cAMP activates hyperpolarization-activated Ca2+ channels in the pollen of Pyrus pyrifolia

Many signal-transduction processes in plant cells have been suggested to be triggered by signal-induced opening of calcium ion (Ca2+) channels in the plasma membrane. Cyclic nucleotides have been proposed to lead to an increase in cytosolic free Ca2+ in pollen. However, direct recordings of cyclic-nucleotide-induced Ca2+ currents in pollen have not yet been obtained. Here, we report that cyclic AMP (cAMP) activated a hyperpolarization-activated Ca2+ channel in the Pyrus pyrifolia pollen tube using the patch-clamp technique, which resulted in a significant increase in pollen tube protoplast cytosolic-Ca2+ concentration. Outside-out single channel configuration identified that cAMP directly increased the Ca2+ channel open-probability without affecting channel conductance. cAMP-induced currents were composed of both Ca2+ and K+. However, cGMP failed to mimic the cAMP effect. Higher cytosolic free-Ca2+ concentration significantly decreased the cAMP-induced currents. These results provide direct evidence for cAMP activation of hyperpolarization-activated Ca2+ channels in the plasma membrane of pollen tubes, which, in turn, modulate cellular responses in regulation of pollen tube growth.