CsSWEET2, a Hexose Transporter from Cucumber (Cucumis sativus L.), Affects Sugar Metabolism and Improves Cold Tolerance in Arabidopsis

Expression Profiling Analysis of the SWEET Gene Family in In Vitro Pitaya Under Low-Temperature Stress and Study of Its Cold Resistance Mechanism

Pitaya (Hylocereus undatus) fruit is an attractive, nutrient-rich tropical fruit with commercial value. However, low-temperature stress severely affects the yield and quality of pitaya. The relevant mechanisms involved in the response of pitaya to low-temperature stress remain unclear. To study whether the SWEET gene family mediates the response of H. undatus to low-temperature stress and the related mechanisms, we performed genome-wide identification of the SWEET gene family in pitaya, and we used 'Baiyulong' tissue-cultured plantlets as material in the present study. We identified 28 members of the SWEET gene family from the H. undatus genome and divided these family members into four groups. Members of this gene family presented some differences in the sequences of introns and exons, but the gene structure, especially the motifs, presented relatively conserved characteristics. The promoter regions of most HuSWEETs have multiple stress- or hormone-related cis-elements. Three duplicated gene pairs were identified, including one tandem duplication gene and two fragment duplication gene pairs. The results revealed that the SWEET genes may regulate the transport and distribution of soluble sugars in plants; indirectly regulate the enzyme activities of CAT, POD, and T-SOD through its expression products; and are involved in the response of pitaya to low-temperature stress and play vital roles in this process. After ABA and MeJA treatment, the expression of HuSWEETs changed significantly, and the cold stress was also alleviated. This study elucidated the molecular mechanism and physiological changes in the SWEET gene in sugar metabolism and distribution of pitaya when it experiences low-temperature stress and provided a theoretical basis for cold-resistant pitaya variety breeding.

Plant membrane transporters function under abiotic stresses: a review

MAIN CONCLUSION

In the present review, we discussed the detailed signaling cascades via membrane transporters that confer plant tolerance to abiotic stresses and possible significant use in plant development for climate-resilient crops. Plant transporters play significant roles in nutrient uptake, cellular balance, and stress responses. They facilitate the exchange of chemicals and signals across the plant's membrane by signal transduction, osmotic adjustment, and ion homeostasis. Therefore, research into plant transporters is crucial for understanding the mechanics of plant stress tolerance. Transporters have potential applications in crop breeding for increased stress resistance. We discuss new results about various transporter families (ABC, MATE, NRAMP, NRT, PHT, ZIP), including their functions in abiotic stress tolerance and plant development. Furthermore, we emphasize the importance of transporters in plant responses to abiotic stresses such as drought, cold, salt, and heavy metal toxicity, low light, flooding, and nutrient deficiencies. We discuss the transporter pathways and processes involved in diverse plant stress responses. This review discusses recent advances in the role of membrane transporters in abiotic stress tolerance in Arabidopsis and other crops. The review contains the genes discovered in recent years and associated molecular mechanisms that improve plants' ability to survive abiotic stress and their possible future applications by integrating membrane transporters with other technologies.

CsLHY positively regulates cold tolerance by activating CsSWEET17 in tea plants

Low temperature is one of the most important environmental factors limiting tea plants' geographic distribution and severely affects spring tea's yield and quality. Circadian components contribute to plant responses to low temperatures; however, comparatively little is known about these components in tea plants. In this study, we identified a core clock component the LATE ELONGATED HYPOCOTYL, CsLHY, which is mainly expressed in tea plants' mature leaves, flowers, and roots. Notably, CsLHY maintained its circadian rhythmicity of expression in summer, but was disrupted in winter and held a high expression level. Meanwhile, we found that CsLHY expression rhythm was not affected by different photoperiods but was quickly broken by cold, and the low temperature induced and kept CsLHY expression at a relatively high level. Yeast one-hybrid and dual-luciferase assays confirmed that CsLHY can bind to the promoter of Sugars Will Eventually be Exported Transporters 17 (CsSWEET17) and function as a transcriptional activator. Furthermore, suppression of CsLHY expression in tea leaves not only reduced CsSWEET17 expression but also impaired the freezing tolerance of leaves compared to the control. Our results demonstrate that CsLHY plays a positive role in the low-temperature response of tea plants by regulating CsSWEET17 when considered together.

