DEVELOPMENTAL EXPRESSION OF THE ARABIDOPSIS CYCLIN GENE CYC1AT

In eukaryotes, the control of cell cycle progression is exercised by h eteromeric protein kinase complexes composed of a cell cycle-dependent , kinase-related subunit (Cdc2) and a cyclin subunit. To explore the p ossibility that cyclin transcription plays a role in the developmental regulation of cell division, we examined the spatial and temporal exp ression of a cyclin gene (cyc1At) in Arabidopsis. In root and shoot ap ical meristems and during embryogenesis, cyc1At expression is almost e xclusively confined to dividing cells. A cell-specific pattern of cyc1 At expression was noticed in root meristems. We examined the effects o f induction of cell division of differentiated cells on cyc1At express ion. During lateral root formation, induction of cyc1At expression is a very early event and was detected before anatomical modifications we re visible. Treatment of roots with oryzalin, which blocks cell divisi on in metaphase, did not inhibit the auxin induction of cyc1At, sugges ting that induction of cyc1At expression precedes the completion of th e fi mt division cycle af ter induction of lateral roots. In tobacco p rotoplasts, an increase in cyc1At expression was observed only when ce ll division was induced. Together, the results suggest that Cyc1At acc umulation in Arabidopsis is transcriptionally regulated and might be o ne of the limiting factors for the activation of cell division.