SIN3 LIKE genes mediate long‐day induction of flowering but inhibit the floral transition in short days through histone deacetylation in Arabidopsis

Summary Day length or photoperiod changes are crucial for plants to align the timing of the floral transition with seasonal changes. Through the photoperiod pathway, day length changes induce the expression of the florigenic FLOWERING LOCUS T (FT) to promote flowering. In the facultative long days (LDs) plant Arabidopsis thaliana, LD signals induce flowering, whereas short days (SDs) inhibit flowering. Here, we show that in Arabidopsis SIN3 LIKE (SNL) family genes, encoding a scaffold protein for assembly of histone deacetylase complexes, directly repress the expression of an FT activator and three FT repressors to regulate the transition to flowering in SDs and LDs, respectively. Under inductive LDs, SNLs including SIN3 LIKE 1 (SNL1) to SNL5, function in partial redundancy to repress the expression of three AP2 family transcription factors that repress FT expression, and therefore mediate LD induction of FT expression and promote the transition to flowering. In contrast, under non‐inductive SDs SNLs act to inhibit the floral transition, partly through direct repression of a MADS box transcriptional factor that promotes FT expression. Therefore, our results reveal that SNLs, through histone deacetylation, play a dual role for the control of flowering in the LD plant Arabidopsis: inhibiting flowering when the day length is shorter and promoting the floral transition when days become longer than a threshold length. Significance Statement Day length changes are crucial for many plants to align the timing of the floral transition with seasonal changes. We report that SIN3 LIKE genes, through histone deacetylation, repress the expression of several FT regulators and play a dual role in flowering‐time regulation in the facultative LD plant Arabidopsis: inhibiting flowering when the day length is shorter and promoting the floral transition when days become longer than a threshold length, to ensure flowering at a proper time for reproductive success.