Sucrose efflux mediated by SWEET proteins as a key step for phloem transport

Functional analysis of the key BrSWEET genes for sugar transport involved in the Brassica rapa-Plasmodiophora brassicae interaction

Plasmodiophora brassicae, the causative agent of clubroot disease, establishes a long-lasting parasitic relationship with its host by inducing the expression of sugar transporters. Previous studies have indicated that most BrSWEET genes in Chinese cabbage are up-regulated upon infection with P. brassicae. However, the key BrSWEET genes responsive to P. brassicae have not been definitively identified. In this study, we selected five BrSWEET genes and conducted a functional analysis of them. These five BrSWEET genes showed a notable up-regulation in roots after P. brassicae inoculation. Furthermore, these BrSWEET proteins were localized to the plasma membrane. Yeast functional complementation assays confirmed transport activity for glucose, fructose, or sucrose in four BrSWEETs, with the exception of BrSWEET2a. Mutants and silenced plants of BrSWEET1a, -11a, and -12a showed lower clubroot disease severity compared to wild-type plants, while gain-of-function Arabidopsis thaliana plants overexpressing these three BrSWEET genes exhibited significantly higher disease incidence and severity. Our findings suggested that BrSWEET1a, BrSWEET11a, and BrSWEET12a play pivotal roles in P. brassicae-induced gall formation, shedding light on the role of sugar transporters in host-pathogen interactions.

GhDOFD45 promotes sucrose accumulation in cotton seeds by transcriptionally activating GhSWEET10 expression

Cotton seed development and fiber elongation are the inseparable and overlapped development processes requiring the continuous supply of sucrose as the direct carbon source. However, little is known about the molecular mechanism of how sucrose is transported from the source tissues (leaves) into growing cotton seeds. Here, we identify the function of a sucrose transporter gene, Sugars Will Eventually be Exported Transporter 10, GhSWEET10 in cotton seed development. GhSWEET10 encodes a functional sucrose transporter, predominantly expressing in the funiculus, inner seedcoat, and endosperm during fiber elongation. GhSWEET10 RNAi plants (GhSWEET10i) accumulated less sucrose and glucose in growing seeds and that led to shorter fibers and smaller seeds, whereas GhSWEET10 overexpressed plants (GhSWEET10OE) had bigger seeds and longer fibers with more sugar accumulation during fiber elongation. GhSWEET10 gene is transcriptionally controlled by the transcription factor GhDOFD45. GhDOFD45 knockout plants (GhDOFD45-KO) possessed the phenotypes of smaller seeds and shorter fibers like those of GhSWEET10i plants. Furthermore, GhSWEET10 mainly exports the sucrose from the funiculus into developing seeds according to the mimic-analysis of sucrose transporting. Collectively, all these findings show that GhDOFD45 positively regulates GhSWEET10 expression to mainly transport sucrose from leaves into developing cotton seeds. Our findings also imply that the sucrose transport into enlarging seeds benefits fiber development, and thus GhSWEET10 can be selected as a target of breeding novel cotton varieties with larger and more vigorous seeds.

Heterologous biosynthesis of betanin triggers metabolic reprogramming in tobacco

Engineering of a specialized metabolic pathway in plants is a promising approach to produce high-value bioactive compounds to address the challenges of climate change and population growth. Understanding the interaction between the heterologous pathway and the native metabolic network of the host plant is crucial for optimizing the engineered system and maximizing the yield of the target compound. In this study, we performed transcriptomic, metabolomic and metagenomic analysis of tobacco (Nicotiana tabacum) plants engineered to produce betanin, an alkaloid pigment that is found in Caryophyllaceae plants. Our data reveals that, in a dose-dependent manor, the biosynthesis of betanin promotes carbohydrate metabolism and represses nitrogen metabolism in the leaf, but enhances nitrogen assimilation and metabolism in the root. By supplying nitrate or ammonium, the accumulation of betanin increased by 1.5-3.8-fold in leaves and roots of the transgenic plants, confirming the pivotal role of nitrogen in betanin production. In addition, the rhizosphere microbial community is reshaped to reduce denitrification and increase respiration and oxidation, assistant to suppress nitrogen loss. Our analysis not only provides a framework for evaluating the pleiotropic effects of an engineered metabolic pathway on the host plant, but also facilitates the development of novel strategies to balance the heterologous process and the native metabolic network for the high-yield and nutrient-efficient production of bioactive compounds in plants.

Sweetpotato sucrose transporter IbSUT1 alters storage roots formation by regulating sucrose transport and lignin biosynthesis

The formation and development of storage roots is the most important physiological process in sweetpotato production. Sucrose transporters (SUTs) regulate sucrose transport from source to sink organs and play important roles in growth and development of plants. However, whether SUTs involved in sweetpotato storage roots formation is so far unknown. In this study, we show that IbSUT1, a SUT, is localized to the plasma membrane. Overexpression of IbSUT1 in sweetpotato promotes the sucrose efflux rate from leaves, leading to increased sucrose levels in roots, thus induces lignin deposition in the stele, which inhibits the storage roots formation and compromises the yield. Heterologous expression of IbSUT1 in Arabidopsis significantly increases the sucrose accumulation and promotes lignification in the inflorescence stems. RNA-seq and biochemical analysis further demonstrated that IbMYB1 negatively regulates the expression of IbSUT1. Overexpression of IbMYB1 in Arabidopsis reduces the sucrose accumulation and lignification degree in the inflorescence stems. Moreover, co-overexpression of IbMYB1 and IbSUT1 restores the phenotype of lignin over-deposition in Arabidopsis. Collectively, our results reveal that IbSUT1 regulates source-sink sucrose transport and participates in the formation of sweetpotato storage roots and highlight the potential application of IbSUT1 in improving sweetpotato yield in the future.

Fast and simple fluorometric measurement of phloem loading exposes auxin-dependent regulation of Arabidopsis sucrose transporter AtSUC2

The rate of sucrose export from leaves is a major factor in balancing whole-plant carbon and energy partitioning. A comprehensive study of its dynamics and relationship to photosynthesis, sink demand, and other relevant processes is hampered by the shortcomings of current methods for measuring sucrose phloem loading. We utilize the ability of sucrose transporter proteins, known as SUCs or SUTs, to specifically transport the fluorescent molecule esculin in a novel assay to measure phloem loading rates. Esculin was administered to source leaves and its fluorescence in the leaf extract was measured after 1 or 2 h. Dicot plants with an active phloem loading strategy showed an export-dependent reduction of esculin fluorescence. Relative leaf esculin export rates correlated with leaf export rates of isotopic carbon and phloem exudate sucrose levels. We used esculin experiments to examine the effects of phytohormones on phloem loading in Arabidopsis, showing, for example, that auxin induces phloem loading while cytokinin reduces it. Transcriptional regulation of AtSUC2 by AUXIN RESPONSE FACTOR1 (ARF1) corroborated the link between auxin signaling and phloem loading. Unlike established methods, the esculin assay is rapid and does not require specialized equipment. Potential applications and limitations of the esculin assay are discussed.

Changes in SWEET-mediated sugar partitioning affect photosynthesis performance and plant response to drought

Sugars, produced through photosynthesis, are at the core of all organic compounds synthesized and used for plant growth and their response to environmental changes. Their production, transport, and utilization are highly regulated and integrated throughout the plant life cycle. The maintenance of sugar partitioning between the different subcellular compartments and between cells is important in adjusting the photosynthesis performance and response to abiotic constraints. We investigated the consequences of the disruption of four genes coding for SWEET sugar transporters in Arabidopsis (SWEET11, SWEET12, SWEET16, and SWEET17) on plant photosynthesis and the response to drought. Our results show that mutations in both SWEET11 and SWEET12 genes lead to an increase of cytosolic sugars in mesophyll cells and phloem parenchyma cells, which impacts several photosynthesis-related parameters. Further, our results suggest that in the swt11swt12 double mutant, the sucrose-induced feedback mechanism on stomatal closure is poorly efficient. On the other hand, changes in fructose partitioning in mesophyll and vascular cells, measured in the swt16swt17 double mutant, positively impact gas exchanges, probably through an increased starch synthesis together with higher vacuolar sugar storage. Finally, we propose that the impaired sugar partitioning, rather than the total amount of sugars observed in the quadruple mutant, is responsible for the enhanced sensitivity upon drought. This work highlights the importance of considering SWEET-mediated sugar partitioning rather than global sugar content in photosynthesis performance and plant response to drought. Such knowledge will pave the way to design new strategies to maintain plant productivity in a challenging environment.

