As the main carbon source and energy substances, soluble sugars such as glucose, fructose and galactose, play essential roles in cytoskeleton formation, plant growth and development, energy metabolism, signal transduction, osmoregulation, and other processes [1, 2]. Sugar transporters are key mediators of sugar transport in plants, and sugars are transported from source to reservoir across the membrane, this process depends on sugar transporters. They can be widely found in fruits, veins, seeds, roots, pollen, and embryos [3]. To date, three main types of sugar transporters have been found in higher plants: SWEETs, MTs (monosaccharide transporters) and SUTs (sucrose transporters). Among them, SUT1 only exists in dicotyledonous plants, while SUT2 and SUT4 coexist in monocotyledonous and dicotyledonous plants; SUT3 and SUT5 only exist in monocotyledons. MTs are a protein superfamily, gene expression can be regulated by environmental factors during cell development and can exhibit specific expression at different stages [4].
The SWEET gene family is a bidirectional transmembrane transporter that can transport fructose, sucrose, and hexose in plants [5]. The typical SWEETs consist of seven transmembrane helices, which contain two tandem 3-TMs repeat units, constituting the MtN3-slv domain or the PQ-Loop domain, and the middle is connected by a less conserved transmembrane helix (TMH), thus comprising the TM SWEET structure of “3-1-3” [6]. In plants, the SWEET gene family can be classified into four subfamilies. The evolutionary sequences of the four subfamilies were as follows: Clade II, Clade I, Clade III and Clade IV [7]. Studies have shown that Clades I and II of the SWEET gene family mainly transport monosaccharides and preferentially transport hexose, while clade III transports disaccharides and mainly transports sucrose, Clade IV functions as fructose transporters in the vacuole [8]. To date, genome-wide analysis has identified SWEET gene family members in many plants, such as 17 in Arabidopsis thaliana, 21 in rice (Oryza sativa) [9], 52 in soybean (Glycine max) [10], 35 in potato (Solanum tuberosum) [11], 29 in tomato (Solanum lycopersicum) [12], and 23 in sorghum (Sorghum bicolor) [13]. Biochemical and functional analyses of the SWEET gene family have revealed that SWEET proteins in plants were involved mainly in growth and development [14], host interactions, sugar compound distribution, transportation and storage, abiotic and biotic stress responses, and other expression regulatory processes [15]. For example, AtSWEET13 can contribute to pollen development [16], AtSWEET16 and AtSWEET17 were shown to be involved in the regulation of root growth and development and the transport of vacuolar sugars in Arabidopsis, and AtSWEET10 was reported to maintain cell viability in the Arabidopsis salt stress environment [17, 18]. SlSWEET15 in S. lycopersicum was considerd as an essential regulator of salt tolerance [19], and SWEET9 in tobacco (Nicotiana attenuata) and Chinese cabbage (Brassica rapa) reportedly regulated nectar secretion [20].
Biotic and abiotic stresses affect plant growth and development and metabolic processes in many ways. In response to stress, plants alter their molecular structure and physiological and biochemical processes through a variety of signaling pathways. Under stress, soluble sugars play an important role in plants. Abiotic stress mainly includes drought, high temperature, low temperature, and ultraviolet radiation. Drought stress affects many physiological and biochemical processes in plants, including damage to the cell membrane, a decrease in the relative water content and chlorophyll content in leaves, and an increase in osmotic adjustment substances; ultimately, plant performance is reduced in terms of yield and quality [22]. Therefore, studying the mechanism of plant growth under drought stress is highly important for Brassica juncea breeding. Previous studies have shown that AtSWEET17 was transcriptionally upregulated under drought stress [23] and thus involved in the drought stress response in Arabidopsis. The Arabidopsis sucrose transporters SWEET11 and SWEET12 were phosphorylated under drought stress, and their gene expression increased significantly; when the environment recovered, their expression remained constant [24]. Under drought induction, OsSWEET11 overexpression significantly enhanced the drought tolerance of rice [25].
Brassica juncea is an allotetraploid of Brassica, that is generated by spontaneous chromosome doubling after hybridization between two progenitor species, Brassica nigra and Brassica rapa [21]. It has been widely grown as an oilseed crop, economic vegetable, and condiment worldwide. To date, SWEET genes have been intensively studied in many species. However, genome-wide identification and analysis of the SWEET gene family have not been performed in Brassica juncea, and whether the BjSWEET genes participate in the drought stress response and the regulatory mechanism remain unknown. Therefore, understanding and studying the SWEET genes and their biological functions are essential for B. juncea breeding and cultivation of resistant varieties. In this study, we performed a genome-wide analysis of SWEET genes in Brassica juncea and the gene was renamed as BjSWEET. We subsequently analyzed the chromosome-based gene mapping, transmembrane helical structure, gene structure, conserved motifs, cis-acting elements in the promoter, phylogenetic relationships, and expression profiles of these genes. These findings lay a foundation for further elucidation of the biological function of SWEET genes in B. juncea.