Actin polymerization promotes the reversal of streaming in the apex of pollen tubes

Abstract

Actin polymerization is important in the control of pollen tube growth. Thus, treatment of pollen tubes with low concentrations of latrunculin B (Lat-B), which inhibits actin polymerization, permits streaming but reversibly blocks oscillatory growth. In the current study, we employ Jasplakinolide (Jas), a sponge cyclodepsipeptide that stabilizes actin microfilaments and promotes polymerization. Uniquely, Jas (2 microM) blocks streaming in the shank of the tube, but induces the formation of a toroidal-shaped domain in the swollen apex, of which longitudinal optical sections exhibit circles of motion. The polarity of this rotary motion is identical to that of reverse fountain motility in control pollen tubes, with the forward direction occurring at the edge of the cell and the rearward direction in the cell interior. Support for the idea that actin polymerization in the apical domain contributes to the formation of this rotary motility activity derives from the appearance therein of aggregates and flared cables of F-actin, using immunofluorescence, and by the reduction in G-actin as indicated with fluorescent DNAse. In addition, Jas reduces the tip-focused Ca2+ gradient. However, the alkaline band appears in the swollen apex and is spatially localized with the reverse fountain streaming activity. Taken together, our results support the idea that actin polymerization promotes reversal of streaming in the apex of the lily pollen tube.

Pollen Tube Growth Oscillations and Intracellular Calcium Levels Are Reversibly Modulated by Actin Polymerization

“…6A), as shown previously (Cárdenas et al, 2005). When DNase-injected pollen tubes are treated with Lat-B (2 nM) the G-actin increases (Fig.…”

“…There is a rich pool of G-actin in the apex of pollen tubes ( Fig. 6; Cárdenas et al, 2005) and root hairs (He et al, 2006) where active Figure 6. Effect of Lat-B on G-actin.…”

“…To test this prediction we have injected cells with fluorescently tagged DNase, which binds G-actin, and serves as a reporter for this form of actin. Because the binding of DNase to G-actin can itself retard actin polymerization and thereby affect pollen tube growth , we have been careful to apply this reagent at levels that are sufficient to provide a good signal but too low to affect growth (Cárdenas et al, 2005; also see ''Materials and Methods''). To gain semiquantitative data we have created a ratio-metric dye by mixing DNase Oregon green with tetramethyl-rhodaminedextran.…”

“…It is well appreciated that acto-myosin drives cytoplasmic streaming, including the transport of the secretory vesicles that are essential for the apical extension of this rapidly growing cell. More recently we have come to realize that actin polymerization also participates in the pollen tube growth process (Gibbon et al, 1999;Vidali et al, 2001;Cárdenas et al, 2005;Chen et al, 2007). Either preventing (Gibbon et al, 1999;Vidali et al, 2001;Chen et al, 2007) or enhancing (Cárdenas et al, 2005) polymerization profoundly affects pollen tube growth.…”

Interdependence of Endomembrane Trafficking and Actin Dynamics during Polarized Growth of Arabidopsis Pollen Tubes

“…Genetically manipulating the activities of the small GTPases ROP (Kost et al, 1999;Fu et al, 2001;Cheung et al, 2008) and Rab (de Graaf et al, 2005), or of actinbinding proteins such as profilin and formin (Staiger et al, 1994;Chen et al, 2002;Cheung and Wu, 2004), disrupted F-actin dynamics and inhibited tube growth and caused apical bulges. Application of drugs such as latrunculin B (LatB) and jasplakinolide (Jas) showed similar effects (Gibbon et al, 1999;Vidali et al, 2001;Cardenas et al, 2005;Hörmanseder et al, 2005;Chen et al, 2007).…”