Serotonin N-acetyltransferase (SNAT) is a crucial enzyme in the melatonin synthesis pathway, playing an essential role in both melatonin biosynthesis and plant resistance to abiotic stress. A bioinformatics approach was employed to identify the members of the citrus SNAT gene family and to analyze their physicochemical properties, gene structure, conserved domains, phylogenetic relationships, and promoter cis-acting elements. Additionally, the tissue-specific expression of trifoliate orange SNAT family members and their expression patterns under stress conditions were examined. This study identified 21 members of the SNAT gene family across five citrus genomes, distributed over five chromosomes, with the majority predicted to localize within chloroplasts. These genes were characterized by having between 1 and 8 exons, 0 and 7 introns, 1 and 2 conserved domains, and 5 and 8 motifs. Phylogenetic analysis classified the genes into four subgroups, demonstrating significant collinearity with SNAT genes in rice. Analysis of the promoter regions revealed 32 cis-acting elements, with those responsive to light, abscisic acid, and drought being the most common. Expression analysis of SNAT genes in trifoliate orange indicated tissue specificity, with the highest expression levels detected in leaves. Quantitative real-time PCR analysis showed that the PtrSNAT1 gene was notably upregulated under various stress conditions, suggesting its role in stress response. Overall, these findings provide critical insights for further functional studies of citrus SNAT genes in relation to abiotic stress responses. Moreover, the PtrSNAT1 gene represents a potential target for developing rootstocks with enhanced resistance to abiotic stress.

Ascorbate oxidase (AAO) plays an important role in maintaining cellular redox homeostasis, thereby influencing plant growth, development, and responses to both biotic and abiotic stresses. However, there has been no systematic characterization of AAO genes in Citrus, especially their roles in response to cold stress. In the present study, nine AAO genes were identified in C. sinensis through bioinformatics analyses, exhibiting uneven distribution across four chromosomes. All CsAAOs possessed three conserved domains and were predicted to localize in the apoplast. The CsAAO gene family displayed varied intron–exon patterns. Phylogenetic analysis categorized the CsAAO family into three main clades (Clade A–C), suggesting distinct biological functions. Collinearity and Ka/Ks analysis revealed three duplicate gene pairs within the CsAAO gene family, with all duplicated CsAAOs primarily evolving under purifying selection. Analysis of cis-acting elements showed the presence of multiple hormone response elements and stress response elements within the CsAAO promoters. The computational analysis of microRNA target transcripts suggested that CsAAO9 may be a target of csi-miR156. RNA-Seq data demonstrated high expression levels of CsAAOs in roots and young fruits, while qRT-PCR analysis showed significant upregulation of six CsAAOs in response to cold treatment. Furthermore, the activities of CsAAOs exhibited a pattern of initial decrease followed by an increase after exposure to low temperatures. These findings offer important insights into the role of CsAAOs in response to cold stress. Furthermore, AAOs could be target genes for breeding crops with better cold resistance.

In most higher plants, sucrose is the significant form of carbohydrate for long-distance transportation. Sucrose transporters/sucrose carriers (SUTs/SUCs) are involved in the loading and unloading of sucrose in phloem and play an important role in the growth and development of plants. In this study, 12 RsSUC genes were first identified from the radish genome, and their phylogenetic relationships, gene structure, and conserved motifs were further analyzed. RT-qPCR results indicated that RsSUC genes exhibited various expression patterns in different tissues and development stages of the radish. Overexpression of RsSUC1b in Arabidopsis significantly improved the uptake efficiency of exogenous sucrose, and promoted leaves and lateral root growth. In addition, the transgenic plants flowered significantly earlier than wild-type (WT) plants, and the soluble sugar contents (SSCs) including sucrose, glucose, and fructose in the mature leaves and pods were increased. It could be inferred that RsSUC1b is a plasma membrane sucrose transporter and plays a vital role in sucrose transportation and sugar accumulation during plant growth and development. These findings provided novel insights into the biological function of RsSUC genes and facilitate dissecting the molecular mechanism underlying sugar transport during radish development.