Development and Stem Cells Research Article

INTRODUCTION The control of final cell and organ size is a fundamental question in the biology of all multicellular organisms. Root length is determined by the number of proliferating cells and their mature final size (Beemster and Baskin, 1998). The root is characterized by consecutive developmental zones along its proximal-distal axis. These zones form a gradient of renewed cells that proliferate, elongate and differentiate (Fig. 1A). In the root meristem zone, cells undergo repeated rounds of cell division. Subsequently, the cells exit the meristematic zone to become part of the elongation/differentiation zone (EDZ) where cells cease dividing, undergo rapid cell expansion and differentiate. The root meristematic zone can be further divided into the apical and basal meristem zones (Beemster et al. The apical meristem is characterized by a high rate of cell proliferation, where cells do not exhibit a significant gain in size. In the basal meristem, also referred to as the transition zone between the apical meristem and the elongation zone, cell proliferation rate slows or stops and cells become larger. The quiescent center (QC, organizing cells), together with their surrounding stem cells, define the stem cell niche. Brassinosteroids (BRs) are essential for normal plant growth and development, and mutants that are unable to synthesize or perceive BRs are dwarfs. BRs are perceived upon direct binding to the extracellular domain of the cell surface receptor kinase BRI1 (He

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