Free ca2+ gradient in growing pollen tubes of<i>lilium</i>

ABSTRACT Fluorescence ratiometric imaging of Lilium pollen tubes loaded with the Ca2+ indicator Fura-2 dextran has revealed a distinct elevation of free intracellular calcium ion concentration ([Ca2+]i) at the extreme tip of actively growing Lilium pollen tubes that declines to a uniform basal level of ∼170 nM throughout the length of the tube. The calcium gradient occurs within the first 10–20 μM proximal to the tip. Experimental inhibition of tip growth, usually achieved through the injection of the Ca2+ buffer 5,5’-dibromo BAPTA, results in the loss of the [Ca2+]igradient. Occasionally these inhibited cells reinitiate growth, and when they do so ratio imaging reveals that the tip gradient of free [Ca2+Ji re-emerges. The results presented here are very different from those previously published by revealing the presence of the [Ca2+]i gradient that is restricted to the 10–20 μM adjacent to the tube tip. Further, these experiments demonstrate a strict correlation between the presence of a [Ca2+Ji gradient, and tip growth in Lilium pollen tubes.