Two chloroplast-localized MORF proteins act as chaperones to maintain tetrapyrrole biosynthesis

Tetrapyrroles have essential functions as pigments and cofactors during plant growth and development, and the tetrapyrrole biosynthesis pathway is tightly controlled. Multiple organellar RNA editing factors (MORFs) are required for editing of a wide variety of RNA sites in chloroplasts and mitochondria, but their biochemical properties remain elusive. Here, we uncovered the roles of chloroplast-localized MORF2 and MORF9 in modulating tetrapyrrole biosynthesis and embryogenesis in Arabidopsis thaliana. The lack or reduced transcripts of MORF2 or MORF9 significantly affected biosynthesis of the tetrapyrrole precursor 5-aminolevulinic acid and accumulation of Chl and other tetrapyrrole intermediates. MORF2 directly interacts with multiple tetrapyrrole biosynthesis enzymes and regulators, including NADPH:PROTOCHLOROPHYLLIDE OXIDOREDUCTASE B (PORB) and GENOMES UNCOUPLED4 (GUN4). Strikingly, MORF2 and MORF9 display holdase chaperone activity, alleviate the aggregation of PORB in vitro, and are essential for POR accumulation in vivo. Moreover, both MORF2 and MORF9 significantly stimulate magnesium chelatase activity. Our findings reveal a previously unknown biochemical property of MORF proteins as chaperones and point to a new layer of post-translational control of the tightly regulated tetrapyrrole biosynthesis in plants.