The role of heterotrimeric G proteins in pollen germination and tube growth was investigated using Arabidopsis thaliana plants in which the gene (GPA) encoding the G-protein α subunit (G α) was null or overexpressed. Pollen germination, free cytosolic calcium concentration ([Ca2+ ] cyt) and Ca2+ channel activity in the plasma membrane (PM) of pollen cells were investigated. Results showed that, compared with pollen grains of the wild type (ecotype Wassilewskija, ws), in vitro germinated pollen of G α null mutants (gpa1-1 and gpa1-2) had lower germination percentages and shorter pollen tubes, while pollen from G α overexpression lines (wG α and cG α) had higher germination percentages and longer pollen tubes. Compared with ws pollen cells, [Ca2+] cyt was lower in gpa1-1 and gpa1-2 and higher in wG α and cG α. In whole-cell patch clamp recordings, a hyperpolarization-activated Ca2+-permeable conductance was identified in the PM of pollen protoplasts. The conductance was suppressed by trivalent cations but insensitive to organic blockers; its permeability to divalent cations was Ba2+>Ca2+>Mg2+>Sr2+>Mn2+. The activity of the Ca2+-permeable channel conductance was down-regulated in pollen protoplasts of gpa1-1 and gpa1-2, and up-regulated in wG α and cG α. The results suggest that G α may participate in pollen germination through modulation of the hyperpolarization-activated Ca2+ channel in the PM of pollen cells.