Loss of Diel Circadian Clock Gene Cycling Is a Part of Grape Berry Ripening

Diel cycles of gene expression are thought to adapt plants to 24-h changes in environmental conditions. The circadian clock contributes to this process, but less is known about circadian programs in developing reproductive organs. While model plants and controlled conditions have contributed greatly to our knowledge of circadian clock function, there is a need to better understand its role in crop plants under field conditions with fluctuating light and temperature. In this study, we investigated changes in the circadian clock during the development of grape berries of Vitis vinifera L. We found that the transcripts of circadian clock homologs had high-amplitude oscillations prior to, but not during, ripening. As ripening progressed, the amplitude and rhythmicity of the diel oscillations decreased until most transcripts tested had no significant fluctuation over the 24-h cycle. Despite this loss of rhythmicity, the majority of circadian clock genes investigated were expressed at or near their abundance at the nadir of their pre-ripening oscillation although the berries remained transcriptionally active. From this, it can be concluded that cycling of the canonical circadian clock appears unnecessary for berry ripening. Our data suggest that changes in circadian clock dynamics during reproductive organ development may have important functional consequences.