Florigen and anti-florigen: flowering regulation in horticultural crops

Abstract

Flowering time regulation has significant effects on the agricultural and horticultural industries. Plants respond to changing environments and produce appropriate floral inducers (florigens) or inhibitors (anti-florigens) that determine flowering time. Recent studies have demonstrated that members of two homologous proteins, FLOWERING LOCUS T (FT) and TERMINAL FLOWER 1 (TFL1), act as florigen and anti-florigen, respectively. Studies in diverse plant species have revealed universal but diverse roles of the FT/TFL1 gene family in many developmental processes. Recent studies in several crop species have revealed that modification of flowering responses, either due to mutations in the florigen/anti-florigen gene itself, or by modulation of the regulatory pathway, is crucial for crop domestication. The FT/TFL1 gene family could be an important potential breeding target in many crop species.

Keywords: FLOWERING LOCUS T (FT); TERMINAL FLOWER 1 (TFL1); anti-florigen; chrysanthemum; florigen; photoperiod.

Fig. 1

Flowering time regulation by florigen and anti-florigen in Arabidopsis, rice, sugar beet, and chrysanthemums. The blue (solid) circles indicate systemic floral inducers, while red (dotted) circles indicate systemic floral inhibitors. TFL1 homologs suppress flowering at the shoot apex.

Fig. 2

Photoperiodic regulation of flowering in chrysanthemums. (A) Over-expression of AFT in Chrysanthemum morifolium results in extremely late flowering under short day (SD) photoperiod. (B) Under SD, FTL3 is produced in leaves to systemically induce flowering. Under non-inductive long day (LD) or night-break (NB), AFT is induced in leaves to systemically inhibit flowering. The red light signal perceived by phyB induces AFT but suppresses FTL3 expression. Induction of AFT by phyB is gated by the circadian clock. TFL1 acts as a constitutive local repressor of flowering. (C) Model for induction of AFT under LD, SD, and NB conditions. The gate for AFT induction opens at a constant time after dusk regardless of the photoperiod. Under LD, the photo-sensitive phase (dotted line) of AFT interacts with red light in the morning and induces AFT expression (solid line) to inhibit flowering. Under NB, illumination at midnight coincides with the photo-sensitive phase of AFT.

Fig. 3

The model for flowering time regulation in two major horticultural crops. (A) Flowering regulation in tomato. In wild species, a systemic anti-florigen SP5G is induced under long days (LD) to inhibit flowering. In day neutral (DN) domesticated species, the induction pathway of SP5G is attenuated. (B) Flowering regulation in wild strawberry (F. vesca). In seasonal flowering of short day (SD) cultivars, a strong floral repressor TFL1 suppresses flowering under LD. In perpetual flowering cultivars in which repressor activity of TFL1 is absent, FT1 and SOC1, act to promote flowering under LD.