Phytochrome B1-dependent control of SP5G transcription is the basis of the night break and red to far-red light ratio effects in tomato flowering

BACKGROUND: Phytochromes are dimeric proteins with critical roles in perceiving day length and the environmental signals that trigger flowering. Night break (NB) and the red to far-red light ratio (R:FR) have been used extensively as tools to study the photoperiodic control of flowering. However, at the molecular level, little is known about the effect of NB and different R:FR values on flowering in day-neutral plants (DNPs) such as tomato. RESULTS: Here, we show that tomato SP5G, SP5G2, and SP5G3 are homologs of Arabidopsis thaliana FLOWERING LOCUS T (FT) that repress flowering in Nicotiana benthamiana. NB every 2 h at intensities of 10 μmol m(− 2) s(− 1) or lower R:FR (e.g., 0.6) caused a clear delay in tomato flowering and promoted SP5G mRNA expression. The promoted SP5G mRNA expression induced by red light NB and low R:FR treatments was reversed by a subsequent FR light stimulus or a higher R:FR treatment. The tomato phyB1 mutation abolished the effects of NB and lower R:FR treatments on flowering and SP5G mRNA expression, indicating that the effects were mediated by phytochrome B1 in tomato. CONCLUSION: Our results strongly suggest that SP5G mRNA suppression is the principal cause of NB and lower R:FR effects on flowering in tomato. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1186/s12870-018-1380-8) contains supplementary material, which is available to authorized users.