After eukaryotic fertilization, gamete nuclei migrate to fuse parental genomes in order to initiate development of the next generation. In most animals, microtubules control female and male pronuclear migration in the zygote. Flowering plants, on the other hand, have evolved actin filament (F-actin)-based sperm nuclear migration systems for karyogamy. Flowering plants have also evolved a unique double-fertilization process: two female gametophytic cells, the egg and central cells, are each fertilized by a sperm cell. The molecular and cellular mechanisms of how flowering plants utilize and control F-actin for double-fertilization events are largely unknown. Using confocal microscopy live-cell imaging with a combination of pharmacological and genetic approaches, we identified factors involved in F-actin dynamics and sperm nuclear migration in Arabidopsis thaliana (Arabidopsis) and Nicotiana tabacum (tobacco). We demonstrate that the F-actin regulator, SCAR2, but not the ARP2/3 protein complex, controls the coordinated active F-actin movement. These results imply that an ARP2/3-independent WAVE/SCAR-signaling pathway regulates F-actin dynamics in female gametophytic cells for fertilization. We also identify that the class XI myosin XI-G controls active F-actin movement in the Arabidopsis central cell. XI-G is not a simple transporter, moving cargos along F-actin, but can generate forces that control the dynamic movement of F-actin for fertilization. Our results provide insights into the mechanisms that control gamete nuclear migration and reveal regulatory pathways for dynamic F-actin movement in flowering plants.

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Published in PNAS, v. 117, issue 51.

© 2020 Published under the PNAS license.

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The document available for download is the authors' post-peer-review final draft of the article.

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This work was supported by the National Science Foundation (IOS-1928836 to TK), the National Institute of Food and Agriculture, United States Department of Agriculture (Hatch Program 1014280 to TK), the National Natural Science Foundation of China (31570317 and 31270362 to XP and MS), and the Ministry of Education, Culture, Sports, Science and Technology of Japan Grants-in-Aid for Scientific Research on Innovative Areas (17H05846 and 19H04869 to DM). MFA, UF, and TK were supported by a start-up fund from the Department of Plant and Soil Sciences and the College of Agriculture, Food and Environment, University of Kentucky.

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This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2015550117/-/DCSupplemental.