Potassium stimulates fruit sugar accumulation by increasing carbon flow in Citrus sinensis

Soluble sugars contribute to the taste and flavor of citrus fruit. Potassium (K), known as a quality element, plays key roles in improving sugar accumulation and fruit quality, but the mechanism is largely unknown. This study aims to elucidate how K improves sugar accumulation by regulating carbon flow from source leaves to fruit in Newhall navel orange. We found that optimal fruit K concentrations around 1.5% improved sugar accumulation and fruit quality in citrus. K application increased the strength of both sink and source, as indicated by the increased fruit growth rate, enzyme activities and expression levels of key genes involved in sucrose (Suc) metabolism in fruit and leaf. K application also facilitated Suc transport from source leaves to fruit, as confirmed by the enhanced 13C-Suc level in fruit. Furthermore, we found that navel orange used the symplastic pathway for transporting Suc from source leaves to fruit, and K application enhanced symplastic loading, as demonstrated by the intensified carboxyfluorescein signal and increased plasmodesmata density in leaves. The findings reveal that K stimulates fruit sugar accumulation by increasing carbon flow from source leaves to fruit in Newhall navel orange.

Effect of Slow-Release Urea on Yield and Quality of Euryale ferox

Slow-release urea, as an environmentally friendly fertiliser, can provide a continuous and uniform supply of nutrients needed by the crop, reduce the amount and frequency of fertiliser application, and promote the uptake and utilisation of nitrogen in crops. The production of E. ferox is often dominated by the application of quick-acting fertilisers, resulting in serious problems of over-fertilisation, inappropriate periods of fertilisation, eutrophication of soil and water due to fertilisation, and difficulties in applying fertilisers. Therefore, in this study, different amounts (CK, T1, T2, T3, T4, T5) of SRU (Slow-release Urea) were first applied, and T3 (18.8 kg·667 m-2) was found to significantly improve both yield and quality. Further, it was found that under different SRU (CK, S1, S2, S3, S4) application period treatments, application of 18.8 kg·667 m-2 at AFP20 (S2) period significantly increased the yield and quality of E. ferox. In the seed kernels of E. ferox, the total yield, soluble sugar content, total starch, and flavonoid content increased significantly by 10.35%, 36.40%, 5.91%, and 22.80%, respectively, compared with CK. In addition, the expression of key sugar transporter genes (EfSWEETs), flavonoid synthesis-related genes (EfPAL, EfDFR, etc.), and starch synthesis-related enzyme activities (SBE, SSS, GBSS) were significantly increased. By exploring the quantity of application and application period of SRU, this study was carried out to investigate the in-depth effect of SRU on the growth and development of E. ferox and to provide technical references for the increase in E. ferox yield, the improvement in E. ferox quality, and the simplification of fertiliser application.

Cell Wall Invertases from Maternal Tissues Modulate Sucrose Flux in Apoplastic Pathways During Rice Anther and Seed Development

Efficient sucrose transport and metabolism are vital for seed and pollen development in plants. Cell wall invertases (CINs) hydrolyze sucrose into glucose and fructose, maintaining a sucrose gradient in the apoplast of sink tissues. In rice, two CIN isoforms, OsCIN1 and OsCIN2, were identified as being specifically expressed in the anthers but not in pollen. Functional analyses through genetic crosses and mutant characterization showed that oscin1/2 double mutants exhibit a sporophytic male-sterile phenotype and produce shrunken seeds. This suggests that CIN activity is essential for proper pollen development and seed formation in rice. Observation of the progeny genotypes and phenotypes from various genetic crosses revealed that the phenotype of oscin1/2 seeds is determined by the genotype of the maternal tissue, indicating the critical role of CIN function in the apoplast between maternal and filial tissues for sucrose transport and metabolism. The CIN activity in the anthers and seeds of wild-type rice was found to be significantly higher-over 500-fold in the anthers and 5-fold in the seeds-than in the leaves, highlighting the importance of CIN in facilitating the efficient unloading of sucrose. These findings suggest that the fine-tuning of CIN activity in the apoplast, achieved through tissue-specific expression and CIN isoform regulation, plays a key role in determining the carbohydrate distribution across different tissues. Understanding this regulatory mechanism could provide opportunities to manipulate carbohydrate allocation to sink organs, potentially enhancing crop yields.

Genome-wide analysis of the SWEET gene family and its response to powdery mildew and leaf spot infection in the common oat (Avena sativa L.)

The nutritional quality and yield of oats (Avena sativa) are often compromised by plant diseases such as red leaf, powdery mildew, and leaf spot. Sugars Will Eventually be Exported Transporters (SWEETs) are newly identified sugar transporters involved in regulating plant growth and stress responses. However, the roles of SWEET genes in biotic stress responses remain uncharacterized in oats. In this study, 13 AsSWEET genes were identified across nine chromosomes of the oat genome, all of which were predicted to contain seven transmembrane regions. Phylogenetic analysis revealed four clades of AsSWEET proteins, with high homology to SWEET proteins in the Poaceae family. Collinearity analysis demonstrated strong relationships between oat and Zea mays SWEETs. Using subcellular localization prediction tools, AsSWEET proteins were predicted to localize to the plasma membrane. Promoter analysis revealed cis-acting elements associated with light response, growth, and stress regulation. Six AsSWEET proteins were predicted to interact in a network centered on AsSWEET1a and AsSWEET11. Gene expression analysis of two oat varieties, 'ForagePlus' and 'Molasses', indicated significant expression differences in several AsSWEET genes following infection with powdery mildew or leaf spot, including AsSWEET1a, AsSWEET1b, AsSWEET2b, AsSWEET3a, AsSWEET11, and AsSWEET16. These SWEET genes are potential candidates for disease resistance in oats. This study provides a foundation for understanding the regulatory mechanisms of AsSWEET genes, particularly in response to powdery mildew and leaf spot, and offers insights for enhancing oat molecular breeding.

Indole-3-Acetic Acid (IAA) and Sugar Mediate Endosperm Development in Rice (Oryza sativa L.)

The yield potential of large-panicle rice is often limited by grain-filling barriers caused by the development of inferior spikelets (IS). Photoassimilates, which are the main source of rice grain filling, mainly enter the caryopsis through the dorsal vascular bundle. The distribution of assimilates between superior spikelets (SS) and IS is influenced by auxin-mediated apical dominance; however, the mechanism involved is still unclear. In this study, the effect of auxin signaling on the grain filling of SS and IS was investigated in two large-panicle japonica rice varieties, W1844 and CJ03. Compared to SS, IS displayed delayed initiation of filling and a significantly lower grain weight. Furthermore, the endosperm development in IS remained stagnant at the coenocytic stage. The development of the dorsal vascular bundle in the IS was also slow, and poor sucrose-unloading was observed during the initial grain filling stage. However, the endosperm development in IS immediately started after the improvement of dorsal vascular bundle development. GUS activity staining further revealed that indole-3-acetic (IAA) was localized in the dorsal vascular bundle and surrounding areas, suggesting that the low IAA content observed in the IS during the initial grain filling stage may have delayed the development of the dorsal vascular bundle. Therefore, these results demonstrate that IAA may control sugar transport and unloading by regulating dorsal vascular bundle development, consequently affecting endosperm development in IS.

Metabolic and transcriptomic analyses of nectaries reveal differences in the mechanism of nectar production between monocots (Ananas comosus) and dicots (Nicotiana tabacum)

BACKGROUND

Nectar is offered by numerous flowering plants to attract pollinators. To date, the production and secretion of nectar have been analyzed mainly in eudicots, particularly rosids such as Arabidopsis. However, due to the enormous diversity of flowering plants, further research on other plant species, especially monocots, is needed. Ananas comosus (monocot) is an economically important species that is ideal for such analyses because it produces easily accessible nectar in sufficient quantities. In addition, the analyses were also carried out with Nicotiana tabacum (dicot, asterids) for comparison.

