Ca2+-induced removal of peri-germ cell membrane in Arabidopsis sperm cells

Immotile sperm cells are delivered to the egg cell through the pollen tube in flowering
plants. A pair of sperm cells are enclosed by a vegetative cell-derived membrane called the perigerm
cell membrane during this sperm cell transport. To achieve gamete fusion, the peri-germ
cell membrane must be removed immediately after sperm release to expose the sperm cell
membrane, but the dynamics and trigger signal of the peri-germ cell membrane breakdown
have not been clarified. Here we report crucial role of Ca2+ in rapid peri-germ cell membrane
breakdown. A time-lapse live imaging of peri-germ cell membrane demonstrated the
fragmentation of peri-germ cell membrane within one minute after the sperm cell release in both
semi-in vivo and in vitro conditions. This peri-germ cell membrane breakdown was significantly
inhibited under Ca2+-depleted condition. To assess the importance of Ca2+ stimulation on perigerm
cell membrane stability, the intact peri-germ cell membrane isolated under Ca2+-depleted
condition were transferred by micromanipulation. Approximately 90% of the intact peri-germ cell
membrane collapsed after transferring them to Ca2+-sufficient medium. Additionally, this perigerm
cell membrane breakdown was suppressed under EGTA-supplemented condition. From
these results, it is confirmed that Ca2+ is a determinant for the breakdown of peri-germ cell
membrane. Recently, microscopy research using calcium sensors demonstrated Ca2+ spikes in
several processes of double fertilization. Here we would like to discuss the significance of Ca2+
spikes for the peri-germ cell membrane breakdown.