From resistance screening to biomarker discovery: the α-linolenic acid–phenylpropanoid pathway as defence indicator against Lymantria xylina in Casuarina equisetifolia

Casuarina equisetifolia L., a key pioneer species in tropical and subtropical coastal shelterbelts, suffers from ecological decline owing to recurrent infestations by Lymantria xylina Swinhoe. To address this, we deciphered the molecular defence mechanisms and identified reliable biomarker metabolites for resistance breeding. Phenotypic analyses suggesting that the resistance to L. xylina was jointly mediated by antixenosis and antibiosis, and three resistant half-sib families (e.g., 3-52, 5-80 and 5-218) were preliminarily identified. Integrated transcriptomic and metabolomic profiling of L. xylina herbivory demonstrated significant enrichment of differentially expressed genes in the circadian rhythm-plant, jasmonic acid signaling and phenylpropanoid biosynthesis pathways-which may mediate the optimization of defence responses. In parallel, differential accumulation metabolites were predominantly enriched in α-linolenic acid metabolism. In particular, the content of α-linolenic acid strongly correlated with resistance, while flavonoids and tannins also exhibited significant positive correlations, confirming their roles in the defence response. These results suggest that the α-linolenic acid to phenylpropanoid cascade may function as a core defence axis in C. equisetifolia against L. xylina. Collectively, α-linolenic acid, flavonoids and tannins serve as candidate biomarkers for breeding insect-resistant C. equisetifolia genotypes.