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

Abstract

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.