Petunia p34 cdc2 protein kinase activity in G 2/M cells obtained with a reversible cell cycle inhibitor, mimosine

Abstract

Protoplasts isolated from petunia leaf mesophyll are non-cycling cells mostly with 2C content. Cells regenerating from protoplast culture enter mitosis after 48 h. This experimental model is used to relate p34 cd2 kinase activity to cell cycle phase. Our results show that the histone Hl phosphorylation, and hence p34 cdc2 kinase activity, peaks with G 2+ early M cell cycle phase. However, a trace kinase activity was already present when most cells were entering S phase. To obtain a maximum of cells in G 1+S phases, the protoplast culture was treated with the rare amino acid, mimosine. Mimosine blocked plant cells derived from protoplast culture both at g 1 and in early and mid S phase. Despite the increased G 1+S level, p34 cdc2 kinase activity did not increase. This suggests that the trace activity appearing when the majority of cells are entering S does not correspond to any putative P34 cdc2 activation at G 1/S transition but to the activation of the minor 4C population initially present in the leaf: the hypothesis remains that p34 cdc2 kinase activity is solely related to G 2+M phase in petunia.