Hexose transporter SWEET5 confers galactose sensitivity to Arabidopsis pollen germination via a galactokinase

SWEET Family Transporters Act as Water-Conducting Carrier Proteins in Plants

Dedicated water channels are involved in the facilitated diffusion of water molecules across cell membranes in plants. Transporter proteins are also known to transport water molecules along with substrates; however, the molecular mechanism of water permeation is not well understood in plant transporters. Here, we show that plant sugar transporters from the SWEET (sugar will eventually be exported transporter) family act as water-conducting carrier proteins via a variety of passive and active mechanisms that allow the diffusion of water molecules from one side of the membrane to the other. This study provides a molecular perspective on how plant membrane transporters act as water carrier proteins, a topic that has not been extensively explored in the literature. Water permeation in membrane transporters could occur via four distinct mechanisms, which form our hypothesis for water transport in SWEETs. These hypotheses are tested using molecular dynamics simulations of the outward-facing, occluded, and inward-facing states of AtSWEET1 to identify the water permeation pathways and the flux associated with them. The hydrophobic gates at the center of the transport tunnel act as barriers that restrict water permeation. We have performed in silico single and double mutations of the hydrophobic gate residues to examine the changes in water conductivity. Surprisingly, the double mutant allows water permeation to the intracellular half of the membrane and forms a continuous water channel. These computational results are validated by experimentally examining the transport of hydrogen peroxide molecules by the AtSWEET family of transporters. We have also shown that the transport of hydrogen peroxide follows a mechanism similar to that of water transport in AtSWEET1. Finally, we conclude that similar water-conduction states are also present in other SWEETs due to the high degree of sequence and structural conservation exhibited by this transporter family.

Transcription factor AtWRKY43 enhances drought tolerance by positively regulating the expression of AtSWEET5

Drought exerts negative effects on plant growth and crop productivity. Sugars play a crucial role in enhancing drought tolerance by acting as osmoprotectants. Sugar transporters AtSWEETs are essential for maintaining sugar distribution equilibrium and facilitating effective adaptation to drought stress. In the study, transcriptomic analysis showed that WRKY transcription factor AtWRKY43 plays a key role in modulating gene expression patterns that enhance the plant's ability to accumulate sugars, which in turn supports osmotic adjustment and overall drought stress resilience. AtWRKY43 was identified as a promoter of sugar accumulation through its induction of transcriptional expression of AtSWEET5, thereby influencing drought tolerance. We observed an upregulation in the expression of the gene AtSWEET5 during drought stress. The mutant atsweet5 exhibited increased sensitivity to drought compared to Col-0, possibly due to increased H2O2 content and disrupted sugar homeostasis. Our research highlights the critical roles of AtWRKY43 and AtSWEET5 in modulating drought stress, providing new gene targets for developing crop varieties with enhanced sugar content. Therefore, it is imperative to understand the impact of drought stresses on sugar transport and transporters at both genetic and molecular levels in order to maintain optimal sugar balance in plants experiencing drought stress.

The sugar transporter proteins in plants: An elaborate and widespread regulation network-A review

In higher plants, sugars are the primary products of photosynthesis, where CO2 is converted into organic carbon within the mesophyll cells of leaves. These sugars serve as a critical source of carbon skeletons for the biosynthesis of essential cellular compounds, energy production, and as osmotic and signaling molecules. Plant sugar transporter proteins play a key role in facilitating the long-distance translocation of sugars from source to sink organs, thereby controlling their distribution and accumulation across the plant. Over the past decade, substantial progress has been achieved in identifying the functions of individual genes linked to sugar transporters; however, the diverse regulatory mechanisms influencing these transporters remain insufficiently explored. This review consolidates current and previous research on the functions of sugar transporter proteins, focusing on their involvement in phloem transport pathways their impact on crop yield, cross-talk with other signals, and plant-microbe interactions. Furthermore, we propose future directions for studying the mechanisms of sugar transporter proteins and their potential applications in agriculture, with the goal of improving sugar utilization efficiency in crops and ultimately increasing crop yield.

