Rice Genetics IV - IRRI books - International Rice Research Institute

tion between the abundance of CDK transcripts and the proliferative state of cells wasdemonstrated in Arabidopsis, maize, and alfalfa. However, it has also been shownthat, in Arabidopsis, transcripts of cdc2aAt are localized not only in dividing cells butalso in differentiated tissues, such as the parenchyma of the vascular cylinder and thepericycle, which contains cells responsible for formation of lateral roots. Moreover,expression of cdc2aAt could be induced without cell division in suspension cultures.These results suggest that at least some CDK transcripts might be correlated with theacquisition of the competence to divide rather than with the actual division of cells.Genes for four different CDKs have been isolated from rice: cdc2Os1, cdc2Os2(Hashimoto et al 1992), cdc2Os3 (Kidou et al 1994, Umeda et al 1998), and R2 (Hata1991), which is similar to the gene for a CDK-activating kinase (CAK) that is requiredfor activation of CDK by phosphorylation of a conserved threonine residue inthe so-called T loop (Morgan 1995). In contrast, a new type of CAK gene was isolatedin Arabidopsis (Umeda et al 1998). The deduced amino acid sequence of Cdc2Os3showed that it is distinct from proteins in the CDC2/CDC28 family (Kidou et al 1994),whereas Cdc2Os1 and Cdc2Os2 are closely related to the homologues of CDC2 thathave been isolated from various organisms. The cdc2Os1 gene was able to partiallycomplement a temperature-sensitive mutation in the cdc28 gene in yeast, but cdc2Os2and R2 were unable to complement the same mutation (Hashimoto et al 1992).We have analyzed levels of transcript of genes for CDKs in rice plants by in situhybridization (Umeda et al 1999b). We found that cdc2Os3, which has an alteredPSTAIRE sequence, was expressed in a cell cycle-dependent manner, whereas theother two CDKs with the conserved PSTAIRE motif were expressed throughout thecell cycle. The transcript and protein product of cdc2Os3 were abundant from the G2to the M phase, an indication that the Cdc2Os3 protein might function in mitosis.We found that cdc2Os1 and cdc2Os2 were also expressed in the sclerenchyma,the pericycle, and the parenchyma of the central cylinder in the differentiated zone ofroots. In parts of Arabidopsis roots beyond the apical meristem, expression of cdc2aAtis restricted to the parenchyma of the vascular cylinder and to the pericycle cells(Martinez et al 1997). Both Cdc2Os1 and Cdc2Os2 of rice are closely related toCdc2aAt of Arabidopsis at the amino acid level, and may also be correlated with thecompetence of these cells to divide. The pericycle is a differentiated tissue but itretains the potential to divide and is responsible for lateral root formation. The expressionof cdc2Os1 and cdc2Os2 in the parenchyma of the central cylinder suggeststhat this cell layer might engage in some mitotic activity that contributes to the thickeningof primary roots in rice plants. The rice-specific expression of these two cdc2genes in the sclerenchyma remains to be investigated.In contrast to transcripts of cdc2Os1, cdc2Os2, and R2, transcripts of cdc2Os3were distributed with a patchy pattern in the dividing region of the root apex. Such apattern was also observed in the region near the shoot meristem and in the primordiafor lateral root formation. Counterstaining of sections with DAPI (4,6-diamidino-2-phenylindole) indicated that almost all cells with mitotic nuclei contained cdc2Os3transcripts, whereas cells forming cell plates had trace levels of transcripts. In doublelabelingexperiments with probes specific for transcripts of a gene for histone H4 andcdc2Os3, signals did not overlap, an indication that expression of cdc2Os3 did not370 Uchimiya et al