Authors
Takenoya, M., Shimizu, T., Miyake, K., Masuda, T.
Abstract
In plant cells, heme is synthesized in the plastid and metabolized to phytochromobilin (P{Phi}B) in two enzymatic steps: heme oxygenase (HO) and P{Phi}B synthase (HY2). In Arabidopsis thaliana, HO1/HY1/GUN2 predominantly functions for heme catabolism among the HO isoforms. Our previous study demonstrated that HO1 localization is altered in plastids or in the cytosol due to transcriptional start-site regulation. Introduction of either plastid- or cytosol-localized HO1 into HO1-deficient mutants (gun2 and hy1-1) resulted in recovery from the long hypocotyl, low pigmentation, and genomes uncoupling (gun) phenotypes, indicating the assembly of functional phytochromes (PHYs), as well as supporting the retrograde heme signaling hypothesis. To dissect the heme signaling and PHY assembly, in this study, we introduced either of the two types of regiospecific bacterial HOs that produce biliverdin IX (BVIX) or BVIX{beta}/{delta} into Arabidopsis hy1-1. Gene introduction of either plastid- or cytosol-localized BVIX-producing HO complemented the long hypocotyl, low pigmentation, and gun phenotypes of hy1-1. Interestingly, the introduction of BVIX{beta}/{delta}-producing HO, either in the plastid or in the cytosol, failed to complement the long hypocotyl and low pigmentation phenotypes, suggesting failure of functional PHY assembly. However, these lines restored the gun phenotype, thus supporting the heme signaling hypothesis. Based on the levels of complementation of the gun phenotype, we found that the expression of photosynthesis-associated nuclear genes (PhANGs) can be separated into PHY-dependent and -independent groups. Our results demonstrate that heme functions as a retrograde mobile biogenic signal from plastids, passing through the cytosol, to regulate the expression of PhANGs, and this regulation is distinct in its dependency of PHY.
Preprint server:
bioRxiv
The authors list and abstract were imported from bioRxiv on 12 Nov 2025.
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