The endogenous time-keeping mechanism is responsible for organizing plant physiology and metabolism according to periodic environmental changes, such as diurnal cycles of light and dark and seasonal progression throughout the year. In plants, circadian rhythms control gene expression, stomatal opening, and the timing component of the photoperiodic responses, leading to enhanced fitness due to increased photosynthetic rates and biomass production. We have investigated the citrus genome databases of expressed sequence tags (EST) in order to identify genes coding for functionally characterized proteins involved in the endogenous time-keeping mechanism in Arabidopsis thaliana. Approximately 180,000 EST sequences from 53 libraries were investigated and 81 orthologs of clock components were identified. We found that the vast majority of Arabidopsis circadian clock genes are present in citrus species, although some important components are absent such as SRR1 and PRR5. Based on the identified transcripts, a model for the endogenous oscillatory mechanism of citrus is proposed. These results demonstrate the power of comparative genomics between model systems and economically important crop species to elucidate several aspects of plant physiology and metabolism.
central oscillator; circadian clock; data mining; photoperiodic responses; regulatory feedback loop
