Dynamics of Chloroplast Translation during Chloroplast Differentiation in Maize

Chloroplast genomes in land plants contain approximately 100 genes, the majority of which reside in polycistronic transcription units derived from cyanobacterial operons. The expression of chloroplast genes is integrated into developmental programs underlying the differentiation of photosynthetic cells from non-photosynthetic progenitors. In C4 plants, the partitioning of photosynthesis between two cell types, bundle sheath and mesophyll, adds an additional layer of complexity. We used ribosome profiling and RNA-seq to generate a comprehensive description of chloroplast gene expression at four stages of chloroplast differentiation, as displayed along the maize seedling leaf blade. The rate of protein output of most genes increases early in development and declines once the photosynthetic apparatus is mature. The developmental dynamics of protein output fall into several patterns. Programmed changes in mRNA abundance make a strong contribution to the developmental shifts in protein output, but output is further adjusted by changes in translational efficiency. RNAs with prioritized translation early in development are largely involved in chloroplast gene expression, whereas those with prioritized translation in photosynthetic tissues are generally involved in photosynthesis. Differential gene expression in bundle sheath and mesophyll chloroplasts results primarily from differences in mRNA abundance, but differences in translational efficiency amplify mRNA-level effects in some instances. In most cases, rates of protein output approximate steady-state protein stoichiometries, implying a limited role for proteolysis in eliminating unassembled or damaged proteins under non-stress conditions. Tuned protein output results from gene-specific trade-offs between translational efficiency and mRNA abundance, both of which span a large dynamic range. Analysis of ribosome footprints at sites of RNA editing showed that the chloroplast translation machinery does not generally discriminate between edited and unedited RNAs. However, editing of ACG to AUG at the rpl2 start codon is essential for translation initiation, demonstrating that ACG does not serve as a start codon in maize chloroplasts.