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Cytokinins Act Directly on Lateral Root Founder Cells to Inhibit Root Initiation^[W]

^a Institut de Recherche pour le Développement, Unité Mixte de Recherche Diversité et Adaptation des Plantes Cultivées (Agro.M/Institut National de la Recherche Agronomique/Institut de Recherche pour le Développement/Université Montpellier 2), Equipe Rhizogenèse, 34394 Montpellier cedex 5, France
^b Department of Plant Systems Biology, Root Development Group, VIaams Instituut voor Biotechnologie, B-9052 Gent, Belgium
^c Department of Molecular Genetics, Ghent University, B-9052 Gent, Belgium
^d Departmento de Ciencias Morfologicas y Biologia Celular y Animal, University of Extramadura, E-06071 Badajoz, Spain
^e Centre for Plant Integrative Biology, School of Biosciences, University of Nottingham, Loughborough, LE12 5RD, United Kingdom
^f Institute of Biology, Leiden University, Wassenaarseweg 64 2333 AL Leiden, The Netherlands
^g Laboratoire de Biologie du Développement des Plantes (Centre National de la Recherche Scientifique/Commissariat à l'Energie Atomique/Aix-Marseille II), Commissariat à l'Energie Atomique Cadarache, 13108 St. Paul les Durance, France
^h Nottingham Arabidopsis Stock Centre, School of Biosciences, University of Nottingham, Loughborough, LE12 5RD, United Kingdom

¹ Address correspondence to laplaze{at}mpl.ird.fr.

In Arabidopsis thaliana, lateral roots are formed from root pericycle cells adjacent to the xylem poles. Lateral root development is regulated antagonistically by the plant hormones auxin and cytokinin. While a great deal is known about how auxin promotes lateral root development, the mechanism of cytokinin repression is still unclear. Elevating cytokinin levels was observed to disrupt lateral root initiation and the regular pattern of divisions that characterizes lateral root development in Arabidopsis. To identify the stage of lateral root development that is sensitive to cytokinins, we targeted the expression of the Agrobacterium tumefaciens cytokinin biosynthesis enzyme isopentenyltransferase to either xylem-pole pericycle cells or young lateral root primordia using GAL4-GFP enhancer trap lines. Transactivation experiments revealed that xylem-pole pericycle cells are sensitive to cytokinins, whereas young lateral root primordia are not. This effect is physiologically significant because transactivation of the Arabidopsis cytokinin degrading enzyme cytokinin oxidase 1 in lateral root founder cells results in increased lateral root formation. We observed that cytokinins perturb the expression of PIN genes in lateral root founder cells and prevent the formation of an auxin gradient that is required to pattern lateral root primordia.

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