Dynamic changes in the osmotic water permeability of protoplast plasma membrane

The osmotic water permeability coefficient (P_f) of plasma membrane of maize (Zea mays) Black Mexican Sweet protoplasts changed dynamically during a hypoosmotic challenge, as revealed using a model-based computational approach. The best-fitting model had three free parameters: initial P _f, P_f rate-of-change (slope_Pf), and a delay, which were hypothesized to reflect changes in the number and/or activity of aquaporins in the plasma membrane. Remarkably, the swelling response was delayed 2 to 11 s after start of the noninstantaneous (but accounted for) bath flush. The P_f during the delay was ≤1 μm s^-1. During the swelling period following the delay, P_f changed dynamically: within the first 15 s P_f either (1) increased gradually to approximately 8 μm s^-1 (in the majority population of low-initial-P_f cells) or (2) increased abruptly to 10 to 20 μm s^-1 and then decreased gradually to 3 to 6 μm s^-1 (in the minority population of high-initial-P_f cells). We affirmed the validity of our computational approach by the ability to reproduce previously reported initial P_f values (including the absence of delay) in control experiments on Xenopus oocytes expressing the maize aquaporin ZmPIP2;5. Although mercury did not affect the P_f in swelling Black Mexican Sweet cells, phloretin, another aquaporin inhibitor, inhibited swelling in a predicted manner, prolonging the delay and slowing P_f increase, thereby confirming the hypothesis that P_f dynamics, delay included, reflected the varying activity of aquaporins.