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

Abstract

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-DGlc)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.