RESULTS

We performed transcriptome sequencing (RNA-Seq) analyses of the nectaries of Ananas comosus and Nicotiana tabacum, to test whether the mechanisms described for nectar production and secretion in Arabidopsis are also present in these plant species. The focus of these analyses is on carbohydrate metabolism and transport (e.g., sucrose-phosphate synthases, invertases, sucrose synthases, SWEETs and further sugar transporters). In addition, the metabolites were analyzed in the nectar, nectaries and leaves of both plant species to address the question of whether concentration gradients for different metabolites exist between the nectaries and nectar The nectar of N. tabacum contains large amounts of glucose, fructose and sucrose, and the sucrose concentration in the nectar appears to be similar to the sucrose concentration in the nectaries. Nectar production and secretion in this species closely resemble corresponding processes in some other dicots, including sucrose synthesis in nectaries and sucrose secretion by SWEET9. The nectar of A. comosus also contains large amounts of glucose, fructose and sucrose and in this species the sucrose concentration in the nectar appears to be higher than the sucrose concentration in the nectaries. Furthermore, orthologs of SWEET9 generally appear to be absent in A. comosus and other monocots. Therefore, sucrose export by SWEETs from nectaries into nectar can be excluded; rather, other mechanisms, such as active sugar export or exocytosis, are more likely.

CONCLUSION

The mechanisms of nectar production and secretion in N. tabacum appear to be largely similar to those in other dicots, whereas in the monocotyledonous species A. comosus, different synthesis and transport processes are involved.

The Role of PLATZ6 in Raffinose Family Oligosaccharides Loading of Leaves via PLATZ Family Characterization in Cucumber

The plant AT protein and zinc-binding protein (PLATZ) genes, a novel cluster of plant-specific zinc-finger-dependent DNA-binding proteins, play a crucial role in regulating stress response and plant development. However, there has been little study focus on the role of the cucumber PLATZ family in assimilating loading in leaves. (1) In this study, a total of 12 PLATZ genes were identified from the cucumber genome. The cucumber PLATZ genes were clustered into five groups, and unevenly distributed on five chromosomes. A single pair of cucumber PLATZ genes underwent segmental duplication. (2) The results of genome-wide expression analysis suggested that the cucumber PLATZ genes were widely expressed in a wide range of cucumber tissues, with three PLATZ (PLATZ2, PLATZ6, and PLATZ12) genes exhibiting high expression in the vascular tissues of cucumber leaves. PLATZ2, PLATZ6, and PLATZ12 proteins were primarily located in cytomembrane and nucleus. (3) In VIGS-PLATZ6 plants, the expression of Galactinol synthase 1 (GolS1) and STACHYOSE SYNTHASE (STS), two genes involved in the synthesis of raffinose family oligosaccharides (RFOs) were observed to be decreased in cucumber leaves. In conclusion, the comprehensive analysis of the cucumber PLATZ family and the preliminary functional verification of PLATZ6 lay the foundation for the molecular and physiological functions of cucumber PLATZ genes.

Screening Germplasms and Detecting Quantitative Trait Loci for High Sucrose Content in Soybean

Sucrose is a desirable component of processed soybean foods and animal feed, and thus, its content is used as an important characteristic for assessing the quality of soybean seeds. However, few studies have focused on the quantitative trait loci (QTLs) associated with sucrose regulation in soybean seeds. This study aims to measure the sucrose content of 1014 soybean accessions and identify genes related to high sucrose levels using QTL analysis. Colorimetric analysis based on the enzymatic reaction of invertase (INV) and glucose oxidase (GOD) was employed to test the germplasms. A total of six high-sucrose genetic resources (IT186230, IT195321, IT263138, IT263276, IT263286, and IT276521) and two low-sucrose genetic resources (IT025668 and IT274054) were identified. Two F2:3 populations, IT186230 × IT025668 and Ilmi × IT186230, were then established from these germplasms. QTL analysis identified four QTLs (qSUC6.1, qSUC11.1, qSUC15.1, and qSUC17.1), explaining 7.3-27.6% of the phenotypic variation in the sugar content. Twenty candidate genes were found at the four QTLs. Notably, Glyma.17G152300, located in the qSUC17.1 QTL region, exhibited a 17-fold higher gene expression in the high-sucrose germplasm IT186230 compared to the control germplasm Ilmi, confirming its role as a major gene regulating the sucrose content in soybean. These results may assist in marker-assisted selection for breeding programs that aim to develop soybean lines with a higher sucrose content.

Integration of Mitoflash and Time-Series Transcriptomics Facilitates Energy Dynamics Tracking and Substrate Supply Analysis of Floral Thermogenesis in Lotus

The high biosynthetic and energetic demands of floral thermogenesis render thermogenic plants the ideal systems to characterize energy metabolism in plants, but real-time tracking of energy metabolism in plant cells remains challenging. In this study, a new method was developed for tracking the mitochondrial energy metabolism at the single mitochondria level by real-time imaging of mitochondrial superoxide production (i.e., mitoflash). Using this method, we observed the increased mitoflash frequencies in the receptacles of Nelumbo nucifera Gaertn. at the thermogenic stages. This increase, combined with the higher expression of antioxidant response-related genes identified through time-series transcriptomics at the same stages, shows us a new regulatory mechanism for plant redox balance. Furthermore, we found that the upregulation of respiratory metabolism-related genes during the thermogenic stages not only correlates with changes in mitoflash frequency but also underscores the critical roles of these pathways in ensuring adequate substrate supply for thermogenesis. Metabolite analysis revealed that sugars are likely one of the substrates for thermogenesis and may be transported over long distances by sugar transporters. Taken together, our findings demonstrate that mitoflash is a reliable tool for tracking energy metabolism in thermogenic plants and contributes to our understanding of the regulatory mechanisms underlying floral thermogenesis.

Soybean hypocotyl elongation is regulated by a MYB33-SWEET11/21-GA2ox8c module involving long-distance sucrose transport

The length of hypocotyl affects the height of soybean and lodging resistance, thus determining the final grain yield. However, research on soybean hypocotyl length is scarce, and the regulatory mechanisms are not fully understood. Here, we identified a module controlling the transport of sucrose, where sucrose acts as a messenger moved from cotyledon to hypocotyl, regulating hypocotyl elongation. This module comprises four key genes, namely MYB33, SWEET11, SWEET21 and GA2ox8c in soybean. In cotyledon, MYB33 is responsive to sucrose and promotes the expression of SWEET11 and SWEET21, thereby facilitating sucrose transport from the cotyledon to the hypocotyl. Subsequently, sucrose transported from the cotyledon up-regulates the expression of GA2ox8c in the hypocotyl, which ultimately affects the length of the hypocotyl. During the domestication and improvement of soybean, an allele of MYB33 with enhanced abilities to promote SWEET11 and SWEET21 has gradually become enriched in landraces and cultivated varieties, SWEET11 and SWEET21 exhibit high conservation and have undergone a strong purified selection and GA2ox8c is under a strong artificial selection. Our findings identify a new molecular pathway in controlling soybean hypocotyl elongation and provide new insights into the molecular mechanism of sugar transport in soybean.

Symplasmic phloem loading and subcellular transport in storage roots are key factors for carbon allocation in cassava

Cassava (Manihot esculenta) is a deciduous woody perennial shrub that stores large amounts of carbon and water in its storage roots. Previous studies have shown that assimilating unloading into storage roots happens symplasmically once secondary anatomy is established. However, mechanisms controlling phloem loading and overall carbon partitioning to different cassava tissues remain unclear. Here, we used a combination of histological, transcriptional, and biochemical analyses on different cassava tissues and at different timepoints to better understand source-sink carbon allocation. We found that cassava likely utilizes a predominantly passive symplasmic phloem loading strategy, indicated by the lack of expression of genes coding for key players of sucrose transport, the existence of branched plasmodesmata in the companion cell/bundle sheath interface of minor leaf veins, and very high leaf sucrose concentrations. Furthermore, we showed that tissue-specific changes in anatomy and non-structural carbohydrate contents are associated with tissue-specific modification in gene expression for sucrose cleavage/synthesis, as well as subcellular compartmentalization of sugars. Overall, our data suggest that carbon allocation during storage root filling is mostly facilitated symplasmically and is likely mostly regulated by local tissue demand and subcellular compartmentalization.

