Cytoplasmic Ca2+ changes dynamically during the interaction of the pollen tube with synergid cells

The directional growth of the pollen tube from the stigma to the embryo sac in the ovules is regulated by pollen-pistil interactions based on intercellular communication. Although pollen tube growth is regulated by the cytoplasmic Ca(2+) concentration ([Ca(2+)](cyt)), it is not known whether [Ca(2+)](cyt) is involved in pollen tube guidance and reception. Using Arabidopsis expressing the GFP-based Ca(2+)-sensor yellow cameleon 3.60 (YC3.60) in pollen tubes and synergid cells, we monitored Ca(2+) dynamics in these cells during pollen tube guidance and reception under semi-in vivo fertilization conditions. In the pollen tube growing towards the micropyle, pollen tubes initiated turning within 150 μm of the micropylar opening; the [Ca(2+)](cyt) in these pollen tube tips was higher than in those not growing towards an ovule in assays with myb98 mutant ovules, in which pollen tube guidance is disrupted. These results suggest that attractants secreted from the ovules affect Ca(2+) dynamics in the pollen tube. [Ca(2+)](cyt) in synergid cells did not change when the pollen tube grew towards the micropyle or entered the ovule. Upon pollen tube arrival at the synergid cell, however, [Ca(2+)](cyt) oscillation began at the micropylar pole of the synergid, spreading towards the chalazal pole. Finally, [Ca(2+)](cyt) in the synergid cell reached a maximum at pollen tube rupture. These results suggest that signals from the pollen tube induce Ca(2+) oscillations in synergid cells, and that this Ca(2+) oscillation is involved in the interaction between the pollen tube and synergid cell.