  Citation: Gautam, T.; Dutta, M.; Jaiswal, V.; Zinta, G.; Gahlaut, V.; Kumar, S. Emerging Roles of SWEET Sugar Transporters in Plant Development and Abiotic Stress Responses. Cells 2022, 11, 1303. https://doi.org/10.3390/ cells11081303 Academic Editors: M. Margarida Oliveira and Tiago Lourenço Received: 23 February 2022 Accepted: 25 March 2022 Published: 12 April 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). cells Review Emerging Roles of SWEET Sugar Transporters in Plant Development and Abiotic Stress Responses Tinku Gautam 1 , Madhushree Dutta 2,3 , Vandana Jaiswal 2,3 , Gaurav Zinta 2,3 , Vijay Gahlaut 2, * and Sanjay Kumar 2,3 1 Department of Genetics and Plant Breeding, Chaudhary Charan Singh University, Meerut 250004, India; tinkugoutam@gmail.com 2 Biotechnology Division, CSIR-Institute of Himalayan Bioresource Technology, Palampur 176061, India; madhushreed89@gmail.com (M.D.); vandana.jaiswal2009@gmail.com (V.J.); gzinta@gmail.com (G.Z.); sanjaykumar@ihbt.res.in (S.K.) 3 Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India * Correspondence: zone4vijay@gmail.com Abstract: Sugars are the major source of energy in living organisms and play important roles in osmotic regulation, cell signaling and energy storage. SWEETs (Sugars Will Eventually be Exported Transporters) are the most recent family of sugar transporters that function as uniporters, facilitating the diffusion of sugar molecules across cell membranes. In plants, SWEETs play roles in multiple physiological processes including phloem loading, senescence, pollen nutrition, grain filling, nectar secretion, abiotic (drought, heat, cold, and salinity) and biotic stress regulation. In this review, we summarized the role of SWEET transporters in plant development and abiotic stress. The gene expression dynamics of various SWEET transporters under various abiotic stresses in different plant species are also discussed. Finally, we discuss the utilization of genome editing tools (TALENs and CRISPR/Cas9) to engineer SWEET genes that can facilitate trait improvement. Overall, recent advancements on SWEETs are highlighted, which could be used for crop trait improvement and abiotic stress tolerance. Keywords: sucrose transport; nectar secretion; phloem loading; gibberellin transport; CRISPR/Cas9 1. Introduction Photosynthetic organisms synthesize sugars during photosynthesis, a primary source of carbon and energy in cells [1]. Synthesized sugars are assimilated, transported, and distributed from source to sink tissues through the process of carbohydrate partitioning [1]. Sucrose is the main product of photosynthetic reactions, synthesized explicitly in the cytosol and transported to the sink organs [2]. Sucrose acts as a signaling molecule to control growth and differentiation [3]. Several review articles provide a detailed account of carbon partitioning, sugar metabolism, and signaling in plants [1,2,4–8]. Sugars are involved in various plant growth and developmental processes by acting as the source of carbon skeletons, the substrate of respiratory reactions, intermediate metabolites in biochemical reactions, storage substances, osmolyte, and signals in biotic and abiotic stresses [9–14]. The demand for sugar increases in the shoot/root apical meristem, flower buds, and seed/fruits organs [2,15,16]. Significant increases in sugar concentrations also occur under biotic and abiotic stresses such as cold, drought, phosphorus starvation, and pathogen attack [9,10,12,17]. In contrast, sugar levels decline under reduced oxygen conditions [9,13]. Additionally, sugars play a crucial role in regulating reproductive events such as pollen germination [18]. Thus, sugar metabolites form the core of the plant metabolism in response to developmental and environmental cues. Sugar transporters across cell membranes mediate sugar translocation. These are evolutionally conserved genes present in bacteria, fungi, archaea, and plants [5,19–21]. Cells 2022, 11, 1303. https://doi.org/10.3390/cells11081303 https://www.mdpi.com/journal/cells