Molecular characterization of clock-associated PSEUDO-RESPONSE REGULATOR 9 gene from Oncidium ‘Gower Ramsey’

The plant circadian clock regulates a number of central activities such as growth, flowering, abiotic and biotic stress responses, and metabolism. The PSEUDO-RESPONSE REGULATORs ( PRRs ) contribute to the regulation of the circadian oscillator and clock output processes. In order to elucidate the role of Oncidium PRR9 , we isolated and characterized a PRR9 homolog ( OnPRR9 ) from commercially important Oncidium ‘Gower Ramsey’. OnPRR9 encodes a 603-amino acid protein with a N-terminal REC domain and a C-terminal CCT domain. Phylogenic analysis displayed that the OnPRR9 was orthologous to other known or deduced PRR proteins at the sequence level. Examination of the transcriptional expression profile of the OnPRR9 revealed that it is subjected to circadian rhythms under the conditions of 12-h light and 12-h dark cycles (12:12LD) and continuous light (LL) but not the continuous dark (DD) conditions. Genetic complementation showed that it can partially rescue the flowering time defect of Arabidopsis mutant prr5/9 grown under the long-day (16L:8D) conditions. Overexpression analysis indicated that the OnPRR9 gene can promote flowering and inhibit hypocotyl elongation under the short-day (8L:16D) conditions. Expression pattern of the OnPRR9 gene in petals during flower development and senescence implied that it may play a role in the initiation of flower senescence. These results extend our understanding of the functional conservation of PRR9 and may provide a means of regulating flowering time of Oncidium .