Genome assembly and population analysis of tetraploid marama bean reveal two distinct genome types

Tylosema esculentum (marama bean), an underutilized orphan legume native to southern Africa, holds significant potential for domestication as a rescue crop to enhance local food security. Well-adapted to harsh desert environments, it offers valuable insights into plant resilience to extreme drought and high temperatures. In this study, k-mer analysis indicated marama as an ancient allotetraploid legume. Using 21.5 Gb of PacBio HiFi data, the genome was assembled with two assemblers, HiCanu and Hifiasm, followed by scaffolding with Omni-C data from Dovetail Genomics (Cantata Bio) using HiRise, resulting in a 558.78 Mb assembly with near chromosome-level continuity (N50 = 22.68 Mb, L50 = 8). Repeats accounted for 58.43% of the genome. Phylogenetic analysis indicated a close relationship with Bauhinia variegata and Cercis canadensis, diverging approximately 27.22 and 31.68 million years ago (Ma), respectively. Whole-genome duplication (WGD) analysis revealed an ancient duplication event in marama. Gene family analysis revealed expanded families enriched in pathways related to stress adaptation, energy metabolism, and environmental signaling, including the spliceosome, citrate cycle, and carbon fixation pathways. These findings highlight marama's resilience to arid environments. In contrast, contracted gene families associated with secondary metabolite biosynthesis and defense pathways suggest a trade-off, potentially due to reduced pathogen pressure. Marama-specific genes were enriched in amino acid catabolism pathways, potentially playing roles in stress signaling and energy regulation. Core gene families shared with other legumes were enriched in conserved pathways, such as photosynthesis and hormone signaling, which are fundamental for plant growth and survival. Population analysis of geographically diverse samples revealed two distinct clusters, though phenotypic differences remain unclear. Overall, this study presents the first high-quality genome assembly of marama bean, offering a valuable genomic reference for understanding its unique biology and highlighting its potential for crop improvement in challenging environments.

Metabolic crosstalk between roots and rhizosphere drives alfalfa decline under continuous cropping

Considerable biological decline of continuously cropped alfalfa may be tightly linked to rhizosphere metabolism. However, plant-soil feedbacks and age-related metabolic changes in alfalfa stands remain unexplored. The aim of this study was to identify the linkages of rhizosphere and root metabolites, particularly autotoxins and prebiotics, to alfalfa decline under continuous cropping. We performed liquid chromatography-mass spectrometry for non-targeted metabolomic profiling of rhizosphere soils and alfalfa roots in 2- and 6-year-old stands. Differentially abundant metabolites that responded to stand age and associated metabolic pathways were identified. Compared with bulk soils, rhizosphere soils were enriched with more triterpenoid saponins (e.g., medicagenic acid glycosides), which showed inhibitory effects on seed germination and seedling growth. These autotoxic metabolites were accumulated in the old stand age, and their relative abundances were negatively correlated with plant growth, yield, and quality traits, as well as soil total nitrogen and alkali-hydrolyzable nitrogen concentrations. In contrast, prebiotic metabolites, represented by glycerolipids (e.g., glycerophosphocholine) and fatty acyls (e.g., colnelenic acid), were depleted in rhizosphere soils in the old stand. The relative abundances of glycerolipids and fatty acyls were positively correlated with plant traits and soil available phosphorus and alkali-hydrolyzable nitrogen concentrations. Age-induced changes in the rhizosphere metabolome mirrored the reprogramming patterns of root metabolome. The pathways of terpenoid backbone biosynthesis and plant hormone signal transduction, as well as metabolism of galactose, glycerophospholipid, and ɑ-linolenic acid in alfalfa roots were affected by stand age. The upregulation of terpenoid backbone biosynthesis in alfalfa roots of old plants, which stimulated triterpenoid saponin biosynthesis and exudation. Rhizosphere accumulation of autotoxins was accompanied by depletion of prebiotics, leading to soil degradation and exacerbating alfalfa decline. This research aids in the development of prebiotics to prevent and manage continuous cropping obstacles in alfalfa.

