[98,99],Antirrhinum[100],Arabidopsis[101–104], maize [105], mothbean [106], Norway spruce [107],
pea [21,97], Petunia[108], rice [109,110], soybean [111] tobacco [112], and tomato [113]. These species
represent plants as diverse as both monocots and dicots in angiosperms and a gymnosperm. The
PSTAIRE motif is conserved in many of the plant homologues but is modified in others. Five types of
Cdks can be identified on the basis of their sequences [66]. A-type Cdks have the conserved PSTAIRE
motif and are most closely related to Cdk1 and Cdk2. B type have a modified PSTAIRE motif (PP-
TALRE). The other three types are represented by only one or two members and are not well character-
ized. An analysis done by Huntley and Murray [114] suggests that B-type Cdks in plants form two sub-
groups, one with a PPTALRE motif and one with a PPTTLRE motif. The protein products of most plant
cdc2 homologues are predicted to be near the expected 34 kDa except alfalfa cdc2MsC, Arabidopsis
cdc2c/At, pea cdkPs3, and rice R2 (Table 4). In yeast, only one Cdk is involved in both G 1 /S and G 2 /M
transitions, but in animals multiple Cdks are involved [115]. At least two different genes have been iso-
lated for many of the plant species (Table 4). Southern analysis in Petuniasuggests the possibility of more
than one homologue [108]. Other tobacco complementary DNA (cDNA) fragments were isolated that had
modified PSTAIRE motifs [112], and Southern analysis in Norway spruce suggests a family of 10 genes,
some of which were identified as psuedogenes [107]. Olomoucine, a chemical inhibitor of Cdk1/Cdk2 ki-
nases, reversibly arrested Petuniaprotoplasts at G 1 andArabidopsiscell suspension cells at G 1 and G 2 ,
suggesting that Cdk1/Cdk2 kinases are involved in both G 1 /S and G 2 /M transitions [116]. Using the same
inhibitor, similar results were obtained in Vicia faba[117]. An alfalfa Cdk, CDC2Ms, was activated at the
G 1 /S transition when phosphate-starved cells reentered the cell cycle and remained active through S, G 2 ,
and M phases, again suggesting that one Cdk is involved in both checkpoints [118]. However, a study of
the cell cycle phase specificity of putative Cdk variants in alfalfa showed a fluctuation of transcript lev-
els and amounts and activities of kinases in different cell cycle phases, which suggests the involvement
of more than one Cdk [99]. Some Cdks may be involved in non–cell cycle functions as has been shown
in yeast [119] and vertebrates [120]. The ArabidopsisCdk, CDC2b, was shown to be involved in seedling
growth via regulation of hypocotyl cell elongation and cotyledon cell development [121].
Expression of plant Cdks has been correlated with proliferative tissues [101,105,107,108,112,122]
and the competence to divide [123,124]. In situ hybridization studies using an Arabidopsiscdc2 cDNA
sequence as a probe showed that transcripts accumulated in leaf primordia, vegetative shoot apical
meristem, flower meristem, root meristematic regions, and pericycle [123]. In root tips, where expres-
sion is high in rapidly proliferating cell files and low in the quiescent center, specific distribution of ex-
pression in the meristems parallels the pattern of mitotic activity. Hemerly et al. [125] introduced a fu-
sion gene consisting of the promoter of the Arabidopsiscdc2 homologue, CDC2a, fused with the
-glucuronidase gene (gus, uidAgene from E. coli) into Arabidopsisplants. Histochemical GUS anal-
ysis showed a positive correlation between CDC2a mRNA levels and the proliferative state of cells.
However,CDC2a expression was not restricted to dividing cells. CDC2a expression was observed in
some nondividing, differentiated tissue. In contrast, in animal cells Cdk1 is expressed in proliferating
cells but not in differentiated, nonproliferating cells [126,127]. It is suggested that the ability of plant
cells to dedifferentiate and reenter the cell cycle may be linked to the low-level expression of cdc2. The
expression pattern of the four rice Cdks showed CDC2Os-1,CDC2Os-2, and R2uniformly in the di-
viding regions of the root apex with CDC2Os-1 and CDC2Os-2 also expressed in differentiated cells
[128].CDC2Os-3 was detected only in patches in the dividing region. In Petuniathe level of the cdc2
homologueCDC2Petwas higher in 4C nuclei than in 2C nuclei even in nonproliferating cells, and so
a high level of Cdk may not indicate the proliferative state in tissues that have a high number of 4C nu-
clei in mature cells [108]. Studies of the expression of plant cdc2 homologues during the cell cycle have
given different results. Arabidopsis CDC2aAtand tobacco CDC2Nt1 mRNA levels were constant
throughout the cell cycle whereas Arabidopsis CDC2bAtwas preferentially expressed during S and G 2
phases of the cell cycle [112,124,129]. Petunia CDC2PetmRNA levels increased during G 2 compared
with G 0 -G 1 , rice CDC2Os-3 levels are highest from G 2 to M phase, and Antirrhinum CDC2candCdc2d
also fluctuate with the cell cycle, being expressed at the highest levels in G 2 /M [100,108,128,130]. Of
the six alfalfa Cdks identified, four showed expression throughout the cell cycle (cdc2MsA–C and
CDC2MsE), while CDC2MsDandFfluctuated with the cell cycle with the highest amount in G 2 to M
phases [99].
Histone H1 kinase (H1K) activity has been associated with fractions from plant cell extracts. Differ-
ent molecular weight fractions of protein from pea cell extracts were analyzed for the presence of a 34-
236 REDDY AND DAY