Genome-wide identification and expression analysis of the StSWEET family genes in potato (Solanum tuberosum L.)

Phytophagy impacts the quality and quantity of plant carbon resources acquired by mutualistic arbuscular mycorrhizal fungi

Arbuscular mycorrhizal (AM) fungi associate with the roots of many plant species, enhancing their hosts access to soil nutrients whilst obtaining their carbon supply directly as photosynthates. AM fungi often face competition for plant carbon from other organisms. The mechanisms by which plants prioritise carbon allocation to mutualistic AM fungi over parasitic symbionts remain poorly understood. Here, we show that host potato plants (Solanum tuberosum cv. Désirée) selectively allocate carbon resources to tissues interacting with AM fungi rather than those interacting with phytophagous parasites (the nematode Globodera pallida). We found that plants reduce the supply of hexoses but maintain the flow of plant-derived fatty acids to AM fungi when concurrently interacting with parasites. Transcriptomic analysis suggest that plants prioritise carbon transfer to AM fungi by maintaining expression of fatty acid biosynthesis and transportation pathways, whilst decreasing the expression of mycorrhizal-induced hexose transporters. We also report similar findings from a different plant host species (Medicago truncatula) and phytophagous pest (the aphid Myzus persicae). These findings suggest a general mechanism of plant-driven resource allocation in scenarios involving multiple symbionts.

Phylogenetic and Expression Analysis of the Sucrose Synthase and Sucrose Phosphate Synthase Gene Family in Potatoes

Sucrose synthase (SUS) and sucrose phosphate synthase (SPS) are essential in plant sucrose metabolism. The potato is an important crop worldwide, but systematic analyses of the StSUS and StSPS gene families in potatoes are still lacking. Ten sucrose metabolism-related genes were identified in this study. The SUSs and SPSs could each be split into three subgroups through phylogenetic analysis. StSUSIc was the most highly expressed gene in different developmental tissues. Ka/Ks analysis showed that StSUSIb and StSUSIc were subjected to more-significant homozygous selection pressure. Our cis-acting element analysis of the StSUS and StSPS promoter sequences showed four elements: defense- and stress-responsive, hormone-responsive, light-responsive, and transcription factor elements. The expression of StSUS and StSPS genes was found to be regulated by circadian rhythm. In the treatments of 1% to 5% sucrose, glucose, and fructose, the expression of StSUS and StSPS family genes was enhanced by sucrose, but inhibited at high-glucose and fructose concentrations. This study identified six StSUS and four StSPS genes and analyzed their gene structure, conserved motifs, chromosome position, promoter elements, phylogenetic tree, and tissue-specific expression patterns. Our results will motivate more research into the biological process underlying the genes of sucrose metabolism in potatoes.

Identification and expression analysis of the SWEET genes in radish reveal their potential functions in reproductive organ development

BACKGROUND

Sugars produced by photosynthesis provide energy for biological activities and the skeletons for macromolecules; they also perform multiple physiological functions in plants. Sugar transport across plasma membranes mediated by the Sugar Will Eventually be Exported Transporter (SWEET) genes substantially affects these processes. However, the evolutionary dynamics and function of the SWEET genes are largely unknown in radish, an important Brassicaceae species.

METHODS AND RESULTS

Genome-wide identification and analysis of the RsSWEET genes from the recently updated radish reference genome was conducted using bioinformatics methods. The tissue-specific expression was analyzed using public RNA-seq data, and the expression levels in the bud, stamens, pistils, pericarps and seeds at 15 and 30 days after flowering (DAF) were determined by RT‒qPCR. Thirty-seven RsSWEET genes were identified and named according to their Arabidopsis homologous. They are unevenly distributed across the nine radish chromosomes and were further divided into four clades by phylogenetic analysis. There are 5-7 transmembrane domains and at least one MtN3_slv domain in the RsSWEETs. RNA-seq and RT‒qPCR revealed that the RsSWEETs exhibit higher expression levels in the reproductive organs, indicating that these genes might play vital roles in reproductive organ development. RsSWEET15.1 was found to be especially expressed in siliques according to the RNA-seq data, and the RT‒qPCR results further confirmed that it was most highly expressed levels in the seeds at 30 DAF, followed by the pericarp at 15 DAF, indicating that it is involved in seed growth and development.

CONCLUSIONS

This study suggests that the RsSWEET genes play vital roles in reproductive organ development and provides a theoretical basis for the future functional analysis of RsSWEETs in radish.