Deletion of the sugar importer gene OsSWEET1b accelerates sugar starvation-promoted leaf senescence in rice

Leaf senescence is a combined response of plant cells stimulated by internal and external signals. Sugars acting as signaling molecules or energy metabolites can influence the progression of leaf senescence. Both sugar starvation and accumulation can promote leaf senescence with diverse mechanisms that are reported in different species. Sugars Will Eventually be Exported Transporters (SWEETs) are proposed to play essential roles in sugar transport, but whether they have roles in senescence and the corresponding mechanism are unclear. Here, we functionally characterized a sugar transporter, OsSWEET1b, which transports sugar and promotes senescence in rice (Oryza sativa L.). OsSWEET1b could import glucose and galactose when heterologously expressed in Xenopus oocytes and translocate glucose and galactose from the extracellular apoplast into the intracellular cytosol in rice. Loss of function of OsSWEET1b decreased glucose and galactose accumulation in leaves. ossweet1b mutants showed accelerated leaf senescence under natural and dark-induced conditions. Exogenous application of glucose and galactose complemented the defect of OsSWEET1b deletion-promoted senescence. Moreover, the senescence-activated transcription factor OsWRKY53, acting as a transcriptional repressor, genetically functions upstream of OsSWEET1b to suppress its expression. OsWRKY53-overexpressing plants had attenuated sugar accumulation, exhibiting a similar phenotype as the ossweet1b mutants. Our findings demonstrate that OsWRKY53 downregulates OsSWEET1b to impair its influx transport activity, leading to compromised sugar accumulation in the cytosol of rice leaves where sugar starvation promotes leaf senescence.

SWEET family transporters act as water conducting carrier proteins in plants

Dedicated water channels are involved in the facilitated diffusion of water molecules across the cell membrane in plants. Transporter proteins are also known to transport water molecules along with substrates, however the molecular mechanism of water permeation is not well understood in plant transporters. Here, we show plant sugar transporters from the SWEET (Sugar Will Eventually be Exported Transporter) family act as water-conducting carrier proteins via a variety of passive and active mechanisms that allow diffusion of water molecules from one side of the membrane to the other. This study provides a molecular perspective on how plant membrane transporters act as water carrier proteins, a topic that has not been extensively explored in literature. Water permeation in membrane transporters could occur via four distinct mechanisms which form our hypothesis for water transport in SWEETs. These hypothesis are tested using molecular dynamics simulations of the outward-facing, occluded, and inward-facing state of AtSWEET1 to identify the water permeation pathways and the flux associated with them. The hydrophobic gates at the center of the transport tunnel act as a barrier that restricts water permeation. We have performed in silico single and double mutations of the hydrophobic gate residues to examine the changes in the water conductivity. Surprisingly, the double mutant allows the water permeation to the intracellular half of the membrane and forms a continuous water channel. These computational results are validated by experimentally examining the transport of hydrogen peroxide molecules by the AtSWEET family of transporters. We have also shown that the transport of hydrogen peroxide follows the similar mechanism as water transport in AtSWEET1. Finally, we conclude that similar water-conduction states are also present in other SWEET transporters due to the high sequence and structure conservation exhibited by this transporter family.

