Arabidopsis Nuclear Architecture related 1 facilitates the floral transition in a process involving molecular hydrogen

• AtNAR1 displays activity of potentially catalyzing electrons and protons (H + ) to form H 2 . • Four cysteine residues in NAR1 is important for the basal inducible catalyzing activity. • Rhythmic inducible NAR1-catalyzed H 2 production in Arabidopsis is observed. • NAR1-dependent early flowering is mainly achieved by H 2 -regulating rhythm oscillators. • NAR1-dependent early flowering maybe achieved by NO in a rhythmic fashion. Although emerging evidence revealed that molecular hydrogen (H 2 ) positively regulates numerous physiological responses, understanding the synthesis of H 2 and its functions is a challenge for biology. The correct timing of flowering is controlled by environmental stimuli and endogenous signals. We report that endogenous H 2 facilitates the floral transition in Arabidopsis. The Nuclear Architecture Related 1 (NAR1) gene of Arabidopsis was prokaryotic expressed and purified, and its H 2 -synthesizing activity was subsequently detected. Furthermore, mutation experiments were used to investigate the catalytic roles of conserved sequences and Cys sites. Biochemistry and molecular approaches were employed to investigate the role of H 2 in regulating floral transition. Similar to Fe-Fe hydrogenase (HYD1) from Chlamydomonas reinhardtii , NAR1 protein is a H 2 -synthesizing enzyme in Arabidopsis thaliana . Protein mutation experiments in vitro show that four Cys residues are important for the inducible catalyzing activity upon hypoxia. Accordingly, NAR1-dependent circadian-rhythmic H 2 was observed under light/dark cycles, which was accompanied with contrasting changes in nitric oxide (NO) signal. Knockdown of NAR1 by CRISPR or RNAi reduced H 2 production and delayed flowering. Whereas, exogenous H 2 supply and overexpressing NAR1 or CrHYD1 promoted early flowering. These early flowering phenotypes were aggravated by the removal of endogenous NO, but abolished by NO addition. Since the mutant ( mms19 ) defective in Fe-S cluster assembly function displays early flowering regardless of H 2 addition, we further deduce that NAR1 control of early flowering is largely achieved by H 2 . Biochemical and genetic evidences show that NAR1-driven floral transition is functionally linked to the modulation of circadian oscillators via NO signaling. Since H 2 production is modulated by circadian rhythms and constitutively produced, it may integrate external and internal cues into floral transition, and the modulation of its production might be a promising strategy for crop breeding cultivation.