Genome-Wide Identification and Expression Analysis of the SWEET Gene Family Juglans regia

To investigate the distribution and characteristics of the Sugars Will Eventually Be Exported Transporters (SWEET) gene family in walnut (Juglans regia), we identified SWEET family members using bioinformatics approaches based on whole-genome sequencing data from 74 walnut accessions. Their sequence and structural features were characterized through phylogenetic analysis, gene structure examination, conserved motif prediction, and chromosomal localization. The results showed that a total of 24 SWEET genes (JrSWEETs) were identified in walnut, which could be classified into four subgroups. Gene structure and conserved motifs exhibited a high degree of conservation within each subgroup. Chromosomal localization analysis indicated that these genes are unevenly distributed across 12 chromosomes. A total of 15 duplicated gene pairs were detected, among which 13 pairs resulted from segmental duplication. The K_a/K_s values of all duplicated gene pairs were less than 1, suggesting that these genes have undergone strong purifying selection during evolution. Promoter analysis revealed the presence of cis-acting elements related to hormone responses, stress responses, and light responsiveness. RNA-seq expression analysis identified SWEET genes associated with fruit development and cold stress responses. In addition, population resequencing-based genetic polymorphism analysis detected numerous SNPs within SWEET genes. Collectively, our findings provide a theoretical basis for further functional studies on the roles of the SWEET gene family in sugar metabolism and stress resistance biology in walnut.