CONFOCAL PH TOPOGRAPHY IN PLANT-CELLS - ACIDIC LAYERS IN THE PERIPHERAL CYTOPLASM AND THE APOPLAST

The distribution of intracellular pH was studied in cultured cells of Gossypium hirsutum by confocal pH topography using the fluorescent probe carboxy SNARF1 and a ratio imaging procedure. The resulting pH maps can visualize pH differences with an accuracy of 0.1 unit in the investigated range between 7.5 and 5.6. They reveal the following characteristic features of the Gossypium cells: - the pH of the cytoplasmic core regions ranges from near 7.4 in younger to near 6.0 in older cells; - vacuoles show the expected acidity with pH < 5.6; - the cell wall/apoplastic region is acidic with a pH near 5.6 or below, especially in young, growing cells; - interestingly, acidic areas appear also at the periphery of the cytoplasm, i.e. beneath the plasma membrane. They remain stable in the presence of 5-mu-mol/l of the protonophore CCCP. Acidic layers of peripheral cytoplasm were also detected in protoplasts of Penicillium cyclopium, i.e. eukaryotic cells of simpler structure, which served as a reference object. This confirms earlier findings obtained with classical fluorescence microscopy and another fluoroprobe (fluorescein diacetate). Through additional experimental support is needed, low pH regions at the cytoplasm/plasma membrane interface should be considered a real contribution to the pH control of plant and fungal cells, facilitating e.g. the maintenance of cytosolic pH in acidic environments.