Evolution and Stress Responses of Gossypium hirsutum SWEET Genes

The SWEET (sugars will eventually be exported transporters) proteins are sugar efflux transporters containing the MtN3_saliva domain, which affects plant development as well as responses to biotic and abiotic stresses. These proteins have not been functionally characterized in the tetraploid cotton, <i>Gossypium hirsutum</i>, which is a widely cultivated cotton species. In this study, we comprehensively analyzed the cotton <i>SWEET</i> gene family. A total of 55 putative <i>G. hirsutum</i><i>SWEET</i> genes were identified. The <i>GhSWEET</i> genes were classified into four clades based on a phylogenetic analysis and on the examination of gene structural features. Moreover, chromosomal localization and an analysis of homologous genes in <i>Gossypium arboreum</i>, <i>Gossypium raimondii</i>, and <i>G. hirsutum</i> suggested that a whole-genome duplication, several tandem duplications, and a polyploidy event contributed to the expansion of the cotton <i>SWEET</i> gene family, especially in Clade III and IV. Analyses of <i>cis</i>-acting regulatory elements in the promoter regions, expression profiles, and artificial selection revealed that the <i>GhSWEET</i> genes were likely involved in cotton developmental processes and responses to diverse stresses. These findings may clarify the evolution of <i>G. hirsutum SWEET</i> gene family and may provide a foundation for future functional studies of SWEET proteins regarding cotton development and responses to abiotic stresses.