Kinetic Characterization of Two Phosphate Uptake Systems with Different Affinities in Suspension-CulturedCatharanthus roseusProtoplasts

摘要

We studied the kinetics of inorganic phosphate (P1) uptake from 0.1–1,000μM P1by protoplasts from suspension-cultured cells ofCatharanthus roseus(L.) G. Don. Concentration dependence of 32PP1uptake revealed two kinetically different uptake systems, a high-affinity system and a low-affinity system, withKmvalues of 3.0 and 47μM, respectively. Protoplasts from cells grown in Pi-rich media had a medium level of the low-affinity activity and a very low level of the high-affinity activity. It appeared low-affinity system is expressed constitutively, while the high-affinity system is regulated by the availability of Pi. When cells grown in a Pi-rich media were transferred to Pi-depleted media, the high-affinity activity increased significantly after 2 d, but the low-affinity activity was barely changed. Upon addition of 10 mM Pi, the high level of the high-affinity activity fell to almost undetectable level in 1d. Both uptake systems exhibited maximum activity between pH 5 an