CELL CYCLE REGULATION IN PLANTS 239TABLE 5 Continued
Dest motifb
Plant Gene name Aaa RXXLX2–4N/PEST Reference
Soybean Glyma;CycA3;1 348 RVVLGELPN 167
Glyma;CycA2;1 469 RAVLSDISN 167
Glyma;CycA1;1 484 RPPLSNLTN 167
Glyma;CycB1;3 440 RRVLQDIGN 167
Glyma;CycB1;2 e RRALGDIGN 146
Glyma;CycB1;1 454 RKALGDIGN 146
Tobacco Nicta;CycA1;1 483 RPALTNISN 169
Nicta;CycA1;2 482 RPALTNISN 174
Nicta;CycA2;1 493 RAVLKDMKN 169
Nicta;CycA3;1 371 RVVLGELIN 174
Nicta;CycA3;2 383 RVVLGEIQN 174
Nicta;CycA3;3 314 RVVLGEIRN 174
Nicta;CycB1;1 447 RRALDIGN 168
Nicta;CycB1;2 473 RKALGDIGN 169
Nicta;CycD2;1 354 PEST 223
Nicta;CycD3;1 373 PEST 223
Nicta;CycD3;2 367 PEST 223
aPredicted number of amino acids in derived polypeptide.
bType of destruction motif present and sequence if RXXLX2–4N.
cSix other cyclin-box sequences were obtained by a PCR analysis of putative positives from a screening of a genomic library.
dTwo other possible start sites following the first stop codon would give polypeptides of different aa number and MW.
eUncertain due to truncated clone.
groups [170]. A third group of cyclins (discussed later), the D-like cyclins, also can be classified into three
groups. In 1996 it was proposed by Renaudin et al. [170] that a uniform naming system be used for plant
cyclins. Cyclin names used in this chapter conform to this system.
Expression of cyclins found in plants has been studied in many different ways and it is hard to com-
pare between systems. However, some overall conclusions can be drawn from the different studies. Anal-
ysis of expression of mitotic-like cyclin mRNA in different plant tissues, cell suspension cultures, and
calli using Northern or RNA dot blots has led to the conclusion that this mRNA is more highly expressed
in tissues that contain meristematic regions (roots, young leaves, flower buds, callus, cell suspension) than
in tissues that do not (stem, old leaves) [151,157,161]. These studies also showed that within a species,
different cyclin mRNAs can be expressed in different amounts in the same tissues. In Brassica,
Brana;CycA2;1 and Brana;CycA1;1 are both expressed in young leaves and apical meristem but only
Brana;CycA1;1 is expressed in roots. Similarly, in Arabidopsis, Arath;CycB2;2 is expressed only in roots
but Arath;CycB2;1 is expressed equally in all tissue [151]. Arath;CycA2;1 and Arath;CycA2;2 are also
expressed largely in roots but Arath;CycA2;2 is also expressed at lower levels in several tissues. In soy-
bean, Glyma;CycB1;1 was expressed at the highest levels in all tissues and Glyma;CycA3;1 and
Glyma;CycA2;1 at the lowest levels [167]. The transcripts of Glyma;CycA3;1 and Glyma;CycB1;1 ap-
peared to be most abundant in root tips and nodules, whereas Glyma;CycA1;1 was more abundant in
shoot apices but the evidence was not conclusive [167].
In situ hybridization has also been used to study expression of plant cyclins. Expression of cyclins in
Antirrhinumshowed that Antma;cycB1;1 and Antma;cycB1;2 were expressed in only some cells in
meristematic regions, suggesting that only cells in specific stages of the cell cycle expressed the genes
[100]. Cells in mitosis expressed both cyclin genes in prophase and metaphase. Some cells in interphase
also expressed cyclin genes but the exact stage (G 1 , S, or G 2 ) could not be determined. In roots of 4-day-
oldArabidopsisseedlings, Arath;CycB2;1&2 and Arath;CycA2;1&2 expression was restricted to the root
apical meristem and strong signals were detected during the formation of lateral roots [151]. In-depth in
situ studies of Arath;CycB1;1 correlate its expression with meristematic tissues such as the root tip, shoot
apices, axillary buds of the inflorescence, and pericycle [171]. In situ hybridization studies of rice A-type
cyclin Orysa;CycA1;1 showed expression from G 2 to early M phase, whereas expression of the B-type
cyclins Orysa;CycB2;1 and Orysa;CycB2;2 lasted until the end of mitosis [164]. In soybean, in situ hy-
bridization has shown that Glyma;CycB1;1 is expressed in G 2 to M phases and Glyma;CycA1;1 is ex-