A Transcriptional Suppressor NTL8 Interacts with CONSTANS to Control Photoperiod-Mediated Flowering Time in <i>Arabidopsis thaliana</i>

Plants perceive rhythmic photoperiodic signals to modulate flowering time. In Arabidopsis thaliana, long-day light conditions accelerate flowering through CONSTANS (CO)-activated FLOWERING LOCUS T (FT) signal pathway. The CO protein abundance presents circadian oscillation, enabling precise regulation of FT transcription. NTL8 belongs to the NAC transcription factor family and is reported to control leaf trichome development. Here, we reported that NTL8 regulated flowering time in Arabidopsis, because overexpressing NTL8 significantly delayed flowering time, whereas loss-of-function mutant of NTL8 accelerated flowering time. NTL8 also presented circadian expression, maintaining elevated transcript levels during the daytime. Biochemical and genetic analyses revealed that NTL8 physically interacted with CO in planta, thereby antagonizing CO activity and repressing FT expression to delay the flowering time. Furthermore, overexpressing NTL8 reduced the protein stability of CO, particularly by attenuating CO accumulation in the morning time and promoting CO degradation during the night. Collectively, our findings indicated that NTL8 delayed photoperiod-dependent flowering time by suppressing CO-mediated activation of FT expression and destabilizing CO protein. This study uncovers a previously unrecognized role for NTL8 in coordinating circadian and photoperiodic signals to fine-tune flowering time in Arabidopsis thaliana.