S-adenosylmethionine synthetase (SAMS), 1-aminocyclopropane-1-carboxylic acid synthetase (ACS) and 1-aminocyclopropane-1-carboxylic acid oxidase (ACO) are three key enzymes in the biosynthesis of ethylene in plants. In this study, the gene families of SAMS, ACS and ACO in Dimocarpus longan (DlSAMS, DlACS and DlACO) were identified at the whole genome level. Bioinformatics analysis, expression pattern analysis at different early stages of somatic embryogenesis (SE), and expression pattern analysis of embryogenic callus (EC) in longan treated with 1-aminocyclopropane-1-carboxylic acid (ACC) at different time and different concentrations were carried out, so as to lay the foundation for in-depth study and utilization of DlSAMS, DlACS and DlACO family genes in longan.

The protein sequences of SAMS, ACS and ACO of Arabidopsis thaliana were downloaded from TAIR as reference sequences. TBtools, NCBI blast and other tools were used to search longan genome database to identify DlSAMS, DlACS and DlACO gene families. ExPASy, PrediSi, TMHMM Server2.0, NetPhos3.1Server, Plant-PLoc, MEME, PlantCARE, STRING, TBtools and other tools were used to predict the basic physicochemical properties, chromosomal localization, gene collinearity and selection pressure, gene structures, protein conservative motifs, cis-acting elements of promoters and protein interaction relationship of DlSAMS, DlACS and DlACO gene family members. The proteins sequences of longan, Arabidopsis thaliana, tomato, rice and maize were aligned by ClustalW software, and the evolutionary trees of SAMS, ACS and ACO were constructed by MEGA-X software. According to the FPKM values of longan transcriptome database, TBtools software was used to draw the expression heat maps of DlSAMS, DlACS and DlACO members at different early stages of SE. The expression of DlSAMS, DlACS and DlACO members in EC of longan under different concentrations of ACC and treatment time were analyzed by real-time fluorescence quantitative PCR (qRT-PCR).

There were 4, 11 and 4 members of SAMS, ACS and ACO families in the third generation genome of longan, respectively, and the number of family members in longan had little difference with those of Arabidopsis and tomato. DlSAMS family members were the most conservative, and all members had the same motifs, DlACS and DlACO family members also had many conservative motifs. A total of 19 members of the three families were distributed on 11 chromosomes, with six pairs of collinearity genes and 30 pairs of collinearity genes with Arabidopsis thaliana, all of which were subjected to purification selection. All the members of the three gene families contained a large number of light, hormone and stress response elements. The proteins interaction within the family was weak, but the interaction between the families were very close. According to the phylogenetic relationship, SAMS, ACS and ACO gene families could be divided into three subgroups. DlSAMS family members were only distributed in SAMS-Ⅰ and SAMS-Ⅱ, SAMS-Ⅲ might be unique to monocotyledons. DlACS and DlACO family members were distributed in all three subgroups, and the distribution was relatively uniform. The three gene families were closely related to SAMS, ACS and ACO in tomato and Arabidopsis thaliana. The analysis of transcriptome FPKM value showed that all members of DlSAMS family and DlACO4B were highly expressed in the early three stages of SE, which might play an important role in SE. DlACS1 and DlACS6B were highly expressed in globular embryo (GE) stage, which might be closely related to the formation of GE. The proliferation of EC and the expression of DlSAMS, DlACS and DlACO members were significantly affected by different concentrations of ACC in subculture medium. After 20 days of culture, the genes were mainly up-regulated, and the higher concentrations of ACC, the higher up regulation of genes. In 25-35 days, the expression of genes were mainly down regulated. However, at 20 d, the genes expression in EC under 0.01 mmol∙L^-1 ACC treatment was mainly down regulated, which might be the reason for its significantly higher proliferation than that of CK.

There were 4, 11 and 4 members of SAMS, ACS and ACO family in the third generation genome of longan, which were relatively conservative in evolution, and they contained a lot of hormones and stress response elements. These three gene family members played important roles in the regulation of SE in longan. 0.01 mmol∙L^-1 ACC treated might regulate and control the proliferation of EC by regulating the expression of DlSAMS, DlACS and DlACO genes.