Cubic plasma membrane domains stabilize and restrict zones for pH band formation in Chara internodal cells

Characean internodal cells display acid and alkaline regions along their surface when exposed to light. The acidification facilitates uptake of carbon required for photosynthesis. In the genus Chara, cubic plasma membrane domains, known as charasomes, were first described five decades ago. Their size and abundance varies along the cell surface and it has been conjectured that the charasome coverage correlates with the pH banding pattern. In this study, we used fluorescein isothiocyanate-dextran (FITC-dextran) and confocal laser scanning microscopy to map the pH profile at high spatial and temporal resolution. In parallel, we stained the charasomes with styryl dyes and investigated their distribution in relation to the fluorescence pattern of FITC-dextran. In both internodal cells of the branchlets and of the main axis, we found that charasomes were small and sparse at the alkaline bands but large and abundant at the acidic regions. Our findings confirm the existence of cell surface regions predisposed to generate either high or low pH zones. We provide evidence that areas with high charasome abundance stabilize and restrict the position of the acidic bands whereas regions with low charasome abundance allow the formation of either stable or unstable alkaline bands.