The eukaryotic proteins of the SWEET family are found in plants, animals, protozoans, bacteria, etc. They have 7 TMSs in a 3+1+3 repeat arrangement. These proteins appear to catalyze facilitated diffusion (entry or export) of sugars across the plant plasma membrane or the endoplasmic reticulum membrane (Takanaga and Frommer, 2010). Plant sweets fall into four subclades (Chen et al., 2010). The tomato genome encodes 29 SWEETs. Feng et al. 2015 analyzed the structures, conserved domains, and phylogenetic relationships of these proteins, and also analyzed the transcript levels of SWEET genes in various tissues, organs, and developmental stages in response to exogenous sugar and adverse environmental stress (e.g., high and low temperatures). The phylogeny of SWEETS has been described (Jia et al. 2017). A database (dbSWEET) of SWEET homologues is freely available to the scientific community at http://bioinfo.iitk.ac.in/bioinfo/dbSWEET/Home (Gupta and Sankararamakrishnan 2018). SWEETs perform diverse physiological functions in plants such as pollen nutrition, nectar secretion, seed filling, phloem loading, and pathogen nutrition (Jeena et al. 2019). Various SWEETS transport various sugars such as sucrose, fructose, glucose, galactose, and mannose (Hu et al. 2019). SWEETS play important roles in sugar efflux, pollen nutrition, nectar secretion, phloem transport, and seed development (Cao et al. 2019). Identification and expression analysis of the SWEET gene family from Poa pratensis under abiotic Stresses has been published (Zhang et al. 2020).
On average, angiosperm genomes contain approximately 20 SWEET paralogs, most of which serve distinct physiological roles. In Arabidopsis, AtSWEET8 and 13 feed the pollen; SWEET 11 and 12 provide sucrose to MFS-type sucrose transporters for phloem loading; AtSWEET11, 12 and 15 have distinct roles in seed filling; AtSWEET16 and 17 are vacuolar hexose transporters; and SWEET9 is essential for nectar secretion (Eom et al. 2015). The remaining family members await characterization, and could play roles in the gametophyte and elsewhere in the plant. In rice and cassava, and possibly other systems, sucrose transporting SWEETs play central roles in pathogen resistance. Plant sweets participate in diverse physiological processes, including pathogen nutrition, seed filling, nectar secretion, and phloem loading. There are 28 SWEET genes in tea (Camellia sinensis), and several members from the CsSWEET gene family have been localized and characterized (Jiang et al. 2021).
Plant SWEETs play crucial roles in cellular sugar efflux processes: phloem loading, pollen nutrition and nectar secretion. Bacterial SemiSWEETs often consist of a triple-helix bundle and form semi-symmetrical, parallel dimers, thereby generating the translocation pathway. Two SemiSWEET isoforms have been crystallized, one in an apparently open state and one in an occluded state, indicating that SemiSWEETs and SWEETs are transporters that undergo rocking-type movements during the transport cycle (Xu et al., 2014). In SemiSWEETs and SWEETs, two triple-helix bundles are arranged in a parallel configuration to produce the 6- and (3 + 1 + 3) -transmembrane-helix pores, respectively. Given the similarity of SemiSWEETs and SWEETs to PQ-loop amino acid transporters and to mitochondrial pyruvate carriers (MPCs), the structures characterized by Xu et al., 2014 may also be relevant to other transporters in the TOG superfamily (Yee et al. 2013). Characterization and expression profiling of the 30 SWEET proteins (8 with one repeat unit, 21 with two, and 1 with 4) in cabbage (Brassica oleracea) revealed their roles in chilling and clubroot disease responses.
Latorraca et al. 2017; captured the translocationprocess by crystallography and unguided molecular dynamics simulations, providing an atomic-level description of alternating access transport. Simulations of a SWEET-family transporter initiated from an outward-open, glucose-bound structure spontaneously adopts occluded and inward-open conformations matching crystal structures. Mutagenesis experiments validated simulation predictions suggesting that state transitions are driven by favorable interactions formed upon closure of extracellular and intracellular 'gates' and by an unfavorable transmembrane helix configuration when both gates are closed. This mechanism leads to tight allosteric coupling between gates, preventing them from opening simultaneously. The substrate appears to take a 'free ride' across the membrane without causing major structural rearrangements in the transporter.
Plant SWEET sugar transporters play roles in phloem transport, nectar secretion, pollen nutrition, stress tolerance, and plant-pathogen interactions (Gao et al. 2017). Fify nine family members have been identified in wheat. Phylogenetic relationships, numbers of TMSs, gene structures, and motifs showed that TaSWEETs have 3-7 TMSs fall into four clades with 10 different types of motifs. Examination of the expression patterns of 18 SWEET genes revealed that a few are tissue-specific while most are ubiquitously expressed. Using a stem rust-susceptible cultivar, 'Little Club' (LC) the expression of five SWEETs tested induced following inoculation (Gao et al. 2017). Sugar is transported via SWEETS and semi-SWEETS from the extracellular side (via an outward-open state) to the intracellular side (inward-open state) through an intermediate occluded state with both extracellular and intracellular gates closed (Bera and Klauda 2018).