Point Mutation of Magnesium Chelatase OsCHLI Gene Dampens the Interaction Between CHLI and CHLD Subunits in Rice

Proper chloroplast development and chlorophyll biosynthesis are essential for the photoautotrophic plants. The insertion of magnesium (Mg²⁺) into protoporphyrin IX (Proto), catalyzed by magnesium chelatase (Mg-chelatase), is the first committed step of chlorophyll biosynthesis. In dicot plants, a proposed model revealed that Mg-chelatase I subunit (CHLI) and Mg-chelatase D subunit (CHLD) can interact directly; however, their relation remains elusive in rice, a monocot model plant. In this study, we characterized a chlorophyll-deficiency mutant, etiolated leaf and lethal (ell), which displayed a yellow leaf in young seedlings and became lethal after three-leaf stage. Chlorophyll content in homozygous ell mutant was approximately 1 % of that in the wild type. Besides, chloroplast development in the mutant was entirely arrested and no thylakoid structure was observed. By map-based cloning, the ell locus was delimited to a 3.9-Mb interval in chromosome 3. A single-base mutation (G529C) in OsCHLI was identified, leading to an amino acid substitution (G177R) in a highly conserved region. Compared with the wild type, more Proto but less magnesium protoporphyrin IX (Mg-Proto) was measured in the ell mutant. Using protoplast transfection and callus transformation, we found that exogenous OsCHLI could consistently recover the lesion of chloroplast in the ell mutant. By subcellar localization analysis, OsCHLI was detected in the chloroplast. Despite the secondary structure of OsCHLI that was predicted to be altered in the mutant, the point mutation did not affect subcellular localization. Real-time PCR demonstrated that the ell mutation induced significantly transcriptional downregulation of the photosynthesis-associated nuclear and plastid genes. Additionally, yeast-two-hybrid experiments indicated that the single amino acid substitution blocked the intrinsic interaction between OsCHLI and OsCHLD. Moreover, OsCHLI showed physical interactions with some thioredoxins (TRXs), suggesting a similar regulatory mechanism of Mg-chelatase activity in both monocot and dicot plants.