Summary.Cyclin proteins are components of the regulatory system that controls the orderly progression of the events of cell division. Their sub-cellular location, as well as their fluctuating abundance and their affinities for the cyclin-dependent kinases (CDKs) to which they bind, determine their successive roles during the cell cycle. Here we employ species-specific antibodies to monitor changes in quantity and location of four maize cyclins and maize Cdc2-kinase in dividing maize root tip cells. Maize cyclin Ia occurs in the nuclear matrix and is released when the nuclear envelope breaks down. In contrast, cyclin Ib is cytoplasmic until prophase; it associates transiently with the nuclear envelope and preprophase band (PPB) just before these structures break down and then associates with the condensed chromosomes and spindle region before declining at anaphase. Cyelin II and Cdc2 also occur in the PPB. Occurrence of cyclin Ib and Cdc2 at the PPB concurrent with initiation of breakdown is consistent with previous studies in which microinjection of cyclin-dependent protein kinase indicated that removal of the PPB at the time of nuclear-envelope breakdown is catalysed by a CDK. While cyclins Ia and III are predominantly nuclear prior to mitosis, cyclins Ib and II are predominantly cytoplasmic until prophase then become nuclear. The initial cytoplasmic retention of cyclins Ib and II correlates with their possession of a sequence similar to the cytoplasmic-retention signal of animal cyclin B1. Cyclin II binds to all microtubule arrays during the cell cycle, becoming markedly concentrated in the phragmoplast, and cyclin III associates with the spindle and then the phragmoplast. Cdc2 also occurs in the phragmoplast. Persistence of mitotic cyclins and CDK after mitosis into the cytokinetic stage, as seen in maize, is not paralleled in animal cells, where the cytokinetic mid-body is not so labelled, presumably reflecting the key role of the phragmoplast apparatus in plant cell division.

We investigated plant cell division by testing for the presence and involvement in progress through the division cycle of the protein p34cdc2, a key participant in division control in other eukaryotes. A protein of the same m, 34,000 has structural similarity indicated by its reaction with three sorts of antibody raised against (1) cell divisionspecific regions within a 16-amino acid interna1 sequence that is perfectly consenred in p34cdc2 from all known sources, (2) the carboxy-terminal 127 amino acids of human p34cdc2 linked to B-galactosidase, and (3) whole p34cdc2 of fission yeast. Participation of p34 in the division cycle of the green plant Chlamydomonas is indicated by phosphorylation of the protein only in proliferating cells. There is a consistent fivefold increase relative to other proteins when cells become committed to division and a maximum of phosphorylation at the time of nuclear division under conditions that alter by twofold the time of these events. A p34 protein is detectable in oats and Arabidopsis and in remote taxa, including red and brown algae. We conclude that the plant kingdom shares a division control involving p34cdc2 that was probably established in the common ancestral eukaryote prior to divergence of any of the major eukaryote taxa.

We investigated plant cell division by testing for the presence and involvement in progress through the division cycle of the protein p34cdc2, a key participant in division control in other eukaryotes. A protein of the same m, 34,000 has structural similarity indicated by its reaction with three sorts of antibody raised against (1) cell divisionspecific regions within a 16-amino acid interna1 sequence that is perfectly consenred in p34cdc2 from all known sources, (2) the carboxy-terminal 127 amino acids of human p34cdc2 linked to B-galactosidase, and (3) whole p34cdc2 of fission yeast. Participation of p34 in the division cycle of the green plant Chlamydomonas is indicated by phosphorylation of the protein only in proliferating cells. There is a consistent fivefold increase relative to other proteins when cells become committed to division and a maximum of phosphorylation at the time of nuclear division under conditions that alter by twofold the time of these events. A p34 protein is detectable in oats and Arabidopsis and in remote taxa, including red and brown algae. We conclude that the plant kingdom shares a division control involving p34cdc2 that was probably established in the common ancestral eukaryote prior to divergence of any of the major eukaryote taxa.

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P34(cdc2) is a key cell-cycle protein in fission yeast that is necessary for progress in the cell cycle from the G1 to the S phase and from G2 through mitosis. Homologues of p34(cdc2) have been found in all eukaryotes that have been investigated. Levels of p34(cdc2)-like protein were studied by quantitative Western blotting in developing cotyledons of Daucus carota L. (carrot) seedlings, in expiants from the same seedlings transferred to tissueculture media with and without 2,4-dichlorophenoxyacetic acid (2,4-D), and in nutrient-starved suspension cultures derived from carrot callus. During the cessation of cell division, which accompanies development of the cotyledon to maturity, there was a 16-fold decline in the level of the p34(cdc2)-like protein. Auxin-stimulated dedifferentiation in excised tissue from mature cotyledons was accompanied by restoration of the level of p34(cdc2)-like protein, and the responding cells formed a callus. These data support our earlier proposition, based upon evidence from wheat leaf, that changes in the level of p34(cdc2)-like protein act in the switch between cycling and differentiation. Persisting high levels of p34(cdc2)-like protein in suspension cultures, when division was stopped by nutrient limitation, indicated that decline of the protein was not an inevitable consequence of the cessation of division. Decline of p34(cdc2) in differentiation may therefore be a regulated process that determines exit from the cell cycle and the converse increase in p34(cdc2) may be a regulated process controlling dedifferentiation and resumption of cell division.