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Plant Physiology Preview Published on November 9, 2007; 10.1104/pp.107.107870
Received August 23, 2007 Diarch symmetry of the vascular bundle in Arabidopsis root encompasses the pericycle and is reflected in distich lateral root initiation
Laboratoire de Biologie du Developpement des Plantes, SBVME, IBEB, DSV, CEA, CNRS, Univ Aix Marseille, Saint Paul lez Durance, F-13108, France; Department of Plant Systems Biology, Root Development group, V.I.B., Technologiepark 927, B-9052 Gent, Belgium and Department of Molecular Genetics, Ghent University, B-9052 Gent, Belgium; Equipe rhizogenese, UMR DIA-PC, Institut de Recherche pour le Developpement (IRD), 911 Av. Agropolis, 34394 Montpellier Cedex 5, France; Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EA, UK; Universite Pierre et Marie Curie, site Ivry-le Raphael, Laboratoire CEMV-EA3494, IFR 83, case 150, 4 place Jussieu, F-75252 Paris, Cedex 05, France; Institute of Biotechnology, University of Helsinki, Helsinki, Finland; Carolyn Lynch Laboratory, Department of Biology, University of Pennsylvania, Philadelphia, PA 19104, USA * Corresponding author; email: http://www-dsv.cea.fr/instituts/.
The outer tissues of dicotyledonous plant roots, i.e. epidermis, cortex and endodermis, are clearly organized in distinct concentric layers in contrast to the diarch to polyarch vascular tissues of the central stele. Up to now the outermost layer of the stele, the pericycle has always been regarded, in accordance with the outer tissue layers, as one uniform concentric layer. However, considering its lateral root forming competence, the pericycle is composed of two different cell types with one subset of cells being associated with the xylem, showing a strong competence to initiate cell division while another group of cells, associated with the phloem, appears to remain quiescent. Here, we established, using detailed microscopy and specific Arabidopsis thaliana reporter lines, the existence of two distinct pericycle cell types. The analysis of two enhancer trap reporter lines further suggests that the specification between these two subsets takes place early during development, in relation with the determination of the vascular tissues. A genetic screen resulted in the isolation of mutants perturbed in pericycle differentiation. Detailed phenotypical analyses of two of these mutants combined with observations made in known vascular mutants revealed an intimate correlation between vascular organization, pericycle fate and lateral root initiation potency and illustrated the independence of pericycle differentiation and lateral root initiation from protoxylem differentiation. Taken together our data show that the pericycle is a heterogeneous cell layer with two groups of cells set up in the root meristem by the same genetic pathway controlling the diarch organization of the vasculature.
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