Induction of cell division-related genes in quiescent (G₀ ) sugar beet cells

Sugar beet cells maintained in the stationary phase of the batch culture cycle for 2 or more days have been shown to exhibit many of the characteristics of quiescent (G₀ ) cells. When such cells were subcultured 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 immediately before entry into the stationary phase. The regulation of gene expression during this extended transition from the G₀ phase back to the cell division cycle was investigated. The cell division cycle-related genes Bvcdc2, Betvu;CycA2, Arath;CycB1;1 histone H4 and Bvcrk1 (a novel cdc2-like gene) showed widely differing patterns of expression. Bvcdc2 transcripts were present at low levels in quiescent cells whereas crk1, cyclin and histone transcripts were not detectable. Expression of both Bvcrk1 and Betvu; CycA2 was induced within 1 h after subculture 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 after subculture. The addition of either sucrose or MS macronutrients to quiescent sugar beet cells was not sufficient to re-initiate cell division but both medium components were able to stimulate the expression of the two 'early' genes (Betvu;CycA2 and Bvcrk1) within 6 h. Furthermore, although the sugar beet cells were habituated, i.e. they were routinely grown without added plant growth regulators, treatment of quiescent cells with IAA and kinetin also induced expression of Betvu;CycA2 and Bvcrk1 without subsequent cell division.