Ion-specific mechanisms of osmoregulation in bean mesophyll cells

Transient kinetics of net H⁺,K⁺, Ca²⁺, and Cl^- fluxes were measured non-invasively, using an ion-selective microelectrode technique, for bean (Vicia faba L.) leaf mesophyll in response to 150 mM mannitol treatment. In a parallel set of experiments, changes in the plasma membrane potential and the total proline content in leaves were monitored. Regardless of the ionic composition of the bath solution, hyperosmotic stress caused a significant increase in the K⁺ and Cl^- uptake into mesophyll cells. At the same time, no significant proline changes were observed for at least 16 h after the onset of stress. Experiments with inhibitors suggested that potassium inward rectifier (KIR) channels, exhibiting mechanosensitive properties and acting as primary receptors of osmotic stress, are likely to be involved. Due to the coupling by membrane potential, changes in K⁺ and Cl^- transport may modify activity of the plasma membrane H⁺-pump. Such coupling may also be responsible for the mannitol-induced oscillations (period of about 4 min) in net ion fluxes observed in 90% of plants. Calculations show that influx of K⁺ and Cl^- observed in response to hyperosmotic treatment may provide an adequate osmotic adjustment in bean mesophyll, which suggests that the activity of the plasma membrane transporters for these ions should be targeted to improve osmotolerance, at least in this crop.