Ratiometric monitoring of transient apoplastic alkalinizations in the leaf apoplast of living Vicia faba plants: chloride primes and PM-H+-ATPase shapes NaCl-induced systemic alkalinizations.

Transient apoplastic alkalinization has been discussed as a general stress factor, and is thought to represent a root-to-shoot signal that transmits information regarding an ongoing NaCl stress event from the site of the trigger to the distant plant tissue. Surprisingly, despite this importance, a number of gaps exist in our knowledge of NaCl-induced apoplastic pH alkalinization. This study was designed in order to shed light onto the mechanisms responsible for the initiation and transiency of leaf apoplastic alkalinization under conditions of NaCl stress as supplied to roots. An H(+)-sensitive fluorescence probe, in combination with ratiometric microscopy imaging, was used for in planta live recording of leaf apoplastic pH. The use of a nonionic solute demonstrated that the alkalinization is induced in response to ionic, and not osmotic, components of NaCl stress. Tests with Cl(-)- or Na(+)-accompanying counter-ions strengthened the idea that the stress factor itself, namely Cl(-), is transferred from root to shoot and elicits the pH alterations. Investigations with a plasma membrane ATPase inhibitor suggest that ATPase activity influences the course of the alkalinization by having a shaping re-acidifying effect on the alkalinization.