Constitutive expression of CsGI alters critical night length for flowering by changing the photo-sensitive phase of anti-florigen induction in chrysanthemum.

Chrysanthemum is a typical short day (SD) flowering plant that requires a longer night period than a critical minimum duration to successfully flower. We identified FLOWERING LOCUS T-LIKE 3 (FTL3) and ANTI-FLORIGENIC FT/TFL1 FAMILY PROTEIN (AFT) as a florigen and antiflorigen, respectively, in a wild diploid chrysanthemum (Chrysanthemum seticuspe). Expression of the genes that produce these proteins, CsFTL3 and CsAFT, is induced in the leaves under SD or a noninductive photoperiod, respectively, and the balance between them determines the progression of floral transition and anthesis. However, how CsFTL3 and CsAFT are regulated to define the critical night length for flowering in chrysanthemum is unclear. In this study, we focused on the circadian clock-related gene GIGANTEA (GI) of C. seticuspe (CsGI) and generated transgenic C. seticuspe plants overexpressing CsGI (CsGI-OX). Under a strongly inductive SD (8 L/16D) photoperiod, floral transition occurred at almost the same time in both wild-type and CsGI-OX plants. However, under a moderately inductive (12 L/12D) photoperiod, the floral transition in CsGI-OX plants was strongly suppressed, suggesting that the critical night length for flowering was lengthened for CsGI-OX plants. Under the 12 L/12D photoperiod, CsAFT was upregulated in CsGI-OX plants. Giving a night break (NB) 10 h after dusk was the most effective time to inhibit flowering in wild-type plants, while the most effective time for NB was extended to dawn (12 and 14 h after dusk) in CsGI-OX plants. In wild-type plants, a red-light pulse delivered 8 or 10 h after dusk induced maximal CsAFT expression, but the length of the time period over which CsAFT could be induced by red light was extended until subjective dawn in CsGI-OX plants. Therefore, CsGI-OX plants required a longer dark period to maintain lower levels of CsAFT, and their critical night length for flowering was thus lengthened. These results suggested that CsGI has an important role in the control of photoperiodic flowering through shaping the gate for CsAFT induction by light in chrysanthemum.