Single-cell RNA-seq reveals a link of ovule abortion and sugar transport in Camellia oleifera

Camellia oleifera is the most important woody oil crop in China. Seed number per fruit is an important yield trait in C. oleifera. Ovule abortion is generally observed in C. oleifera and significantly decreases the seed number per fruit. However, the mechanisms of ovule abortion remain poorly understood at present. Single-cell RNA sequencing (scRNA-seq) was performed using mature ovaries of two C. oleifera varieties with different ovule abortion rates (OARs). In total, 20,526 high-quality cells were obtained, and 18 putative cell clusters were identified. Six cell types including female gametophyte, protoxylem, protophloem, procambium, epidermis, and parenchyma cells were identified from three main tissue types of ovule, placenta, and pericarp inner layer. A comparative analysis on scRNA-seq data between high- and low-OAR varieties demonstrated that the overall expression of CoSWEET and CoCWINV in procambium cells, and CoSTP in the integument was significantly upregulated in the low-OAR variety. Both the infertile ovule before pollination and the abortion ovule producing after compatible pollination might be attributed to selective abortion caused by low sugar levels in the apoplast around procambium cells and a low capability of hexose uptake in the integument. Here, the first single-cell transcriptional landscape is reported in woody crop ovaries. Our investigation demonstrates that ovule abortion may be related to sugar transport in placenta and ovules and sheds light on further deciphering the mechanism of regulating sugar transport and the improvement of seed yield in C. oleifera.

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.

Adaptive responses of eelgrass (Zostera marina L.) to ocean warming and acidification

Ocean warming (OW) and ocean acidification (OA), driven by rapid global warming accelerating at unprecedented rates, are profoundly impacting the stability of seagrass ecosystems. Yet, our current understanding of the effects of OW and OA on seagrass remains constrained. Herein, we investigated the response of eelgrass (Zostera marina L.), a representative seagrass species, to OW and OA through comprehensive transcriptomic and metabolomic analyses. The results showed notable variations in plant performance under varying conditions: OW, OA, and OWA (a combination of both conditions). Specifically, under average oceanic temperature conditions for eelgrass growth over the past 20 years -from May to November-OA promoted the production of differentially expressed genes and metabolites associated with alanine, aspartate, and glutamate metabolism, as well as starch and sucrose metabolism. Under warming condition, eelgrass was resistant to OA by accelerating galactose metabolism, along with glycine, serine, and threonine metabolism, as well as the tricarboxylic acid (TCA) cycle. Under the combined OW and OA condition, eelgrass stimulated fructose and mannose metabolism, glycolysis, and carbon fixation, in addition to galactose metabolism and the TCA cycle to face the interplay. Our findings suggest that eelgrass exhibits adaptive capacity by inducing different metabolites and associated genes, primarily connected with carbon and nitrogen metabolism, in response to varying degrees of OW and OA. The data generated here support the exploration of mechanisms underlying seagrass responses to environmental fluctuations, which hold critical significance for the future conservation and management of these ecosystems.

Genome-wide transcriptional responses to water deficit during seed development in Pisum sativum, focusing on sugar transport and metabolism

Agriculture is particularly impacted by global changes, drought being a main limiting factor of crop production. Here, we focus on pea (Pisum sativum), a model legume cultivated for its seed nutritional value. A water deficit (WD) was applied during its early reproductive phase, harvesting plant organs at two key developmental stages, either at the embryonic or the seed-filling stages. We combined phenotypic, physiological and transcriptome analyses to better understand the adaptive response to drought. First, we showed that apical growth arrest is a major phenotypic indicator of water stress. Sugar content was also greatly impacted, especially leaf fructose and starch contents. Our RNA-seq analysis identified 2001 genes regulated by WD in leaf, 3684 genes in root and 2273 genes in embryonic seed, while only 80 genes were regulated during seed-filling. Hence, a large transcriptional reprogramming occurred in response to WD in seeds during early embryonic stage, but no longer during the later stage of nutritional filling. Biological processes involved in transcriptional regulation, carbon transport and metabolism were greatly regulated by WD in both source and sink organs, as illustrated by the expression of genes encoding transcription factors, sugar transporters and enzymes of the starch synthesis pathway. We then looked at the transcriptomic changes during seed development, highlighting a transition from monosaccharide utilization at the embryonic stage to sucrose transport feeding the starch synthesis pathway at the seed-filling stage. Altogether, our study presents an integrative picture of sugar transport and metabolism in response to drought and during seed development at a genome-wide level.

