Digitonin-aided loading of Fluo-3 into embryogenic plant cells

This paper describes a method to load embryogenic plant cells with Fluo-3 in its cell impermeant form with the aid of digitonin. Attempts to load cells with Fluo-3/AM were all unsuccessful. Presumably the indicator is cleaved outside the cells and cannot penetrate in its acidic form. At a low pH, Fluo-3 enters the plant cells but normal Ca2+ homeostasis seems to be disturbed. Successful loading of Fluo-3 was achieved by adding 0.1% digitonin during incubation with the Ca(2+)-indicator. A bright fluorescence was observed in the epidermal layer of heart and torpedo shaped somatic embryos of carrot with confocal scanning laser microscopy. Vacuoles were always without fluorescence which indicates that the dye, after loading, remains in the cytosol and does not leak out. The fluorescence intensity was sensitive to treatments with A23187 and EGTA. We conclude that Fluo-3 can be effectively loaded, with the aid of digitonin, into plant embryogenic cells in liquid culture. Therefore, we expect this technique to be very useful for the study of changes in cytosolic free Ca2+ levels during plant growth and development.