Summary

A new hypothesis for the formation of the quiescent center is presented. Reported data support a mechanism for the establishment and maintenance of the quiescent center. The quiescent center is located at the most distal part of the root proper, the most terminal location in the root proper on the path of polar transport from the shoot. Of the many substances polarly transported in the root, auxin is one of the best studied and has been shown to affect root meristem organization. In our mechanism, polar auxin is directly linked to quiescence through the action of ascorbate oxidase and ascorbic acid. Immunolocalization of auxin in the root tip of Zea mays showed that auxin levels in the quiescent center were high compared to the levels in the immediately surrounding meristematic cells. Isolated quiescent centers were shown to have high levels of ascorbate oxidase mRNA and ascorbate oxidase activity relative to proximal meristem tissue. Exogenous auxin caused an increase in ascorbate oxidase mRNA levels and ascorbate oxidase enzyme activity in cultured root tissue. Immunolocalization of ascorbate oxidase in Zea root tips showed high levels of the protein in the quiescent center relative to surrounding cells. This is the first report of a positive marker and activity for the quiescent center. Histochemical detection of ascorbic acid in Zea root tips showed that quiescent center cells have low or undetectable levels of ascorbic acid, presumably due to the high levels of ascorbate oxidase in the quiescent center. As ascorbic acid is a compound known to be necessary for the transition from G1 to S in the cell cycle, its low levels in the quiescent center may be directly responsible for holding these rarely dividing cells in the extended G1 state in which they are mainly found. We propose that our mechanism complements published mathematical modeling of the anatomical structure of root apices, and further propose that the control of relative growth rates in this focal region of the root apex by this mechanism is a determining aspect in generating anatomical patterning in the root apex.