Structural and molecular basis of pollen germination

Pollen germination is a prerequisite for double fertilization of flowering plants. A comprehensive understanding of the structural and molecular basis of pollen germination holds great potential for crop yield improvement. The pollen aperture serves as the foundation for most plant pollen germination and pollen aperture formation involves the establishment of cellular polarity, the formation of distinct membrane domains, and the precise deposition of extracellular substances. Successful pollen germination requires precise material exchange and signal transduction between the pollen grain and the stigma. Recent cytological and mutant analysis of pollen germination process in Arabidopsis and rice has expanded our understanding of this biological process. However, the overall changes in germination site structure and energy-related metabolites during pollen germination remain to be further explored. This review summarizes and compares the recent advances in the processes of pollen aperture formation, pollen adhesion, hydration, and germination between eudicot Arabidopsis and monocot rice, and provides insights into the structural basis and molecular mechanisms underlying pollen germination process.

Transcriptome analysis provides insight into the regulatory mechanisms underlying pollen germination recovery at normal high ambient temperature in wild banana (Musa itinerans)

Introduction

Cultivated banana are polyploid, with low pollen fertility, and most cultivars are male sterile, which leads to difficulties in banana breeding research. The selection of male parent with excellent resistance and pollen fertility is therefore essential for banana breeding. Wild banana (Musa itinerans) have developed many good characteristics during natural selection and constitute an excellent gene pool for breeding. Therefore, research on wild banana breeding is very important for banana breeding.

Results

In the current analysis, we examined the changes in viability of wild banana pollens at different temperatures by in vitro germination, and found that the germination ability of wild banana pollens cultured at 28°C for 2 days was higher than that of pollens cultured at 23°C (pollens that could not germinate normally under low temperature stress), 24°C (cultured at a constant temperature for 2 days) and 32°C (cultured at a constant temperature for 2 days). To elucidate the molecular mechanisms underlying the germination restoration process in wild banana pollens, we selected the wild banana pollens that had lost its germination ability under low temperature stress (23°C) as the control group (CK) and the wild banana pollens that had recovered its germination ability under constant temperature incubation of 28°C for 2 days as the treatment group (T) for transcriptome sequencing. A total of 921 differentially expressed genes (DEGs) were detected in CK vs T, of which 265 were up-regulated and 656 were down-regulated. The combined analysis of Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) revealed that the activation, metabolism of various substances (lipids, sugars, amino acids) play a major role in restoring pollen germination capacity. TCA cycle and the sesquiterpenoid and triterpenoid biosynthetic pathways were also significantly enriched in the KEGG pathway. And we found that some DEGs may be associated with pollen wall formation, DNA methylation and DNA repair. The cysteine content, free fatty acid (FFA) content, H2O2 content, fructose content, and sucrose content of pollen were increased at treatment of 28°C, while D-Golactose content was decreased. Finally, the GO pathway was enriched for a total of 24 DEGs related to pollen germination, of which 16 DEGs received targeted regulation by 14 MYBs.

Discussions

Our study suggests that the balance between various metabolic processes, pollen wall remodelling, DNA methylation, DNA repairs and regulation of MYBs are essential for germination of wild banana pollens.

