Genetic Characterization of SWEET Genes in Coconut Palm

Sugar-Will-Eventually-be-Exported Transporters (SWEETs) play a crucial role in sugar transport in plants, mediating both plant development and stress responses. Despite their importance, there has been limited research characterizing the functional characteristics of CnSWEET genes in coconut (Cocos nucifera). In this study, we conducted a systematic analysis of SWEET genes in coconut using bioinformatics, subcellular localization studies, in silico promoter analysis, and functional assays with yeast mutants. A total of 16 CnSWEET genes were identified and grouped into four clades. Clade I contained the highest number of genes (eight), derived from four pairs of duplicated genomic segments. In contrast, the other clades had fewer genes (one to four) compared to those in Arabidopsis and other species in the Arecaceae family. An extensive analysis of gene expansion using MSCanX indicated significant divergence in gene expansion patterns, both between and within monocots and dicots, as well as among closely related species within the same family. Notable variations in conserved protein motifs and the number of transmembrane helices (TMHs) were detected within Clade I compared to other clades, affecting the subcellular localization of CnSWEET proteins. Specifically, seven TMHs were associated with proteins located in the cell membrane, while CnSWEET2A, which had five TMHs, was found in both the cell membrane and cytosol. Promoter analysis revealed that some CnSWEET genes contained endosperm or seed specific motifs associated with specific endosperm expression, consistent with expression patterns observed in transcriptome data. Functional analysis of five CnSWEET genes, with transcript sequences supported by transcriptome data, was conducted using yeast mutant complementation assays. This analysis demonstrated diverse transport activities for sucrose, fructose, glucose, galactose, and mannose. Our findings provide valuable insights into the functional diversity of SWEET genes in coconuts and their potential roles in stress responses and plant development.