Compartmentation of fluorescent tracers injected into the epidermal cells of Egeria densa leaves

Abstract

We have compared the movement of a series of fluorescent tracers of increasing molecular weight injected into the cytoplasm in the epidermal cells of leaves of Egeria densa Planch. In general, the tracers showed major movement into three cellular compartments: first, to the cytoplasm of adjacent cells; secondly, from the cytoplasm, to the vacuole (irreversible); and thirdly, from the cytoplasm to the nucleus (reversible). No visible accumulation in chloroplasts or mitochondria, or loss across the plasmalemma was observed. No evidence for metabolic breakdown was found in extracts from injected leaves. The time course of accumulation of the dye in the three major compartments (cytoplasm, nucleus, vacuole) was monitored using fluorescence microscopy. The rate measurements and the quantified geometry of the cells were used to generate a model of compartmentation during intercellular transport. Permeability coefficients were calculated and related to the molecular sizes of the tracers. The coefficients for the tonoplast and nuclear envelope were independent of the molecular sizes of the tracers, and were in the range 2.4·10⁻⁶–4.1· 10⁻⁶ cm·s⁻¹ for the tonoplast, and 2.6·10⁻⁵-9.4.10⁻⁵ cm· s⁻¹ for the nuclear envelope. For intercellular movement, permeabilities were strongly dependent on molecular size, and ranged from 1.1·10⁻⁴ cm·s⁻¹ for 6-carboxyfluorescein (376 daltons (Da)) to 9·10⁻⁹ cm·s⁻¹ for fluorescein leucyldiglutamylleucine (874 Da). Thus, the differences in cell-to-cell movement of these tracers are based upon their differing ability to cross the intercellular walls, not upon differences in their intracellular compartmentation.