Exocytosis Precedes and Predicts the Increase in Growth in Oscillating Pollen Tubes

Abstract

We examined exocytosis during oscillatory growth in lily (Lilium formosanum and Lilium longiflorum) and tobacco (Nicotiana tabacum) pollen tubes using three markers: (1) changes in cell wall thickness by Nomarski differential interference contrast (DIC), (2) changes in apical cell wall fluorescence in cells stained with propidium iodide (PI), and (3) changes in apical wall fluorescence in cells expressing tobacco pectin methyl esterase fused to green fluorescent protein (PME-GFP). Using PI fluorescence, we quantified oscillatory changes in the amount of wall material from both lily and tobacco pollen tubes. Measurement of wall thickness by DIC was only possible with lily due to limitations of microscope resolution. PME-GFP, a direct marker for exocytosis, only provides information in tobacco because its expression in lily causes growth inhibition and cell death. We show that exocytosis in pollen tubes oscillates and leads the increase in growth rate; the mean phase difference between exocytosis and growth is -988 6 38 in lily and -1248 6 48 in tobacco. Statistical analyses reveal that the anticipatory increase in wall material predicts, to a high degree, the rate and extent of the subsequent growth surge. Exocytosis emerges as a prime candidate for the initiation and regulation of oscillatory pollen tube growth.

Finite Element Model of Polar Growth in Pollen Tubes

“…In most of the mentioned models, the thickness of the cell wall is only indirectly considered, whereas in the FE model, this variable can be manipulated either globally or, with minor modifications, locally, to accommodate phenomena such as the temporal variation in the thickness of the apical cell wall observed in lily and tobacco (Nicotiana tabacum) pollen tubes (McKenna et al, 2009;Zerzour et al, 2009). Remarkably, this experimentally observed change in cell wall thickness is associated with temporary growth arrest or periodic temporal changes in the growth rate.…”

Exocyst SEC3 and phosphoinositides define sites of exocytosis in pollen tube initiation and growth

“…To further study the function of SEC3a, we examined its localization relative to PI, which labels esterified pectins in pollen cell walls (Rounds et al, 2014) and has been used previously as a marker for exocytosis in the pollen tube (McKenna et al, 2009). PI was applied to growing pollen at a concentration of 10 mM, and imaging was performed immediately, since under our experimental conditions, pollen tube elongation was inhibited 3 to 5 min after the addition of the dye.…”

“…It is well established that oscillating polarized exocytosis is fundamental for pollen tube development and determines growth rate (Bove et al, 2008;McKenna et al, 2009;Chebli et al, 2013). Exocytosis is required for the delivery of membrane and cell wall components to the growing tip.…”

Quantifying growth mechanics of living, growing plant cells in situ using microrobotics

“…The time window for successful fertilization is relatively short. Lily pollen tubes elongate at a moderate but still amazing rate of about 2 mm/h [23][24][25]. Since cellular growth is restricted to the tip of the pollen tube, the cell wall in this zone must be deform, whereas in more distal parts (the shank) the main function of the wall is to resist turgor pressure.…”