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

Abstract

Pollen tubes grown in vitro require an intracellular tip-high gradient of Ca2+ in order to elongate. Moreover, after about 2 h in vitro both the tip Ca2+ 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 +/- 0.2 s out of average periods of 38.7 +/- 1.8 s. Also, by collecting images of the light output of aequorin, we find that the magnitude of the [Ca2+] at the tip oscillates between 3 and 10 microM, 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.

Imaging of the Yellow Cameleon 3.6 Indicator Reveals That Elevations in Cytosolic Ca2+ Follow Oscillating Increases in Growth in Root Hairs of Arabidopsis

“…Our observation that Ca 21 concentrations increased after growth parallels results obtained with pollen tubes, in which it has even been suggested that pulsatile expansion may actually be divorced from the oscillating Ca 21 gradient (Messerli et al, 2000). To further assess the relationship between Ca 21 and tip growth, we therefore attempted to manipulate cytoplasmic Ca 21 levels while simultaneously monitoring cell expansion.…”

“…1). Crosscorrelation analysis indicates that the oscillations of the Ca 21 gradient lagged oscillations in growth by 5 s. A similar phenomenon has been observed in pollen tubes, in which the cytosolic Ca 21 increase also lagged growth by 4 s (Messerli et al, 2000;Messerli and Robinson, 2007).…”

“…1D). Crosscorrelation analysis comparing the temporal kinetics of Ca 21 and growth oscillations (Messerli et al, 2000) confirmed that growth peaks most likely preceded Ca 21 increases by 5.3 6 0.3 s (Fig. 1E).…”

“…In this system, growth oscillations were accompanied by oscillations in the tip-focused Ca 21 gradient (HoldawayClarke et al, 1997;Messerli and Robinson, 1997;Messerli et al, 2000;Watahiki et al, 2004). Interestingly, the periodic increases in the Ca 21 gradient actually lagged the periodic increases in growth by about 4 s (Messerli et al, 2000).…”

“…It has even been suggested that a burst of secretion may precede elongation in Agapanthus pollen (Coelho and Malhó, 2006). Lily pollen tubes in culture (for review, see Messerli et al, 2000) grow five to seven times faster than Arabidopsis (Arabidopsis thaliana) root hairs in culture (Monshausen et al, 2007), and it is unclear whether an equivalent relationship between oscillatory growth and Ca 21 changes exists for the root hair. Although a stretch-activated Ca 21 influx channel has been identified in pollen tubes (Dutta and Robinson, 2004), the Ca 21 channel sustaining the tip-focused Ca 21 gradient in root hairs is thought to be gated by membrane voltage and reactive oxygen species (ROS; Foreman et al, 2003).…”