydrazone, no detectable Pi uptake was found. These results were used to propose a H+-ATPase related transport mechanism for phosphate uptake and compartmentation in corn root cells.