Plant SWEETs: from sugar transport to plant-pathogen interaction and more unexpected physiological roles

Sugars Will Eventually be Exported Transporters (SWEETs) have important roles in numerous physiological mechanisms where sugar efflux is critical, including phloem loading, nectar secretion, seed nutrient filling, among other less expected functions. They mediate low affinity and high capacity transport, and in angiosperms this family is composed by 20 paralogs on average. As SWEETs facilitate the efflux of sugars, they are highly susceptible to hijacking by pathogens, making them central players in plant-pathogen interaction. For instance, several species from the Xanthomonas genus are able to up-regulate the transcription of SWEET transporters in rice (Oryza sativa), upon the secretion of TAL-effectors. Other pathogens, such as Botrytis cinerea or Erysiphe necator are also capable of increasing SWEET expression. However, the opposite behavior has been observed in some cases, as over-expression of the tonoplast AtSWEET2 during Pythium irregulare infection restricted sugar availability to the pathogen, rendering plants more resistant. Therefore, a clear-cut role for SWEET transporters during plant-pathogen interactions has so far been difficult to define, as the metabolic signatures and their regulatory nodes, which decide the susceptibility or resistance responses, remain poorly understood. This fuels the still ongoing scientific question: what roles can SWEETs play during plant-pathogen interaction? Likewise, the roles of SWEET transporters in response to abiotic stresses are little understood. Here, in addition to their relevance in biotic stress, we also provide a small glimpse of SWEETs importance during plant abiotic stress, and briefly debate their importance in the particular case of grapevine (Vitis vinifera) due to its socioeconomic impact.
Fundação para a Ciência e Tecnologia (FCT), under the strategic programmes UID/AGR/04033/2020 and UID/BIA/04050/2020. This work was also supported by FCT and European Funds (FEDER/POCI/COMPETE2020) through the research project “MitiVineDrought—Combining ‘omics’ with molecular, biochemical, and physiological analyses as an integrated effort to validate novel and easy-to-implement drought mitigation strategies in grapevine while reducing water use” with ref. PTDC/BIA-FBT/30341/2017 and ref. POCI-01-0145-FEDER-030341, respectively; through the research project “BerryPlastid—Biosynthesis of secondary compounds in the grape berry: unlocking the role of the plastid” with ref. POCI-01-0145-FEDER-028165 and ref. PTDC/BIA-FBT/28165/2017, respectively; and also through the FCT-funded research project “GrapeInfectomics” (PTDC/ASPHOR/28485/2017). A.C. was supported with a post-doctoral researcher contract/position within the project “MitiVineDrought” (PTDC/BIA-FBT/30341/2017 and POCI-01-0145-FEDER-030341). R.B. was supported by a PhD student grant (PD/BD/113616/2015) under the Doctoral Programme “Agricultural Production Chains—from fork to farm” (PD/00122/2012) funded by FCT. H.B. was supported by a PhD fellowship funded by FCT (SFRH/BD/144638/2019). This work also benefited from the networking activities within the European Union-funded COST Action CA17111 “INTEGRAPE—Data Integration to maximize the power of omics for grapevine improvement”
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