Microtubules and microfilaments coordinate to direct a fountain streaming pattern in elongating conifer pollen tube tips

Abstract

This study investigates how microtubules and microfilaments control organelle motility within the tips of conifer pollen tubes. Organelles in the 30-microm-long clear zone at the tip of Picea abies (L.) Karst. (Pinaceae) pollen tubes move in a fountain pattern. Within the center of the tube, organelles move into the tip along clearly defined paths, move randomly at the apex, and then move away from the tip beneath the plasma membrane. This pattern coincides with microtubule and microfilament organization and is the opposite of the reverse fountain seen in angiosperm pollen tubes. Application of latrunculin B, which disrupts microfilaments, completely stops growth and reduces organelle motility to Brownian motion. The clear zone at the tip remains intact but fills with thin tubules of endoplasmic reticulum. Applications of amiprophosmethyl, propyzamide or oryzalin, which all disrupt microtubules, stop growth, alter organelle motility within the tip, and alter the organization of actin microfilaments. Amiprophosmethyl inhibits organelle streaming and collapses the clear zone of vesicles at the extreme tip together with the disruption of microfilaments leading into the tip, leaving the plasma membrane intact. Propyzamide and oryzalin cause the accumulation of membrane tubules or vacuoles in the tip that reverse direction and stream in a reverse fountain. The microtubule disruption caused by propyzamide and oryzalin also reorganizes microfilaments from a fibrillar network into pronounced bundles in the tip cytoplasm. We conclude that microtubules control the positioning of organelles into and within the tip and influence the direction of streaming by mediating microfilament organization.

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

“…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). In contrast, the direction and speed of cytoplasmic streaming in MG132-treated tubes was markedly affected in a time-dependent manner.…”

“…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 ).…”

Cadmium toxicity

“…22,23 In plant cells, however, vesicles are transported along actin fibers by myosin motor molecules. [24][25][26][27] Fan et al labeled the endocytic vesicles using the fluorescent membrane dye FM4-64 to observe whether cadmium treatment influenced endocytosis and vesicular transport in root hair cells. 15 Their time-lapse images of cadmium-treated root hairs showed that the FM dye staining pattern was obviously distinct from that of control cells, suggesting that membrane endocytosis and vesicular trafficking were perturbed during cadmium stress and this perturbation might result from the cadmium-induceddisruption of the "tracks" (actin filaments) for vesicle transport (Fig.…”

Kinesins Have a Dual Function in Organizing Microtubules during Both Tip Growth and Cytokinesis in Physcomitrella patens

“…In many angiosperm pollen tubes, disruption of the MTs has no apparent effect on growth (Cai and Cresti, 2010), but it has recently been reported to affect both endocytosis and exocytosis in the tip of tobacco (Nicotiana tabacum) pollen tubes (Idilli et al, 2013). The growth of gymnosperm pollen tubes is inhibited by MT-depolymerizing drugs (Anderhag et al, 2000;Justus et al, 2004). In growing root hairs, the loss of MTs leads to wavy growth and bifurcated tips (Bibikova et al, 1999).…”