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Auxin and ethylene interactions control mitotic activity of the quiescent centre, root cap size, and

Auxin and ethylene interactions control mitotic activity of the quiescent centre, root cap size, and

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Plant, Cell and Environment (2005) 28 , 719–732 © 2005 Blackwell Publishing Ltd 719 Blackwell Science, LtdOxford, UKPCEPlant, Cell and Environment0016-8025Blackwell Science Ltd 2005? 2005 28?719732 Original Article Regulation of root apical meristem development by auxin and ethylene G. Ponce et al. Correspondence: Gladys I. Cassab. Fax: 52 777313 99 88; e-mail: [email protected] Auxin and ethylene interactions control mitotic activity of the quiescent centre, root cap size, and pattern of cap cell differentiation in maize GEORGINA PONCE 1 , PETER W. BARLOW 2 , LEWIS J. FELDMAN 3 & GLADYS I. CASSAB 1 1 Departamento de Biología Molecular de Plantas, Instituto de Biotecnología, Apdo. Postal 510–3, Cuernavaca, Mor., 62210, México, 2 School of Biological Sciences, University of Bristol, Woodland Road, Bristol BS8 1UG, UK and 3 Department of Plant and Microbial Biology, 111 Koshland Hall, University of California, Berkeley, CA 94720, USA ABSTRACT Root caps (RCs) are the terminal tissues of higher plant roots. In the present study the factors controlling RC size, shape and structure were examined. It was found that this control involves interactions between the RC and an adja- cent population of slowly dividing cells, the quiescent centre, QC. Using the polar auxin transport inhibitor 1- N - naphthylphthalamic acid (NPA), the effects of QC activa- tion on RC gene expression and border cell release was characterized. Ethylene was found to regulate RC size and cell differentiation, since its addition, or the inhibition of its synthesis, affected RC development. The stimulation of cell division in the QC following NPA treatment was reversed by ethylene, and quiescence was re-established. Moreover, inhibition of both ethylene synthesis and auxin polar transport triggered a new pattern of cell division in the root epidermis and led to the appearance of supernu- merary epidermal cell files with cap-like characteristics. The data suggest that the QC ensures an ordered internal distribution of auxin, and thereby regulates not only the planes of growth and division in both the root apex proper and the RC meristem, but also regulates cell fate in the RC. Ethylene appears to regulate the auxin redistribution sys- tem that resides in the RC. Experiments with Arabidopsis roots also reveal that ethylene plays an important role in regulating the auxin sink, and consequently cell fate in the RC. Key-words : auxin; auxin polar transport inhibitors; ethylene; maize root; quiescent centre; root cap. INTRODUCTION Root caps (RC) are the terminal tissue of roots of most plants (Barlow 2003). Some basic functions relating to root biology, such as lubrication of root growth and gravitro- pism, were ascribed to RCs by Haberlandt (1914, reviewed in Barlow 1975). Within the past 25 years, however, the functions of the RC have been shown to be considerably more diverse and to include regulation of many aspects of root development (Scheres et al . 1996; Tsugeki & Federoff 1999). The RC perceives and processes many environmen- tal stimuli, and mediates the direction of root growth accordingly. Gravity (gravitropism), light (phototropism),
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