Surface pH changes suggest a role for H⁺/OH^- channels in salinity response of Chara australis

To understand salt stress, the full impact of salinity on plant cell physiology has to be resolved. Electrical measurements suggest that salinity inhibits the proton pump and opens putative H⁺/OH^- channels all over the cell surface of salt sensitive Chara australis (Beilby and Al Khazaaly 2009; Al Khazaaly and Beilby 2012). The channels open transiently at first, causing a characteristic noise in membrane potential difference (PD), and after longer exposure remain open with a typical current-voltage (I/V) profile, both abolished by the addition of 1 mM ZnCl₂, the main known blocker of animal H⁺ channels. The cells were imaged with confocal microscopy, using fluorescein isothiocyanate (FITC) coupled to dextran 70 to illuminate the pH changes outside the cell wall in artificial fresh water (AFW) and in saline medium. In the early saline exposure, we observed alkaline patches (bright fluorescent spots) appearing transiently in random spatial distribution. After longer exposure, some of the spots became fixed in space. Saline also abolished or diminished the pH banding pattern observed in the untreated control cells. ZnCl₂ suppressed the alkaline spot formation in saline and the pH banding pattern in AFW. The osmotic component of the saline stress did not produce transient bright spots or affect banding. The displacement of H⁺ from the cell wall charges, the H⁺/OH^- channel conductance/density, and self-organization are discussed. No homologies to animal H⁺ channels were found. Salinity activation of the H⁺/OH^- channels might contribute to saline response in roots of land plants and leaves of aquatic angiosperms.