Genome-Wide Bioinformatics Analysis of SWEET Gene Family and Expression Verification of Candidate PaSWEET Genes in Potentilla anserina

Sugars act as the main energy sources in many fruit and vegetable crops. The biosynthesis and transportation of sugars are crucial and especially contribute to growth and development. SWEET is an important gene family that plays a vital role in plants' growth, development, and adaptation to various types of stresses (biotic and abiotic). Although SWEET genes have been identified in numerous plant species, there is no information on SWEETs in Potentilla anserina. In the present study, we performed a comprehensive genome-wide bioinformatics analysis and identified a total of 23 candidate PaSWEETs genes in the Potentilla anserina genome, which were randomly distributed on ten different chromosomes. The phylogenetic analysis, chromosomal location, gene structure, specific cis-elements, protein interaction network, and physiological characteristics of these genes were systematically examined. The identified results of the phylogenetic relationship with Arabidopsis thaliana revealed that these PaSWEET genes were divided into four clades (I, II, III, and IV). Moreover, tissue-specific gene expression through quantitative real-time polymerase chain reaction (qRT-PCR) validation exposed that the identified PaSWEETs were differentially expressed in various tissues (roots, stems, leaves, and flowers). Mainly, the relative fold gene expression in swollen and unswollen tubers effectively revealed that PaSWEETs (7, 9, and 12) were highly expressed (300-, 120-, and 100-fold) in swollen tubers. To further elucidate the function of PaSWEETs (7, 9, and 12), their subcellular location was confirmed by inserting them into tobacco leaves, and it was noted that these genes were present on the cell membrane. On the basis of the overall results, it is suggested that PaSWEETs (7, 9, and 12) are the candidate genes involved in swollen tuber formation in P. anserina. In crux, we speculated that our study provides a valuable theoretical base for further in-depth function analysis of the PaSWEET gene family and their role in tuber development and further enhancing the molecular breeding of Potentilla anserina.

Functional Characterization of CsSWEET5a, a Cucumber Hexose Transporter That Mediates the Hexose Supply for Pollen Development and Rescues Male Fertility in Arabidopsis

Pollen cells require large amounts of sugars from the anther to support their development, which is critical for plant sexual reproduction and crop yield. Sugars Will Eventually be Exported Transporters (SWEETs) have been shown to play an important role in the apoplasmic unloading of sugars from anther tissues into symplasmically isolated developing pollen cells and thereby affect the sugar supply for pollen development. However, among the 17 CsSWEET genes identified in the cucumber (Cucumis sativus L.) genome, the CsSWEET gene involved in this process has not been identified. Here, a member of the SWEET gene family, CsSWEET5a, was identified and characterized. The quantitative real-time PCR and β-glucuronidase expression analysis revealed that CsSWEET5a is highly expressed in the anthers and pollen cells of male cucumber flowers from the microsporocyte stage (stage 9) to the mature pollen stage (stage 12). Its subcellular localization indicated that the CsSWEET5a protein is localized to the plasma membrane. The heterologous expression assays in yeast demonstrated that CsSWEET5a encodes a hexose transporter that can complement both glucose and fructose transport deficiencies. CsSWEET5a can significantly rescue the pollen viability and fertility of atsweet8 mutant Arabidopsis plants. The possible role of CsSWEET5a in supplying hexose to developing pollen cells via the apoplast is also discussed.

MdSWEET23, a sucrose transporter from apple (Malus × domestica Borkh.), influences sugar metabolism and enhances cold tolerance in tomato

Photosynthetic products in most fleshy fruits are unloaded via the apoplasmic pathway. Sugar transporters play an important role in the apoplasmic unloading pathway and are involved in sugar transport for fruit development. The MdSWEET23, cloned from ''Hanfu'' apple (Malus × domestica Borkh.) fruits, belongs to Clade III of the SWEET family. Subcellular localization revealed that MdSWEET23 is localized on the plasma membrane. β-glucuronidase activity assays showed that MdSWEET23 was primarily expressed in the sepal and carpel vascular bundle of apple fruits. Heterologous expression assays in yeast showed that MdSWEET23 functions in sucrose transport. The overexpression of MdSWEET23 in the ''Orin" calli increased the soluble sugar content. The silencing of MdSWEET23 significantly reduced the contents of sucrose and sorbitol in apple fruits. Ectopic overexpression of MdSWEET23 in tomato altered sugar metabolism and distribution in leaves and fruits, causing a reduction in photosynthetic rates and plant height, enhanced cold stress tolerance, and increased the content of sucrose, fructose, and glucose in breaking color fruits, but did not increase sugar sink potency of tomato fruits.