Genome-wide analysis of the SWEET genes in Taraxacum kok-saghyz Rodin: An insight into two latex-abundant isoforms

Taraxacum kok-saghyz Rodin (Tk) is a promising alternative rubber-producing grass. However, low biomass and rubber-producing capability limit its commercial application. As a carbon source transporter in plants, sugar will eventually be exported transporters (SWEETs) have been reported to play pivotal roles in diverse physiological events in the context of carbon assimilate transport and utilization. Theoretically, SWEETs would participate in Tk growth, development and response to environmental cues with relation to the accumulation of rubber and biomass, both of which rely on the input of carbon assimilates. Here, we identified 22 TkSWEETs through homology searching of the Tk genomes and bioinformatics analyses. RNA-seq and qRT-PCR analysis revealed these TkSWEETs to have overlapping yet distinct tissue expression patterns. Two TkSWEET isofroms, TkSWEET1 and TkSWEET12 expressed substantially in the latex, the cytoplasm of rubber-producing laticifers as well as the rubber source. As revealed by the transient expression analysis using Tk mesophyll protoplasts, both TkSWEET1 and TkSWEET12 were located in the plasma membrane. Heterologous expressions of the two TkSWEETs in a yeast mutant revealed that only TkSWEET1 exhibited apparent sugar transport activities, with a preference for monosaccharides. Interestingly, TkSWEET12, the latex-predominant TkSWEET isoform, seemed to have evolved from a tandem duplication event that results in a cluster of six TkSWEET genes with the TkSWEET12 therein, suggesting its specialized roles in the laticifers.

Comprehensive Analysis of StSRO Gene Family and Its Expression in Response to Different Abiotic Stresses in Potato

As a highly conserved family of plant-specific proteins, SIMILAR-TO-RCD-ONE (SROs) play an essential role in plant growth, development and response to abiotic stresses. In this study, six StSRO genes were identified by searching the PARP, RST and WWE domains based on the genome-wide data of potato database DM v6.1, and they were named StSRO1–6 according to their locations on chromosomes. StSRO genes were comprehensively analyzed using bioinformatics methods. The results showed that six StSRO genes were irregularly distributed on five chromosomes. Phylogenetic analysis showed that 30 SRO genes of four species were distributed in three groups, while StSRO genes were distributed in groups II and III. The promoter sequence of StSRO genes contained many cis-acting elements related to hormones and stress responses. In addition, the expression level of StSRO genes in different tissues of doubled monoploid (DM) potato, as well as under salt, drought stresses and hormone treatments, was analyzed by RNA-seq data from the online database and quantitative real-time polymerase chain reaction (qRT-PCR) analysis. Furthermore, the expression level of StSRO genes was analyzed by transcriptome analysis under mild, moderate and severe salt stress. It was concluded that StSRO genes could respond to different abiotic conditions, but their expression level was significantly different. This study lays a foundation for further studies on the biological functions of the StSRO gene family.

Genome-wide identification of the SWEET gene family mediating the cold stress response in Prunus mume

The Sugars Will Eventually be Exported Transporter (SWEET) gene family encodes a family of sugar transporters that play essential roles in plant growth, reproduction, and biotic and abiotic stresses. Prunus mume is a considerable ornamental wood plant with high edible and medicinal values; however, its lack of tolerance to low temperature has severely limited its geographical distribution. To investigate whether this gene family mediates the response of P. mume to cold stress, we identified that the P. mume gene family consists of 17 members and divided the family members into four groups. Sixteen of these genes were anchored on six chromosomes, and one gene was anchored on the scaffold with four pairs of segmental gene duplications and two pairs of tandem gene duplications. Cis-acting regulatory element analysis indicated that the PmSWEET genes are potentially involved in P. mume development, including potentially regulating roles in procedure, such as circadian control, abscisic acid-response and light-response, and responses to numerous stresses, such as low-temperature and drought. We performed low-temperature treatment in the cold-tolerant cultivar 'Songchun' and cold-sensitive cultivar 'Zaolve' and found that the expression of four of 17 PmSWEETs was either upregulated or downregulated with prolonged treatment times. This finding indicates that these family members may potentially play a role in cold stress responses in P. mume. Our study provides a basis for further investigation of the role of SWEET proteins in the development of P. mume and its responses to cold stress.

