Periodic Increases in Elongation Rate Precede Increases in Cytosolic Ca super(2+) during Pollen Tube Growth

Pollen tubes grown in vitro require an intracellular tip-high gradient of Ca super(2+) in order to elongate. Moreover, after about 2 h in vitro both the tip Ca super(2+) and the elongation rate of lily tubes begin to oscillate regularly with large amplitudes. This raises the question of the phase relation between these two oscillations. Previous studies lacked the temporal resolution to accurately establish this relationship. We have studied these oscillations with a newly developed, high temporal resolution system and the complementary use of both luminescent and fluorescent calcium reporters. We hereby show that the periodic increases in elongation rate during oscillatory growth of Lilium longiflorum pollen tubes clearly precede those in subtip calcium and do so by 4.1 plus or minus 0.2 s out of average periods of 38.7 plus or minus 1.8 s. Also, by collecting images of the light output of aequorin, we find that the magnitude of the [Ca super(2+)] at the tip oscillates between 3 and 10 mu M, which is considerably greater than that reported by fluorescent indicators. We propose an explanatory model that features cyclic growth and secretion in which growth oscillations give rise to secretion that is essential for the subsequent growth oscillation. We also critically compile data on L. longiflorum stylar growth rates, which show little variation from in vitro rates of pollen tubes grown in optimal medium.