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

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Abstract
Summary: 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.