Abstract

Drought stress severely restricts citrus growth and production, making the exploration of drought resistance mechanisms crucial for agricultural sustainability. Melatonin (MT) has emerged as a key regulator of plant abiotic stress responses, but its molecular mechanisms in enhancing drought tolerance in trifoliate orange ( Poncirus trifoliata L., also known as Citrus trifoliata L.) remain unclear. In this study, we integrated physiological, transcriptomic, and molecular approaches to investigate the role of MT in drought resistance of trifoliate orange. Exogenous MT significantly improved drought tolerance by mitigating oxidative damage (reduced EL, MDA, and H 2 O 2 ) and enhancing MT accumulation. Genome-wide analysis identified key MT biosynthesis families (TDC, T5H, SNAT, ASMT), with SNAT and ASMT showing significant expansion. Transcriptomic and WGCNA analyses indicated that MT treatment modulated abscisic acid (ABA) and jasmonic acid (JA) biosynthesis pathways, upregulating key genes such as SDR , AAO , PLA , and LOX . Further, PtrSNAT38 (a SNAT family member) was identified as a hub gene responsive to dehydration, with its overexpression enhancing citrus drought tolerance by increasing MT content. Yeast one-hybrid, dual-luciferase assays, and EMSA assays confirmed that PtrMYB13 directly bind to the PtrSNAT38 promoter, promoting its transcription. These findings reveal that MT enhances drought resistance via a regulatory network involving the PtrMYB13-PtrSNAT38 module and coordinated ABA/JA signaling.