The Apical Actin Fringe Contributes to Localized Cell Wall Deposition and Polarized Growth in the Lily Pollen Tube    

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.

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

“…Yet, the exact location where exocytosis takes place is under debate. Ultrastructural studies showing the accumulation of vesicles at the tip suggested that exocytosis takes place at the tip (Lancelle et al, 1987;Lancelle and Hepler, 1992;Derksen et al, 1995), which was further supported by studies on the dynamics of cell wall thickness (Rojas et al, 2011), secretion of pectin methyl esterase (PME) and PME inhibitor, and staining of pectin by propidium iodide (PI; Röckel et al, 2008;Rounds et al, 2014). Conversely, based on colabeling with FM1-43 and FM4-64, it was concluded that exocytosis takes place in a subapical collar located in the transition zone between the tip and the shank, as well as at the shank, but not at the tip (Bove et al, 2008;Zonia and Munnik, 2008).…”

“…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.…”

Interplay between Ions, the Cytoskeleton, and Cell Wall Properties during Tip Growth

“…1A). In growing pollen tubes expressing Lifeact-GFP, the fringe was observed to rapidly remodel and move forward with growth (Rounds et al, 2014). Additionally, when pollen tube growth was inhibited with cyanide, the apical fringe degraded more rapidly than the actin cables in the shank of the tube (Winship et al, 2016).…”

Rice Morphology Determinant-Mediated Actin Filament Organization Contributes to Pollen Tube Growth

“…Pollen tube expansion depends on the deposition of newborn cell wall materials at the tip, and the deposited materials can be maintained by the polarized behaviors of F-actin (Rounds and Bezanilla, 2013;Rounds et al, 2014). To test whether the rmd pollen tube has a defective deposition of cell wall components, pectin labeling was performed with the carbohydrate antibody JIM5, which is used to indicate pectins with a low level of methylesterification (also called callose; Clausen et al, 2003).…”

“…At the subapex, actin filaments form regular structures in different species, such as the collar, fringe, mesh, or funnel, and some observations indicated that subapical F-actin could regulate the accumulation of vesicles and the growth of the tip region (Gibbon et al, 1999;Fu et al, 2001;Vidali et al, 2001;Chen et al, 2002;LovyWheeler et al, 2005;Qu et al, 2017). At the apex, actin filaments are generally less abundant but are highly dynamic with dense actin structures, which mediates the interaction between the apical/subapical PM and the cytoplasm during rapid tube elongation (Fu et al, 2001; Lee and Yang, 2008;Cai and Cresti, 2009;Cheung et al, 2010;Qu et al, 2013;Rounds et al, 2014;Zhou et al, 2015;Qu et al, 2017). Recently, Qu et al, (2017) demonstrated that F-actin originating from the apical membrane forms a specialized structure consisting of longitudinally aligned actin bundles at the cortex and inner cytoplasmic filaments with a distinct distribution, which plays a pivotal role in the regulation of the growth of the pollen tube tip in Arabidopsis (Arabidopsis thaliana).…”