Periodic increases in elongation rate precede increases in cytosolic Ca2+ during pollen tube growth

Pollen tubes grown in vitro require an intracellular tip-high gradient of C a2+ in order to elongate. Moreover, after about 2 h in vitro both the tip C a2+ and the elongation rate of lily tubes begin to oscillate regularly with large amplitudes, This raises the question of the phase relation between t hese two oscillations. Previous studies lacked the temporal resolution to a ccurately establish this relationship. We have studied these oscillations w ith a newly developed, high temporal resolution system and the complementar y use of both luminescent and fluorescent calcium reporters. We hereby show that the periodic increases in elongation rate during oscillatory growth o f Lilium longiflorum pollen tubes clearly precede those in subtip calcium a nd do so by 4.1 +/- 0.2 s out of average periods of 38.7 +/- 1.8 s. Also, b y collecting images of the light output of aequorin, we find that the magni tude of the [Ca2+] at the tip oscillates between 3 and 10 mu M, which is co nsiderably greater than that reported by fluorescent indicators. We propose an explanatory model that features cyclic growth and secretion in which gr owth oscillations give rise to secretion that is essential for the subseque nt growth oscillation, We also critically compile data on L. longiflorum st ylar growth rates, which show little variation from in vitro rates of polle n tubes grown in optimal medium. (C) 2000 Academic Press.