Uptake and Compartmentation of Fluorescent Probes by Plant Cells

Several fluorescent compounds are now being used as probes for studying plant transport processes. This review considers the potential mechanisms of uptake of such probes with particular emphasis on their subsequent compartmentation within the cell. Physico-chemical parameters, such as the dissociation constant (pKa) and polarity (log kow) of the dye molecule provide important guides as to the likely permeability of the plasmalemma to different fluorochromes and an ion-trap mechanism may explain the accumulation of many fluorescent probes by plant cells. However, physico-chemical parameters alone do not always explain the subsequent compartmentation of fluorescent probes within the cell. Evidence is accumulating that many anionic fluorescent probes may cross the plasmalemma in the undissociated state, followed by carrier-mediated transport of the anion across the tonoplast. In the specialized case of the highly dissociated dye, Lucifer Yellow CH (LYCH), the physico-chemical properties of the molecule would predict that it should be unable to cross membranes. Despite this, there have been several reports of the movement of LYCH from the apoplast to the vacuole of plant cells. Fluid-phase endocytosis has been implicated in the vacuolar accumulation of LYCH and also a range of high-molecular weight, purified fluorescent conjugates. This evidence is discussed in the light of some reports that membrane-impermeant dyes, including LYCH, may cross the tonoplast following their microinjection into the cytoplasm.