Résumé
The proton gradient across a biological membrane is a matter of great importance for plant development and nutrition. To gain a better understanding of proton distribution in the plant root apoplast as well as across the plasma membrane, we generated Arabidopsis plants expressing stable membrane‐anchored ratiometric fluorescent sensors based on pHluorin. These sensors enabled non‐invasive and specific pH measurements in mature root cells from the medium‐epidermis interface up to the inner cell layers that lie beyond the Casparian strip. We found that the membrane‐associated apoplastic pH was much more alkaline than in the overall apoplastic space. In apparent contradiction with the direct connection between root intercellular space and the external medium, proton concentration associated with the plasma membrane was very stable, even when the growth medium pH was altered. The plasma membrane‐associated pH in the stele was the most preserved, and displayed the lowest apoplastic pH (6.0 to 6.1) and the highest transmembrane delta pH (1.5 to 2.2). Both pH values also correlated well with optimal activities of channels and transporters involved in ion uptake and redistribution from the root to the aerial part. In growth medium where ionic content is minimized, the root plasma membrane‐associated pH was more affected by environmental proton changes, especially for the most external cell layers. Calcium concentration appears to play a major role in apoplastic pH under these restrictive conditions, supporting a role for the cell wall in pH homeostasis of the unstirred surface layer of plasma membrane in mature roots. Alexandre Martinière, Rémy Gibrat, Hervé Sentenac, Xavier Dumont, Isabelle Gaillard and Nadine Paris. Shedding light on pH at both sides of the root plasma membrane interface using non‐invasive imaging. PNAS, in press under embargo.