Abstract
BackgroundRiboflavin is the precursor of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), essential cofactors for many metabolic enzymes that catalyze a variety of biochemical reactions. Previously we showed that free flavin (riboflavin, FMN, and FAD) concentrations were decreased in leaves of transgenic Arabidopsis plants expressing a turtle riboflavin-binding protein (RfBP). Here, we report that flavin downregulation by RfBP induces the early flowering phenotype and enhances expression of floral promoting photoperiod genes.ResultsEarly flowering was a serendipitous phenomenon and was prudently characterized as a constant phenotype of RfBP-expressing transgenic Arabidopsis plants in both long days and short days. The phenotype was eliminated when leaf free flavins were brought back to the steady-state levels either by the RfBP gene silencing and consequently nullified production of the RfBP protein, or by external riboflavin feeding treatment. RfBP-induced early flowering was correlated with enhanced expression of floral promoting photoperiod genes and the florigen gene FT in leaves but not related to genes assigned to vernalization, autonomous, and gibberellin pathways, which provide flowering regulation mechanisms alternative to the photoperiod. RfBP-induced early flowering was further correlated with increased expression of the FD gene encoding bZIP transcription factor FD essential for flowering time control and the floral meristem identity gene AP1 in the shoot apex. By contrast, the expression of FT and photoperiod genes in leaves and the expression of FD and AP1 in the shoot apex were no longer enhanced when the RfBP gene was silenced, RfBP protein production canceled, and flavin concentrations were elevated to the steady-state levels inside plant leaves.ConclusionsToken together, our results provide circumstantial evidence that downregulation of leaf flavin content by RfBP induces early flowering and coincident enhancements of genes that promote flowering through the photoperiod pathway.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-014-0237-z) contains supplementary material, which is available to authorized users.
Highlights
Riboflavin is the precursor of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), essential cofactors for many metabolic enzymes that catalyze a variety of biochemical reactions
riboflavin-binding protein (RfBP) reduces leaf flavin content in long days and short days Recently we showed that leaf flavin concentrations were significantly reduced in the Arabidopsis RfBP+ line than in WT or RfBP− plants under a 12hour light/12-hour dark cycle [11]
Free riboflavin, FMN, and FAD concentrations were decreased by 60%, 52%, and 69%, respectively, in leaves of RfBP+ compared to WT, but in RfBP−, flavins were retrieved to approximations of WT levels (Figure 1c)
Summary
Riboflavin is the precursor of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), essential cofactors for many metabolic enzymes that catalyze a variety of biochemical reactions. The induction of disease resistance accompanies elevated cytosolic levels of hydrogen peroxide (H2O2), a cellular signal that can regulate defense responses [7,10,11,16] All of these RfBP-conferred responses can be eliminated by nullifying RfBP expression and abolishing production of the RfBP protein. The RfBPsilenced (RfBP−) Arabidopsis line generated under RfBP+ background resembles the wild-type (WT) plant in the leaf flavin content, disease resistance, and H2O2 production [11]. These findings support the notion that changing flavin concentrations has biological consequences [7,10,11]
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