Changes in cell cycle status and expression of p34^cdc2 kinase during potato tuber meristem dormancy

Potato (Solanum tuberosum L. cv. Russet Burbank) tubers undergo a period of endodormancy that is characterized by cell division arrest. At the time of harvest, the tubers used in this study were completely endodormant (i.e. 0% sprouting). After 120 days of storage at 3°C, tubers transferred to 20°C had begun to exit endodormancy and exhibited ca 50% sprouting. After 223 days of 3°C storage, tubers transferred to 20°C were completely nondormant and exhibited 100% sprouting. Based on flow cytometry, about 70% of nuclei isolated from endodormant meristems are arrested in the G1/G₀ phase of the cell cycle. Storage of tubers at 3°C did not alter the cell cycle position nor did transfer of tubers from 3 to 20°C for 7 days prior to analysis unless tubers had been stored for at least 223 days. After 223 days of cold (3°C) storage, tubers transferred to 20°C for 7 days showed sprout growth in excess of 5 mm and an increase in the percentage of nuclei in the G₂ phase of the cell cycle. Uptake and incorporation of ³H-thymidine into DNA was low in all tubers up until 120 days postharvest. After that time, only tubers incubated at 20°C for 7 days prior to analysis exhibited an increase in ³H-thymidine incorporation. This increase coincided with visible sprout growth, demonstrating that cell cycle shifts in tuber meristems relate directly to sprout growth and not the breakage of the endodormancy per se. Using degenerate primers, a portion of a p34^cdc2 homolog was amplified from RNA isolated from log-phase potato suspension culture cells by polymerase chain reaction. Northern analysis with this probe demonstrated that mRNA levels for two p34^cdc2 homologs were present throughout the endodormant period. Imrnunoblot analysis demonstrated that levels of at least four proteins containing a PSTAIRE epitope (i.e. cdc2-like) were present at reduced levels in endodormant meristems and increased in reactivated tuber meristems that showed a shift in cell cycle kinetics based on flow cytometry and increased ³H-thymidine incorporation. These results indicate that the temporal shift in competence for cell division in potato meristems induced by dormancy is not accompanied by alterations in the level of mRNA for p34^cdc2 homologues but is correlated with a change in the level of PSTAIRE-containing proteins. This suggests that during endodormancy cell division in potato tuber meristems is regulated indirectly by post-transcriptional regulation of genes controlling the cell cycle.