Ca2+ channels control the rapid expansions in pulsating growth of Petunia hybrida pollen tubes

Pollen tubes exhibit a rip-focused gradient of the cytosolic Ca2+ concentration, which is presumably governed by the activity of Ca2+ channels in the plasma membrane. The putative role of Ca2+ as a signalling factor in pulsating growth of Petunia hybrida pollen tubes was examined. The application of inorganic Ca2+ channel inhibitors La3+ and Gd3+ caused the pulsating pollen tubes to abandon their rhythm and continue growth with rather steady rates. This effect was reversible upon removal of these ions. The organic inhibitors of Ca2+ channels nifedipine and verapamil caused the arrest of overall pollen tube growth at higher concentrations, whereas intermediate concentrations caused a slowdown of pulse frequency, but not the inhibition of pulses. This leads to the hypothesis, that the activity of a certain group of Ca2+ channels, which is sensitive to La3+ and Gd3+ would be responsible for the control of pulsating growth. Another group of Ca2+ channels, which is sensitive to organic inhibitors, would be required for pollen tube growth in general and provide the basic Ca2+ influx, but would nor be directly involved in the growth oscillations. Based on these findings a hypothetical model for the mechanism that controls pulsating growth in pollen tubes is proposed.