Role of 1,4,5-inositol triphosphate-induced Ca2+ release in pollen tube orientation

Pollen tube reorientation is a dynamic cellular event crucial for successful fertilization. Previously, it was shown that reorientation is preceded by an asymmetric increase of cytosolic free calcium ([Ca2+](c)) in the side of the apex to which the cell will bend. In order to find the targets for this signal transduction pathway, the effects of inositol 1,4,5-trisphosphate [Ins(1,4,5)P-3] in the reorientation process were analyzed. Ins(1,4,5)P-3 was artificially increased in different cell domains by localized photoactivation of caged Ins(1,4,5)P-3 and its effects on [Ca2+](c) monitored by ion confocal microscopy. It was found that photolysis of caged Ins(1,4,5)P-3 in the nuclear or subapical region resulted in a transient increase in [Ca2+](c) and reorientation of the growth axis, while photolysis in the apex frequently resulted in disturbed growth or tip bursting. Perfusion of the cells with the Ins(1,4,5)P-3 receptor blocker heparin prior to photoactivation inhibited the increase in [Ca2+](c) and no reorientation was observed. Ca2+ release from Ins(1,4,5)P-3-dependent stores localized in the shank of the tube thus seems to be part of the signal transduction pathway that controls tube guidance, although not the primary stimulus leading to reorientation.