Elucidating the role of SWEET13 in phloem loading of the C4 grass Setaria viridis

Photosynthetic efficiency and sink demand are tightly correlated with rates of phloem loading, where maintaining low cytosolic sugar concentrations is paramount to prevent the downregulation of photosynthesis. Sugars Will Eventually be Exported Transporters (SWEETs) are thought to have a pivotal role in the apoplastic phloem loading of C₄ grasses. SWEETs have not been well studied in C₄ species, and their investigation is complicated by photosynthesis taking place across two cell types and, therefore, photoassimilate export can occur from either one. SWEET13 homologues in C₄ grasses have been proposed to facilitate apoplastic phloem loading. Here, we provide evidence for this hypothesis using the C₄ grass Setaria viridis. Expression analyses on the leaf gradient of C₄ species Setaria and Sorghum bicolor show abundant transcript levels for SWEET13 homologues. Carbohydrate profiling along the Setaria leaf shows total sugar content to be significantly higher in the mature leaf tip compared with the younger tissue at the base. We present the first known immunolocalization results for SvSWEET13a and SvSWEET13b using novel isoform-specific antisera. These results show localization to the bundle sheath and phloem parenchyma cells of both minor and major veins. We further present the first transport kinetics study of C₄ monocot SWEETs by using a Xenopus laevis oocyte heterologous expression system. We demonstrate that SvSWEET13a and SvSWEET13b are high-capacity transporters of glucose and sucrose, with a higher apparent V_max for sucrose, compared with glucose, typical of clade III SWEETs. Collectively, these results provide evidence for an apoplastic phloem loading pathway in Setaria and possibly other C₄ species.

Sugar transporters of the SWEET family and their role in arbuscular mycorrhiza

Plant sugar transporters play an essential role in the organism’s productivity by carrying out carbohydrate transportation from source cells in the leaves to sink cells in the cortex. In addition, they aid in the regulation of a substantial part of the exchange of nutrients with microorganisms in the rhizosphere (bacteria and fungi), an ty essential to the formation of symbiotic relationships. This review pays special attention to carbohydrate nutrition during the development of arbuscular mycorrhiza (AM), a symbiosis of plants with fungi from the Glomeromycotina subdivision. This relationship results in the host plant receiving micronutrients from the mycosymbiont, mainly phosphorus, and the fungus receiving carbon assimilation products in return. While the eff icient nutrient transport pathways in AM symbiosis are yet to be discovered, SWEET sugar transporters are one of the three key families of plant carbohydrate transporters. Specif ic AM symbiosis transporters can be identif ied among the SWEET proteins. The survey provides data on the study history, structure and localization, phylogeny and functions of the SWEET proteins. A high variability of both the SWEET proteins themselves and their functions is noted along with the fact that the same proteins may perform different functions in different plants. A special role is given to the SWEET transporters in AM development. SWEET transporters can also play a key role in abiotic stress tolerance, thus allowing plants to adapt to adverse environmental conditions. The development of knowledge about symbiotic systems will contribute to the creation of microbial preparations for use in agriculture in the Russian Federation.

Sugar and Hormone Dynamics and the Expression Profiles of SUT/SUC and SWEET Sweet Sugar Transporters during Flower Development in Petunia axillaris

Flowering is the first committed step of plant sexual reproduction. While the developing flower is a strong sink requiring large quantity of sugars from photosynthetic source tissues, this process is under-temper-spatially controlled via hormone signaling pathway and nutrient availability. Sugar transporters SUT/SUC and SWEET mediate sugars movement across membranes and play a significant role in various physiological processes, including reproductive organ development. In Petunia axillaris, a model ornamental plant, 5 SUT/SUC and 36 SWEET genes are identified in the current version of the genome. Analysis of their gene structure and chromosomal locations reveal that SWEET family is moderately expanded. Most of the transporter genes are abundantly expressed in the flower than in other organs. During the five flower developmental stages, transcript levels of PaSUT1, PaSUT3, PaSWEET13c, PaSWEET9a, PaSWEET1d, PaSWEET5a and PaSWEET14a increase with the maturation of the flower and reach their maximum in the fully open flowers. PaSWEET9c, the nectar-specific PhNEC1 orthologous, is expressed in matured and fully opened flowers. Moreover, determination of sugar concentrations and phytohormone dynamics in flowers at the five developmental stages shows that glucose is the predominant form of sugar in young flowers at the early stage but depletes at the later stage, whereas sucrose accumulates only in maturated flowers prior to the corolla opening. On the other hand, GA₃ content and to a less extent IAA and zeatin decreases with the flower development; however, JA, SA and ABA display a remarkable peak at mid- or later flower developmental stage.