Key enzyme in the synthesis of the chromophore of the phytochrome family of plant photoreceptors. Catalyzes the opening of the heme ring to form the open-chain tetrapyrrole biliverdin IX with the release of iron and carbon monoxide (CO). Produces specifically the biliverdin IX-alpha isomer. Can form complex with heme, is ferredoxin-dependent and its activity is increased in the presence of ascorbate. Plays a role in salt acclimation signaling. May affect the plastid-to-nucleus signaling pathway by perturbing tetrapyrrole synthesis. The plastid-to-nucleus signal plays an important role in the coordinated expression of both nuclear- and chloroplast-localized genes that encode photosynthesis-related proteins. heme b + 3 O2 + 3 reduced [NADPH--hemoprotein reductase] = biliverdin IXalpha + CO + Fe(2+) + H(+) + 3 H2O + 3 oxidized [NADPH--hemoprotein reductase] Activated by ascorbate. Optimum pH is 7.2. Widely expressed. By salt treatment. Down-regulated by jasmonate. Long hypocotyl, incomplete chloroplast and leaf development, lack of photoreversible phytochromes and lack of phytochromobilin, the phytochrome chromophore. No cotyledon expansion in response to bright-red light. Increased levels of jasmonate (JA) and constitutive expression of JA-inducible defense genes. Increased sensitivity to salt stress and no acclimation response to salinity. Belongs to the heme oxygenase family. Truncated N-terminus.