Phytochrome-interacting factors interact with transcription factor CONSTANS to suppress flowering in rose

As day-neutral (DN) woody perennial plants, the flowering time of roses (Rosa spp.) is assumed to be independent of the photoperiodic conditions; however, light responses of rose plants are not well understood. Chinese rose (Rosa chinensis) plants were grown under two light intensities (low light [LL], 92 μmol·m-2·s-1; or high light [HL], 278 μmol·m-2·s-1), and either with or without an end-of-day far-red (EOD-FR) treatment. Flowering was significantly delayed in the LL condition compared with the HL, but was not affected by EOD-FR treatment. The time until flowering positively corresponded with the mRNA and protein levels of phytochrome-interacting factors (PIFs; RcPIFs). The heterologous expression of RcPIF1, RcPIF3, or RcPIF4 in the Arabidopsis (Arabidopsis thaliana) pifq quadruple mutant partially rescued the mutant's shorter hypocotyl length. Simultaneous silencing of three RcPIFs in R. chinensis accelerated flowering under both LL and HL, with a more robust effect in LL, establishing RcPIFs as flowering suppressors in response to light intensity. The RcPIFs interacted with the transcription factor CONSTANS (RcCO) to form a RcPIFs-RcCO complex, which interfered with the binding of RcCO to the promoter of FLOWERING LOCUS T (RcFT), thereby inhibiting its expression. Furthermore, this inhibition was enhanced when RcPIFs were stabilized by LL, leading to delayed flowering under LL compared with HL. Our results not only revealed another layer of PIF functioning in the flowering of woody perennial plants, but also established a mechanism of light response in DN plants.