Tansley insight Autocrine regulation of root hair size by the RALF-FERONIA-RSL4 signaling pathway Authors for correspondence: Jos e M. Estevez Tel: +54 1152387500 ext. 3206 Email: jestevez@leloir.org.ar Feng Yu Tel: +1 305 516 4913 Email: feng_yu@hnu.edu.cn Received: 7 January 2020 Accepted: 7 February 2020 Sirui Zhu 1 *, Javier Mart  ınez Pacheco 2 *, Jos  e M. Estevez 2,3,4 and Feng Yu 1,5 1 College of Biology, State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan Key Laboratory of Plant Functional Genomics and Developmental Regulation, Hunan University, Changsha, China; 2 Fundaci on Instituto Leloir and Instituto de Investigaciones Bioqu  ımicas de Buenos Aires (IIBBA-CONICET), Av. Patricias Argentinas 435, Buenos Aires CP C1405BWE, Argentina; 3 Centro de Biotecnolog  ıa Vegetal (CBV), Facultad de Ciencias de la Vida, Universidad Andr  es Bello Santiago, Santiago 8370186, Chile; 4 Millennium Institute for Integrative Biology (iBio), Santiago 8331150, Chile; 5 State Key Laboratory of Hybrid Rice, Hunan Hybrid Rice Research Center, Changsha 410125, China Contents Summary 45 I. Introduction 45 II. The RALF1-FERONIA-eIF4E1 module promotes RSL4 protein synthesis 46 III. Transcriptional regulatory networks driving RSL4 expression 46 IV. FER as a central hub orchestrating complex intracellular and extracellular signals 47 V. Concluding remarks 48 Acknowledgements 48 Author contributions 48 New Phytologist (2020) 227: 45–49 doi: 10.1111/nph.16497 Key words: Arabidopsis, eIF4E1, FERONIA, RALF1, root hair size, RSL4. Summary Root hair (RH) size has vital physiological implications, since it influences the surface area of the root and thus the ability of the plant to absorb water and nutrients from the soil. Arabidopsis ROOT HAIR DEFECTIVE 6-LIKE 4 (RSL4), a bHLH transcription factor, controls the expression of hundreds of RH genes, and RSL4 expression itself can trigger ectopic RH growth. Recent studies reveal an autocrine mechanism governing plant RH cell growth in which the extracellular peptide RAPID ALKALINIZATION FACTOR 1 (RALF1) and receptor FERONIA (FER) act as a central hub between the cell surface and downstream signaling events. RALF1-FER promotes the phosphorylation of eIF4E1. Then, phosphorylated eIF4E1 further regulates the synthesis of RH proteins, including RSL4, to promote RH growth. High levels of RSL4 exert a negative feedback on RALF1 expression via directly binding to the RALF1 gene promoter, slowing RH growth and determining final RH cell size. I. Introduction Cell size is determined by a well-defined developmental process that integrates cell-intrinsic factors and external environmental cues. Autocrine signaling, in which signals are perceived by the same cells from which they arise, contributes to plant cell growth. Unicellular root hairs (RHs), which extend to several hundred times their original cell bulge size within hours of emerging, provide a unique model system in which to study cell size regulation. The hairs on the surface of vascular plant roots, which evolved from much simpler structures, are exposed to heterogeneous soil microenvironments, including highly variable nutrient and water conditions and beneficial and pathogenic microorganism interactions. Root hair size has vital physiological implications, since it influences the surface area of the root and thus the plant’s ability to absorb water and nutrients from the soil. The study of the Arabidopsis ROOT HAIR DEFECTIVE 6-LIKE 4 (RSL4), a *These authors contributed equally to this work. Ó 2020 The Authors New Phytologist Ó 2020 New Phytologist Trust New Phytologist (2020) 227: 45–49 45 www.newphytologist.com Review