Characterization of an enigmatic tubular ultrastructure in the bacterial defensive symbiont of the Asian citrus psyllid

"Candidatus Profftella armatura" (Betaproteobacteria) is a unique organelle-like defensive symbiont harbored intracellularly within the symbiotic organ of a devastating citrus pest, the Asian citrus psyllid Diaphorina citri (Insecta: Hemiptera). Our previous two-dimensional transmission electron microscopy identified an unprecedented ultrastructure that appeared tubular in Profftella, but their detailed architecture, three-dimensional arrangement, and components were unknown. To address these issues, this study conducted serial block-face scanning electron microscopy, high-voltage electron tomography, and fluorescence in situ hybridization. The results revealed that highly elongated (2.8-136 {micro}m observed), string-shaped Profftella cells contain tubes of various numbers (1-43 per cell observed) and length (up to 45 m observed), depending on the cell length. The tubes were evenly distributed throughout the cells, occupying an average of 6.3% of the total cell volume. Each tube consisted of five or six thin fibers twisted into a right-handed helix, maintaining a consistent diameter of approximately 230 nm along its entire length. Even without fixation or embedding, the tubes retained their shape under high vacuum conditions in electron microscopes, demonstrating their remarkable stability and robustness. These findings suggest that the tubes may help provide mechanical stability to the highly elongated and potentially vulnerable Profftella cells. Further analysis showed that the tubes are closely associated with ribosomes, suggesting a role in protein synthesis. Overall, these results offer new insights into the structural and functional evolution of bacteria, with potential implications for developing novel pest control strategies. Significance statementBacteria generally have simple intracellular structures, lacking organelles. Here, we report a highly elongated, elaborate organelle-like structure with a tubular shape in a bacterial symbiont of an important agricultural pest. The tube, which exhibits high durability and robustness, may contribute to the mechanical stability of the notably elongated symbiont cell. They are closely associated with numerous ribosomes, suggesting a potential role in gene expression. These findings not only enhance our understanding of bacterial evolution but may also provide clues for developing novel strategies for pest control.