Down-regulation of specific plastid ribosomal proteins suppresses thf1 leaf variegation, implying a role of THF1 in plastid gene expression.

Chloroplast development is regulated by many biological processes. However, these processes are not fully understood. Leaf variegation mutants have been used as powerful models to elucidate the genetic network of chloroplast development since the degree of leaf variegation is regulated by developmental and environmental cues. The thylakoid formation 1 (thf1) mutant is unique for its variegation in both leaves and cotyledons. Here, we reported a new suppressor gene of thf1 leaf variegation, designated sot8. Map-based cloning and DNA sequencing results showed that a single nucleotide mutation from G to A was detected in the second exon of the gene encoding the ribosomal protein small subunit 9 (PRPS9) in sot8-1, resulting in an amino acid change and a partial loss of PRPS9 function. However, sot8-1 was unable to rescue the thf1 phenotype in low photosystem II activity (Fv/Fm). In addition, we identified two T-DNA insertion mutants defective in plastid-specific ribosomal proteins (PSRPs), psrp2-1, and psrp5-1. Genetic analysis showed that knockdown of PSRP5 expression but not PSRP2 expression suppressed leaf variegation. Northern blotting results showed that precursors of plastid rRNAs over-accumulated in prps9-1 and psrp5-1, indicating that mutations in PRPS9 and PSRP5 cause a defect in rRNA processing. Consistently, inhibition of plastid protein synthesis by spectinomycin led to increased levels of plastid rRNA precursors in wild-type plants, suggesting that rRNA processing and plastid ribosomal assembly are coupled. Taken together, our data indicate that downregulating the expression of specific plastid ribosomal proteins suppresses thf1 leaf variegation, and provide new insights into a role of THF1 in plastid gene expression.