Calcium is well known as a signal transduction molecule. In animals, membrane fusion process between egg cell and sperm cell, in other words, fertilization causes calcium spike and its oscillation in the egg cell. On the other hand, calcium dynamics during more complex fertilization process which involves not only egg cell but central cell fertilization in flowering plants were not investigated well. We succeeded in imaging cytosolic calcium in these two cells, and in the two synergid cells that are important for pollen tube attraction. A calcium sensor protein called YC3.60 expressed in each cell was used to observe calcium dynamics during semi-in vivo fertilization (Figure 1) under high-sensitive laser microscopes. Following pollen tube discharge and membrane fusion the egg and central cells showed transient calcium spikes, but not oscillations. Only the second calcium spike in the egg cell correlated with the membrane fusion (Figures 2 and 3). By contrast, the synergid cells displayed calcium oscillations upon pollen tube arrival (Figure 4). These calcium dynamics in the fertilization responsible cells seem to represent highly specific signatures that coordinate successful fertilization in the flowering plants (Figure 5).
Figure 1 Diagram of Arabidopsis flower, fertilization process and semi-in vivo fertilization assay
Figure 2 Calcium dynamics in egg cells during fertilization
Figure 3 Relationship between second calcium spike in egg cell and sperm cell fusion to the egg cell
Figure 4 Calcium dynamics in two synergid cells during fertilization
Figure 5 Diagram of calcium dynamics in four cells responsible for double fertilization
Author: Yuki Hamamura, Moe Nishimaki, Hidenori Takeuchi, Anja Geitmann, Daisuke Kurihara and Tetsuya Higashiyama
Title: “Live imaging of calcium spikes during double fertilization in Arabidopsis ”
Nature Communications, Published online 22nd August 2014
Tetsuya Higashiyama Ph.D.
Director of ERATO Higashiyama Live-Holonics Project
Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University
Department of Research Project, JST