Low temperature and mechanical stresses differently gate aquaporins of root cortical cells of chilling-sensitive cucumber and -resistant figleaf gourd

The inhibition of the hydraulic conductivity of individual cortical cells (Lp) of young roots of cucumber and figleaf gourd by low root temperature (8 degrees C, LRT) was measured using a cell pressure probe. When LRT was imposed, the Lp of the two species responded differently. Water permeability of cortical cells of chilling-sensitive cucumber decreased by a factor of 10, but there was only a small effect in the chilling-resistant figleaf gourd. Mechanical stresses (pulses of cell turgor pressure) resulted in a similar inhibition for both species by a factor of 6.5. When applied at LRT, abscisic acid (ABA) partially or even completely reversed the effects of chilling and mechanical stresses of both species. At the control temperature of 22 degrees C, 50 mu M of the aquaporin (AQP) inhibitor HgCl2 acted on root cells of both species, although the effect on root cells of figleaf gourd was small. There was no effect of HgCl2, when AQPs were already closed either by LRT or by mechanical stress. The effect of mechanical stress (pressure pulses) was substantially bigger than that of HgCl2. When AQPs were closed by big pulses in the presence of 50 mu M HgCl2, they could be partially re-opened in the presence of the inhibitor by applying small pulses, suggesting that there are at least two different types of channels present, which respond differently to mechanical stress or to the heavy metal. The presence of 1 mu M ABA in the root medium prevented the effects of LRT and mechanical stress, namely an increase in the half-times of water exchange (T-1/2(w) proportional to 1/Lp). In the absence of stresses at short T-1/2(w), there was no effect of ABA. It is concluded that the responsiveness of AQPs of the two species differs in the presence of LRT but not under conditions of mechanical stress. In both cases, however, ABA has an ameliorative effect. The results suggest that the presence of ABA reduces the activation energy of changes of the conformation of AQPs, when switching between open and closed states. Mechanisms of the gating of AQP activity by LRT and mechanical stresses and the possible role of the stress hormone ABA are discussed.