Recovery from the 5-AU induced blockage in interphase: Evidence for differential recovery.

Abstract

Treatment with 5-aminouracil (5-AU) has two effects on proliferating cells: 1) it induces a temporary block at the S-G2 transition or at the end of S phase (Diez, Gonzalez-Fernandez and Lopez-Saez 1976, Scheuermann and Klaffke-Lobsien 1973, Socher and Davidson 1971 and 2) it reduces the rate of DNA synthesis (Jakob and Trosko 1965, Scheuermann andKlaffke-Lobsien 1973). The results of treatment are, first, a fall in mitotic index (MI) and, subsequently, partial synchronization of cells entering mitosis (refs. in Davidson, Golding and Armstrong 1979).The response to 5-AU is as rapid in root primordia of V. faba as in primary roots and fully emerged laterals. This shows that: 1) there is no barrier to the movement of 5-AU through the root cortex and into the primordia (Rudolph and Davidson 1975, Socher andDavidson 1971); 2) the initial response to 5-AU is the same in all meristems of a root system. The rate of recovery from 5-AU, however, as shown by the re-establishment of mitotic activity, is faster in primordia than in primary or lateral roots (Rudolph and Davidson 1975); it is also faster in some cells of a meristem than in other cells (Clowes 1965, Mattingly 1966.One problem in analysing the overall response to 5-AU, i.e., both inhibition and recovery, is that meristems are asynchronous; their cells are distributed through out G1, S, G2 and mitosis. Thus, it is difficult to estimate the extent of the inhibi tion imposed by 5-AU on cells at different stages of interphase because, in normal meristems, cells in these stages cannot be identified. Because of this difficulty we cannot estimate the number of cycles completed by a cell in the presence of 5-AU. One approach to these problems is to follow the behaviour of a marked subpopula tion of cells. Mattingly (1966) induced tetraploid cell formation, with colchicine, treated with 5-AU and showed that the tetraploid cells underwent a complete cell cycle in the presence of 5-AU. Furthermore, these tetraploid cells entered mitosis at the same time as the first diploid cells to recover from the 5-AU blockade.The sensitivity of G, cells to 5-AU has been studied here by treating with col chicine, to induce tetraploid cells, and then treating these cells for two hours during G, with 5-AU. Other root systems were treated with colchicine and then with 5-AU for up to 26 hours. Recovery from 5-AU treatment occurred more rapidly in pri mordia than in primary roots or fully emerged lateral roots, even with a continuous exposure to 5-AU. These results, reported here, suggest that the ability to overcome the block imposed by 5-AU is achieved by whole meristems, not merely by individual cells acting independently. In turn, this suggests a change in the physiological state of a meristem that involves cell-cell interations. This conclusion is largely