Journal Article

Dynamic F-actin movement is essential for fertilization in Arabidopsis thaliana

Hamamura, Yuki, Berger, Frédéric, Maruyama, Daisuke, Yelagandula, Ramesh, Kawashima, Tomokazu, Shagirov, Murat, Li, Jing, Toyama, Yusuke

2014

Overview

In animals, microtubules and centrosomes direct the migration of gamete pronuclei for fertilization. By contrast, flowering plants have lost essential components of the centrosome, raising the question of how flowering plants control gamete nuclei migration during fertilization. Here, we use Arabidopsis thaliana to document a novel mechanism that regulates F-actin dynamics in the female gametes and is essential for fertilization. Live imaging shows that F-actin structures assist the male nucleus during its migration towards the female nucleus. We identify a female gamete-specific Rho-GTPase that regulates F-actin dynamics and further show that actin–myosin interactions are also involved in male gamete nucleus migration. Genetic analyses and imaging indicate that microtubules are dispensable for migration and fusion of male and female gamete nuclei. The innovation of a novel actin-based mechanism of fertilization during plant evolution might account for the complete loss of the centrosome in flowering plants. Sexual reproduction involves combining the genetic material from two parents to create an offspring. The genetic material in the male sperm cell and the female egg cell is contained in the nucleus of each cell. Once these two cells fuse at fertilization, their nuclei must then navigate towards each other and fuse. When an animal egg cell is fertilized, cable-like protein filaments called microtubules guide the two nuclei into contact. These microtubules are organized by a cellular structure called a centrosome. However, flowering plants do not have centrosomes; as such, it was unclear how genetic material from the sperm and egg cells is brought together after fertilization in flowering plants. To investigate this, Kawashima et al. turned to a flowering plant commonly used in research, called Arabidopsis thaliana, and found that microtubules are not needed to guide the nuclei of the sperm and the egg cell after fertilization. Instead, another cable-forming protein—called F-actin—fulfills a similar role in Arabidopsis cells. F-actin filaments often connect together to form a network; and when Kawashima et al. disrupted the F-actin in Arabidopsis egg cells, the nucleus of the sperm cell failed to fuse with that of the female. Pollen from Arabidopsis plants actually contains two sperm cells. One sperm cell fertilizes the egg cell; the other fertilizes the so-called ‘central cell’, which develops into a tissue that nourishes the plant embryo. Kawashima et al. found that the fertilization of both of these cells requires an intact F-actin network. By looking more closely at F-actin networks in the larger central cell, Kawashima et al. discovered that the sperm nucleus becomes surrounded by a star-shaped structure of F-actin cables and that this F-actin structure migrates together with the sperm nucleus. The F-actin network constantly moves inward, from the edges of the cell towards the nucleus, prior to fertilization. This movement is essential for guiding the sperm nucleus towards the central cell nucleus. Kawashima et al. also found that this continual movement of the F-actin network depends on a small signaling protein found in the central cell, called ROP8. It also involves a motor protein that normally transports “cargo”, such as proteins and other molecules, inside cells by walking along the F-actin networks. However, rather than transporting the sperm nucleus as cargo, Kawashima et al. believe that the motor protein instead helps to maintain the inward movement of the F-actin network. One of the next challenges will be to investigate the molecular mechanism that underlies this motor protein's involvement in this dynamic F-actin network.

• Publisher: eLife Sciences Publications Ltd,eLife Sciences Publications, Ltd
• Subject: Actin / Actins - metabolism / Arabidopsis / Arabidopsis - cytology / Arabidopsis - metabolism / Arabidopsis Proteins - metabolism / Cell Nucleus - metabolism / Centrosomes / Cytoskeleton / Defects / F-actin / Fertilization / Flowering / gamete nuclear migration / Gametes / Gene expression / Guanine nucleotide-binding protein / Guanosine triphosphatases / Microtubules / Microtubules - metabolism / Migration / Myosin / Myosins - metabolism / Nuclei / Ovule - metabolism / Plant Biology / Pollen - metabolism / Probability distribution / Protein Transport / reproduction / rho GTP-Binding Proteins - metabolism / Rho-GTPase / Seeds / Sperm / Time-Lapse Imaging / Velocity