Differences in volumes of sister nuclei in binucleate cells: evidence for asymmetry of mitosis

Abstract

Roots of Zea mays cv. Golden Bantam were treated with a 0.1% solution of caffeine for 1–4 h and allowed to recover for up to 17 h. Binucleate cells were present in the root cap and in the meristem; nuclear volumes were determined for the pairs of sister nuclei in the binucleate cells. In row I cells of the root cap, 41%of pairs of sister nuclei had different volumes in roots treated with caffeine for 1–4 h or for 2 h and then allowed to recover for 5 h. In the period 2 + 6 to 2 + 17 h, however, 70% of the pairs differed in volume. Volumes of sister nuclei present in the same cell differ soon after they enter G₁; they also differ in the period 2 + 6 to 2 + 17 h when mean nuclear volume increases from 145 to 243 μm³. Both aspects suggest that sister nuclei differ in their ability to grow. In most binucleate cells, the larger nucleus was in the position it would have occupied if it lay in a row I cell. It is suggested, therefore, that a gradient exists, in a row I cell, of some factor that regulates nuclear growth. The consequence of this gradient is that sister nuclei grow differentially even though they share a common cytoplasm. Mitosis in a row I cell appears to be asymmetrical. Cells induced to become binucleate were inhibited by the caffeine treatment. Only 9% of the binucleate cells completed a cell cycle and divided and, for some of them, cycle time was increased.