Extensive import of nucleus-encoded tRNAs into chloroplasts of the photosynthetic lycophyte, Selaginella kraussiana

Over the course of evolution, land plant mitochondrial genomes have lost many transfer RNA (tRNA) genes and the import of nucleus-encoded tRNAs is essential for mitochondrial protein synthesis. By contrast, plastidial genomes of photosynthetic land plants generally possess a complete set of tRNA genes and the existence of plastidial tRNA import remains a long-standing question. The early vascular plants of the Selaginella genus show an extensive loss of plastidial tRNA genes while retaining photosynthetic capacity, and represent an ideal model for answering this question. Using purification, northern blot hybridization, and high-throughput tRNA sequencing, a global analysis of total and plastidial tRNA populations was undertaken in Selaginella kraussiana. We confirmed the expression of all plastidial tRNA genes and, conversely, observed that nucleus-encoded tRNAs corresponding to these plastidial tRNAs were generally excluded from the chloroplasts. We then demonstrated a selective and differential plastidial import of around forty nucleus-encoded tRNA species, likely compensating for the insufficient coding capacity of plastidial-encoded tRNAs. In-depth analysis revealed differential import of tRNA isodecoders, leading to the identification of specific situations. This includes the expression and import of nucleus-encoded tRNAs expressed from plastidial or bacterial-like genes inserted into the nuclear genome. Overall, our results confirm the existence of molecular processes that enable tRNAs to be selectively imported not only into mitochondria, as previously described, but also into chloroplasts, when necessary.