Ca^2+ Dynamics in a Pollen Grain and Papilla Cell during Pollination of Arabidopsis

Ca^2+ dynamics in the growing pollen tube have been well documented in vitro using germination assays and Ca^2+ imaging techniques. However, very few in vivo studies of Ca^2+ in the pollen grain and papilla cell during pollination have been performed. We expressed yellow cameleon, a Ca^2+ indicator based on green fluorescent protein, in the pollen grains and papilla cells of Arabidopsis (Arabidopsis thaliana) and monitored Ca^2+ dynamics during pollination. In the pollen grain, [Ca^2+]cyt increased at the potential germination site soon after hydration and remained augmented until germination. As in previous in vitro germination studies, [Ca^2+]cyt oscillations were observed in the tip region of the growing pollen tube, but the oscillation frequency was faster and [Ca^2+]cyt was higher than had been observed in vitro. In the pollinated papilla cell, remarkable increases in [Ca^2+]cyt occurred three times in succession, just under the site of pollen-grain attachment. [Ca^2+]cyt increased first soon after pollen hydration, with a second increase occurring after pollen protrusion. The third and most remarkable [Ca^2+]cyt increase took place when the pollen tube penetrated into the papilla cell wall.