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In most plants, sucrose is the major transported carbon source. Carbon source availability in the form of sucrose is likely to be a major determinant of cell division, and mechanisms must exist for sensing sugar levels and mediating appropriate control of the cell cycle. We show that sugar availability plays a major role during the G 1 phase by controlling the expression of CycD cyclins in Arabidopsis. CycD2 mRNA levels increase within 30 min of the addition of sucrose; CycD3 is induced after 4 h. This corresponds to induction of CycD2 expression early in G 1 and CycD3 expression in late G 1 near the S-phase boundary. CycD2 and CycD3 induction is independent both of progression to a specific point in the cell cycle and of protein synthesis. Protein kinase activity of CycD2-and CycD3-containing cyclin-dependent kinases is consistent with the observed regulation of their mRNA levels. CycD2 and CycD3 therefore …