Reshaping Organellar Translation and tRNA Metabolism: The Consequences of Photosynthesis Loss and Massive Horizontal Gene Transfer
The transition to holoparasitism in plants precipitates the loss of photosynthesis, fundamentally altering the selective landscape acting on organellar genomes. These changes raise questions about the mechanisms by which the essential, coevolved machinery of translation responds to extreme genomic erosion and metabolic dependency. Integrating comparative genomics, tRNA sequencing, and subcellular localization assays, we elucidate the extensive rewiring of organellar translation systems and the tRNA-dependent tetrapyrrole biosynthesis pathway in the holoparasitic angiosperm family Balanophoraceae, which exhibits extreme reduction of tRNA content in plastid and mitochondrial genomes. We identified a rare evolutionary event: the putative intracellular transfer of the plastid initiator tRNA (tRNA-iMet) to the nucleus, which compensates for its loss from the plastid genome. We also demonstrate that the unusual UAG-to-Trp reassignment in the Balanophora plastid genetic code is driven by the loss of release factor pRF1 and the recruitment of a mutated nuclear tRNA-Trp. Furthermore, we reveal that the retention of organellar nuclear-encoded aminoacyl-tRNA synthetases is dictated by the presence/absence of cognate organellar tRNAs, which appear to be functional regardless of their foreign (horizontal transfer from the host plant) or native origins. Finally, we uncover a striking evolutionary asymmetry in nuclear-encoded ribosomal proteins: while plastid subunits exhibit elevated substitution rates consistent with relaxed selection and compensatory coevolution, mitochondrial subunits display high sequence conservation, likely maintaining compatibility with the extensive horizontal gene transfer observed in this lineage. Collectively, these findings represent some of the most extreme changes ever identified in the anciently conserved machinery of plant organellar translation.