Chapter 7 Cooperation Between Auxin and Actin During the Process of Plant Polar Growth Jie Liu and Markus Geisler Abstract Polar growth is provided by rapid cell expansion that spatially focuses at the tip. The regulation and maintenance of polar growth requires two important intracellular events: intensive exocytosis in the tip region and a highly dynamic cytoskeleton system. The selective transport of secretory vesicles and their accumu- lation in the apical region, which is driven by motor proteins that move along actin cables, is critical for plant polar growth. The regulation of vesicle trafficking and actin cytoskeleton turnover is affected by several intracellular components and signaling pathways. Auxin as one of the most important intracellular elements regulating plant growth and development involved in numerous signaling pathways that can affect the organization and dynamics of cell cytoskeleton is a potential candidate as a polar growth regulator. The current knowledge is summarized here in order to highlight the role of auxin in plant polar growth regulation and the cooperation between auxin and the actin cytoskeleton during this process. 7.1 Introduction Polar growth, also known as tip growth, is a spatially focused cell expansion that exists in a few types of cells, including hyphae in fungi, pollen tubes and root hairs in plants, and neurites in animals (Pierson et al. 1996; Galway et al. 1997; Gomez and Spitzer 1999; Geitmann and Emons 2000). Root hairs develop at hair-forming cells of the plant root epidermis in order to increase the surface area for absorption of water and nutrients, to support the symbiotic interaction with soil microorganisms. They are single cells and are rarely branched (Gilroy and Jones 2000). Pollen tubes develop from the germinating pollen grains and deliver sperm cells from pollen grains to the ovule through the female tissue of the pistil for double fertilization (Taylor and Hepler 1997; Lord 2000). Pollen tubes and root hairs are different in origin, function, and growth rate but share a common polarity growth pattern (Hepler et al. 2001). In the two cell types, all growth is focused on the specific tip region. J. Liu · M. Geisler (*) Department of Biology, University of Fribourg, Fribourg, Switzerland e-mail: markus.geisler@unifr.ch © Springer Nature Switzerland AG 2019 V. P. Sahi, F. Baluška (eds.), The Cytoskeleton, Plant Cell Monographs 24, https://doi.org/10.1007/978-3-030-33528-1_7 101