The biosynthetic pathway of tetrapyrrole is under stringent regulation in living systems. Heme, one of the products of this branched pathway is well known to regulate the pathway through feedback inhibition. Since the accumulation of intermediates of tetrapyrrole biosynthetic pathway is found to be cytotoxic and inducer of plant programmed cell death through retrograde signalling, certain role of heme in regulating plant cell death is quite logical. In the present study, we report the in vivo hemebinding property of OsHFP, a novel hemopexin fold protein from the rice (Oryza sativa) plant, and document the effect of heme–OsHFP interaction on oxidative stress responses in bacterial and plant systems. The bound heme is retained by the OsHFP even in presence of SDS, and the amount of bound heme to OsHFP increases upon exogenous application of heme biosynthetic precursor in Escherichia coli. In heme-replete condition, the recombinant OsHFP alters the oxidative stress response in E. coli. Ectopic expression of the OsHFP generates oxidative stress in tobacco leaves, which is visualized through the induction of superoxide dismutase activity and localized accumulation of H2O2. On the basis of the findings from the present study and the available literature, we propose a possible mode of action of OsHFP in regulating cell death in plant system, which might signify the yet-to-be-deciphered physiological role of hemopexin fold proteins in plants.
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