INDUCTION OF CELL DIVISION-RELATED GENES IN QUIESCENT (G(0)) SUGAR-BEET CELLS

Sugar beet cells maintained in the stationary phase of the batch cultu re cycle for 2 or more days have been shown to exhibit many of the cha racteristics of quiescent (G(0)) cells. When such cells were subcultur ed into fresh medium they progressed through a period of DNA synthesis to a highly synchronised first division, 6 days after subculture. The onset of DNA synthesis and cell division were each delayed by 2 days relative to the timing of the events when the cells were subcultured i mmediately before entry into the stationary phase. The regulation of g ene expression during this extended transition from the G(0) phase bac k to the cell division cycle was investigated. The cell division cycle -related genes Bvcdc2, Betvu;CycA2, Arath;CycB1;1 histone H4 and Bvcrk 1 (a novel cdc2-like gene) showed widely differing patterns of express ion. Bvcdc2 transcripts were present at low levels in quiescent cells whereas crk1, cyclin and histone transcripts were not detectable. Expr ession of both Bvcrk1 and Betvu; CycA2 was induced within 1 h after su bculture into fresh medium, whereas histone H4 gene expression was not detectable for 24 h and showed a marked increase between 24 and 48 h. B-type cyclin transcripts were not detectable until more than 48 h af ter subculture. The addition of either sucrose or MS macronutrients to quiescent sugar beet cells was not sufficient to re-initiate cell div ision but both medium components were able to stimulate the expression of the two 'early' genes (Betvu;CycA2 and Bvcrk1) within 6 h. Further more, although the sugar beet cells were habituated, i.e. they were ro utinely grown without added plant growth regulators, treatment of quie scent cells with IAA and kinetin also induced expression of Betvu;CycA 2 and Bvcrk1 without subsequent cell division.