The role of sugar transporters in the battle for carbon between plants and pathogens

In photosynthetic cells, plants convert carbon dioxide to sugars that can be moved between cellular compartments by transporters before being subsequently metabolized to support plant growth and development. Most pathogens cannot synthesize sugars directly but have evolved mechanisms to obtain plant-derived sugars as C resource for successful infection and colonization. The availability of sugars to pathogens can determine resistance or susceptibility. Here, we summarize current progress on the roles of sugar transporters in plant-pathogen interactions. We highlight how transporters are manipulated antagonistically by both host and pathogens in competing for sugars. We examine the potential application of this target in resistance breeding and discuss opportunities and challenges for the future.

LcINH1 as an inhibitor of cell wall invertase LcCWIN5 regulates early seed development in Litchi chinensis Sonn

Sugar signal mediated by Cell wall invertase (CWIN) plays a central role in seed development. In higher plants, invertase inhibitors (INHs) suppress CWIN activities at a post-translational level. In Litchi chinensis cultivar 'Nuomici', impaired CWIN expression is associated with seed abortion. Here, the expression of LcINH1 was significantly higher in the funicle of seed-aborting cultivar 'Nuomici' than big-seeded cultivar 'Heiye'. Promoter analyses found LcINH1 contained a 404 bp repeat fragment with an endosperm regulatory element of Skn-1_motif. LcINH1 and LcCWIN2/5 were located in plasma membrane. LcINH1 was able to interact with LcCWIN5, but not with LcCWIN2. In vitro enzyme activity assay demonstrated that LcINH1 could inhibit CWIN activity. Silencing LcINH1 in 'Nuomici' resulted in normal seed development, paralleled increased CWIN activities and glucose levels. Transcriptome analysis identified 1079 differentially expressed genes (DEGs) in LcINH1-silenced fruits. KEGG analysis showed significant enrichment of DEGs in pathways related to transporters and plant hormone signal transduction. Weighted gene co-expression network analysis indicated that the turquoise module was highly correlated with fructose content, and LcSWEET3b was closely associated with early seed development. These findings suggest that LcINH1 regulate LcCWIN5 activity at the post-translational level to alter sucrose metabolism, thereby affecting early seed development in litchi.

Genome-wide identification and expression analyses of SWEET gene family reveal potential roles in plant development, fruit ripening and abiotic stress responses in cranberry (Vaccinium macrocarpon Ait)

The sugars will eventually be exported transporter (SWEET) family is a novel class of sugar transporters that play a crucial role in plant growth, development, and responses to stress. Cranberry (Vaccinium macrocarpon Ait.) is a nutritious berry with economic importance, but little is known about SWEET gene family functions in this small fruit. In this research, 13 VmSWEET genes belonging to four clades were identified in the cranberry genome for the first time. In the conserved domains, we observed seven phosphorylation sites and four amino acid residues that might be crucial for the binding function. The majority of VmSWEET genes in each clade shared similar gene structures and conserved motifs, showing that the VmSWEET genes were highly conserved during evolution. Chromosomal localization and duplication analyses showed that VmSWEET genes were unevenly distributed in eight chromosomes and two pairs of them displayed synteny. A total of 79 cis-acting elements were predicted in the promoter regions of VmSWEETs including elements responsive to plant hormones, light, growth and development and stress responses. qRT-PCR analysis showed that VmSWEET10.1 was highly expressed in flowers, VmSWEET16 was highly expressed in upright and runner stems, and VmSWEET3 was highly expressed in the leaves of both types of stems. In fruit, the expression of VmSWEET14 and VmSWEET16 was highest of all members during the young fruit stage and were downregulated as fruit matured. The expression of VmSWEET4 was higher during later developmental stages than earlier developmental stages. Furthermore, qRT-PCR results revealed a significant up-regulation of VmSWEET10.2, under osmotic, saline, salt-alkali, and aluminum stress conditions, suggesting it has a crucial role in mediating plant responses to various environmental stresses. Overall, these results provide new insights into the characteristics and evolution of VmSWEET genes. Moreover, the candidate VmSWEET genes involved in the growth, development and abiotic stress responses can be used for molecular breeding to improve cranberry fruit quality and abiotic stress resistance.

PLASMODESMATA-LOCATED PROTEIN 6 regulates plasmodesmal function in Arabidopsis vasculature

Plasmodesmata connect adjoining plant cells, allowing molecules to move between the connected cells for communication and sharing resources. It has been well established that the plant polysaccharide callose is deposited at plasmodesmata, regulating their aperture and function. Among proteins involved in maintaining callose homeostasis, PLASMODESMATA-LOCATED PROTEINSs (PDLPs) promote callose deposition at plasmodesmata. This study explored the function of PDLP5 and PDLP6 in different cell types. We discovered that PDLP5 and PDLP6 are expressed in nonoverlapping cell types in Arabidopsis (Arabidopsis thaliana). The overexpression of PDLP5 and PDLP6 results in the overaccumulation of plasmodesmal callose at different cell interfaces, indicating that PDLP5 and PDLP6 are active in different cell types. We also observed 2 distinct patterns of starch accumulation in mature leaves of PDLP5 and PDLP6 overexpressors. An enzyme-catalyzed proximity labeling approach was used to identify putative functional partners of the PDLPs. We identified SUCROSE SYNTHASE 6 (SUS6) as a functional partner of PDLP6 in the vasculature. We further demonstrated that PDLP6 physically and genetically interacts with SUS6. In addition, CALLOSE SYNTHASE 7 (CALS7) physically interacts with SUS6 and PDLP6. Genetic interaction studies showed that CALS7 is required for PDLP6 function. We propose that PDLP6 functions with SUS6 and CALS7 in the vasculature to regulate plasmodesmal function.

Genome‑wide identification of the SWEET gene family in grape (Vitis vinifera L.) and expression analysis of VvSWEET14a in response to water stress

Sugars are considered primary metabolites that determine the flavor and quality of grape berries, also playing a crucial role in the plants to resist stress. Sugars Will Eventually be Exported Transporters (SWEETs) gene family has been previously reported to be involved in the growth and development of grape, while the changes in transcriptional levels under water stress remain unclear. In this study, sixteen grape SWEETs members were identified and annotated based on their homologous genes in Arabidopsis and tomato, they were classified into four clades (Clades I to IV) with VvSWEETs by phylogenetic analysis. The highly conserved motifs and gene structures of VvSWEETs indicate that they are closely evolutionary conservation. Chromosomal localization and synteny analysis found that VvSWEETs were unevenly distributed on 11 chromosomes, and the VvSWEET5a, VvSWEET5b, VvSWEET14b and VvSWEET14c existed a relatively recent evolutionary relationship. Promoter cis-acting elements showed that the clade III has more ABRE motif, especially the VvSWEET14a. The regulation of VvSWEETs is mainly influenced by the Dof and MYB families, which are associated with grape ripening, while VvSWEET14a is closely related to the bHLH, MYB, NAC, and bZIP families. RT-qPCR data and subcellular localization show that VvSWEET14a was highly induced under early water stress and is located in the vacuole membrane. The instantaneous transformation assay identified that this gene could promote to transport hexose in the vacuole to maintain normal osmotic pressure. In summary, our study provides a basis for further research on SWEET genes function and regulatory mechanism in the future, and lays the foundation for stress resistance breeding of Vitis vinifera.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-024-01501-1.

Regulation of nitrogen metabolism by COE2 under low sulfur stress in Arabidopsis

The interplay between nitrogen and sulfur assimilation synergistically supports and sustains plant growth and development, operating in tandem to ensure coordinated and optimal outcomes. Previously, we characterized Arabidopsis CHLOROPHYLL A/B-BINDING (CAB) overexpression 2 (COE2) mutant, which has a mutation in the NITRIC OXIDE-ASSOCIATED (NOA1) gene and exhibits deficiency in root growth under low nitrogen (LN) stress. This study found that the growth suppression in roots and shoots in coe2 correlates with decreased sensitivity to low sulfur stress treatment compared to the wild-type. Therefore, we examined the regulatory role of COE2 in nitrogen and sulfur interaction by assessing the expression of nitrogen metabolism-related genes in coe2 seedlings under low sulfur stress. Despite the notable upregulation of nitrate reductase genes (NIA1 and NIA2), there was a considerable reduction in nitrogen uptake and utilization, resulting in a substantial growth penalty. Moreover, the elevated expression of miR396 perhaps complemented growth stunting by selectively targeting and curtailing the expression levels of GROWTH REGULATING FACTOR 2 (GRF2), GRF4, and GRF9. This study underscores the vital role of COE2-mediated nitrogen signaling in facilitating seedling growth under sulfur deficiency stress.

