ROLE OF 1,4,5-INOSITOL TRIPHOSPHATE-INDUCED CA2-TUBE ORIENTATION( RELEASE IN POLLEN)

Pollen tube reorientation is a dynamic cellular event crucial for succ essful fertilization. Previously, it was shown that reorientation is p receded 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 fin d the targets for this signal transduction pathway, the effects of ino sitol 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 i ts effects on [Ca2+](c) monitored by ion confocal microscopy. It was f ound that photolysis of caged Ins(1,4,5)P-3 in the nuclear or subapica l region resulted in a transient increase in [Ca2+](c) and reorientati on of the growth axis, while photolysis in the apex frequently resulte d in disturbed growth or tip bursting. Perfusion of the cells with the Ins(1,4,5)P-3 receptor blocker heparin prior to photoactivation inhib ited 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 tha t controls tube guidance, although not the primary stimulus leading to reorientation.