Much progress has been made in identifying and functionally characterizing components of the plant cell division apparatus (Inze and Veylder 2006), and the mechanisms involved in cell expansion are also beginning to be quite well understood (Carol and Dolan 2006; Cosgrove 2005; Tsukaya 2006). In contrast, cell growth control is mechanistically still less well understood. Based on the preceding analysis of plant growth processes, I propose the existence of two major growth control points in plants likely to be sensitive to developmental or environmental inputs. The first is suggested to co-regulate cell growth and the onset of the cell cycle; the second is the switch of growth mechanisms at the phase I/II boundary to suppress mitosis and activate cell expansion. The identification of components involved in these control points, the mechanisms by which they operate and how they are coupled to cues will be major milestones to improve our understanding of plant growth control.
Recently, a few candidates for such components were identified. They are considered in detail below, because each one is a possible target or component of growth control pathways responsive to environmental or developmental cues. However, it will require more extensive analysis to unambiguously establish their specific function in growth control networks.
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