An update on sugar allocation and accumulation in fruits

Fruit sweetness is determined by the amount and composition of sugars in the edible flesh. The accumulation of sugar is a highly orchestrated process that requires coordination of numerous metabolic enzymes and sugar transporters. This coordination enables partitioning and long-distance translocation of photoassimilates from source tissues to sink organs. In fruit crops, sugars ultimately accumulate in the sink fruit. Whereas tremendous progress has been achieved in understanding the function of individual genes associated with sugar metabolism and sugar transport in non-fruit crops, there is less known about the sugar transporters and metabolic enzymes responsible for sugar accumulation in fruit crop species. This review identifies knowledge gaps and can serve as a foundation for future studies, with comprehensive updates focusing on (1) the physiological roles of the metabolic enzymes and sugar transporters responsible for sugar allocation and partitioning and that contribute to sugar accumulation in fruit crops; and (2) the molecular mechanisms underlying the transcriptional and posttranslational regulation of sugar transport and metabolism. We also provide insights into the challenges and future directions of studies on sugar transporters and metabolic enzymes and name several promising genes that should be targeted with gene editing in the pursuit of optimized sugar allocation and partitioning to enhance sugar accumulation in fruits.

Genome-Wide Identification, Expression, and Response to Fusarium Infection of the SWEET Gene Family in Garlic (Allium sativum L.)

Proteins of the SWEET (Sugar Will Eventually be Exported Transporters) family play an important role in plant development, adaptation, and stress response by functioning as transmembrane uniporters of soluble sugars. However, the information on the SWEET family in the plants of the Allium genus, which includes many crop species, is lacking. In this study, we performed a genome-wide analysis of garlic (Allium sativum L.) and identified 27 genes putatively encoding clade I-IV SWEET proteins. The promoters of the A. sativum (As) SWEET genes contained hormone- and stress-sensitive elements associated with plant response to phytopathogens. AsSWEET genes had distinct expression patterns in garlic organs. The expression levels and dynamics of clade III AsSWEET3, AsSWEET9, and AsSWEET11 genes significantly differed between Fusarium-resistant and -susceptible garlic cultivars subjected to F. proliferatum infection, suggesting the role of these genes in the garlic defense against the pathogen. Our results provide insights into the role of SWEET sugar uniporters in A. sativum and may be useful for breeding Fusarium-resistant Allium cultivars.

The impact of boron nutrient supply in mulberry (Morus alba) response to metabolomics, enzyme activities, and physiological parameters

Boron (B) is essential for normal and healthy plant growth. Therefore, Boron stress is a common abiotic stress that limits plant growth and productivity. However, how mulberry copes with boron stress remains unclear. In this study, seedlings of the Morus alba cultivar, Yu-711, were treated with five different concentrations of boric acid (H₃BO₃), including deficient (0 and 0.02 mM), sufficient (0.1 mM) and toxic (0.5 and 1 mM) levels. Physiological parameters, enzymatic activities and non-targeted liquid chromatography-mass spectrometry (LC-MS) technique were employed to evaluate the effects of boron stress on the net photosynthetic rate (Pn), chlorophyll content, stomatal conductance (Gs), transpiration rate (Tr), intercellular CO₂ concentration (Ci) and metabolome signatures. Physiological analysis revealed that Boron deficiency and toxicity induced a decline in P_n, C_i, G_s, T_r, and chlorophyll content. Also, enzymatic activities, including catalase (CAT) and superoxide dismutase (SOD), decreased, while POD activity increased in response to Boron stress. Osmotic substances such as soluble sugars, soluble proteins, and proline (PRO) presented elevated levels under all Boron concentrations. Metabolome analysis indicated that differential metabolites, including amino acids, secondary metabolites, carbohydrates, and lipids, played a key role in Yu-711's response to Boron stress. These metabolites were mainly involved in amino acid metabolism, biosynthesis of other secondary metabolites, lipid metabolism, metabolism of cofactors and vitamins, and metabolism of other amino acids pathways. Our findings reveal the various metabolites pathways in mulberry response to boron nutrient supply and may serve as fundamental knowledge in breeding resistance mulberry plants, so that it can cope with climate changes.

