Extracellular calcium modulates pollen tube growth and guidance in Arabidopsis thaliana

In angiosperms, pollen tubes deliver sperm cells to the ovule and communicate with the external environment as they elongate through the pistils. Although pollination alters Ca2+ conditions within the pistil, the effects of extracellular Ca2+ fluctuations on pollen tube growth and guidance remain largely unknown. In this study, we visualized intracellular Ca2+ dynamics using a semi-in vivo assay with the Ca2+-sensitive fluorescent protein GCaMP6s to investigate how pollen tubes respond to changes in extracellular Ca2+ levels. We found that the Ca2+ levels in the apical region of the pollen tubes reflected the extracellular Ca2+ concentrations. The pollen tube growth rate increased depending on the Ca2+ concentration in the growth medium. However, excessive Ca2+ affected the polar growth of pollen tubes. At elevated Ca2+ concentrations of 10 mM, the pollen tube exhibited coiling behavior and failed to maintain directional growth toward the ovule. Moreover, we provided the first evidence that Ca2+ oscillations are not restricted to the apical region but propagate as a wave, reaching 30-50 m from the apex toward the basal regions. As the pollen tube approached the ovule, it coincided with a substantial elevation in Ca2+ levels, which appeared to drive the accelerated nuclear migration toward the tube apex. Our findings demonstrate that the extracellular Ca2+ environment directly regulates intracellular Ca2+ levels in pollen tubes, thereby influencing their growth and guidance.