Host plant quality reshapes symbiotic organ architecture without altering symbiont density

Obligate nutritional symbioses enable sap-feeding insects to overcome essential amino acid limitations, yet the extent to which these associations exhibit plasticity across host plants remains unclear. We tested whether host plant quality alters the Bactericera cockerelli - Carsonella symbiosis by comparing psyllids developing on tomato and pepper. Tomato sap contained significantly higher essential amino acid concentrations than pepper. Despite these nutritional differences, only 42 genes were differentially expressed in the bacteriome, a symbiotic organ composed of insect cells (bacteriocytes) that house Carsonella . Tomato-fed psyllids upregulated genes associated with cell communication and structural organization, whereas pepper-fed psyllids upregulated metabolic and translational pathways. We also observed that bacteriocyte number was higher on tomato and in females, while bacteriocyte symbiont density remained unchanged. Psyllid fitness was also higher on tomato. These findings support a condition-dependent model in which plant nutrition shapes bacteriome structural and transcriptional investment in symbiosis without altering symbiont density per bacteriocyte.