Extracellular calcium modulates pollen tube growth and guidance in <i>Arabidopsis thaliana</i>

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-<i>in vivo</i> 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 μm 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.