Circular F-actin bundles and a G-actin gradient in pollen and pollen tubes of Lilium davidii

Abstract

The distribution of and relationship between F-actin and G-actin were investigated in pollen grains and pollen tubes of Lilium davidii Duch. using a confocal laser scanning microscope after fluorescence and immunofluorescence labeling. Circular F-actin bundles were found to be the main form of microfilament cytoskeleton in pollen grains and pollen tubes. Consistent with cytoplasmic streaming in pollen tubes, there were no obvious F-actin bundles in the 10- to 20-microm tip region of long pollen tubes, only a few short F-actin fragments. Labeling with fluorescein isothiocyanate (FITC)-DNase I at first established the presence of a tip-focused gradient of intracellular G-actin concentration at the extreme apex of the tube, the concentration of G-actin being about twice as high in the 10- to 20-microm region of the tip as in other regions of the pollen tube. We also found that the distribution of G-actin was related negatively to that of the F-actin in pollen tubes of L. davidii. Caffeine treatment caused the G-actin tip-focused gradient to disappear, and F-actin to extend into the pollen tube tip. Based on these results, we speculate that the circular F-actin bundles may be the track for bidirectional cytoplasmic streaming in pollen tubes, and that in the pollen tube tip most of the F-actin is depolymerized into G-actin, leading to the absence of F-actin bundles in this region.

Roles of the Ubiquitin/Proteasome Pathway in Pollen Tube Growth with Emphasis on MG132-Induced Alterations in Ultrastructure, Cytoskeleton, and Cell Wall Components

“…To further confirm the MG132-induced disruption of the cytoskeleton, confocal microscopy was used to visualize the cytoplasmic streaming in pollen tubes, a phenomenon that depends on intact cytoskeleton elements, especially actin microfilaments (Taylor and Hepler, 1997;Li et al, 2001;Justus, et al, 2004). As shown in Figure 10A (see also Supplemental Video 1), cytoplasm in control tubes streams in a fountain pattern or, rarely, in a reverse fountain pattern, similar to a previous report in Picea abies pollen tubes (Justus, et al, 2004).…”

“…It has long been appreciated that AFs control cytoplasmic streaming and hence the transport of secretory vesicles (Taylor and Hepler, 1997;Li et al, 2001;Justus et al, 2004). More recent evidence indicates that, in addition to actomyosin-driven streaming, actin polymerization itself also contributes to pollen tube growth, either as a force generator (Baluška and Volkmann, 2002) or as an organizer of the apical cytoplasm ).…”

Effects of low-energy argon ion implantation on the dynamic organization of the actin cytoskeleton during maize pollen germination

“…When the pollen tube elongates, the actin filaments organize into longer bundles and gradually enter the emerging tube (Cai et al, 1997). When the pollen tube grows, the actin cytoskeleton form thick bundles parallel to the pollen tubes (Li et al, 2001). In our research, such dynamic organization of actin arrays occurred in both treated and control pollen grains, but the actin cytoskeleton in pollen implanted by 5.2 × 10 15 ions/cm 2 Ar + organized much earlier than that in the control pollen, and the reorganization of the actin cytoskeleton in pollen implanted with 9.1 × 10 15 ions/cm 2 Ar + was comparatively slower than that in the control pollen.…”

“…The first and most parsimonious explanation is that actin bundles control cytoplasmic streaming and hence the delivery of secretory vesicles essential for growth (Wang et al, 2006;Samaj et al, 2006). Li et al (1998) found that the microfilaments just extended to the region 10~20 μm from the pollen tube apex, while cytoplasmic streaming also changes in direction at this region. Additional support for a role of actin microfilaments in intracellular motility comes from studies using the fungal toxin cytochalasin B.…”

Arabidopsis Vacuolar H+-ATPase (V-ATPase) B Subunits Are Involved in Actin Cytoskeleton Remodeling via Binding to, Bundling, and Stabilizing F-actin

“…The actin cytoskeleton is a highly organized and dynamic structure present in all eukaryotic cells, where it plays a central role and is involved in numerous cellular processes, including intracellular transport, cell growth, cell division, cytoplasmic streaming, organelle positioning, and cell-to-cell communication (15)(16)(17)(18)(19). For such processes, a regulatory system is required that contains actin filaments and actin-binding proteins (ABPs) that directly interact with G-actin and/or F-actin to promote the nucleation, polymerization, depolymerization, stabilization, severing, capping, bundling, and cross-linking of actin filaments (20 -23).…”