Objective MYC was a core subfamily member of the bHLH transcription factor family and played a significant role in plant growth and development, stress response, and secondary metabolism regulation. This study aimed to identify and systematically analyze MYC family members in three cultivars of longan (Dimocarpus longan), namely HHZ, JDB, and SX, as well as in litchi (Litchi chinensis), and to explore their expression characteristics based on the transcriptome database of longan HHZ. The study further investigated the functions of the MYC family in Sapindaceae.

Method Based on genomic data, MYC family members in longan and litchi were identified through Blast comparison and HMMsearch methods. Protein physicochemical properties, conserved motifs, gene structure, phylogenetic evolution, chromosomal location, collinearity, and cis-acting elements in the promoter region were analyzed using Expasy, MEME, TBtools, and MEGA software. Based on transcriptome data, the expression patterns of MYC genes in longan HHZ at different stages of somatic embryogenesis and under different treatments (NPA, PEG, and temperature) were analyzed.

Result A total of 11, 10, 11, and 15 MYC family members were identified in longan HHZ, JDB, SX, and litchi, respectively. All members contained bHLH_MYC_N and bHLH domains. Except for DlMYC10(SX), which was a neutral protein, all MYC proteins were acidic and hydrophilic, and most were located in the nucleus without signal peptides or transmembrane structures. Phylogenetic analysis divided the MYC family into five subgroups. Members within the same subgroup had similar gene structures and motif compositions. The promoter regions harbored abundant cis-elements associated with light response, hormone response (such as methyl jasmonate, gibberellin, and auxin), abiotic stress response (such as drought, low temperature, and hypoxia), and tissue-specific expression-related elements. Collinearity analysis showed that there were 14 pairs of collinear genes between longan HHZ, JDB, and litchi MYC members, indicating their high conservation in the evolution of Sapindaceae. Expression analysis indicated that DlMYC1(HHZ), DlMYC8(HHZ), and DlMYC9(HHZ) were highly expressed at the GE stage; DlMYC6(HHZ) and DlMYC8(HHZ) expression levels were downregulated under NPA treatment, and the downregulation was significant at 15 days of treatment; DlMYC2(HHZ), DlMYC3(HHZ), and DlMYC6(HHZ) had the highest expression level under 25℃ treatment; and DlMYC9(HHZ) expression levels were downregulated under different concentrations of PEG treatment.

Conclusion MYC family members in longan and litchi are highly conserved in evolution and have functional diversity. MYC family members in longan HHZ may be involved in methyl jasmonate signaling response and the early stage of somatic embryogenesis. This study lays a theoretical foundation for further understanding the gene functions of MYC family members in Sapindaceae plants.