Yellow-Green Leaf 19 Encoding a Specific and Conservative Protein for Photosynthetic Organisms Affects Tetrapyrrole Biosynthesis, Photosynthesis, and Reactive Oxygen Species Metabolism in Rice

Chlorophyll is the main photosynthetic pigment and is crucial for plant photosynthesis. Leaf color mutants are widely used to identify genes involved in the synthesis or metabolism of chlorophyll. In this study, a spontaneous mutant, yellow-green leaf 19 (ygl19), was isolated from rice (Oryza sativa). This ygl19 mutant showed yellow-green leaves and decreased chlorophyll level and net photosynthetic rate. Brown necrotic spots appeared on the surface of ygl19 leaves at the tillering stage. And the agronomic traits of the ygl19 mutant, including the plant height, tiller number per plant, and total number of grains per plant, were significantly reduced. Map-based cloning revealed that the candidate YGL19 gene was LOC_Os03g21370. Complementation of the ygl19 mutant with the wild-type CDS of LOC_Os03g21370 led to the restoration of the mutant to the normal phenotype. Evolutionary analysis revealed that YGL19 protein and its homologues were unique for photoautotrophs, containing a conserved Ycf54 functional domain. A conserved amino acid substitution from proline to serine on the Ycf54 domain led to the ygl19 mutation. Sequence analysis of the YGL19 gene in 4726 rice accessions found that the YGL19 gene was conserved in natural rice variants with no resulting amino acid variation. The YGL19 gene was mainly expressed in green tissues, especially in leaf organs. And the YGL19 protein was localized in the chloroplast for function. Gene expression analysis via qRT-PCR showed that the expression levels of tetrapyrrole synthesis-related genes and photosynthesis-related genes were regulated in the ygl19 mutant. Reactive oxygen species (ROS) such as superoxide anions and hydrogen peroxide accumulated in spotted leaves of the ygl19 mutant at the tillering stage, accompanied by the regulation of ROS scavenging enzyme-encoding genes and ROS-responsive defense signaling genes. This study demonstrates that a novel yellow-green leaf gene YGL19 affects tetrapyrrole biosynthesis, photosynthesis, and ROS metabolism in rice.