Regulation of Aquaporin-Mediated Water Transport in Arabidopsis Roots Exposed to NaCl

The effects of Ca(NO3)2, KF and okadaic acid (OA) on cell hydraulic responses to NaCl were examined in roots of Arabidopsis thaliana wild-type plants and compared with plants overexpressing plasma membrane intrinsic protein PIP2;5. Root treatment with 10 mM NaCl rapidly and sharply reduced cell hydraulic conductivity (L(p)) in the wild-type Arabidopsis plants, but had no effect on L(p) in Arabidopsis plants overexpressing PIP2;5, suggesting that changes in protein and aquaporin gene expression were among the initial targets responsible for the inhibition of L(p) by NaCl. The down-regulation of PIP transcripts after 1 h exposure to 10 mM NaCl was likely a significant factor in the reduction of L(p). The effect of NaCl on L(p) in the wild-type plants was abolished when the NaCl-treated roots were subsequently exposed to 5 mM KF, 5 mM Ca(NO3)2 and 5 µM OA. The reduction of L(p) by 5 mM KF could not be prevented by treatment with 5 mM Ca(NO3)2 in both wild-type and PIP2;5-overexpressing plants. However, 5 µM OA, which was added following NaCl or KF treatment, completely reversed L(p) within several minutes. The results provide evidence for high sensitivity of aquaporin-mediated water transport to relatively low NaCl concentrations and point to the phosphorylation and/or dephosphorylation processes as those that are likely responsible for the protection of L(p) by fluoride and calcium treatments against the effects of NaCl.