Physiological implications of SWEETs in plants and their potential applications in improving source-sink relationships for enhanced yield

The sugars will eventually be exported transporters (SWEET) family of transporters in plants is identified as a novel class of sugar carriers capable of transporting sugars, sugar alcohols and hormones. Functioning in intercellular sugar transport, SWEETs influence a wide range of physiologically important processes. SWEETs regulate the development of sink organs by providing nutritional support from source leaves, responses to abiotic stresses by maintaining intracellular sugar concentrations, and host-pathogen interactions through the modulation of apoplastic sugar levels. Many bacterial and fungal pathogens activate the expression of SWEET genes in species such as rice and Arabidopsis to gain access to the nutrients that support virulence. The genetic manipulation of SWEETs has led to the generation of bacterial blight (BB)-resistant rice varieties. Similarly, while the overexpression of the SWEETs involved in sucrose export from leaves and pathogenesis led to growth retardation and yield penalties, plants overexpressing SWEETs show improved disease resistance. Such findings demonstrate the complex functions of SWEETs in growth and stress tolerance. Here, we review the importance of SWEETs in plant-pathogen and source-sink interactions and abiotic stress resistance. We highlight the possible applications of SWEETs in crop improvement programmes aimed at improving sink and source strengths important for enhancing the sustainability of yield. We discuss how the adverse effects of the overexpression of SWEETs on plant growth may be overcome.

Sucrose rather than GA transported by AtSWEET13 and AtSWEET14 supports pollen fitness at late anther development stages

Both sugar and the hormone gibberellin (GA) are essential for anther-enclosed pollen development and thus for plant productivity in flowering plants. Arabidopsis (Arabidopsis thaliana) AtSWEET13 and AtSWEET14, which are expressed in anthers and associated with seed yield, transport both sucrose and GA. However, it is still unclear which substrate transported by them directly affects anther development and seed yield. Histochemical staining, cross-sectioning and microscopy imaging techniques were used to investigate and interpret the phenotypes of the atsweet13;14 double mutant during anther development. Genetic complementation of atsweet13;14 using AtSWEET9, which transports sucrose but not GA, and the GA transporter AtNPF3.1, respectively, was conducted to test the substrate preference relevant to the biological process. The loss of both AtSWEET13 and AtSWEET14 resulted in reduced pollen viability and therefore decreased pollen germination. AtSWEET9 fully rescued the defects in pollen viability and germination of atsweet13;14, whereas AtNPF3.1 failed to do so, indicating that AtSWEET13/14-mediated sucrose rather than GA is essential for pollen fertility. AtSWEET13 and AtSWEET14 function mainly at the anther wall during late anther development stages, and they probably are responsible for sucrose efflux into locules to support pollen development to maturation, which is vital for subsequent pollen viability and germination.

Comparative Metabolic Analysis Reveals a Metabolic Switch in Mature, Hydrated, and Germinated Pollen in Arabidopsis thaliana

Pollen germination is an essential process for pollen tube growth, pollination, and therefore seed production in flowering plants, and it requires energy either from remobilization of stored carbon sources, such as lipids and starches, or from secreted exudates from the stigma. Transcriptome analysis from in vitro pollen germination previously showed that 14 GO terms, including metabolism and energy, were overrepresented in Arabidopsis. However, little is understood about global changes in carbohydrate and energy-related metabolites during the transition from mature pollen grain to hydrated pollen, a prerequisite to pollen germination, in most plants, including Arabidopsis. In this study, we investigated differential metabolic pathway enrichment among mature, hydrated, and germinated pollen using an untargeted metabolomic approach. Integration of publicly available transcriptome data with metabolomic data generated as a part of this study revealed starch and sucrose metabolism increased significantly during pollen hydration and germination. We analyzed in detail alterations in central metabolism, focusing on soluble carbohydrates, non-esterified fatty acids, glycerophospholipids, and glycerolipids. We found that several metabolites, including palmitic acid, oleic acid, linolenic acid, quercetin, luteolin/kaempferol, and γ-aminobutyric acid (GABA), were elevated in hydrated pollen, suggesting a potential role in activating pollen tube emergence. The metabolite levels of mature, hydrated, and germinated pollen, presented in this work provide insights on the molecular basis of pollen germination.