Rice transcription factor bHLH25 confers resistance to multiple diseases by sensing HO.

Hydrogen peroxide (HO) is a ubiquitous signal regulating many biological processes, including innate immunity, in all eukaryotes. However, it remains largely unknown that how transcription factors directly sense HO in eukaryotes. Here, we report that rice basic/helix-loop-helix transcription factor bHLH25 directly senses HO to confer resistance to multiple diseases caused by fungi or bacteria. Upon pathogen attack, rice plants increase the production of HO, which directly oxidizes bHLH25 at methionine 256 in the nucleus. Oxidized bHLH25 represses miR397b expression to activate lignin biosynthesis for plant cell wall reinforcement, preventing pathogens from penetrating plant cells. Lignin biosynthesis consumes HO causing accumulation of non-oxidized bHLH25. Non-oxidized bHLH25 switches to promote the expression of Copalyl Diphosphate Synthase 2 (CPS2), which increases phytoalexin biosynthesis to inhibit expansion of pathogens that escape into plants. This oxidization/non-oxidation status change of bHLH25 allows plants to maintain HO, lignin and phytoalexin at optimized levels to effectively fight against pathogens and prevents these three molecules from over-accumulation that harms plants. Thus, our discovery reveals a novel mechanism by which a single protein promotes two independent defense pathways against pathogens. Importantly, the bHLH25 orthologues from available plant genomes all contain a conserved M256-like methionine suggesting the broad existence of this mechanism in the plant kingdom. Moreover, this Met-oxidation mechanism may also be employed by other eukaryotic transcription factors to sense HO to change functions.