Signaling in Polar Auxin Transport Anindya Ganguly and Hyung-Taeg Cho Abstract Polar auxin transport, directionally delivering auxin and generating local auxin gradients, is pivotal to fundamental developmental processes in plants. The directional auxin flow is attributable to polar-localized auxin transporters. In partic- ular, over the past several years, the cytological and molecular mechanisms of how auxin transporters distribute asymmetrically in the plasma membrane have become greatly understood. The cellular polarity players for auxin transporters include intracellular trafficking components, cargo phosphorylation cues and corresponding kinases, membrane lipid composition, ubiquitylation, and extracellular auxin receptor-mediated modulation of cytoskeleton. In addition to these internal polarity regulators, other major plant hormones, signaling molecules, environmental stimuli, and nutrients have been implicated in regulation of auxin transporters, reflecting the communication between diverse developmental/environmental signals and auxin- mediated plant development. In this chapter, we review the recent studies that have elucidated the regulatory mechanisms of auxin transport. 1 Introduction He got up early as 6:40 A.M. in the summer morning, walked to a quite sizable dark box on the table, and carefully opened the top. He slowly looked over the stems and leaves of a verbena plant inside the dark box and then cautiously placed the fifth dot on the glass plate precisely above the tiny bead affixed through a glass filament to the verbena stem apex. Over the night, the bead had traveled from the east to the southwest (Fig. 1). A. Ganguly • H.-T. Cho (*) Department of Biological Sciences and Plant Genomics and Breeding Institute, Seoul National University, Seoul 151-742, South Korea e-mail: htcho@snu.ac.kr R. Chen and F. Balus ˇka (eds.), Polar Auxin Transport, Signaling and Communication in Plants 17, DOI 10.1007/978-3-642-35299-7_1, # Springer-Verlag Berlin Heidelberg 2013 1