Author ORCID Identifier

Year of Publication


Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation


Agriculture, Food and Environment


Plant and Soil Sciences

First Advisor

Dr. Tomokazu Kawashima


Fertilization is the process of fusion of haploid male and female gametes to develop a new individual. In most animals, microtubules drive the migration of the female pronucleus toward the male pronucleus for fertilization. By contrast, the fertilization process in flowering plants is dependent on actin filament (F-actin) dynamics; F-actin, not microtubules, is responsible for sperm nuclear migration. In one of the female gamete cells, the central cell, constant F-actin meshwork inward movement towards the nucleus takes place to transfer the sperm nucleus for karyogamy. This inward F-actin dynamic is controlled by a unique ARP2/3 (Actin Related Protein 2/3)-independent WAVE/SCAR [Wiskott–Aldrich syndrome protein (WASP) family verprolin-homologous/suppressor of cAMP receptor] signaling in Arabidopsis thaliana. In addition, plant-specific class XI myosins are involved in controlling the F-actin dynamics for fertilization, and the involvement of XIG in the active movement of F-actin is essential for sperm nuclear migration. The primary function of plant myosins is cargo transportation along F-actin, and we discover a non-canonical function of the myosin XIG that can generate forces for the dynamic movement of F-actin for fertilization. However, how XIG plays its role in the unique F-actin dynamics in the female gametophyte remains unknown. In this study, we demonstrate that a member of the vacuole localized membrane protein, connects myosin XIG with the vacuole membrane and provides a base to hook globular tail domain (GTD) of myosin XIG so that XIG can slide F-actin and generate dynamics with its motor domain. Genetic tests on an Arabidopsis mutant revealed that XIG regulates the meshwork movement of F-actin in a female gamete's central cell but not to the degree of myosin inhibitory chemical. Additionally, fertilization of the xig mutant Arabidopsis ovule was successful, indicating that other myosins may function redundantly with XIG to maintain F-actin meshwork movement in the Arabidopsis central cell. In this study, we characterized the meshwork movement of central cell F-actin in mutants of XID, the second most abundant myosin in the central cell, and XIH, the paralogous pair of XIG. Our study indicates that neither XIG nor XIH has additive role on central cell F-actin movement. Instead we found a complex relationship between XIG and XID in regulating central cell F-actin meshwork movement, in which XIG acts as a F-actin movement accelerator and, in contrast, XID as an inhibitor. On the other hand, myosin XIH works additively with XIG to maintain synergid cell morphology and female gamete nuclei position. Collectively, our findings point to the functional specialization of class XI myosins, as well as new form of interaction between myosin members. In a broader aspect, knowledge from these projects will shed light on our profound understanding of fertilization and cytoskeleton usage in flowering plants. In addition, understanding the precise mechanism of the early events of fertilization dynamics will also help us tackle the reduced crop fertility crisis that is predicted for the future due to climate instability.

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