The Regulatory-Associated Protein of the Target of Rapamycin Complex, RAPTOR1B, interconnects with the photoperiod pathway to promote flowering in Arabidopsis

The transition from vegetative to reproductive growth (floral transition) is a strictly regulated energy-demanding process. In Arabidopsis, light perception coupled with internal circadian rhythms allows sensing changes in the duration of the light period (photoperiod) to accelerate flowering under long days (LD) in spring. This photoperiod-mediated floral induction relies on the accumulation of CONSTANS (CO) at dusk, a transcription factor that upregulates FLOWERING LOCUS T (FT) in leaves. Subsequently, FT protein moves into the shoot apical meristem to trigger the floral transition. Light and circadian clock-related signals are known to control CO at the genetic and protein levels; however, less is known about how energy sensing regulates components of the photoperiod pathway to modulate flowering. Here, we found that RAPTOR, a component of the Target Of Rapamycin complex (TORC), contributes to the induction of specific flowering genes that are under CO control. While transcription of CO remains intact in raptor mutants, its protein levels are reduced at dusk compared to wild-type (Col-0). This is due to increased protein degradation. Remarkably, GIGANTEA (GI) protein levels, which contribute to CO stabilization at dusk, are likewise hampered in the mutant. We show that RAPTOR interacts with and co-localizes at the nucleus with GI, altering GI levels through an unknown posttranscriptional mechanism. Phenotypic and molecular analysis of genetic crosses placed RAPTOR upstream of CO and GI. Since TORC is an energy sensor, our work suggests that RAPTOR could convey energy status information into the photoperiod sensing mechanism to fine-tune flowering behavior