SlGRAS17 negatively regulates chlorophyll biosynthesis in tomato

Chlorophyll content is a pivotal agronomic trait that directly determines photosynthetic efficiency and visual quality in tomato (Solanum lycopersicum). Although positive regulators of chlorophyll biosynthesis have been extensively characterized, the mechanisms of their transcriptional repression remain less understood. Here, we report that the GRAS family transcription factor SlGRAS17 functions as a key negative regulator of chlorophyll accumulation in tomato. We isolated a high-chlorophyll mutant, hcm1, from an ethyl methanesulfonate (EMS)-mutagenized population and identified the causative lesion as a premature stop codon in SlGRAS17. Both CRISPR/Cas9-generated slgras17 mutants and the original hcm1 allele exhibited dark green leaves with significantly elevated chlorophyll content, whereas SlGRAS17-overexpressing plants displayed a pale green phenotype with reduced chlorophyll levels. We demonstrated that SlGRAS17 localizes to the nucleus and directly binds to a distal region of the GOLDEN-like 1 (SlGLK1) promoter to repress its transcription. Genetic evidence from virus-induced gene silencing confirmed that the high-chlorophyll phenotype of slgras17 mutants is dependent on SlGLK1. Furthermore, we elucidated that SlGRAS17 physically interacts with the transcriptional co-repressor SEUSS (SlSEU3), which, in turn, bridges the interaction with LEUNIG (SlLUG) in a configuration that is proposed to form a complex with repressive function. Genetic analyses revealed that SlLUG is essential for SlGRAS17-mediated repression, as the loss of SlLUG abolished the repressive effect of SlGRAS17 overexpression on chlorophyll accumulation. Our study unveils a previously unidentified transcriptional regulatory module in which SlGRAS17 recruits the SlSEU3-SlLUG co-repressor complex to suppress SlGLK1 expression, thereby fine-tuning chlorophyll biosynthesis in tomato.