The defective chloroplast and leaf-mutable (dcl-m) mutation of tomato blocks chloroplast differentiation in leaf mesophyll cells and a signaling system that appears to be required for morphogenesis of palisade cells during leaf growth. To dissect the function of DCL, mutants with stable dcl alleles (dcl-s) were generated and examined for their phenotype. DCL/dcl-s plant produce dcl-s/dcl-s seeds with embryos arrested at the globular stage of development. The levels of several chloroplast- and nuclear-encoded proteins are strongly reduced in dcl-m mutant leaf sectors without significant changes in their corresponding mRNAs. The 4.5S rRNA fails to be processed efficiently, however, suggesting that DCL has a direct or indirect function in rRNA processing or correct ribosome assembly. Accordingly, chloroplasts in dcl-m sectors are impaired in polysome assembly, which can explain the reduced accumulation of chloroplast-encoded proteins. These results suggest that DCL is required for chloroplast rRNA processing, and emphasize the importance of plastid function during embryogenesis.