Sugar transporters PpSWEET9a and PpSWEET14 synergistically mediate peach sucrose allocation from source leaves to fruit

Sugar content is a critical indicator of fruit quality and is mainly controlled by sugar transporters. Sugars will eventually be exported transporters (SWEET) proteins play an indispensable role in sugar allocation between and within plant organs. Sucrose is the major sugar in many fruits and the predominant form of sugar translocated in peach (Prunus persica). However, the role of the multiple peach SWEET genes in sucrose allocation to fruit remains elusive. In this study, a total of 19 SWEET candidates have been identified in the peach genome, and two Clade III SWEET genes, PpSWEET9a and PpSWEET14, are found to be highly expressed in mature source leaves and branches. Complementation assays, transgene manipulations, and protein interaction studies reveal that PpSWEET9a and PpSWEET14 serve as sucrose efflux proteins and form a heterooligomer that synergistically directs sucrose allocation from source leaves to fruits. Our findings provide insights into the effect of SWEETs on sugar accumulation in peach fruit and identify genetic candidates for improving fruit quality.

Using Transcriptomics to Determine the Mechanism for the Resistance to Fusarium Head Blight of a Wheat-Th. elongatum Translocation Line

Fusarium head blight (FHB), caused by the Fusarium graminearum species complex, is a destructive disease in wheat worldwide. The lack of FHB-resistant germplasm is a barrier in wheat breeding for resistance to FHB. Thinopyrum elongatum is an important relative that has been successfully used for the genetic improvement of wheat. In this study, a translocation line, YNM158, with the YM158 genetic background carrying a fragment of diploid Th. elongatum 7EL chromosome created using 60Co-γ radiation, showed high resistance to FHB under both field and greenhouse conditions. Transcriptome analysis confirmed that the horizontal transfer gene, encoding glutathione S-transferase (GST), is an important contributor to FHB resistance in the pathogen infection stage, whereas the 7EL chromosome fragment carries other genes regulated by F. graminearum during the colonization stage. Introgression of the 7EL fragment affected the expression of wheat genes that were enriched in resistance pathways, including the phosphatidylinositol signaling system, protein processing in the endoplasmic reticulum, plant-pathogen interaction, and the mitogen-activated protein kinase (MAPK) signaling pathway at different stages after F. graminearium infection. This study provides a novel germplasm for wheat resistance to FHB and new insights into the molecular mechanisms of wheat resistance to FHB.

Source-sink synergy is the key unlocking sweet potato starch yield potential

Sweet potato starch is in high demand globally for food and industry. However, starch content is negatively correlated with fresh yield. It is urgent to uncover the genetic basis and molecular mechanisms underlying the starch yield of sweet potato. Here we systematically explore source-sink synergy-mediated sweet potato starch yield formation: the production, loading, and transport of photosynthates in leaves, as well as their unloading and allocation in storage roots, lead to starch content divergence between sweet potato varieties. Moreover, we find that six haplotypes of IbPMA1 encoding a plasma membrane H+-ATPase are significantly linked with starch accumulation. Overexpression of IbPMA1 in sweet potato results in significantly increased starch and sucrose contents, while its knockdown exhibits an opposing effect. Furthermore, a basic helix-loop-helix (bHLH) transcription factor IbbHLH49 directly targets IbPMA1 and activates its transcription. Overexpression of IbbHLH49 notably improves source-sink synergy-mediated fresh yield and starch accumulation in sweet potato. Both IbbHLH49 and IbPMA1 substantially influence sugar transport and starch biosynthesis in source and sink tissues. These findings expand our understanding of starch yield formation and provide strategies and candidate genes for high starch breeding in root and tuber crops.

Sugar Transporter HmSWEET8 Cooperates with HmSTP1 to Enhance Powdery Mildew Susceptibility in Heracleum moellendorffii Hance

The powdery mildew caused by Eeysiphe heraclei is a serious concern in Heracleum moellendorffii Hance. Therefore, exploring the mechanisms underlying sugar efflux from host cells to the fungus during the plant-fungus interaction showed great significance. The study successfully cloned HmSWEET8 and HmSTP1 genes based on RNA-seq technology. The complementation assays in yeast EBY.VW4000 found HmSWEET8 and HmSTP1 transporting hexose. Over-expressing or silencing HmSWEET8 in H. moellendorffii leaves increased or decreased powdery mildew susceptibility by changing glucose concentration in infective sites. Meanwhile, over-expressing HmSTP1 in H. moellendorffii leaves also increased powdery mildew susceptibility by elevating the glucose content of infective areas. Additionally, HmSTP1 expression was up-regulated obviously in HmSWEET8 over-expressed plants and inhibited significantly in HmSWEET8 silenced plants. Co-expressing HmSWEET8 and HmSTP1 genes significantly increased powdery mildew susceptibility compared with over-expressed HmSWEET8 or HmSTP1 plants alone. The results demonstrated that HmSTP1 may assist with HmSWEET8 to promote E. heraclei infection. Consequently, the infection caused by E. heraclei resulted in the activation of HmSWEET8, leading to an increased transfer of glucose to the apoplasmic spaces at the sites of infection, then, HmSTP1 facilitated the transport of glucose into host cells, promoting powdery mildew infection.

Single-cell network analysis reveals gene expression programs for Arabidopsis root development and metabolism

Single-cell RNA-sequencing datasets of Arabidopsis roots have been generated, but related comprehensive gene co-expression network analyses are lacking. We conducted a single-cell gene co-expression network analysis with publicly available scRNA-seq datasets of Arabidopsis roots using a SingleCellGGM algorithm. The analysis identified 149 gene co-expression modules, which we considered to be gene expression programs (GEPs). By examining their spatiotemporal expression, we identified GEPs specifically expressed in major root cell types along their developmental trajectories. These GEPs define gene programs regulating root cell development at different stages and are enriched with relevant developmental regulators. As examples, a GEP specific for the quiescent center (QC) contains 20 genes regulating QC and stem cell niche homeostasis, and four GEPs are expressed in sieve elements (SEs) from early to late developmental stages, with the early-stage GEP containing 17 known SE developmental regulators. We also identified GEPs for metabolic pathways with cell-type-specific expression, suggesting the existence of cell-type-specific metabolism in roots. Using the GEPs, we discovered and verified a columella-specific gene, NRL27, as a regulator of the auxin-related root gravitropism response. Our analysis thus systematically reveals GEPs that regulate Arabidopsis root development and metabolism and provides ample resources for root biology studies.

Effect of drought acclimation on sugar metabolism in millet

Drought stress triggers sugar accumulation in plants, providing energy and aiding in protection against oxidative damage. Plant hardening under mild stress conditions has been shown to enhance plant resistance to severe stress conditions. While sugar accumulation and metabolism under drought stress have been well-documented in crop plants, the effect of drought acclimation treatment on sugar accumulation and metabolism has not yet been explored. In this study, we investigated the impact of drought stress acclimation on sugar accumulation and metabolism in the leaves and root tissues of two commonly cultivated foxtail millet (Setaria italica L.) genotypes, 'PI 689680' and 'PI 662292'. Quantification of total sugars (soluble sugar, fructose, glucose, and sucrose), their related enzymes (SPS, SuSy, NI, and AI), and the regulation of their related transcripts (SiSPS1, SiSuSy1, SiSWEET6, SiA-INV, and SiC-INV) revealed that drought-acclimated (DA) plants exhibited levels of these indicators comparable to those of control plants. However, under subsequent drought stress conditions, both the leaves and roots of non-acclimated plants accumulated higher levels of total sugars, displayed increased activity of sugar metabolism enzymes, and showed elevated expression of sugar metabolism-related transcripts compared to drought-acclimated plants. Thus, acclimation-induced restriction of sugar accumulation, transport, and metabolism could be one of the metabolic processes contributing to enhanced drought tolerance in millet. This study advocates for the use of acclimation as an effective strategy to mitigate the negative impacts of drought-induced metabolic disturbances in millet, thereby enhancing global food security and promoting sustainable agricultural systems.

