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

Cytoplasmic calcium concentration ([Ca2+](i)) and extracellular calcium (Ca -o(2+)) influx has been studied in pollen tubes of Lilium longliflorum in w hich the processes of cell elongation and exocytosis have been uncoupled by use of Yariv phenylglycoside ((beta-D-Glc)(3)). Growing pollen tubes were pressure injected with the ratio dye fura-2 dextran and imaged after applic ation of (beta-D-Glc)(3), which binds arabinogalactan proteins (AGPs). Appl ication of (P-D-Glc)3 inhibited growth but not secretion. Ratiometric imagi ng of [Ca2+](i) revealed an initial spread in the locus of the apical [Ca2](i) gradient acid substantial elevations in basal [Ca2+](i) followed by th e establishment of new regions of elevated [Ca2+](i) on the flanks of the t ip 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. Ca-o(2+) influx at the tip of (beta-D- glc)(3)-treated po llen tubes was not significantly different to that of control tubes. Taken together these data indicate that regions of elevated [Ca2+](i), probably r esulting from Ca-o(2+) influx across the plasma membrane, stimulate exocyto sis in pollen tubes independent of cell elongation.