It is commonly accepted in plant and animal cells that Ca2+ facilitates secretion (Battey et al. 1999). Thus, a strong candidate function for the tip-focused Ca2+ gradient would be the stimulation of exocytosis of the apically accumulated vesicles, which contain the cell wall precursors needed for cell elongation (Fig. 4; chapter by Malho, this volume). Experimental probing of this idea shows that elevation of the Ca2+ levels, brought about by local uncaging of a light-sensitive Ca2+-containing reagent, stimulates exocytosis, with the latter being inferred from a reduction in FM1-43 fluorescence (Camacho and Malho 2003). However, in parallel studies, increasing the concentration of GTPy S, a nonhydrolyzable analog of GTP, increased exocytosis concomitant with a slight decrease in Ca2+. While these results may seem inconsistent with the idea that Ca2+ facilitates secretion, it must be realized that the pollen tubes still exhibit an apical gradient, and thus at the point of secretion the local concentration of Ca2+ is well above basal levels. Studies in maize coleop-tiles have shown that the secretory process saturates above 1.5 ^M, with half maximal stimulation at ~ 0.9 ^M (Sutter et al. 2000). If the pollen tube is similar, then even when the gradient is at its low point (~ 0.75 ^M), there is still sufficient Ca2+ to stimulate the secretory process (Holdaway-Clarke and Hepler 2003).
The connection between Ca2+ and secretion is also supported by studies using Yariv reagent to block pollen tube growth (Roy et al. 1999). Under these conditions, elongation stops, whereas secretion continues with irregular wall thickenings arising, which are accompanied by elevated levels of Ca2+ in the adjacent cytoplasm (Roy et al. 1999). It is possible that stretch-activated channels are involved, with the deformation resulting from secretion causing mechanical strain that opens a Ca2+ channel. The recent work of Pickard and Fujiki (2005) supports this contention. In cultured BY-2 cells, they show by patch-clamp analysis that inactivation of the wall-associated arabinogalactan proteins with Yariv reagent deregulates the activity of the stretch-activated Ca2+ channels, allowing uncontrolled ion influx. These authors promote the idea of a cortical plasma membrane-associated reticulum, in which a complex of cytoskeleton and arabinogalactan proteins creates a force-focusing system that spatially confines and regulates the activity of stretch-activated channels (Pickard and Fujiki 2005).
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