The maize sugar transporters ZmSWEET15a and ZmSWEET15b positively regulate salt tolerance in plants

The SWEETs (sugars will eventually be exported transporter) family comprises a class of recently identified sugar transporters that play diverse roles in regulating plant development. Beyond those fundamental functions, emerging evidence suggests that SWEETs may also be involved in plant stress responses, such as salt tolerance. However, the specific role of maize SWEETs in regulating salt tolerance remains unexplored. In this study, we demonstrate that two maize SWEET family members, ZmSWEET15a and ZmSWEET15b, are typical sugar transporters with seven transmembrane helices localized in the cell membrane. The heterologous expression of ZmSWEET15a and ZmSWEET15b in the yeast mutant strain confirms their role as sucrose transporters. Overexpression of ZmSWEET15a and ZmSWEET15b in Arabidopsis resulted in improved NaCl resistance and significant increase in seed germination rate compared to the wild type. Furthermore, by generating maize knockout mutants, we observe that the absence of ZmSWEET15a and ZmSWEET15b affects both plant growth and grain development. The salt treatment results indicate that the knockout mutants of these two genes are more sensitive to salt stress. Comparative analyses revealed that wild-type maize plants outperformed the knockout mutants in terms of growth parameters and physiological indices. Our findings unravel a novel function of ZmSWEET15a and ZmSWEET15b in the salt stress response, offering a theoretical foundation for enhancing maize salt resistance.

Variability in soybean yield responses to elevated atmospheric CO2: Insights from non-structural carbohydrate remobilisation during seed filling

The increasing atmospheric CO2 concentration (e[CO2]) has mixed effects on soybean most varieties' yield. This study elucidated the effect of e[CO2] on soybean yield and the underlying mechanisms related to photosynthetic capacity, non-structural carbohydrate (NSC) accumulation, and remobilisation. Four soybean cultivars were cultivated in open-top chambers at two CO2 levels. Photosynthesis rates were determined from R2 to R6. Plants were sampled at R5 and R8 to determine carbohydrate concentrations. There were significant variations in yield responses among the soybean cultivars under e[CO2], from no change in DS1 to a 22% increase in SN14. DS1 and SN14 had the smallest and largest increase, respectively, in daily carbon assimilation capacity. Under e[CO2], DS1, MF5, and XHJ had an increase in Ci, at which point the transition from Rubisco-limited to ribulose-1,5-bisphosphate regeneration-limited photosynthesis occurred, in contrast with SN14. Thus, the cultivars might have distinct mechanisms that enhance photosynthesis under e[CO2] conditions. A positive correlation was between daily carbon assimilation response to e[CO2] and soybean yield, emphasising the importance of enhanced photosynthate accumulation before the R5 stage in determining yield response to e[CO2]. E[CO2] significantly influenced NSC accumulation in vegetative organs at R5, with variation among cultivars. There was enhanced NSC remobilisation during seed filling, indicating cultivar-specific responses to the remobilisation of sucrose and soluble sugars, excluding sucrose and starch. A positive correlation was between leaf and stem NSC remobilisation and yield response to e[CO2], emphasising the role of genetic differences in carbohydrate remobilisation mechanisms in determining soybean yield variation under elevated CO2 levels.

Functional Characterization of JcSWEET12 and JcSWEET17a from Physic Nut

Physic nut (Jatropha curcas L.) has attracted extensive attention because of its fast growth, easy reproduction, tolerance to barren conditions, and high oil content of seeds. SWEET (Sugar Will Eventually be Exported Transporter) family genes contribute to regulating the distribution of carbohydrates in plants and have great potential in improving yield and stress tolerance. In this study, we performed a functional analysis of the homology of these genes from physic nut, JcSWEET12 and JcSWEET17a. Subcellular localization indicated that the JcSWEET12 protein is localized on the plasma membrane and the JcSWEET17a protein on the vacuolar membrane. The overexpression of JcSWEET12 (OE12) and JcSWEET17a (OE17a) in Arabidopsis leads to late and early flowering, respectively, compared to the wild-type plants. The transgenic OE12 seedlings, but not OE17a, exhibit increased salt tolerance. In addition, OE12 plants attain greater plant height and greater shoot dry weight than the wild-type plants at maturity. Together, our results indicate that JcSWEET12 and JcSWEET17a play different roles in the regulation of flowering time and salt stress response, providing a novel genetic resource for future improvement in physic nut and other plants.

Genome-wide analysis of the SWEET gene family in Hemerocallis citrina and functional characterization of HcSWEET4a in response to salt stress

Sugars will be eventually effluxed transporters (SWEETs) have been confirmed to play diverse physiological roles in plant growth, development and stress response. However, the characteristics and functions of the SWEET genes in Hemerocallis citrina remain unclear and poorly elucidated. In this study, the whole genome of Hemerocallis citrina was utilized to conduct bioinformatics analysis and a total of 19 HcSWEET genes were successfully identified. Analysis of the physicochemical properties indicated dominant differences among these HcSWEETs. A phylogenetic analysis revealed that HcSWEET proteins can be divided into 4 clades ranging from Clade I to IV, where proteins within the same clade exhibited shared conserved motifs and gene structures. Five to six exons were contained in the majority of HcSWEET genes, which were unevenly distributed across 11 chromosomes. The gene duplication analysis showed the presence of 4 gene pairs. Comparative syntenic maps revealed that the HcSWEET gene family might present more closed homology in monocotyledons than dicotyledons. Cis-acting element analysis of HcSWEET genes indicated key responsiveness to various hormones, light, and stresses. Additionally, transcriptome sequencing analysis suggested that most HcSWEET genes had a relatively higher expression in roots, and HcSWEET4a was significantly up-regulated under salt stress. Overexpression further verified the possibility that HcSWEET4a was involved in response to salt stress, which provides novel insights and facilitates in-depth studies of the functional analysis of HcSWEETs in resistance to abiotic stress.

Light signaling regulates root-knot nematode infection and development via HY5-SWEET signaling

BACKGROUND

Meloidogyne incognita is one of the most important plant-parasitic nematodes and causes tremendous losses to the agricultural economy. Light is an important living factor for plants and pathogenic organisms, and sufficient light promotes root-knot nematode infection, but the underlying mechanism is still unclear.

RESULTS

Expression level and genetic analyses revealed that the photoreceptor genes PHY, CRY, and PHOT have a negative impact on nematode infection. Interestingly, ELONGATED HYPOCOTYL5 (HY5), a downstream gene involved in the regulation of light signaling, is associated with photoreceptor-mediated negative regulation of root-knot nematode resistance. ChIP and yeast one-hybrid assays supported that HY5 participates in plant-to-root-knot nematode responses by directly binding to the SWEET negative regulatory factors involved in root-knot nematode resistance.

CONCLUSIONS

This study elucidates the important role of light signaling pathways in plant resistance to nematodes, providing a new perspective for RKN resistance research.

Brown planthoppers manipulate rice sugar transporters to benefit their own feeding

The feeding of piercing-sucking insect herbivores often elicits changes in their host plants that benefit the insect.1 In addition to thwarting a host's defense responses, these phloem-feeding insects may manipulate source-sink signaling so as to increase resources consumed.2,3 To date, the molecular mechanisms underlying herbivore-induced resource reallocation remain less investigated. Brown planthopper (BPH), an important rice pest, feeds on the phloem and oviposits into leaf sheaths. BPH herbivory increases sugar accumulations 5-fold in the phloem sap of leaf sheaths and concurrently induces the expression of two clade III SWEET genes, SWEET13 and SWEET14, in leaf tissues, but not in leaf sheaths of attacked rice plants. Mutations of both genes by genome editing attenuate resistance to BPH without alterations of known chemical and physical defense responses. Moreover, BPH-elicited sugar levels in the phloem sap were significantly reduced in sweet13/14 mutants, which is likely to attenuate BPH feeding behavior on sweet13/14 mutants. In one of the two field seasons tested, the sweet13/14 mutants showed comparable yield to wild types, and in the other season, the mutants demonstrated stronger BPH resistance. These preliminary results suggested that the mutations in these SWEET transporters could enhance BPH resistance without yield penalties. Given that sweet13/14 mutants also exhibit resistance to bacterial blight pathogen, Xanthomonas oryzae pv. oryzae, these SWEET genes could serve as excellent molecular targets for the breeding of resistant rice cultivars.

