Cytoplasmic auxin signaling 
Zhenbiao Yang 

Department of Botany and Plant Sciences and Center for Plant Cell Biology, University of California at Riverside, CA 92521, USA;
Shanghai Institute of Plant Physiology and Ecology, Shanghai Institutes of Biological Sciences, Chinese Academy of Sciences, Shanghai, China.
Corresponding Author：yang@ucr.edu

Abstract 
Auxin regulates the formation of various developmental and morphogenetic patterns in plants, but the molecular and cellular mechanisms for the auxin actions are not well understood. We investigate these mechanisms using Arabidopsis leaf epidermal pavement cells (PCs) as a model system, which form the jigsaw puzzle cell pattern with interdigitated lobes and indentation.  We have shown that pavement cell interdigitation is controlled by localized ROP GTPase signaling, which impinges on the organization of cortical actin microfilaments and microtubules to generate interdigitating lobes and indentations^1,2.  Our recent studies indicate that auxin is a signal that activates the generation of multi-polarity to form lobes in pavement cells and coordinates lobe formation with indentation formation by activating ROP GTPase signaling through an Auxin-Binding Protein 1 (ABP1) dependent auxin perception system^2,3. This cytoplasmic auxin signaling system, which is distinct from the well-established TIR1-dependent nuclear auxin signaling system that regulates gene transcription, modulates local coordination of lobe and indentation formation within a PC and between adjacent PCs through its modulation of PIN protein polarization³.  The ROP-based cytoplasmic auxin signaling also regulates the subcellular distribution of PINs in root cells, supporting a widespread role for this new auxin signaling system in plants ⁴. Recent findings of additional components in this new auxin signaling system will be discussed.

Key Words: AUXIN SIGNALING, ABP1, PIN1, PIN2