Transcript profiling of an Arabidopsis PSEUDO RESPONSE REGULATOR arrhythmic triple mutant reveals a role for the circadian clock in cold stress response.

Arabidopsis PSEUDO RESPONSE REGULATOR (PRR) genes are components of the circadian clock mechanism. In order to understand the scope of genome-wide transcriptional regulation by PRR genes, a comparison survey of gene expression in wild-type Arabidopsis and a prr9-11 prr7-10 prr5-10 triple mutant (d975) using mRNA collected during late daytime was conducted using an Affymetrix ATH-1 GeneChip. The expression of 'night genes' increased and the expression of 'day genes' decreased toward the end of the diurnal light phase, but expression of these genes was essentially constant in d975. The expression levels of 'night genes' were lower, whereas the expression of 'day genes' was higher in d975 than in the wild type. Bioinformatics approaches have indicated that the set of up-regulated genes in d975 and the set of cold-responsive genes have significant overlap. We found that d975 is more tolerant to cold, high salinity and drought stresses than the wild type. In addition, dehydration-responsive element B1/C-repeat-binding factor (DREB1/CBF), which is expressed around mid-day, is more highly expressed in d975. Raffinose and L-proline accumulated at higher levels in d975 even when plants were grown under normal conditions. These results suggest that PRR9, PRR7 and PRR5 are involved in a mechanism that anticipates diurnal cold stress and which initiates a stress response by mediating cyclic expression of stress response genes, including DREB1/CBF.