SWEET transporters and their potential roles in response to abiotic and biotic stresses in mulberry

Mulberry (Morus spp., Moraceae) is a traditional economic crop plant and is also gradually utilized as beverage plants. SWEETs (Sugars Will Eventually be Exported Transporter) are important sugar transporters involved in various biological processes and responses to various stresses. However, SWEETs in mulberry are still poorly studied without a comprehensive functional analysis of SWEETs. In the present study, a total of 24 SWEETs were identified using Morus alba (Ma) genome. Phylogenetic analysis showed that these 24 MaSWEETs were clustered with SWEETs from Arabidopsis, Populus and Oryza and fell into four clades. MaSWEETs in the same clade are likely to pose similar intron/exon patterns. These MaSWEETs distributed on 12 chromosomes and tandem duplication and block duplication were responsible for the expansion of SWEETs in mulberry. Transmembrane domains and conserved active sites of Tyr and Asp were observed in MaSWEETs. Cis-elements in promoter regions of MaSWEETs indicated the possible function of MaSWEETs in response to hormones and environment stimulus. MaSWEETs showed quite different expression preference in tissues and organs indicating the possible function divergence. In addition, most MaSWEETs showed a disturbed expression levels in response to various abiotic stresses and Ciboria shiraiana infection. MaSWEET1a was functionally characterized as a negative regulator of resistance to C. shiraiana infection based on in vivo transient overexpression of MaSWEET1a in tobacco and down-regulation of MaSWEET1a/b in mulberry. Our results provided foundation for further functional dissection of SWEETs in mulberry and a potential regulator for genetic modification.