Uptake of Inorganic Phosphate by Suspension-Cultured Tobacco Cells: Kinetics and Regulation by PiStarvation

摘要

The kinetics of Piuptake by phosphate-starved and non-starved tobacco cells (Nicotiana tabacumBY-2) suspension culture was investigated. The kinetic parameters of Piuptake were determined by computer simulation of the curve that represented the time-dependent loss of Pifrom the culture medium. The uptake profile could be completely explained by assuming the existence of only one kind of Michaelis-Menten-type Pi-transport system with an affinity for Pi(Km) of about 2.5μM (the lowest value reported to date) in both Pi-starved and non-starved cells. No evidence was obtained suggesting the existence of a“low-affinity”Pi-uptake system that has been postulated to exist in several other plant materials. TheVmaxfor uptake of Piby non-starved cells was 12 nmol per minute per milliliter of packed cell. Phosphate starvation increased theVmaxmore than 5-fold, while it had no effect on the affinity for Pi.Vmaxbegan to increase (at an almost constant rate) just after loss of all Pifrom the culture medium and it reached a maximum about 16 hours later. This induction process was completely prevented by the addition of cycloheximide to the culture medium. All these results suggest that Pistarvation increases the synthesis of a phosphate-carrier complex that is postulated to be involved in the Pi-uptake pro