Transcriptome-wide identification and expression profiling of the SWEET family in Bletilla striata and regulation analysis with non-coding RNAs

Polysaccharides are major bioactive ingredients in the medicinal tubers of Bletilla striata (Thunb.) Rchb.f. The SWEET (Sugars Will Eventually Be Exported Transporters) family is widely identified as bidirectional transporters mediating sugar translocation between source and sink tissues in plants. However, to date, SWEET family members have not been reported in B. striata, as well as the roles in sugar transport and B. striata polysaccharide (BSP) accumulation have not been elucidated. Here, by combining multiple RNA-seq datasets, a de novo transcriptome was assembled to identify SWEET genes in B. striata. Seventeen BsSWEETs were found, which were phylogenetically divided into four clades. Most BsSWEETs contain seven transmembrane domains (TMs) composed of two MtN3/saliva domains. Four of the seven TMs were conserved in the B. striata SWEET family. The expression profiles showed that BsSWEET1, 2b and 3a were the most dominant in tubers, leaves, and roots, respectively. Additionally, BsSWEET4, 5, 11 and 12 responded to exogenous MeJA. According to the co-expression pattern and interaction network prediction, BsSWEET2a-BsSWEET2b, BsSWEET4-BsSWEET5, and BsSWEET11-BsSWEET12 pairs were potential dimers formed during functioning. Furthermore, Bst-miR396g/h and a number of long non-coding RNAs (lncRNAs) were predicted to be involved in BsSWEETs regulation. In particularly, based on sequence alignment and qRT-PCR analysis, lncRNA MSTRG.63923 was likely to act as a competitive endogenous RNA (ceRNA) to compete with BsSWEET1 for Bst-miR396g/h. In conclusion, this study, provides comprehensive information about BsSWEETs and is beneficial for further functional analysis of the SWEET family in B. striata.