Identification of hyperpolarization‐activated calcium channels in apical pollen tubes of Pyrus pyrifolia

Abstract

• The pollen tube has been widely used to study the mechanisms underlying polarized tip growth in plants. A steep tip-to-base gradient of free cytosolic calcium ([Ca²⁺]_cyt) is essential for pollen-tube growth. Local Ca²⁺ influx mediated by Ca²⁺-permeable channels plays a key role in maintaining this [Ca²⁺]_cyt gradient. • Here, we developed a protocol for successful isolation of spheroplasts from pollen tubes of Pyrus pyrifolia and identified a hyperpolarization-activated cation channel using the patch-clamp technique. • We showed that the cation channel conductance displayed a strong selectivity for divalent cations, with a relative permeability sequence of barium (Ba²⁺) ≈ Ca²⁺ > magnesium (Mg²⁺) > strontium (Sr²⁺) > manganese (Mn²⁺). This channel conductance was selective for Ca²⁺ over chlorine (Cl⁻) (relative permeability P_Ca/P_Cl = 14 in 10 mm extracellular Ca²⁺). We also showed that the channel was inhibited by the Ca²⁺ channel blockers lanthanum (La³⁺) and gadolinium (Gd³⁺). Furthermore, channel activity depended on extracellular pH and pollen viability. • We propose that the Ca²⁺-permeable channel is likely to play a role in mediating Ca²⁺ influx into the growing pollen tubes to maintain the [Ca²⁺]_cyt gradient.