The apical actin fringe contributes to localized cell wall deposition and polarized growth in the lily pollen tube.

Peter K. Hepler,Caleb M. Rounds,Lawrence J. Winship

doi:10.1104/pp.114.242974

Peter K. Hepler, Caleb M. Rounds + Show 1 more

Open Access

https://doi.org/10.1104/pp.114.242974

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Abstract

In lily (Lilium formosanum) pollen tubes, pectin, a major component of the cell wall, is delivered through regulated exocytosis. The targeted transport and secretion of the pectin-containing vesicles may be controlled by the cortical actin fringe at the pollen tube apex. Here, we address the role of the actin fringe using three different inhibitors of growth: brefeldin A, latrunculin B, and potassium cyanide. Brefeldin A blocks membrane trafficking and inhibits exocytosis in pollen tubes; it also leads to the degradation of the actin fringe and the formation of an aggregate of filamentous actin at the base of the clear zone. Latrunculin B, which depolymerizes filamentous actin, markedly slows growth but allows focused pectin deposition to continue. Of note, the locus of deposition shifts frequently and correlates with changes in the direction of growth. Finally, potassium cyanide, an electron transport chain inhibitor, briefly stops growth while causing the actin fringe to completely disappear. Pectin deposition continues but lacks focus, instead being delivered in a wide arc across the pollen tube tip. These data support a model in which the actin fringe contributes to the focused secretion of pectin to the apical cell wall and, thus, to the polarized growth of the pollen tube.

Highlights

In lily (Lilium formosanum) pollen tubes, pectin, a major component of the cell wall, is delivered through regulated exocytosis
Recent studies suggest that the targeted exocytosis increases the extensibility of the cell wall matrix at the tip, which yields to the existing turgor pressure, permitting the tip to extend or grow (McKenna et al, 2009; Hepler et al, 2013)
There are many factors that influence exocytosis in growing pollen tubes; in this study, we investigate the role of the apical actin fringe

Summary

Introduction

In lily (Lilium formosanum) pollen tubes, pectin, a major component of the cell wall, is delivered through regulated exocytosis. Pectin deposition continues but lacks focus, instead being delivered in a wide arc across the pollen tube tip These data support a model in which the actin fringe contributes to the focused secretion of pectin to the apical cell wall and, to the polarized growth of the pollen tube. It has long been assumed that these represent exocytotic vesicles destined to deliver new cell wall material This model of polarized secretion has been challenged in recent years in studies using FM dyes. Experiments using tobacco (Nicotiana tabacum) pollen and a receptor-like kinase fused to GFP indicate that exocytosis occurs largely at the apical polar axis (Lee et al, 2008)

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