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

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 contributes 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. Significance statementFor annual plants, such as Arabidopsis, the correct timing of flowering in spring/summer is critical for reproductive success. Molecular mechanisms through which plants perceive and integrate day length with internal rhythms to accelerate flowering under long days are well described. However, little is known about the pathways sensing and conveying energy availability to the flowering programs. We found that RAPTOR, the regulatory unit of Target of Rapamycin complex (TORC), regulates CONSTANS post-transcriptionally through GIGANTEA. Both proteins are components of the photoperiod pathway of the flowering network and their miss-regulation in raptor mutants hampers the upregulation of genes promoting flowering. Our work suggests that the high energy available in long days is sensed and integrated into the photoperiod pathway by TORC.