Plant melatonin is widely recognized as a pleiotropic regulator. As a growth-regulating hormone, it extensively participates in various growth and developmental processes and has significant functions in stress responses and disease resistance. Plant melatonin is synthesized primarily through the catalytic actions of five enzymes: TDC (tryptophan decarboxylase), T5H (tryptamine-5-hydroxylase), SNAT (serotonin N-acetyltransferase), ASMT (N-acetylserotonin methyltransferase), and COMT (caffeic acid-O-methyltransferase). There are multiple genes for each of these five enzymes in citrus genomes, however, with the exception of COMT5—whose function has recently been elucidated—and SNAT, which has only been preliminarily identified, the remaining genes have not been unequivocally characterized or functionally annotated. Hence, we carried out a genome-wide analysis of melatonin biosynthesis enzyme-related gene families in trifoliate orange (Poncirus trifoliata), one of the most common citrus rootstock varieties. Through bioinformatics approaches, we identified 96 gene family members encoding melatonin biosynthetic enzymes and characterized their protein sequence properties, phylogenetic relationships, gene structures, chromosomal distributions, and promoter cis-acting elements. Furthermore, by analyzing expression patterns in different tissues and under various stresses, we identified multiple stress-responsive melatonin synthase genes. These genes likely participate in melatonin synthesis under adverse conditions, thereby enhancing stress adaptation. Specifically, PtCOMT5, PtASMT11, and PtTDC9 were significantly induced by low temperature; PtSNAT1, PtSNAT14, PtSNAT18, and PtTDC10 were markedly responsive to drought; and PtASMT15, PtSNAT15, PtASMT16, and PtSNAT3 were strongly induced by ABA. Among them, PtASMT23 expression was induced up to 120-fold under low temperature, while PtSNAT18 showed over 100-fold upregulation under dehydration treatment. These findings strongly suggest that PtASMT23 and PtSNAT18 play critical roles in regulating melatonin biosynthesis in response to cold and drought stress, respectively. Collectively, these findings pinpoint novel genetic targets for enhancing stress resilience in citrus breeding programs and lay the foundation for the functional characterization of specific melatonin biosynthesis pathway gene family members in citrus and other horticultural crop species.

Citrus fruits are widely cultivated all over the world. Due to climatic conditions, citrus fruits are frequently exposed to periodic low temperatures, which poses a serious threat to their yield and quality. Cold not only restricts plant growth and deteriorates fruit quality but also leads to fruit abscission and tree mortality, posing severe constraints on large-scale citrus production. The TIFY family gene plays crucial roles in plant development and stress adaptation. However, the genome-wide identification and functional analysis of TIFY genes in cold stress adaptation of citrus plants remain largely unexplored. Here, we performed a systematic genome-wide analysis of the TIFY family in sweet orange (Citrus sinensis (L.) Osbeck) and identified 14 CsTIFY members. We conducted a comprehensive study on the protein characteristics, phylogenetic relationships, gene structure, chromosome distribution, promoter cis-acting elements, and subcellular localization of these genes. Phylogenetic analysis classified the CsTIFYs into ZML (ZML1–ZML4), JAZ (JAZ1–JAZ7), PPD (JAZ8, JAZ9), and TIFY (TIFY1) subfamilies, and they are distributed on seven chromosomes. Collinearity analysis revealed that segmental duplication is the primary driver for CsTIFY family expansion. Expression profiling under cold stress identified JAZ1, JAZ2, and JAZ3 as the most cold-inducible members. All three CsTIFY proteins are targeted to the nucleus, as confirmed by subcellular localization analysis. Overexpression of JAZ1, JAZ2, or JAZ3 in citrus calli significantly enhanced cold sensibility. Collectively, this study elucidates the gene function of CsTIFYs under cold stress and provides new insight for molecular breeding of cold-tolerant citrus varieties.