The transcription factor CitZAT5 modifies sugar accumulation and hexose proportion in citrus fruit

Sugars are fundamental to plant developmental processes. For fruits, the accumulation and proportion of sugars play crucial roles in the development of quality and attractiveness. In citrus (Citrus reticulata Blanco.), we found that the difference in sweetness between mature fruits of "Gongchuan" and its bud sport "Youliang" is related to hexose contents. Expression of a SuS (sucrose synthase) gene CitSUS5 and a SWEET (sugars will eventually be exported transporter) gene CitSWEET6, characterized by transcriptome analysis at different developmental stages of these 2 varieties, revealed higher expression levels in "Youliang" fruit. The roles of CitSUS5 and CitSWEET6 were investigated by enzyme activity and transient assays. CitSUS5 promoted the cleavage of sucrose to hexoses, and CitSWEET6 was identified as a fructose transporter. Further investigation identified the transcription factor CitZAT5 (ZINC FINGER OF ARABIDOPSIS THALIANA) that contributes to sucrose metabolism and fructose transportation by positively regulating CitSUS5 and CitSWEET6. The role of CitZAT5 in fruit sugar accumulation and hexose proportion was investigated by homologous transient CitZAT5 overexpression, -VIGS, and -RNAi. CitZAT5 modulates the hexose proportion in citrus by mediating CitSUS5 and CitSWEET6 expression, and the molecular mechanism explained the differences in sugar composition of "Youliang" and "Gongchuan" fruit.

Genome-Wide Identification of the Rose SWEET Gene Family and Their Different Expression Profiles in Cold Response between Two Rose Species

Sugars Will Eventually be Exported Transporter (SWEET) gene family plays indispensable roles in plant physiological activities, development processes, and responses to biotic and abiotic stresses, but no information is known for roses. In this study, a total of 25 RcSWEET genes were identified in Rosa chinensis 'Old Blush' by genome-wide analysis and clustered into four subgroups based on their phylogenetic relationships. The genomic features, including gene structures, conserved motifs, and gene duplication among the chromosomes of RcSWEET genes, were characterized. Seventeen types of cis-acting elements among the RcSWEET genes were predicted to exhibit their potential regulatory roles during biotic and abiotic stress and hormone responses. Tissue-specific and cold-response expression profiles based on transcriptome data showed that SWEETs play widely varying roles in development and stress tolerance in two rose species. Moreover, the different expression patterns of cold-response SWEET genes were verified by qRT-PCR between the moderately cold-resistant species R. chinensis 'Old Blush' and the extremely cold-resistant species R. beggeriana. Especially, SWEET2a and SWEET10c exhibited species differences after cold treatment and were sharply upregulated in the leaves of R. beggeriana but not R. chinensis 'Old Blush', indicating that these two genes may be the crucial candidates that participate in cold tolerance in R. beggeriana. Our results provide the foundation for function analysis of the SWEET gene family in roses, and will contribute to the breeding of cold-tolerant varieties of roses.

Special Issue “Sugar Transport, Metabolism and Signaling in Plants”

Sucrose and its derivative hexoses are key metabolites of the plant metabolism, structural units of cell walls and stored reserves (e [...]

Plasma Membrane-Localized Transporter MdSWEET12 Is Involved in Sucrose Unloading in Apple Fruit

Sugar content is an important factor determining the flavor in apple fruit. Sugar unloading is a prerequisite step for sugar accumulation. However, little is known about sugar unloading mechanisms in apple. Transcriptomic sequencing of two apple varieties, "Envy" and "Pacific Rose," with significantly different sugar content was performed. MdSWEET12a from the SWEET transporter family was differentially expressed. Further study of the MdSWEET12a showed that this plasma membrane-localized transporter protein-encoding gene was mainly expressed in sieve element-companion cells (SE-CC) in the fruit, which was positively correlated with the sucrose accumulation during the development of "Envy" apple. Consistently manipulating the gene expression through either transient overexpression or silencing significantly increased or decreased the sugar content in apple fruit, respectively. Complementary growth experiments in mutant yeast cells indicated that MdSWEET12a transported sucrose. Heterologous expression of MdSWEET12a in tomato increased the expression of genes related to sugar metabolism and transport, leading to increased sugar content. These findings underpin the involvement of MdSWEET12a in sugar unloading in apple fruit.

Genome-Wide Identification and Expression Analysis of SWEET Family Genes in Sweet Potato and Its Two Diploid Relatives

Sugar Will Eventually be Exported Transporter (SWEET) proteins are key transporters in sugar transportation. They are involved in the regulation of plant growth and development, hormone crosstalk, and biotic and abiotic stress responses. However, SWEET family genes have not been explored in the sweet potato. In this study, we identified 27, 27, and 25 SWEETs in cultivated hexaploid sweet potato (Ipomoea batatas, 2n = 6x = 90) and its two diploid relatives, Ipomoea trifida (2n = 2x = 30) and Ipomoea triloba (2n = 2x = 30), respectively. These SWEETs were divided into four subgroups according to their phylogenetic relationships with Arabidopsis. The protein physiological properties, chromosome localization, phylogenetic relationships, gene structures, promoter cis-elements, protein interaction networks, and expression patterns of these 79 SWEETs were systematically investigated. The results suggested that homologous SWEETs are differentiated in sweet potato and its two diploid relatives and play various vital roles in plant growth, tuberous root development, carotenoid accumulation, hormone crosstalk, and abiotic stress response. This work provides a comprehensive comparison and furthers our understanding of the SWEET genes in the sweet potato and its two diploid relatives, thereby supplying a theoretical foundation for their functional study and further facilitating the molecular breeding of sweet potato.

