Alternative localization of HEME OXYGENASE 1 in plant cells regulates cytosolic heme catabolism.

This article focuses on the discovery and characterization of alternative transcription start sites (TSSs) in the plant HEME OXYGENASE 1 (HO1) gene, which lead to the production of two distinct HO1 isoforms with different subcellular localizations and functions in heme catabolism. In both Arabidopsis thaliana and rice (Oryza sativa), HO1 has two TSSs generating a long transcript (HO1L) encoding a plastid-targeted protein and a short transcript (HO1S) encoding a cytosolic protein. Light signaling via phytochromes and cryptochromes regulates the HO1L/HO1S ratio through key transcription factors, with HO1L predominating in light-grown tissues and HO1S in roots and etiolated seedlings. Functional analyses demonstrated that both HO1L and HO1S can complement HO1-deficient mutants, restoring normal photomorphogenesis, pigment content, and retrograde signaling phenotypes, indicating that cytosolic heme catabolism constitutes a bypass pathway alongside the plastid-localized pathway. This finding challenges the prevailing view that heme catabolism occurs exclusively in plastids and suggests a cytosolic heme degradation route important during early development and under varying light conditions.