the presence of hormones, they can undergo cell division and regenerate into whole plants. Pericycle cells
retain the ability to divide and are responsible for the formation of lateral roots at vascular poles. Unlike
animal cells, plant cells are unique in that they are totipotent. In several plant systems terminally differ-
entiated nondividing somatic cells can dedifferentiate, divide, and regenerate into a whole plant. Reiniti-
ation of cell division in differentiated and nondividing cells is a central feature in plant regeneration. Cy-
tokinesis, a process by which cytoplasm is divided, is considerably different in plants as compared with
other organisms. In plant cells, cytokinesis is initiated by forming a phragmoplast (made of mircrotubules
and actin) between daughter cells, which is followed by deposition of cell wall material.
Some of the proteins involved in cell cycle control have appeared to be involved in development. A
D-type cyclin, Arath;CycD4;1, was expressed along with CDC2aAt upon mitogenic stimulation follow-
ing starvation [156]. In situ hybridization with Arath;CycD4;1 showed expression during vascular tissue
development, embryogenesis, and formation of lateral roots. In pea auxilary buds, Pissa;CycD3;1 was
found in dormant buds while Pissa;CycB1;2 and Cdc2 proteins were not [235]. Pissa;CycD3;1 interacted
with PCNA during dormancy but not in growing buds, suggesting a means of regulation of this D-type
cyclin. An Arabidopsiscdc2 homologue (CDC2cAt) was isolated that is divergent from the other cdc2
homologues in Arabidopsis, having unusual N- and C-terminal ends [104]. Its expression is restricted to
flowers, weakly in buds and strongly in mature flowers. Its promoter has high homologies with a tran-
scription factor that was previously immunolocalized in the epidermal cell layer of petals, suggesting that
CDC2cAt is involved in flower development [104]. Two tomato Cdks were found to be expressed be-
tween anthesis and 5 days after anthesis (DPA) but their maximum kinase activity was obtained between
5 and 20 DPA, suggesting a posttranslational regulation of Cdk at the temporal and spatial levels during
early tomato fruit development [113].
The retinoblastoma protein (Rb) is a Cdk substrate that when phosphorylated is involved in regula-
tion of the cell cycle [224,330,331]. Rb has been shown to be involved in differentiation in animals by re-
pressing transcription via E2F, and it also seems to act as a transcriptional coactivator in differentiating
cells, possibly through its interaction with other proteins involved in transcriptional activation [332]. The
Rb pathway may be involved in differentiation and development in plants [229,333]. ZmRb showed a gra-
dient of accumulation that correlated with the gradient of cell proliferation in maize leaves, being abun-
dant in the more differentiated cells whereas it is almost undetectable in the basal proliferative zone [229].
Plant Rb protein can interact with RbAp48, a protein that binds Rb and is present in chromatin assembly
and histone deacetylation complexes and thus may also negatively regulate the expression of E2F-regu-
lated genes by directing chromatin alterations [334].
Analysis of p34 protein kinase mRNA in roots has shown high levels of p34 protein kinase mRNA
in meristem and all pericycle cells but not in the quiescent center. In pericycle, p34 protein kinase mRNA
is expressed uniformly in all the prericycle cells, although lateral roots are initiated only at the vascular
poles [123]. These results suggest that lateral root initiation opposite to vascular poles is controlled by a
mechanism other than p34 protein kinase transcription. In situ hybridization studies during lateral root
formation showed that induction of Arath;CycB1;1 was a very early event and that its accumulation might
be one of the limiting factors for activation of cell division [171]. Before stem nodule development in Ses-
bania rostrata, Sesro;CycB1;1 transcripts were absent in cortical cells whereas Cdk gene Cdc2-1Srtran-
scripts were found in all cells [166]. After infection with Azorhizobium caulinodans, Sesro;CycB1;1 tran-
scripts were expressed in a patchy pattern in the cortex of the root primordium. As discussed in Sec. III.B,
cyclin transcripts appeared in only some cells of meristematic tissues [100,128,171], suggesting that cy-
clin accumulation could influence cell cycle timing. Doerner et al. [335] studied the affect of the expres-
sion of an Arabidopsiscyclin (Arath;CycB1;1) under the control of the cdc2a promoter. In roots, this ec-
topic expression accelerated root growth without altering the pattern of lateral root development. Normal
and mutant forms of an Arabidopsiscdc2 homologue were expressed in Arabidopsisand tobacco [336].
Overexpression of normal Cdc2a did not affect cell division. Thus, it appears that cyclin expression may
be the limiting factor for growth [335]. Expressed mutant Cdc2aAt completely abolished cell division in
Arabidopsis. When expressed in tobacco, a few mutants were able to survive. They had reduced H1K ac-
tivity and a lower number of cells than normal plants. The morphogenesis, histogenesis, and develop-
mental timing were not affected, indicating that the developmental controls defining shape can act inde-
pendently of cell division rates [336].
On the other hand, evidence suggests that cell division can be regulated by cell fate specification
CELL CYCLE REGULATION IN PLANTS 251