Formation of a plant involves generation of new cells by the division cycle and development in these of specialised structure and metabolism. Specialisation is accompanied by a decreasing capacity for division, which declines with particular rapidity in cells of monocotyledonous plants such as the cereals. Here we report that in wheat leaves a homologue of the cell cycle control protein p34(cdc2) participates in the control of these developmental programmes. Accumulation of p34(cdc2) to a maximum level in dividing cells and the cessation of its accumulation during subsequent cell growth and expansion indicate that it contributes specifically to division. There is a decline in p34(cdc2) level as cell differentiation proceeds, in close parallel with the previously established decline of cell division in response to auxin hormones. A basal level of p34(cdc2) in fully differentiated cells that is one-sixteenth of that in dividing cells correlates with their loss of capacity to divide. We conclude that p34(cdc2) level is controlled in diverse multicellular eukaryotes and suggest that it is an important element in the switch from cell division to differentiation.

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