Integrated Metabolomic and Transcriptomic Profiling Reveals the Defense Response of Tea Plants (<i>Camellia sinensis</i>) to <i>Toxoptera aurantii</i>

The tea plant (<i>Camellia sinensis</i>) is a unique beverage crop worldwide, but its yield and quality are adversely affected by <i>Toxoptera aurantii</i>. However, the response mechanisms of tea plants to <i>T. aurantii</i> stress remain poorly known. Herein, we present the life table of <i>T. aurantii</i> on resistant (W016) and susceptible (HJY) tea cultivars, demonstrating that the fitness of <i>T. aurantii</i> on W016 was lower than that on HJY. Integrated metabolic and transcriptomic analyses revealed that <i>T. aurantii</i> feeding activated pathways associated with phenylpropanoid biosynthesis, plant hormone signal transduction, and ATP-binding cassette (ABC) transporters. Notably, <i>T. aurantii</i> feeding significantly upregulated the levels of brassinolide and <i>p</i>-coumaryl alcohol in W016 but not in HJY. Furthermore, <i>in vitro</i> enzymatic assays indicated that <i>C. sinensis</i> cinnamyl alcohol dehydrogenase (CsCAD1) catalyzes the formation of <i>p</i>-coumaryl alcohol participation in lignin synthesis. Our findings highlight the role of brassinolide-mediated lignin biosynthesis of the tea plant in response to <i>T. aurantii</i> feeding.