Endo/exocytosis in the pollen tube apex is differentially regulated by Ca2+and GTPases

Pollen tube growth relies on an extremely fast delivery of new membrane and wall material to the apical region where growth takes place. Despite the obvious meaning of this fact, the mechanisms that control this process remain very much unknown. It has previously been shown that apical growth is regulated by cytosolic free calcium ([Ca2+]c) so it was decided to test how changes in [Ca2+]c affect endo/exocytosis in pollen tube growth and reorientation. The endo/exocytosis was assayed in living cells using confocal imaging of FM 1-43. It was found that growing pollen tubes exhibited a higher endo/exocytosis activity in the apical region whereas in nongrowing cells FM 1-43 is uniformly distributed. During pollen tube reorientation, a spatial redistribution of exocytotic activity was observed with the highest fluorescence in the side to which the cell will bend. Localized increases in [Ca2+]c induced by photolysis of caged Ca2+ increased exocytosis. In order to find if [Ca2+]c changes were modulating endo/exocytosis directly or through a signalling cascade, tests were conducted to find how changes in GTP levels and GTPase activity (primary regulators of the secretory pathway) affect the apical [Ca2+]c gradient and endo/exocytosis. It was found that increases in GTP levels could promote exocytosis (and growth). Interestingly, the increase in [GTP] did not significantly affect [Ca2+]c distribution, thus suggesting that the apical endo/exocytosis is regulated in a concerted but differentiated manner by the Ca2+ gradient and the activity of GTPases. Rop GTPases are likely candidates to mediate the Ca2+/GTP cross-talk as shown by knock-down experiments in growing pollen tubes.