Meristem-specific suppression of mitosis and a global switch in gene expression in the root cap of pea by endogenous signals

Two functionally distinct sets of meristematic cells exist within root tips of pea (Pisum sativum): the root apical meristem, which gives rise to the body of the root; and the root cap meristem, which gives rise to cells that differentiate progressively through the cap and separate ultimately from i ts periphery as border cells. When a specific number of border cells has ac cumulated on the root cap periphery, mitosis within the root cap meristem, but not the epical meristem, is suppressed. When border cells are removed b y immersion of the root tip in water, a transient induction of mitosis in t he root cap meristem can be detected starting within 5 min. A corresponding switch in gene expression throughout the root cap occurs in parallel with the increase in mitosis, and new border cells begin to separate from the ro ot cap periphery within 1 h. The induction of renewed border cell productio n is inhibited by incubating root tips in extracellular material released f rom border cells. The results are consistent with the hypothesis that opera tion of the root cap meristem and consequent turnover of the root cap is se lf-regulated by a signal from border cells.