title: A mutually inhibitory interaction between auxin and cytokinin specifies vascular pattern in roots.
abstract: BACKGROUND
  Whereas the majority of animals develop toward a predetermined body plan, plants show iterative growth and continually produce new organs and structures from actively dividing meristems. This raises an intriguing question: How are these newly developed organs patterned? In Arabidopsis embryos, radial symmetry is broken by the bisymmetric specification of the cotyledons in the apical domain. Subsequently, this bisymmetry is propagated to the root promeristem.
  
  
  RESULTS
  Here we present a mutually inhibitory feedback loop between auxin and cytokinin that sets distinct boundaries of hormonal output. Cytokinins promote the bisymmetric distribution of the PIN-FORMED (PIN) auxin efflux proteins, which channel auxin toward a central domain. High auxin promotes transcription of the cytokinin signaling inhibitor AHP6, which closes the interaction loop. This bisymmetric auxin response domain specifies the differentiation of protoxylem in a bisymmetric pattern. In embryonic roots, cytokinin is required to translate a bisymmetric auxin response in the cotyledons to a bisymmetric vascular pattern in the root promeristem.
  
  
  CONCLUSIONS
  Our results present an interactive feedback loop between hormonal signaling and transport by which small biases in hormonal input are propagated into distinct signaling domains to specify the vascular pattern in the root meristem. It is an intriguing possibility that such a mechanism could transform radial patterns and allow continuous vascular connections between other newly emerging organs.
authors.affiliation: Institute of Biotechnology and Department of Biosciences, University of Helsinki, FIN-00014 Helsinki, Finland.
authors.forename: Anthony
authors.initials: A
authors.lastname: Bishopp
authors.forename: Hanna
authors.initials: H
authors.lastname: Help
authors.forename: Sedeer
authors.initials: S
authors.lastname: El-Showk
authors.forename: Dolf
authors.initials: D
authors.lastname: Weijers
authors.forename: Ben
authors.initials: B
authors.lastname: Scheres
authors.forename: Jiří
authors.initials: J
authors.lastname: Friml
authors.forename: Eva
authors.initials: E
authors.lastname: Benková
authors.forename: Ari Pekka
authors.initials: AP
authors.lastname: Mähönen
authors.forename: Ykä
authors.initials: Y
authors.lastname: Helariutta
mesh_terms: D017360:Arabidopsis / Q000033:anatomy & histology / Q000254:growth & development* / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D019521:Body Patterning; D003583:Cytokinins / Q000378:metabolism*; D025461:Feedback, Physiological; D007210:Indoleacetic Acids / Q000378:metabolism*; D018519:Meristem / Q000502:physiology; D008954:Models, Biological; D010937:Plant Growth Regulators / Q000378:metabolism* / Q000502:physiology; D018517:Plant Roots / Q000033:anatomy & histology / Q000254:growth & development / Q000378:metabolism; D052584:Xylem / Q000254:growth & development / Q000378:metabolism
chemical_list: D029681:Arabidopsis Proteins; D003583:Cytokinins; D007210:Indoleacetic Acids; D010937:Plant Growth Regulators