ROSCOVITINE, A NOVEL CYCLIN-DEPENDENT KINASE INHIBITOR, CHARACTERIZESRESTRICTION POINT AND G2 M TRANSITION IN TOBACCO BY-2 CELL-SUSPENSION/

Although the developmental programs of plants and animals differ, key regulatory components of their cell cycle have been conserved. Particu lar attention has been paid to the role of the complexes between highl y conserved cyclin and cyclin-dependent kinases in regulating progress ion through the cell cycle. The recent demonstration that roscovitine is a potent and selective inhibitor of the animal cyclin-dependent kin ases cdc2 (CDK1), CDK2 and CDK5 prompted an investigation into its eff ects on progression through the plant cell cycle. Roscovitine induced arrests both in late G1 and late G2 phase in BY-2 tobacco cell suspens ions. Both blocks were fully reversible when roscovitine was used at c oncentrations similar to those used in the animal system. Stationary-p hase cells subcultured in the presence of roscovitine were arrested at a 2C DNA content, This arrest was more efficient without exogenous ad dition of plant growth regulator. Roscovitine induced a block in G1 ea rlier than that induced by aphidicolin. S-phase synchronized cells tre ated with roscovitine were arrested at a 4C DNA content at the G2/M tr ansition. The expression analysis of a mitotic cyclin (NTCYC1) indicat ed that the roscovitine-induced G2 block probably occurs in late G2. F inally, cells in metaphase were insensitive to roscovitine. The purifi ed CDK/cyclin kinase activities of late G1 and early M arrested cells were inhibited in vitro by roscovitine. The implications of these expe rimental observations for the requirement for CDK activity during prog ression through the plant cell cycle are discussed.