MAC3A and MAC3B Modulate FLOWERING LOCUS M Splicing to Repress Photoperiod-Dependent Floral Transition.

Plants adjust their flowering time by integrating environmental cues through complex regulatory networks. RNA splicing plays a crucial role in modulating gene expression in response to flowering signals. The MOS4-associated complex (MAC), consisting of the evolutionarily conserved E3 ubiquitin ligases MAC3A and MAC3B, is pivotal in splicing regulation. However, their involvement in floral transition remained unclear. This study observed that mac3a/mac3b mutants flowered significantly earlier under short-day (SD) conditions, a phenotype absent under long-day (LD) conditions. This early flowering correlated with upregulation of FLOWERING LOCUS T (FT) and SUPPRESSOR OF OVEREXPRESSION OF CO 1 (SOC1) compared to wild-type plants. Transcriptomic analysis revealed alterations in transcript levels and splicing profiles of key floral regulators across different flowering pathways. Further investigation identified the thermosensory flowering regulator FLOWERING LOCUS M (FLM) as being influenced by MAC3A and MAC3B. Subsequently, we found that MAC3A and MAC3B exhibited higher expression in SD and were associated with FLM transcripts to modulate their splicing. This study elucidates how the MAC complex, through RNA splicing regulation, integrates environmental signals to modulate flowering, unveiling a new layer of complexity in flowering pathways crosstalk under non-inductive photoperiods.