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 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.