Polarized cell growth is crucial for development and morphogenesis of eukaryotic organisms. Pollen tubes provide a passage for the delivery of two nonmotile sperm cells to the ovule in order to facilitate double fertilization in higher plants, and this system is ideal for study of the molecular mechanisms underlying polarized cell growth. Previous studies suggest that the actin cytoskeleton, along with small GTPase molecular switches, and a tip-focused calcium gradient, constitutes an intricate system known as the LENS (for localization-enhancing network, self-sustaining) signaling network to define the tip growing domain in the pollen tube. This network may represent a unifying mechanism that facilitates polarized growth in tip-growing cells. Understanding the state of actin filaments and the regulation of actin dynamics in the apical cytoplasm will provide insight into the integrated role of the actin cytoskeleton in the LENS signaling network.

 

In the newly published paper, Huang and coauthors performed the high spatiotemporal live-cell imaging technology to visualize the dynamics of actin filaments within the apical cytoplasm of pollen tubes, and provide the first detailed analysis of parameters associated with actin filament dynamics in the apical cytoplasm of pollen tubes, and identified villin as the major player that drives rapid turnover of actin filaments at pollen tube tips (Qu et al., 2013, Plant Cell). Thus, this study provides insight into the molecular mechanism underpins the control of tip growth.

 

Figure 1. Villins Promote Actin Turnover at Pollen Tube Tips
(A) Actin filaments accumulate at pollen tube apex and apical flank. (a) Dynamics of apical actin filaments during pollen tube growth. (b) Kymographs were plotted based on lines parallel to the growth axes of pollen tubes, demonstrating that actin filaments accumulate in vln2 vln5 pollen tube tips.
(B) Simplified Model Describing VILLIN-Mediated Regulation of Actin Dynamics at Pollen Tube Tips.