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Exocytosis Precedes and Predicts the Increase in Growth in Oscillating Pollen Tubes^[W]

^a Department of Biology, Long Island University, Brooklyn, New York 11201
^b Department of Biology, University of Massachusetts, Amherst, Massachusetts 01003
^c Institute of Biological, Environmental, and Rural Sciences, Aberystwyth University Plas Gogerddan, Aberystwyth, SY23 3EB, United Kingdom
^d Department of Biology and Biotechnology, Worcester Polytechnic Institute, Worcester, Massachusetts 01609
^e School of Natural Science, Hampshire College, Amherst, Massachusetts 01002

¹ Address correspondence to hepler{at}bio.umass.edu.

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 –98° ± 3° in lily and –124° ± 4° 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.

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