Low concentration of LatB dramatically changes the microtubule organization and the timing of vegetative nucleus/generative cell entrance in tobacco pollen tubes

Latrunculin B facilitates gravitropic curvature of Arabidopsis root by inhibiting cell elongation, especially the cells in the lower flanks of the transition and elongation zones

Gravitropism plays a critical role in the growth and development of plants. Previous reports proposed that the disruption of the actin cytoskeleton resulted in enhanced gravitropism; however, the mechanism underlying these phenomena is still unclear. In the present study, real-time observation on the effect of Latrunculin B (Lat B), a depolymerizing agent of microfilament cytoskeleton, on gravitropism of the primary root of Arabidopsis was undertaken using a vertical stage microscope. The results indicated that Lat B treatment prevented the growth of root, and the growth rates of upper and lower flanks of the horizontally placed root were asymmetrically inhibited. The growth of the lower flank was influenced by Lat B more seriously, resulting in an increased differential growth rate between the upper and lower flanks of the root. Further analysis indicated that Lat B affected cell growth mainly in the transition and elongation zones. Briefly, the current data revealed that Lat B treatment inhibited cell elongation, especially the cells in the lower flanks of the transition and elongation zones, which finally manifested as the facilitation of gravitropic curvature of the primary root.

Microtubules play a role in trafficking prevacuolar compartments to vacuoles in tobacco pollen tubes

Fine regulation of exocytosis and endocytosis plays a basic role in pollen tube growth. Excess plasma membrane secreted during pollen tube elongation is known to be retrieved by endocytosis and partially reused in secretory pathways through the Golgi apparatus. Dissection of endocytosis has enabled distinct degradation pathways to be identified in tobacco pollen tubes and has shown that microtubules influence the transport of plasma membrane internalized in the tip region to vacuoles. Here, we used different drugs affecting the polymerization state of microtubules together with SYP21, a marker of prevacuolar compartments, to characterize trafficking of prevacuolar compartments in Nicotiana tabacum pollen tubes. Ultrastructural and biochemical analysis showed that microtubules bind SYP21-positive microsomes. Transient transformation of pollen tubes with LAT52-YFP-SYP21 revealed that microtubules play a key role in the delivery of prevacuolar compartments to tubular vacuoles.

The cytoskeleton in the pollen tube

The cytoskeleton in pollen tubes has been intensively studied, because of its abundance and prominent roles and because the pollen tube is an excellent experimental system for cell biological studies. Pollen actin microfilaments (MFs) exist as multiple distinct populations, each participating in a specific cellular trafficking or organization process. Consequently, MFs are essential for pollen tube growth and are tightly regulated in response to various signals. Pollen microtubules (MTs) are non-essential and less characterized, but recent studies have implicated MTs in vesicle trafficking and cell wall construction in pollen tubes. This review summarizes recent advances in understanding the organization and regulation of both MFs and MTs and discusses their roles in cellular trafficking and the modulation of pollen-tube tip growth.

Emerging roles for microtubules in angiosperm pollen tube growth highlight new research cues

In plants, actin filaments have an important role in organelle movement and cytoplasmic streaming. Otherwise microtubules (MTs) have a role in restricting organelles to specific areas of the cell and in maintaining organelle morphology. In somatic plant cells, MTs also participate in cell division and morphogenesis, allowing cells to take their definitive shape in order to perform specific functions. In the latter case, MTs influence assembly of the cell wall, controlling the delivery of enzymes involved in cellulose synthesis and of wall modulation material to the proper sites. In angiosperm pollen tubes, organelle movement is generally attributed to the acto-myosin system, the main role of which is in distributing organelles in the cytoplasm and in carrying secretory vesicles to the apex for polarized growth. Recent data on membrane trafficking suggests a role of MTs in fine delivery and repositioning of vesicles to sustain pollen tube growth. This review examines the role of MTs in secretion and endocytosis, highlighting new research cues regarding cell wall construction and pollen tube-pistil crosstalk, that help unravel the role of MTs in polarized growth.

Endocytic Pathways and Recycling in Growing Pollen Tubes

Pollen tube growth is based on transport of secretory vesicles into the apical region where they fuse with a small area of the plasma membrane. The amount of secretion greatly exceeds the quantity of membrane required for growth. Mechanisms of membrane retrieval have recently been demonstrated and partially characterized using FM (Fei Mao) dyes or charged nanogold. Both these probes reveal that clathrin-dependent and -independent endocytosis occur in pollen tubes and are involved in distinct degradation pathways and membrane recycling. Exocytosis, internalization and sorting of PM proteins/lipids depend on the integrity of the actin cytoskeleton and are involved in actin filament organization. However, some kinds of endocytic and exocytic processes occurring in the central area of the tip still need to be characterized. Analysis of secretion dynamics and data derived from endocytosis highlight the complexity of events occurring in the tip region and suggest a new model of pollen tube growth.