Abstract
BackgroundRecently we showed that de novo expression of a turtle riboflavin-binding protein (RfBP) in transgenic Arabidopsis increased H2O2 concentrations inside leaf cells, enhanced the expression of floral regulatory gene FD and floral meristem identity gene AP1 at the shoot apex, and induced early flowering. Here we report that RfBP-induced H2O2 presumably results from electron leakage at the mitochondrial electron transport chain (METC) and this source of H2O2 contributes to the early flowering phenotype.ResultsWhile enhanced expression of FD and AP1 at the shoot apex was correlated with early flowering, the foliar expression of 13 of 19 METC genes was repressed in RfBP-expressing (RfBP+) plants. Inside RfBP+ leaf cells, cytosolic H2O2 concentrations were increased possibly through electron leakage because similar responses were also induced by a known inducer of electron leakage from METC. Early flowering no longer occurred when the repression on METC genes was eliminated by RfBP gene silencing, which restored RfBP+ to wild type in levels of FD and AP1 expression, H2O2, and flavins. Flowering was delayed by the external riboflavin application, which brought gene expression and flavins back to the steady-state levels but only caused 55% reduction of H2O2 concentrations in RfBP+ plants. RfBP-repressed METC gene expression remedied the cytosolic H2O2 diminution by genetic disruption of transcription factor NFXLl and compensated for compromises in FD and AP1 expression and flowering time. By contrast, RfBP resembled a peroxisomal catalase mutation, which augments the cytosolic H2O2, to enhance FD and AP1 expression and induce early flowering.ConclusionsRfBP-repressed METC gene expression potentially causes electron leakage as one of cellular sources for the generation of H2O2 with the promoting effect on flowering. The repressive effect on METC gene expression is not the only way by which RfBP induces H2O2 and currently unappreciated factors may also function under RfBP+ background.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-014-0381-5) contains supplementary material, which is available to authorized users.
Highlights
We showed that de novo expression of a turtle riboflavin-binding protein (RfBP) in transgenic Arabidopsis increased H2O2 concentrations inside leaf cells, enhanced the expression of floral regulatory gene FLOWERING LOCUS D (FD) and floral meristem identity gene AP1 at the shoot apex, and induced early flowering
Because flavin mononucleotide (FMN)/FMNH2 and flavin adenine dinucleotide (FAD)/FADH2 serve as redox centers in complexes I and II, respectively, flavins are likely to play a pivotal role in electron leakage and H2O2 generation from mitochondrial electron transport chain (METC) [13,21,26]. In agreement with this notion, recently we demonstrated that cell cytosolic H2O2 concentrations could be altered by modulating concentrations of free flavins in leaves of Arabidopsis thaliana [13]
To simplify experimental conditions in this study, we investigated those plants grown in typical long days and under this condition we confirmed de novo expression of the RfBP gene in RfBP+ and gene silencing in RfBP−
Summary
We showed that de novo expression of a turtle riboflavin-binding protein (RfBP) in transgenic Arabidopsis increased H2O2 concentrations inside leaf cells, enhanced the expression of floral regulatory gene FD and floral meristem identity gene AP1 at the shoot apex, and induced early flowering. We report that RfBP-induced H2O2 presumably results from electron leakage at the mitochondrial electron transport chain (METC) and this source of H2O2 contributes to the early flowering phenotype. Riboflavin (vitamin B2) is the precursor of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), essential cofactors for many metabolic enzymes involved in multiple cellular processes, such as mitochondrial electron transport chain (METC) and cellular redox regulation in other cellular compartments [1,2,3]. H2O2 is a more stable ROS form, than O2– for example, and frequently functions as a cellular signal to regulate multiple aspects of plant development [10,11]. If METC functions normally, an electron tetrad (four electrons as a group) in each transport round is transferred through the carrier-protein complexes to a single O2 accepter, which reduces O2 to form H2O with protons from
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