Catalyzes the opening of the heme ring to form the open-chain tetrapyrrole biliverdin IX with the release of iron and carbon monoxide (CO). Is a key enzyme in the synthesis of the chromophore of the phytochrome family of plant photoreceptors. Essential for photoperiod response and repression of flowering through cytochromes that inhibit flowering by affecting both HD1 and EHD1 flowering pathways. heme b + 3 O2 + 3 reduced [NADPH--hemoprotein reductase] = biliverdin IXalpha + CO + Fe(2+) + H(+) + 3 H2O + 3 oxidized [NADPH--hemoprotein reductase] Pale green and very early flowering phenotype and complete deficiency in photoperiodic response. Lack of photoreversible phytochromes and responses of coleoptile elongation by light pulses. Belongs to the heme oxygenase family.