UPTAKE OF INORGANIC-PHOSPHATE BY SUSPENSION-CULTURED TOBACCO CELLS - KINETICS AND REGULATION BY P-I STARVATION

The kinetics of P-i uptake by phosphate-starved and non-starved tobacco cells (Nicotiana tabacum BY-2)suspension culture was investigated. The kinetic parameters of P-i uptake were determined by computer simulation of the curve that represented the time-dependent loss of P-i from the culture medium. The uptake profile could be completely explained by assuming the existence of only one kind of Michaelis-Menten-type P-i-transport system with an affinity for P-i (K-m) of about 2.5 mu M (the lowest value reported to date) in both P-i-starved and non-starved cells. No evidence was obtained suggesting the existence of a ''low-affinity'' P-i-uptake system that has been postulated to exist in several other plant materials. The V-max for uptake of P-i by non-starved cells was 12 nmol per minute per milliliter of packed cell. Phosphate starvation increased the V-max more than 5-fold, while it had no effect on the affinity for P-i. V-max began to increase (at an almost constant rate) just after loss of all P-i from 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 P-i starvation increases the synthesis of a phosphate-carrier complex that is postulated to be involved in the P-i-uptake process.