Sugars Will Eventually be Exported Transporters (SWEETs) are central for sugar allocation in plants. The SWEET family has approximately 20 homologs in most plant genomes, and despite extensive research on their structures and molecular functions, it is still unclear how diverse SWEETs recognize different substrates. Previous work using SweetTrac1, a biosensor constructed by the intramolecular fusion of a conformation-sensitive fluorescent protein in the plasma membrane transporter SWEET1 from Arabidopsis thaliana, identified common features in the transporter’s substrates. Here, we report SweetTrac2, a new biosensor based on the Arabidopsis vacuole membrane transporter SWEET2, and use it to explore the substrate specificity of this second protein. Our results show that SWEET1 and SWEET2 recognize similar substrates but some with different affinities. Sequence comparison and mutagenesis analysis support the conclusion that the differences in affinity depend on nonspecific interactions involving previously uncharacterized residues in the substrate-binding pocket. Furthermore, SweetTrac2 can be an effective tool for monitoring sugar transport at vacuolar membranes that would be otherwise challenging to study.
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When SWEETs Turn Tweens: Updates and Perspectives
Sugar translocation between cells and between subcellular compartments in plants requires either plasmodesmata or a diverse array of sugar transporters. Interactions between plants and associated microorganisms also depend on sugar transporters. The sugars will eventually be exported transporter (SWEET) family is made up of conserved and essential transporters involved in many critical biological processes. The functional significance and small size of these proteins have motivated crystallographers to successfully capture several structures of SWEETs and their bacterial homologs in different conformations. These studies together with molecular dynamics simulations have provided unprecedented insights into sugar transport mechanisms in general and into substrate recognition of glucose and sucrose in particular. This review summarizes our current understanding of the SWEET family, from the atomic to the whole-plant level. We cover methods used for their characterization, theories about their evolutionary origins, biochemical properties, physiological functions, and regulation. We also include perspectives on the future work needed to translate basic research into higher crop yields.
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- Award ID(s):
- 1942722
- NSF-PAR ID:
- 10355189
- Date Published:
- Journal Name:
- Annual Review of Plant Biology
- Volume:
- 73
- Issue:
- 1
- ISSN:
- 1543-5008
- Page Range / eLocation ID:
- 379 to 403
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Summary Plant breeders have developed crop plants that are resistant to pests, but the continual evolution of pathogens creates the need to iteratively develop new control strategies. Molecular tools have allowed us to gain deep insights into disease responses, allowing for more efficient, rational engineering of crops that are more robust or resistant to a greater number of pathogen variants. Here we describe the roles of
SWEET andSTP transporters, membrane proteins that mediate transport of sugars across the plasma membrane. We discuss how these transporters may enhance or restrict disease through controlling the level of nutrients provided to pathogens and whether the transporters play a role in sugar signaling for disease resistance. This review indicates open questions that require further research and proposes the use of genome editing technologies for engineering disease resistance. -
In maize, starch mutants have facilitated characterization of key genes involved in endosperm starch biosynthesis such as large subunit of AGPase Shrunken2 ( Sh2 ) and isoamylase type DBE Sugary1 ( Su1 ). While many starch biosynthesis enzymes have been characterized, the mechanisms of certain genes (including Sugary enhancer1 ) are yet undefined, and very little is understood about the regulation of starch biosynthesis. As a model, we utilize commercially important sweet corn mutations, sh2 and su1 , to genetically perturb starch production in the endosperm. To characterize the transcriptomic response to starch mutations and identify potential regulators of this pathway, differential expression and coexpression network analysis was performed on near-isogenic lines (NILs) (wildtype, sh2 , and su1 ) in six genetic backgrounds. Lines were grown in field conditions and kernels were sampled in consecutive developmental stages (blister stage at 14 days after pollination (DAP), milk stage at 21 DAP, and dent stage at 28 DAP). Kernels were dissected to separate embryo and pericarp from the endosperm tissue and 3′ RNA-seq libraries were prepared. Mutation of the Su1 gene led to minimal changes in the endosperm transcriptome. Responses to loss of sh2 function include increased expression of sugar (SWEET) transporters and of genes for ABA signaling. Key regulators of starch biosynthesis and grain filling were identified. Notably, this includes Class II trehalose 6-phosphate synthases, Hexokinase1 , and Apetala2 transcription factor-like (AP2/ERF) transcription factors. Additionally, our results provide insight into the mechanism of Sugary enhancer1 , suggesting a potential role in regulating GA signaling via GRAS transcription factor Scarecrow-like1 .more » « less
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Endosymbiotic associations between hornworts and nitrogen-fixing cyanobacteria form when the plant is limited for combined nitrogen (N). We generated RNA-seq data to examine temporal gene expression patterns during the culturing of N-starved Anthoceros punctatus in the absence and the presence of symbiotic cyanobacterium Nostoc punctiforme. In symbiont-free A. punctatus gametophytes, N starvation caused downregulation of chlorophyll content and chlorophyll fluorescence characteristics as well as transcription of photosynthesis-related genes. This downregulation was reversed in A. punctatus cocultured with N. punctiforme, corresponding to the provision by the symbiont of N 2 -derived NH 4 + , which commenced within 5 days of coculture and reached a maximum by 14 days. We also observed transient increases in transcription of ammonium and nitrate transporters in a N. punctiforme–dependent manner as well as that of a SWEET transporter that was initially independent of N 2 -derived NH 4 + . The temporal patterns of differential gene expression indicated that N. punctiforme transmits signals that impact gene expression to A. punctatus both prior to and after its provision of fixed N. This study is the first illustrating the temporal patterns of gene expression during establishment of an endosymbiotic nitrogen-fixing association in this monophyletic evolutionary lineage of land plants. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .more » « less
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Summary SWEETs play important roles in intercellular sugar transport. Induction of SWEET sugar transporters by Transcription Activator‐Like effectors (TALe) of
Xanthomonas ssp. is key for virulence in rice, cassava and cotton.We identified OsSWEET11b with roles in male fertility and potential bacterial blight (BB) susceptibility in rice. While single
ossweet11a or11 b mutants were fertile, double mutants were sterile. As clade III SWEETs can transport gibberellin (GA), a key hormone for spikelet fertility, sterility and BB susceptibility might be explained by GA transport deficiencies. However, in contrast with the Arabidopsis homologues, OsSWEET11b did not mediate detectable GA transport. Fertility and susceptibility therefore are likely to depend on sucrose transport activity.Ectopic induction of
OsSWEET11b by designer TALe enabled TALe‐freeXanthomonas oryzae pv.oryzae (Xoo ) to cause disease, identifyingOsSWEET11b as a potential BB susceptibility gene and demonstrating that the induction of host sucrose uniporter activity is key to virulence ofXoo . Notably, only three of six clade III SWEETs are targeted by knownXoo strains from Asia and Africa.The identification of OsSWEET11b is relevant for fertility and for protecting rice against emerging
Xoo strains that targetOsSWEET11b .
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1146/annurev-arplant-070621-093907
