Balancing forces in the photoperiodic control of flowering.

In many plant species, the duration of the daily exposure to light (photoperiod) provides a seasonal cue that helps to adjust flowering time to the most favourable time of the year. In Arabidopsis thaliana, the core mechanism of acceleration of flowering by long days involves the stabilisation of the CONSTANS (CO) protein by light reaching the leaves, the direct induction of the expression of FLOWERING LOCUS T (FT) by CO and the migration of FT to the apex to promote flowering. In rice (Oryza sativa), the promotion of flowering by short days depends on the interplay between light conditions, and the genes Grain number, plant height and heading date locus 7 (Ghd7) and Early heading date 1 (Ehd1). In both cases, other day length-induced changes reinforce the core photoperiodic pathway of promotion of flowering. However, there are regulators of flowering time, quantitatively less important than the core pathways but still significant, which impact in the opposite direction, i.e. favouring rice flowering under long days or Arabidopsis flowering under short days. We show, for instance, that short days enhance leaf expression of SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 3 (SPL3), which stimulates Arabidopsis flowering under these conditions. We propose that fine tuning of flowering time depends on the balance of a hierarchy of multiple points of action of photoperiod on the network controlling flowering.