The CsbHLH124‐CsTPS5 Regulatory Module Governs d ‐Limonene Biosynthesis and Gray Blight Resistance in Tea Plant ( Camellia sinensis )

d-limonene is a key monoterpene that contributes to the characteristic aroma of tea (Camellia sinensis) and plays a defensive role against pathogenic infections. However, the regulatory mechanisms underlying its biosynthesis and associated disease resistance remain largely elusive. In this study, d-limonene levels were found to be significantly elevated in pathogen-infected tea leaves, and exogenous application of d-limonene markedly enhanced resistance to gray blight disease. Transcriptomic analysis identified CsTPS5, encoding a terpene synthase, as bening strongly induced upon pathogen challenge. Subcellular localization, in vitro enzyme assays, and in vivo functional verification demonstrated that CsTPS5 catalyzes the conversion of geranyl diphosphate (GPP) into d-limonene. Furthermore, yeast one-hybrid screening, electrophoretic mobility shift assays (EMSA), and dual-luciferase reporter assays revealed that the bHLH transcription factor CsbHLH124 directly binds to the promoter of CsTPS5 and activates its expression. Gene silencing experiments further confirmed that the CsbHLH124-CsTPS5 regulatory module plays a crucial role in promoting d-limonene biosynthesis and suppressing lesion formation caused by Pseudopestalotiopsis camelliae-sinensis (Ps.cs.). These findings elucidate a previously uncharacterized regulatory pathway controlling terpene metabolism and provide a promising molecular framework for enhancing both aroma quality and disease resistance in tea breeding programmes.