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DEVELOPMENT AND HORMONE ACTION

Diarch Symmetry of the Vascular Bundle in Arabidopsis Root Encompasses the Pericycle and Is Reflected in Distich Lateral Root Initiation^1,[W]

Laboratoire de Biologie du Développement des Plantes, Department of Plant Biology and Environmental Microbiology, The Institute of Environmental Biology and Biotechnology, Direction des Sciences du Vivant, Commissariat à l'Energie Atomique, Centre National de la Recherche Scientifique, Université Aix-Marseille, Saint Paul lez Durance F–13108, France (B.P., L.R., V.B., L.N.); Department of Plant Systems Biology, Root Development Group, Flanders Interuniversity Institute of Biotechnology and Department of Molecular Genetics, Ghent University, B–9052 Ghent, Belgium (B.P., I.D.S., T.B.); Equipe Rhizogenèse, Université Mixte de Recherche DIA-PC, Institut de Recherche pour le Développement, 34394 Montpellier cedex 5, France (L.L.); Department of Plant Sciences, University of Cambridge, Cambridge CB2 3EA, United Kingdom (L.R., J.H.); Université Pierre et Marie Curie, F–75252 Paris cedex 05, France (E.B.-D., D.C.); Plant Molecular Biology Laboratory, Institute of Biotechnology, University of Helsinki, FI–00014 Helsinki, Finland (M.B., Y.H.); and Carolyn Lynch Laboratory, Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19104 (S.R.P.)

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 strong competence to initiate cell division, whereas 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. 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.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Laurent Nussaume (lnussaume{at}cea.fr).

^[W] The online version of this article contains Web-only data.

www.plantphysiol.org/cgi/doi/10.1104/pp.107.107870

^* Corresponding author; e-mail lnussaume{at}cea.fr.

Received August 23, 2007; accepted November 1, 2007; published November 9, 2007.

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