Inactivation of DNA replication origins by the cell cycle regulator, trigonelline, in root meristems of Lactuca sativa

The effects of trigonelline (TRG) on the cell cycle in root meristems of La ctuca sativa L. were examined in the knowledge that TRG is a cell cycle reg ulator that causes cell arrest in G2, and prevents ligation of replicons in S-phase. The hypothesis was tested that continuous exposure to TRG would p erturb DNA replication which, in turn, would lengthen the cell cycle and im pair root elongation. Using DNA fibre autoradiography, mean replicon size w as 31 and 13 mu m in the TRG (3 mM) and control treatments, respectively. T rigonelline also resulted in a lengthening of both S-phase and the cell cyc le and a decrease in primary root elongation. Hence, replicon inactivation was responsible for the protracted S-phase. Trigonelline treatment also res ulted in a 1.6-fold increase in fork rate (13.8 mu m h(-1)) compared with t he control (8.4 m h(-1)). The faster fork rate in the larger replicons is i n accord with the highly significant positive relationship already establis hed between fork rate and replicon size for various unrelated higher plants .