Sugar import mediated by sugar transporters and cell wall invertases for seed development in Camellia oleifera

Seed development and yield depend on the transport and supply of sugar. However, an insufficient supply of nutrients from maternal tissues to embryos results in seed abortion and yield reduction in Camellia oleifera. In this study, we systematically examined the route and regulatory mechanisms of sugar import into developing C. oleifera seeds using a combination of histological observations, transcriptome profiling, and functional analysis. Labelling with the tracer carboxyfluorescein revealed a symplasmic route in the integument and an apoplasmic route for postphloem transport at the maternal-filial interface. Enzymatic activity and histological observation showed that at early stages [180-220 days after pollination (DAP)] of embryo differentiation, the high hexose/sucrose ratio was primarily mediated by acid invertases, and the micropylar endosperm/suspensor provides a channel for sugar import. Through Camellia genomic profiling, we identified three plasma membrane-localized proteins including CoSWEET1b, CoSWEET15, and CoSUT2 and one tonoplast-localized protein CoSWEET2a in seeds and verified their ability to transport various sugars via transformation in yeast mutants and calli. In situ hybridization and profiling of glycometabolism-related enzymes further demonstrated that CoSWEET15 functions as a micropylar endosperm-specific gene, together with the cell wall acid invertase CoCWIN9, to support early embryo development, while CoSWEET1b, CoSWEET2a, and CoSUT2 function at transfer cells and chalazal nucellus coupled with CoCWIN9 and CoCWIN11 responsible for sugar entry in bulk into the filial tissue. Collectively, our findings provide the first comprehensive evidence of the molecular regulation of sugar import into and within C. oleifera seeds and provide a new target for manipulating seed development.

Stem Cells and Differentiation in Vascular Tissues

Plant vascular tissues are crucial for the long-distance transport of water, nutrients, and a multitude of signal molecules throughout the plant body and, therefore, central to plant growth and development. The intricate development of vascular tissues is orchestrated by unique populations of dedicated stem cells integrating endogenous as well as environmental cues. This review summarizes our current understanding of vascular-related stem cell biology and of vascular tissue differentiation. We present an overview of the molecular and cellular mechanisms governing the maintenance and fate determination of vascular stem cells and highlight the interplay between intrinsic and external cues. In this context, we emphasize the role of transcription factors, hormonal signaling, and epigenetic modifications. We also discuss emerging technologies and the large repertoire of cell types associated with vascular tissues, which have the potential to provide unprecedented insights into cellular specialization and anatomical adaptations to distinct ecological niches.

Orchids acquire fungal carbon for seed germination: pathways and players

To germinate in nature, orchid seeds strictly rely on seed germination-promoting orchid mycorrhizal fungi (sgOMFs) for provision of carbon nutrients. The underlying delivery pathway, however, remains elusive. We develop here a plausible model for sugar transport from sgOMFs to orchid embryonic cells to fuel germination. Orchids exploit sgOMFs to induce the formation of pelotons, elaborate intracellular hyphal coils in orchid embryos. The colonized orchid cells then obtain carbon nutrients by uptake from living hyphae and peloton lysis, primarily as glucose derived from fungal trehalose hydrolyzed by orchid-specific trehalases. The uptake of massive fungally derived glucose is likely to be mediated by two classes of membrane proteins, namely, sugars will eventually be exported transporters (SWEETs) and H+-hexose symporters. The proposed model serves as a launch pad for further research to better understand and improve orchid seed germination and conservation.

Co-overexpression of SWEET sucrose transporters modulates sucrose synthesis and defence responses to enhance immunity against bacterial blight in rice

Enhancing carbohydrate export from source to sink tissues is considered to be a realistic approach for improving photosynthetic efficiency and crop yield. The rice sucrose transporters OsSUT1, OsSWEET11a and OsSWEET14 contribute to sucrose phloem loading and seed filling. Crucially, Xanthomonas oryzae pv. oryzae (Xoo) infection in rice enhances the expression of OsSWEET11a and OsSWEET14 genes, and causes leaf blight. Here we show that co-overexpression of OsSUT1, OsSWEET11a and OsSWEET14 in rice reduced sucrose synthesis and transport leading to lower growth and yield but reduced susceptibility to Xoo relative to controls. The immunity-related hypersensitive response (HR) was enhanced in the transformed lines as indicated by the increased expression of defence genes, higher salicylic acid content and presence of HR lesions on the leaves. The results suggest that the increased expression of OsSWEET11a and OsSWEET14 in rice is perceived as a pathogen (Xoo) attack that triggers HR and results in constitutive activation of plant defences that are related to the signalling pathways of pathogen starvation. These findings provide a mechanistic basis for the trade-off between plant growth and immunity because decreased susceptibility against Xoo compromised plant growth and yield.

Key transcription factors regulate fruit ripening and metabolite accumulation in tomato

Fruit ripening is a complex process involving dynamic changes to metabolites and is controlled by multiple factors, including transcription factors (TFs). Several TFs are reportedly essential regulators of tomato (Solanum lycopersicum) fruit ripening. To evaluate the effects of specific TFs on metabolite accumulation during fruit ripening, we combined CRISPR/Cas9-mediated mutagenesis with metabolome and transcriptome analyses to explore regulatory mechanisms. Specifically, we generated various genetically engineered tomato lines that differed regarding metabolite contents and fruit colors. The metabolite and transcript profiles indicated that the selected TFs have distinct functions that control fruit metabolite contents, especially carotenoids and sugars. Moreover, a mutation to ELONGATED HYPOCOTYL5 (HY5) increased tomato fruit fructose and glucose contents by approximately 20% (relative to the wild-type levels). Our in vitro assay showed that HY5 can bind directly to the G-box cis-element in the Sugars Will Eventually be Exported Transporter (SWEET12c) promoter to activate expression, thereby modulating sugar transport. Our findings provide insights into the mechanisms regulating tomato fruit ripening and metabolic networks, providing the theoretical basis for breeding horticultural crops that produce fruit with diverse flavors and colors.

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.

The Arabidopsis AtSWEET13 transporter discriminates sugars by selective facial and positional substrate recognition

Transporters are targeted by endogenous metabolites and exogenous molecules to reach cellular destinations, but it is generally not understood how different substrate classes exploit the same transporter's mechanism. Any disclosure of plasticity in transporter mechanism when treated with different substrates becomes critical for developing general selectivity principles in membrane transport catalysis. Using extensive molecular dynamics simulations with an enhanced sampling approach, we select the Arabidopsis sugar transporter AtSWEET13 as a model system to identify the basis for glucose versus sucrose molecular recognition and transport. Here we find that AtSWEET13 chemical selectivity originates from a conserved substrate facial selectivity demonstrated when committing alternate access, despite mono-/di-saccharides experiencing differing degrees of conformational and positional freedom throughout other stages of transport. However, substrate interactions with structural hallmarks associated with known functional annotations can help reinforce selective preferences in molecular transport.

Pyramiding of bacterial blight resistance genes into promising restorer BRRI31R line through marker-assisted backcross breeding and evaluation of agro-morphological and physiochemical characteristics of developed resistant restorer lines

BRRI31R is one of the Bangladesh's most promising restorer lines due to its abundant pollen producing capacity, strong restoring ability, good combining ability, high outcrossing rate and genetically diverse from cytoplasmic male sterile (CMS) line. But the drawback of this line is that it is highly susceptible to bacterial blight (BB) disease of rice caused by Xanthomonas oryzae pv. oryzae. The present study highlighted the pyramiding of effective BB resistance genes (xa5, xa13 and Xa21) into the background of BRRI31R, through marker-assisted backcrossing (MABC). Backcross progenies were confirmed and advanced based on the foreground selection of target genes. Pyramided lines were used for pathogenicity test against five Bangladeshi Xanthomonas oryzae (BXo) races (BXo93, BXo220, BXo822, BXo826, BXo887) and confirmed the dominant fertility restore genes, Rf3 and Rf4 and further validated against SNP markers for more confirmation of target resistance genes. All pyramided restorer lines consisted of Xa4 (in built), xa5, xa13, Xa21, and Chalk5 with two fertility restorer genes, Rf3, Rf4. and these restorer lines showed intermediate amylose content (<25%). Restorer lines BRRI31R-MASP3 and BRRI31R-MASP4 showed high levels of resistance against five virulent BXo races and SNP genotyping revealed that these lines also contained a blast resistance gene Pita races. Gene pyramided restorer lines, BRRI31R-MASP3 and BRRI31R-MASP4 can directly be used as a male parent for the development of new BB resistant hybrid rice variety or could be used as a replacement of restorer line of BRRI hybrid dhan5 and 7 to enhance the quality of hybrid seeds as well as rice production in Bangladesh.

