In Arabidopsis the quiescent center (QC) consists of four cells and is surrounded by meristematic initials. In maize roots the QC can constitute upwards of 800-1200 cells, most of which spend an extended period of time (180-200 hours) in the G1 phase of the cell cycle. How the QC forms and is maintained is not known.
The quiescent center regulates the differentiation of neighboring cells. If one or 2 QC cells were ablated, the columnella initials that were no longer in direct contact with a QC cell ceased dividing and underwent differentiation as a columnella cell. Similarly, cortical initial cells that were no longer in direct contact with the a QC cell behaved as cortical daughter cells and underwent an asymmetric division to generate cortical and endodermal cell files. Therefore, direct contact with the QC inhibits the differentiation of initial cells in the RAM (van den Berg, 1997).
Auxin and RAM organizationEdit
An GUS reporter for auxin concentration shows a concentration of auxin just at the distal end of the vasculature, just proximal to the RAM. Disruption of this localized accumulation by auxin transport inhibitors, exogenous auxin application or mutations in genes involved with auxin response, disrupts the organization of cell types in the root apex, as visualized by promoter trap cell markers. Thus the proper localization and perception of this auxin maximum are important for organizing the RAM (Sabatini, 1999). It has been shown AtPIN4 is expressed in the quiescent center region (Friml et al 2002).
In maize it has been reported that cells of the QC are characterized by their highly oxidized status. Glutathione and ascorbic acid occur predominately in the oxidized forms in the QC. This is contrasted with the status of these redox intermediates in adjacent, rapidly dividing cells in the root meristem, in which the reduced forms of these two species are favored. Using a redox sensitive fluorescent dye it is possible to visualize an overall oxidizing environment in the QC, and make comparisons with the adjacent, rapidly dividing cells in the root meristem. Altering the distribution of auxin and the location of the auxin maximum in the root tip activates the QC, and cells leave G1 and enter mitosis. Commencement of relatively more rapid cell division in the QC is preceded by changes in the overall redox status of the QC, which becomes less oxidizing. (Jiang, et al, 2003)