Molecular Mechanism ofTtrapyrrole Distribution and Transportation in Higher Plants

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In plants, tetrapyrroles are classified into four major groups, chlorophylls, bilins, hemes, and siroheme. These tetrapyrroles are indispensable for various vital activities of plant cells. These tetrapyrroles are synthesized in plastids, subsequently distributed to various organelles. Chlorophylls exist in only plastids, while hemes are required for numerous apo-heme proteins existing in various organelles in the cell. Furthermore, recent studies on retrograde signaling from the chloroplast suggest that some chlorophyll intermediates, such as Mg-protoporphyrin IX and Mg-protoporphyrin IX monomethyl ester, act as signals transferred from plastids to the nucleus to control nuclear-encoded photosynthetic genes. However, the mechanisms of distribution and transportation of these tetrapyrroles in plant cells are not well understood. Since tetrapyrroles are poorly soluble in aqueous solutions under physiological conditions, it is assumed that there must be tetrapyrrole transportation system that are responsible for tetrapyrrole trafficking in the plant cell.In this research project, to clarify the molecular mechanism of tetrapyrrole distribution and transportation in higher plants, we first developed extremely sensitive heme assay by reconstitution with horseradish peroxidase apo-enzyme, which allows to detect effused heme from plastids. We then performed biochemical characterization of Arabidopsis cytosolic heme-binding proteins (cHBPs) homologous to the p22HBP/SOUL family. Furthermore, we clarified the redox-dependent regulation of Mg-chelatase via chloroplast thioredoxins, whish is responsible for production of retrograde signals, Mg-protoporphyrin IX.These contributions significantly enhanced our understandings of regulations and functions of tetrapyrroles in plant cells.