The kinetics associated with phosphate absorption by Arabidopsis and its regulation by phosphorus status

The effects of phosphorus status on the kinetics of phosphate absorption by 3-day-old Arabidopsis thaliana (L.) Heynh. seedlings were studied. Seeds were germinated for 3 days on agarose gel containing different amounts of phosphate and the absorption of P-32-labelled phosphate by these seedlings was measured at a range of phosphate concentrations. The absorption rates for seeds germinated in the absence of phosphate were up to four times faster than for seedlings germinated on gels containing 1, 5 or 10 mM phosphate. The kinetic constants K-m and V-max were determined by fitting the data to models with one and two mechanisms operating in parallel. A model with two mechanisms was needed to provide a good fit to the data for concentrations ranging from 0.5 mu M to 5 mM. For the high affinity mechanism, K-m,K-1 (mean value 12.3 mu M) was not affected as phosphorus status increased but V-max,V-1 decreased. For the low affinity mechanism the values for both K-m,K-2 (5697-16547 mu M) and V-max,V-2 increased with increasing phosphorus status. Arsenate inhibited phosphate absorption but only moderately. Varying the pH over the range 4.8 to 7.3 did not affect the rate of absorption. It is concluded that a kinetic approach may not have sufficient resolution to discriminate between different mechanisms unless there are large quantitative differences in the constants K-m or V-max.