p34^cdc2 Related Proteins in Control of Cell Cycle Progression, the Switch Between Division and Differentiation in Tissue Development, and Stimulation of Division by Auxin and Cytokinin

Abstract

Progress through the plant cell cycle involves changes of phosphorylation state and catalytic activity in p34^cdc2 -like protein. The plant protein has protein kinase activity, binds p13^suc1, copurifies with cyclin B-like protein and its catalytic activation at mitosis is shown, by a metaphase arresting mutant, to be tightly coupled to cyclin accumulation and breakdown. These properties correlate with findings in other laboratories that some plant cdc2 genes can complement the yeast cdc2 gene. Activation of p34^cdc2 at mitosis is proposed to trigger preprophase band breakdown since these two events continue in cells inhibited with okadaic acid although completion of chromosome condensation and spindle formation, which normally precede breakdown, do not occur and are therefore not triggers. In plant development the level of this key division protein relative to other proteins declines in cells entering division to less than one-tenth of that in dividing cells and resumption of division follows restoration of the level in all tissues that have been tested, including cotyledon, stem pith and root. Hormonal induction of division revealed tissue-specific differences in effects of individual hormones on p34^cdc2-like protein accumulation and its catalytic activation. In tobacco pith, auxin could induce synthesis of p34^cdc2-like protein but cytokinin was necessary for its activation and for cell cycle activity. Within the cell cycle, hormone-dependent control points were identified at late G1 and at mitotic initiation by measurement of nuclear DNA content and the timing of activation of p34^cdc2 and cell division, in Nicotiana suspension culture cells restimulated with hormone. Auxin was required at both control points, but cytokinin was required only at mitotic initiation. A direct effect of auxin on cdc2 gene expression is indicated by an increase of cdc2 mRNA levels within 10 min of exposing root tissue to 3-indolylacetic acid (IAA).