Uncoupling secretion and tip growth in lily pollen tubes: evidence for the role of calcium in exocytosis

Cytoplasmic calcium concentration (Ca²⁺]_i) and extracellular calcium (Ca²⁺_o) influx has been studied in pollen tubes of Lilium longliflorum in which the processes of cell elongation and exocytosis have been uncoupled by use of Yariv phenylglycoside ((β-D-Glc)₃). Growing pollen tubes were pressure injected with the ratio dye fura-2 dextran and imaged after application of (β-D-Glc)₃, which binds arabinogalactan proteins (AGPs). Application of (β-D-Glc)₃ inhibited growth but not secretion. Ratiometric imaging of Ca²⁺]_i revealed an initial spread in the locus of the apical Ca²⁺]_i gradient and substantial elevations in basal Ca²⁺]_i followed by the establishment of new regions of elevated Ca²⁺]_i on the flanks of the tip region. Areas of elevated Ca²⁺]_i corresponded to sites of pronounced exocytosis, as evidenced by the formation of wall ingrowths adjacent to the plasma membrane. Ca²⁺_o influx at the tip of (β-D-Glc)₃-treated pollen tubes was not significantly different to that of control tubes. Taken together these data indicate that regions of elevated Ca²⁺]_i, probably resulting from Ca²⁺_o influx across the plasma membrane, stimulate exocytosis in pollen tubes independent of cell elongation.