Characterization of the SWEET Gene Family in Longan (Dimocarpus longan) and the Role of DlSWEET1 in Cold Tolerance

Sugars will eventually be exported transporters (SWEET), a group of relatively novel sugar transporters, that play important roles in phloem loading, seed and fruit development, pollen development, and stress response in plants. Longan (Dimocarpus longan), a subtropic fruit tree with high economic value, is sensitive to cold. However, whether the SWEET gene family plays a role in conferring cold tolerance upon longan remains unknown. Here, a total of 20 longan SWEET (DlSWEET) genes were identified, and their phylogenetic relationships, gene structures, cis-acting elements, and tissue-specific expression patterns were systematically analyzed. This family is divided into four clades. Gene structures and motifs analyses indicated that the majority of DlSWEETs in each clade shared similar exon–intron organization and conserved motifs. Tissue-specific gene expression suggested diverse possible functions for DlSWEET genes. Cis-elements analysis and quantitative real-time PCR (qRT-PCR) analysis revealed that DlSWEET1 responded to cold stress. Notably, the overexpression of DlSWEET1 improved cold tolerance in transgenic Arabidopsis, suggesting that DlSWEET1 might play a positive role in D. longan’s responses to cold stress. Together, these results contribute to a better understanding of SWEET genes, which could serve as a foundation for the further functional identification of these genes.

Photosynthesis Mediated by RBOH-Dependent Signaling Is Essential for Cold Stress Memory

Cold tolerance is improved by cold stress acclimation (CS-ACC), and the cold tolerance level is ‘remembered’ by plants. However, the underlying signaling mechanisms remain largely unknown. Here, the CS memory mechanism was studied by bioinformation, plant physiological and photosynthetic parameters, and gene expression. We found that CS-ACC induced the acquisition of CS memory and enhanced the maintenance of acquired cold tolerance (MACT) in cucumber seedlings. The H₂O₂ content and NADPH oxidase activity encoded by CsRBOH was maintained at higher levels during recovery after CS-ACC and inhibition of RBOH-dependent signaling after CS-ACC resulted in a decrease in the H₂O₂ content, NADPH oxidase activity, and MACT. CsRBOH2, 3, 4, and 5 showed high expression during recovery after CS-ACC. Many BZR-binding sites were identified in memory-responsive CsRBOHs promoters, and CsBZR1 and 3 showed high expression during recovery after CS-ACC. Inhibition of RBOH-dependent signaling or brassinosteroids affected the maintenance of the expression of these memory-responsive CsRBOHs and CsBZRs. The photosynthetic efficiency (PE) decreased but then increased with the prolonged recovery after CS-ACC, and was higher than the control at 48 h of recovery; however, inhibition of RBOH-dependent signaling resulted in a lower PE. Further etiolated seedlings experiments showed that a photosynthetic capacity was necessary for CS memory. Therefore, photosynthesis mediated by RBOH-dependent signaling is essential for CS memory.

Genome-Wide Identification, Expression Patterns and Sugar Transport of the Physic Nut SWEET Gene Family and a Functional Analysis of JcSWEET16 in Arabidopsis

The Sugars Will Eventually be Exported Transporters (SWEET) family is a class of sugar transporters that play key roles in phloem loading, seed filling, pollen development and the stress response in plants. Here, a total of 18 JcSWEET genes were identified in physic nut (Jatropha curcas L.) and classified into four clades by phylogenetic analysis. These JcSWEET genes share similar gene structures, and alternative splicing of messenger RNAs was observed for five of the JcSWEET genes. Three (JcSWEET1/4/5) of the JcSWEETs were found to possess transport activity for hexose molecules in yeast. Real-time quantitative PCR analysis of JcSWEETs in different tissues under normal growth conditions and abiotic stresses revealed that most are tissue-specifically expressed, and 12 JcSWEETs responded to either drought or salinity. The JcSWEET16 gene responded to drought and salinity stress in leaves, and the protein it encodes is localized in both the plasma membrane and the vacuolar membrane. The overexpression of JcSWEET16 in Arabidopsis thaliana modified the flowering time and saline tolerance levels but not the drought tolerance of the transgenic plants. Together, these results provide insights into the characteristics of SWEET genes in physic nut and could serve as a basis for cloning and further functional analysis of these genes.