Transcriptomic Analysis Reveals Immune Signaling Pathways Orchestrate “Lantern-like” Flower Formation Induced by Contarinia citri Barnes in Citrus grandis ‘Tomentosa’

Contarinia citri Barnes is a major pest of Citrus grandis ‘Tomentosa’, damaging flowers, including abnormal development with lantern-like morphology, and substantially reducing yield. However, the molecular mechanisms underlying this abnormal development remain unclear. Structural and anatomical observations combined with transcriptome analyses of normal and lantern-like flowers were performed to elucidate host regulatory pathways in response to C. citri. Infestation increased levels of salicylic acid, indole-3-acetic acid, and cis-zeatin, as well as chlorophyll and total flavonoid accumulation in petals. Simultaneously, an increased number of transverse petal cell layers led to petal thickening and lantern-like flower formation. Transcriptome sequencing identified 5601 differentially expressed genes. C. citri induced genes associated with increased petal cell number and enhanced photosynthesis and amino acid synthesis, likely providing nutrients for larvae. Most genes in the jasmonic acid, salicylic acid, and mitogen-activated protein kinase signaling pathways were up-regulated, promoting the synthesis of resistance-related compounds, including terpenoids, flavonoids, lignin, and wax, thereby enhancing petal resistance to C. citri. These findings elucidate plant–insect interactions and provide a new framework for understanding insect-induced plant developmental reprogramming, while identifying potential targets for breeding resistant C. grandis ‘Tomentosa’ varieties and developing novel C. citri control strategies.