Uptake of Lucifer Yellow CH into plant-cell protoplasts: a quantitative assessment of fluid-phase endocytosis.

The highly fluorescent dye Lucifer Yellow CH (LYCH), now in common use in microinjection studies, has been shown to enter the vacuole of a range of plant-cell protoplasts from the external medium. Uptake was quantified by lysing the protoplasts following incubation and determining the amount of LYCH incorporated by spectrofluorimetry. Uptake was biphasic with respect to both time and substrate concentration, enhanced at low pH and inhibited by low temperature and metabolic inhibitors. The kinetics of uptake showed several similarities with those reported for the fluid-phase endocytosis of LYCH in animal cells and yeast cells. A calculated membrane permeability coefficient for LYCH, based on the observed rates of uptake, was too high to be consistent with simple diffusion of the undissociated form of the molecule and inconsistent with the membrane-impermeant properties of the dye. The data are discussed in the light of the possibility of fluid-phase endocytosis versus active transmembrane transport.