Cytosolic heme metabolism by alternative localization of heme oxygenase 1 in plant cells

Abstract

AbstractHeme, an organometallic molecule, is widely engaged in oxygen transport, electron delivery, enzymatic reactions, and signal transduction. Additionally, heme serves as a precursor to phytochromobilin, the chromophore of plant phytochrome. Heme oxygenase (HO) initiates the first committed step in heme metabolism. Our transcription start site-sequencing (TSS-seq) revealed thatHO1inArabidopsis thalianaandOryza sativa(rice) has two TSSs, producing long (HO1L) and short (HO1S) transcripts, with or without an intact N-terminal plastid transit peptide.HO1LandHO1Sproducts localize in plastids and the cytosol, respectively. In Arabidopsis,HO1Lis prevalent in light-exposed shoots, whileHO1Sis clearly detected in roots and etiolated seedlings. During de-etiolation and early development,HO1Lratio gradually rises andHO1Sratio decreases. Light perception via phytochrome and cryptochrome elevatesHO1Lratio and reduceHO1Sratio through the functioning of HY5 and HYH transcription factors, and the suppression of DET1, E3 ubiquitin ligase COP1, and PIFs transcription factors. As expected,HO1Lproduct was able to complement theHO1-deficient mutantgun2(hy1), but surprisingly,HO1Sexpression could also restore the short hypocotyl phenotype and high pigment content, and make the mutant recover from thegunphenotype. This indicates the formation of functional holo-phytochrome within these lines. Our work highlights the presence of a cytosolic pathway for heme metabolism, especially during etiolation and early development. Furthermore, it supports the hypothesis that a mobile heme signal is involved in the mediation of retrograde signaling from the chloroplast.Significance StatementIn this research, through both TSS-seq and CAGE-seq, we discovered that theHO1(GUN2orHY1) gene in both Arabidopsis and rice has two TSSs, generatingHO1LandHO1Stranscripts. We reveal that theHO1TSS regulation pathway is the same as the light signaling pathway. Significantly, our study identifies that a cytosolic heme metabolism pathway is existent in plant cells.