ROLE OF CYTOSOLIC-FREE CALCIUM IN THE REORIENTATION OF POLLEN-TUBE GROWTH

Growing pollen tubes of Agapanthus umbellatus exhibited a tip to-base gradient in cytosolic free calcium ([Ca2+](c)). Although this gradient is believed to be involved in pollen tube growth, its role in specifying reorientation is unknown. The direction of pollen tube growth could be modified by iontophoretic microinjection, electrical fields (EFs) or photolysis of caged Ca2+. Iontophoretic injection resulted in a temporary cessation of growth, an increase in [Ca2+](c) and, upon recovery, reoriented growth. Weak EFs also elevated [Ca2+](c), reduced growth rates and resulted in the reorientation of pollen tubes towards the cathode. Treatment with very low concentrations of the Ca2+-channel blocker lanthanum chloride, prior to exposure to an EF, inhibited both the increase in [Ca2+](c) and reorientation whilst only slightly affecting growth rates. The responses of growth inhibition and reorientation were mimicked when [Ca2+](c) was artificially elevated. by photoactivating caged Ca2+ (Nitr-5). Our data suggest that [Ca2+](c) is part of a transduction mechanism which enables growing pollen tubes to successfully reorient to directional signals in the style and thus accomplish eventual fertilization of the egg.