Conserved Daily Transcriptional Programs in

Most organisms have internal circadian clocks that mediate responses to daily environmental changes in order to synchronize biological functions to the correct times of the day. Previous studies have focused on plants found in temperate and sub-tropical climates, and little is known about the circadian transcriptional networks of plants that typically grow under conditions with relatively constant day lengths and temperatures over the year. In this study we conducted a genomic and computational analysis of the circadian biology of , a tropical tree. We found that predicted papaya circadian clock genes cycle with the same phase as genes. The patterns of time-of-day overrepresentation of circadian-associated promoter elements were nearly identical across papaya, , rice, and poplar. Evolution of promoter structure predicts the observed morning- and evening-specific expression profiles of the papaya paralogs. The strong conservation of previously identified circadian transcriptional networks in papaya, despite its tropical habitat and distinct life-style, suggest that circadian timing has played a major role in the evolution of plant genomes, consistent with the selective pressure of anticipating daily environmental changes. Further studies could exploit this conservation to elucidate general design principles that will facilitate engineering plant growth pathways for specific environments.