Enigmatic tubular ultrastructure in the bacterial defensive symbiont of the Asian citrus psyllid Diaphorina citri

Candidatus Profftella armatura (Betaproteobacteria) is an organelle-like defensive symbiont inhabiting the symbiotic organ of a devastating citrus pest, the Asian citrus psyllid Diaphorina citri. Previous two-dimensional electron microscopy hinted at unprecedented ultrastructures in Profftella, but their precise architecture and composition were unknown. Here, using serial block-face scanning electron microscopy, high-voltage electron tomography, and fluorescence in situ hybridization, we show that elongated Profftella cells (2.8-136 μm observed) contain multiple tubes (1-43 per cell) up to 45 μm long. These tubes, occupying ~6.3% of the cell volume, are composed of five or six fibers twisted into a right-handed helix with a consistent diameter of ~230 nm. Their stability under high vacuum suggests a mechanical support role in elongated Profftella. Close association with ribosomes implies a possible role in protein synthesis. Our findings provide insight into the structural adaptations of intracellular symbionts and may inform strategies for controlling citrus pests.