A polarity crossroad in the transition growth zone of maize root apices: Cytoskeletal and developmental implications

Due to their simple and regular anatomy, root apices represent a unique model object for studying growth, polarity, and morphogenesis. This advantageous anatomy has been exploited to characterize the developmental changes that occur as root cells progress from their origin in the meristem tip to their final nongrowing state at the proximal limit of the elongation region. A new growth region located between the apical meristem and the distal portion of the region of rapid cell elongation was discovered and designated as the 'transition zone.' Cells of this zone accomplish a developmental transition recently from cytoplasmically driven expansion to vacuome-driven elongation. Cells traversing the transition zone use cytoskeletal elements to regulate both growth polarity and the maintenance of cellular growth per se. Transition zone cells are also sensitive to diverse endogenous clues and exogenous factors such as auxin, ethylene, extracellular calcium, mechanical pressure, aluminum, and microorganisms. This high sensitivity of transition zone cells, which are not engaged in mitotic divisions, seems to be related to their specific cytoarchitecture whereby postmitotic nuclei occupy a central position within the cell, with their radial perinuclear microtubules extending to the cell periphery. Future studies are challenged to identify genes and proteins that determine the various sensory behaviors of cells in this transitional phase of development, and which, in turn, drive directed growth responses (tropisms) of root apices in response to diverse external stimuli.