AtFH5-labeled secretory vesicles-dependent calcium oscillation drives exocytosis and stepwise bulge during pollen germination.

Pollen germination is an essential step for delivering sperm cells to the embryo sac for double fertilization in flowering plants. The cytosolic Ca concentration ([Ca]) and vesicle dynamics are critical for pollen germination, but their potential correlation in pollen grains is not fully understood. Here, we report that [Ca] oscillates periodically at the prospective germination sites during pollen germination. The [Ca] is mainly from extracellular Ca ([Ca]) influx, which implicates the Ca-permeable ion channel cyclic nucleotide-gated channel 18 (CNGC18). The [Ca] oscillations spatiotemporally correlate with the accumulation of secretory vesicles labeled by a formin protein AtFH5, and disruption of vesicle accumulation inhibits the [Ca] oscillations. In turn, the [Ca] oscillations promote exocytosis, which leads to stepwise cell extension during pollen germination. Together, these data provide a timeline of vesicle dynamics, calcium oscillation, and exocytosis during pollen germination and highlight the importance of the correlation of these events for pollen germination.