The Nuclear Endoreduplication Cycle in Metaxylem Cells of Primary Roots of Zea mays L.

Abstract

The nuclear DNA content of metaxylem cells in roots of Zea mays cv. Golden Bantam reaches 16C or 32C by successive rounds of DNA endoreduplication. Each phase of endoreduplication (endo-S) is separated by a non-DNA synthetic phase (endo-G). These phases seem to occur in zones at fixed distances from the root tip. The duration of the phases in two of the endoreduplication cycles (4C–8C, 8C–16C) has been estimated in two ways. The first makes use of the rate of movement of cells through the positions along the root where the different phases of the cycle are occurring, the second uses labelling with methyl-[3H]thymidine and autoradiography. Both methods indicate that the endo-S phases which cause the nuclear DNA content to rise from 4C to 8C and from 8C to 16C last 8–10 h, and that the intervening endo-G phase lasts 8–12 h. DNA endoreduplication keeps pace with the increase of nuclear volume; cell volume increases at a more rapid rate, however. Comparison of the endoreduplication cycle in the metaxylem with the mitotic cycle in the adjoining files of parenchyma cells shows that the mitotic cells complete their cycle more slowly.