Expression analysis of four pseudo-response regulator (PRR) genes in Chrysanthemum morifolium under different photoperiods

Genes encoding pseudo-response regulator (PRR) proteins play significant roles in plant circadian clocks. In this study, four genes related to flowering time were isolated from Chrysanthemum morifolium. Phylogenetic analysis showed that they are highly homologous to the counterparts of PRRs of Helianthus annuus and named as CmPRR2, CmPRR7, CmPRR37, and CmPRR73. Conserved motifs prediction indicated that most of the closely related members in the phylogenetic tree share common protein sequence motifs, suggesting functional similarities among the PRR proteins within the same subtree. In order to explore functions of the genes, we selected two Chrysanthemum varieties for comparison; that is, a short-day sensitive Zijiao and a short-day insensitive Aoyunbaixue. Compared to Aoyunbaixue, Zijiao needs 13 more days to complete the flower bud differentiation. Evidence from spatio-temporal gene expression patterns demonstrated that the CmPRRs are highly expressed in flower and stem tissues, with a growing trend across the Chrysanthemum developmental process. In addition, we also characterized the CmPRRs expression patterns and found that CmPRRs can maintain their circadian oscillation features to some extent under different photoperiod treatment conditions. These lines of evidence indicated that the four CmPRRs undergo circadian oscillation and possibly play roles in regulating the flowering time of C. morifolium.