Chapman, KellyIvanovici, ArielTaleski, MichaelSturrock, Craig J.Ng, JasonMohd-Radzman, Nadiatul A.Frugier, FlorianBennett, Malcolm J.Mathesius, UlrikeDjordjevic, Michael2021-02-080028-646Xhttp://hdl.handle.net/1885/222394Root system architecture (RSA) influences the effectiveness of resources acquisition from soils but the genetic networks that control RSA remain largely unclear. We used rhizoboxes, X‐ray computed tomography, grafting, auxin transport measurements and hormone quantification to demonstrate that Arabidopsis and Medicago CEP (C‐TERMINALLY ENCODED PEPTIDE)‐CEP RECEPTOR signalling controls RSA, the gravitropic set‐point angle (GSA) of lateral roots (LRs), auxin levels and auxin transport. We showed that soil‐grown Arabidopsis and Medicago CEP receptor mutants have a narrower RSA, which results from a steeper LR GSA. Grafting showed that CEPR1 in the shoot controls GSA. CEP receptor mutants exhibited an increase in rootward auxin transport and elevated shoot auxin levels. Consistently, the application of auxin to wild‐type shoots induced a steeper GSA and auxin transport inhibitors counteracted the CEP receptor mutant's steep GSA phenotype. Concordantly, CEP peptides increased GSA and inhibited rootward auxin transport in wild‐type but not in CEP receptor mutants. The results indicated that CEP-CEP receptor‐dependent signalling outputs in Arabidopsis and Medicago control overall RSA, LR GSA, shoot auxin levels and rootward auxin transport. We propose that manipulating CEP signalling strength or CEP receptor downstream targets may provide means to alter RSAThis work was supported by an Australian Research Council grant to MAD (DP150104250); AI was supported by an A.W. Howard fellowship and Nicolas Baudin travel award, and FF was supported by Centre National de la Recherche Scientifique, the Agence Nationale de la Recherche Labex ‘Saclay Plant Science’ and Lidex ‘Plant Phenotyping Pipeline’; MT, AI and KC were supported by an Australian Postgraduate awards. Financial support by the Access to Research Infrastructures activity in the Horizon 2020 Programme of the EU (EPPN2020 Grant Agreement 731013) is gratefully acknowledged. NAMR’s current address is the Sainsbury Laboratory, University of Cambridge, Cambridge, UK.application/pdfen-AU© 2020 The Authors New Phytologist © 2020 New Phytologist TrustCEPCEPR1CRA2gravitropic set-point anglelateral rootpeptide hormoneroot system architecturerootward auxin transport2020-02-1110.1111/nph.164832020-11-08