Uptake of Lucifer Yellow CH into intact barley roots: Evidence for fluid-phase endocytosis

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Abstract
Intact barley (Hordeum vulgare L.) roots have been shown to take up the highly fluorescent dye Lucifer Yellow CH (LYCH) into their cell vacuoles. In the apical 1 cm of root tip, differentiating and dividing cells showed a prolific uptake of LYCH into their provacuoles. The LYCH was retained during fixation, apparently becoming bound to electron-dense material in the vacuoles. The dye freely entered the apoplast of roots in which the Casparian band was not developed, being taken up into the vacuoles of cells in both the cortex and stele. However, when LYCH was applied to a 1-cm zone approx. 6 cm behind the root tip the Casparian band on the radial walls of the endodermis completely prevented the dye from entering the cells of the stele, only the cell walls and vacuoles of the cortical cells taking up the dye. The inability of LYCH to cross the plasmalemma of the endodermal cells and enter the stele via the symplast substantiates previous claims that the dye is unable to cross the plasmalemma of plant cells. The results are discussed in the light of recent demonstrations that LYCH is a particularly effective marker for fluid-phase endocytosis in animal and yeast cells. A calculation of the energetic requirements for LYCH uptake into barley roots supports the contention that LYCH is taken up into the vacuoles of plant cells by fluid-phase endocytosis.