A genetic screen identifies the E3 ubiquitin ligase MPSR1 as a suppressor of early flowering of the sensitivity to red light reduced 1 mutant

The timing of floral transition is critical for plants to maximize reproductive success. In Arabidopsis thaliana, SENSITIVITY TO RED LIGHT REDUCED 1 (SRR1) delays the onset of flowering in non-favourable environmental conditions. Furthermore, SRR1 is required for the function of the circadian clock. In a genetic suppressor screen for mutants that alleviate early flowering of srr1-1 mutants, we identified ssm136 (suppressor of srr1-1 mutant 136) that represses srr1-1 early flowering in short photoperiods. Bulk segregant analysis and full-genome sequencing identified MISFOLDED PROTEIN SENSING RING E3 LIGASE 1 (MPSR1) as the causal gene. Outcrossing the mpsr1-ssm allele to wild-type showed that MPSR1 represses flowering independently of SRR1, but the effect is stronger in srr1-1, suggesting that MPSR1 and SRR1 act together in flowering time control. Co-immunoprecipitation showed that SRR1 and MPSR1 are able to physically interact in vivo. Expression analysis revealed that MPSR1 is regulated by the circadian clock but not required for maintaining the clock itself. Furthermore, MPSR1 expression is repressed by light and induced in short days, consistent with its role in floral repression under noninductive photoperiodic conditions.