Abstract
The maintenance of a cytosolic free calcium gradient (Ca2+]c) and vesicle secretion in the apex of pollen tubes is essential for growth. It has been postulated that high [Ca2+]c levels promote and confine vesicle fusion with the apical plasma membrane and in this study we performed a correlative analysis of both events using specific fluorescent dyes and confocal scanning microscopy. [Ca2+]c was imaged with Calcium Green-1 10 kDa dextran (CG-1) while secretory events were followed with FM1–43 or FM4–64 in pollen tubes undergoing normal growth and reorientation events. During straight growth (no modification in direction), we found that changes in apical [Ca2+]c accompany changes in apical FM fluorescence indicating a tight coupling between [Ca2+]c and apical secretion. This coupling seems however to be perturbed during periods of reorientation of the pollen tube growth axis. Analysis of apical and sub-apical fluorescent signals during the reorientation events and subsequent re-entry in straight growth indicate that the increase in secretory events (higher fusion rate) precede the increases in [Ca2+]c that should be required for the transduction of the signal. Based on these findings, we discuss a model for membrane secretion and recycling which considers the apical and sub-apical region as a functional area containing all the elements required to promote and sustain growth.
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