From acidity to sweetness: a comprehensive review of carbon accumulation in grape berries

Most of the carbon found in fruits at harvest is imported by the phloem. Imported carbon provide the material needed for the accumulation of sugars, organic acids, secondary compounds, in addition to the material needed for the synthesis of cell walls. The accumulation of sugars during fruit development influences not only sweetness but also various parameters controlling fruit composition (fruit "quality"). The accumulation of organic acids and sugar in grape berry flesh cells is a key process for berry development and ripening. The present review presents an update of the research on grape berry development, anatomical structure, sugar and acid metabolism, sugar transporters, and regulatory factors.

MORF9-dependent specific plastid RNA editing inhibits root growth under sugar starvation in Arabidopsis

Multiple organellar RNA editing factor (MORF) complex was shown to be highly associated with C-to-U RNA editing of vascular plant editosome. However, mechanisms by which MORF9-dependent plastid RNA editing controls plant development and responses to environmental alteration remain obscure. In this study, we found that loss of MORF9 function impaired PSII efficiency, NDH activity, and carbohydrate production, rapidly promoted nuclear gene expression including sucrose transporter and sugar/energy responsive genes, and attenuated root growth under sugar starvation conditions. Sugar repletion increased MORF9 and MORF2 expression in wild-type seedlings and reduced RNA editing of matK-706, accD-794, ndhD-383 and ndhF-290 in the morf9 mutant. RNA editing efficiency of ndhD-383 and ndhF-290 sites was diminished in the gin2/morf9 double mutants, and that of matK-706, accD-794, ndhD-383 and ndhF-290 sites were significantly diminished in the snrk1/morf9 double mutants. In contrast, overexpressing HXK1 or SnRK1 promoted RNA editing rate of matK-706, accD-794, ndhD-383 and ndhF-290 in leaves of morf9 mutants, suggesting that HXK1 partially impacts MORF9 mediated ndhD-383 and ndhF-290 editing, while SnRK1 may only affect MORF9-mediated ndhF-290 site editing. Collectively, these findings suggest that sugar and/or its intermediary metabolites impair MORF9-dependent plastid RNA editing resulting in derangements of plant root development.

SlZIP11 mediates zinc accumulation and sugar storage in tomato fruits

Background

Zinc (Zn) is a vital micronutrient essential for plant growth and development. Transporter proteins of the ZRT/IRT-like protein (ZIP) family play crucial roles in maintaining Zn homeostasis. Although the acquisition, translocation, and intracellular transport of Zn are well understood in plant roots and leaves, the genes that regulate these pathways in fruits remain largely unexplored. In this study, we aimed to investigate the function of SlZIP11 in regulating tomato fruit development.

Methods

We used Solanum lycopersicum L. 'Micro-Tom' SlZIP11 (Solanum lycopersicum) is highly expressed in tomato fruit, particularly in mature green (MG) stages. For obtaining results, we employed reverse transcription-quantitative polymerase chain reaction (RT-qPCR), yeast two-hybrid assay, bimolecular fluorescent complementation, subcellular localization assay, virus-induced gene silencing (VIGS), SlZIP11 overexpression, determination of Zn content, sugar extraction and content determination, and statistical analysis.

Results

RT-qPCR analysis showed elevated SlZIP11 expression in MG tomato fruits. SlZIP11 expression was inhibited and induced by Zn deficiency and toxicity treatments, respectively. Silencing SlZIP11 via the VIGS technology resulted in a significant increase in the Zn content of tomato fruits. In contrast, overexpression of SlZIP11 led to reduced Zn content in MG fruits. Moreover, both silencing and overexpression of SlZIP11 caused alterations in the fructose and glucose contents of tomato fruits. Additionally, SlSWEEET7a interacted with SlZIP11. The heterodimerization between SlSWEET7a and SlZIP11 affected subcellular targeting, thereby increasing the amount of intracellularly localized oligomeric complexes. Overall, this study elucidates the role of SlZIP11 in mediating Zn accumulation and sugar transport during tomato fruit ripening. These findings underscore the significance of SlZIP11 in regulating Zn levels and sugar content, providing insights into its potential implications for plant physiology and agricultural practices.

Sugar delivery at the tomato root and root galls after Meloidogyne incognita infestation

Root-knot nematodes (RKNs) infect host plants and obtain nutrients such as sugars for their own development. Therefore, inhibiting the nutrient supply to RKNs may be an effective method for alleviating root-knot nematode disease. At present, the pathway by which sucrose is unloaded from the phloem cells to giant cells (GCs) in root galls and which genes related to sugar metabolism and transport play key roles in this process are unclear. In this study, we found that sugars could be unloaded into GCs only from neighboring phloem cells through the apoplastic pathway. With the development of galls, the contents of sucrose, fructose and glucose in the galls and adjacent tissue increased gradually. SUT1, SUT2, SWEET7a, STP10, SUS3 and SPS1 may provide sugar sources for GCs, while STP1, STP2 and STP12 may transport more sugar to phloem parenchyma cells. At the early stage of Meloidogyne incognita infestation, the sucrose content in tomato roots and leaves increased, while the glucose and fructose contents decreased. SWEET7a, SPS1, INV-INH1, INV-INH2, SUS1 and SUS3 likely play key roles in root sugar delivery. These results elucidated the pathway of sugar unloading in tomato galls and provided an important theoretical reference for eliminating the sugar source of RKNs and preventing root-knot nematode disease.

The AP2/ERF transcription factor MdDREB2A regulates nitrogen utilisation and sucrose transport under drought stress

Drought stress is one of the main environmental factors limiting plant growth and development. Plants adapt to changing soil moisture by modifying root architecture, inducing stomatal closure, and inhibiting shoot growth. The AP2/ERF transcription factor DREB2A plays a key role in maintaining plant growth in response to drought stress, but the molecular mechanism underlying this process remains to be elucidated. Here, it was found that overexpression of MdDREB2A positively regulated nitrogen utilisation by interacting with DRE cis-elements of the MdNIR1 promoter. Meanwhile, MdDREB2A could also directly bind to the promoter of MdSWEET12, which may enhance root development and nitrogen assimilation, ultimately promoting plant growth. Overall, this regulatory mechanism provides an idea for plants in coordinating with drought tolerance and nitrogen assimilation to maintain optimal plant growth and development under drought stress.

A coordinated switch in sucrose and callose metabolism enables enhanced symplastic unloading in potato tubers

One of the early changes upon tuber induction is the switch from apoplastic to symplastic unloading. Whether and how this change in unloading mode contributes to sink strength has remained unclear. In addition, developing tubers also change from energy to storage-based sucrose metabolism. Here, we investigated the coordination between changes in unloading mode and sucrose metabolism and their relative role in tuber sink strength by looking into callose and sucrose metabolism gene expression combined with a model of apoplastic and symplastic unloading. Gene expression analysis suggests that callose deposition in tubers is decreased by lower callose synthase expression. Furthermore, changes in callose and sucrose metabolism are strongly correlated, indicating a well-coordinated developmental switch. Modelling indicates that symplastic unloading is not the most efficient unloading mode per se. Instead, it is the concurrent metabolic switch that provides the physiological conditions necessary to potentiate symplastic transport and thereby enhance tuber sink strength .

Sugar transporters spatially organize microbiota colonization along the longitudinal root axis of Arabidopsis

Plant roots are functionally heterogeneous in cellular architecture, transcriptome profile, metabolic state, and microbial immunity. We hypothesized that axial differentiation may also impact spatial colonization by root microbiota along the root axis. We developed two growth systems, ArtSoil and CD-Rhizotron, to grow and then dissect Arabidopsis thaliana roots into three segments. We demonstrate that distinct endospheric and rhizosphere bacterial communities colonize the segments, supporting the hypothesis of microbiota differentiation along the axis. Root metabolite profiling of each segment reveals differential metabolite enrichment and specificity. Bioinformatic analyses and GUS histochemistry indicate microbe-induced accumulation of SWEET2, 4, and 12 sugar uniporters. Profiling of root segments from sweet mutants shows altered spatial metabolic profiles and reorganization of endospheric root microbiota. This work reveals the interdependency between root metabolites and microbial colonization and the contribution of SWEETs to spatial diversity and stability of microbial ecosystem.