Plasmodesmata and their role in the regulation of phloem unloading during fruit development

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.

Sugar Transport and Signaling in Shoot Branching

The source-sink relationship is critical for proper plant growth and development, particularly for vegetative axillary buds, whose activity shapes the branching pattern and ultimately the plant architecture. Once formed from axillary meristems, axillary buds remain dormant or become active to grow into new branches. This transition is notably driven by the regulation of the bud sink strength, which is reflected in the ability to unload, metabolize and store photoassimilates. Plants have so far developed two main mechanisms for unloading sugars (sucrose) towards sink organs, a symplasmic pathway and an apoplasmic pathway, but so far limited investigations have been reported about the modes of sugar uptake during the transition from the dormant to the active outgrowth state of the bud. The available data indicate that the switch from dormant bud to active outgrowing state, requires sugar and is shortly preceded by an increase in bud metabolic activity and a remobilization of the stem starch reserves in favor of growing buds. This activation of the bud sink strength is accompanied by an up-regulation of the main markers of apoplasmic unloading, such as sugar transporters (sucrose transporters-SUTs; sugar will eventually be exported transporters-SWEETs), sucrose hydrolyzing enzymes (cell wall invertase-CWINV) and sugar metabolic pathways (glycolysis/tricarboxylic cycle-TCA; oxidative pentose phosphate pathway-OPPP). As these results are limited to a few species, they are not sufficient to provide a complete and accurate picture of the mode(s) of sugar unloading toward axillary buds and deserve to be complemented by additional studies in a wide variety of plants using systems integration, combining genetic, molecular and immunolocalization approaches. Altogether, we discuss here how sugar is a systemic regulator of shoot branching, acting both as an energy-rich molecule and a signaling entity in the establishment of the bud sink strength.

Novel resources to investigate leaf plasmodesmata formation in C3 and C4 monocots

Plasmodesmata (PD) are nanochannels that facilitate cell-to-cell transport in plants. More productive and photosynthetically efficient C4 plants form more PD at the mesophyll (M)-bundle sheath (BS) interface in their leaves than their less efficient C3 relatives. In C4 leaves, PD play an essential role in facilitating the rapid metabolite exchange between the M and BS cells to operate a biochemical CO2 concentrating mechanism, which increases the CO2 partial pressure at the site of Rubisco in the BS cells and hence photosynthetic efficiency. The genetic mechanism controlling PD formation in C3 and C4 leaves is largely unknown, especially in monocot crops, due to the technical challenge of quantifying these nanostructures with electron microscopy. To address this issue, we have generated stably transformed lines of Oryza sativa (rice, C3) and Setaria viridis (setaria, C4) with fluorescent protein-tagged PD to build the first spatiotemporal atlas of leaf pit field (cluster of PD) density in monocots without the need for electron microscopy. Across leaf development, setaria had consistently more PD connections at the M-BS wall interface than rice while the difference in M-M pit field density varied. While light was a critical trigger of PD formation, cell type and function determined leaf pit field density. Complementary temporal mRNA sequencing and gene co-expression network analysis revealed that the pattern of pit field density correlated with differentially expressed PD-associated genes and photosynthesis-related genes. PD-associated genes identified from our co-expression network analysis are related to cell wall expansion, translation and chloroplast signalling.

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.

Sustained carbon import supports sugar accumulation and anthocyanin biosynthesis during fruit development and ripening in blueberry (Vaccinium ashei)

Fruit ripening is a highly coordinated process involving molecular and biochemical changes that collectively determine fruit quality. The underlying metabolic programs and their transitions leading to fruit ripening remain largely under-characterized in blueberry (Vaccinium sp.), which exhibits atypical climacteric behavior. In this study, we focused on sugar, acid and anthocyanin metabolism in two rabbiteye blueberry cultivars, Premier and Powderblue, during fruit development and ripening. Concentrations of the three major sugars, sucrose (Suc), glucose (Glc), and fructose (Fru) increased steadily during fruit development leading up to ripening, and increased dramatically by around 2-fold in 'Premier' and 2- to 3-fold in 'Powderblue' during the final stage of fruit ripening. Starch concentration was very low throughout fruit development in both cultivars indicating that it does not serve the role of a major transitory carbon (C) storage form in blueberry fruit. Together, these patterns indicate continued import of C, likely in the form of Suc, throughout blueberry fruit development. Concentrations of the predominant acids, malate and quinate, decreased during ripening, and may contribute to increased shikimate biosynthesis which, in-turn, allows for downstream phenylpropanoid metabolism leading to anthocyanin synthesis. Consistently, anthocyanin concentrations were highest in fully ripened blue fruit. Weighted gene co-expression network analysis (WGCNA) was performed using a 'Powderblue' fruit ripening transcriptome and targeted fruit metabolite concentration data. A 'dark turquoise' module positively correlated with sugars and anthocyanins, and negatively correlated with acids (malate, quinate), was identified. Gene Ontology (GO) enrichment analysis of this module identified transcripts related to sugar, acid, and phenylpropanoid metabolism pathways. Among these, increased transcript abundance of a VACUOLAR INVERTASE during ripening was consistent with sugar storage in the vacuole. In general, transcript abundance of the glycolysis pathway genes was upregulated during ripening. The transcript abundance of PHOSPHOENOLPYRUVATE (PEP) CARBOXYKINASE increased during fruit ripening and was negatively correlated with malate concentration, suggesting increased malate conversion to PEP, which supports anthocyanin production during fruit ripening. This was further supported by the co-upregulation of several anthocyanin biosynthesis-related genes. Together, this study provides insights into important metabolic programs, and their underlying gene expression patterns during fruit development and ripening in blueberry.

Investigating the ecological implications of nanomaterials: Unveiling plants' notable responses to nano-pollution

The rapid advancement of nanotechnology has led to unprecedented innovations; however, it is crucial to analyze its environmental impacts carefully. This review thoroughly examines the complex relationship between plants and nanomaterials, highlighting their significant impact on ecological sustainability and ecosystem well-being. This study investigated the response of plants to nano-pollution stress, revealing the complex regulation of defense-related genes and proteins, and highlighting the sophisticated defense mechanisms in nature. Phytohormones play a crucial role in the complex molecular communication network that regulates plant responses to exposure to nanomaterials. The interaction between plants and nano-pollution influences plants' complex defense strategies. This reveals the interconnectedness of systems of nature. Nevertheless, these findings have implications beyond the plant domain. The incorporation of hyperaccumulator plants into pollution mitigation strategies has the potential to create more environmentally sustainable urban landscapes and improve overall environmental resilience. By utilizing these exceptional plants, we can create a future in which cities serve as centers of both innovation and ecological balance. Further investigation is necessary to explore the long-term presence of nanoparticles in the environment, their ability to induce genetic changes in plants over multiple generations, and their overall impact on ecosystems. In conclusion, this review summarizes significant scientific discoveries with broad implications beyond the confines of laboratories. This highlights the importance of understanding the interactions between plants and nanomaterials within the wider scope of environmental health. By considering these insights, we initiated a path towards the responsible utilization of nanomaterials, environmentally friendly management of pollution, and interdisciplinary exploration. We have the responsibility to balance scientific advancement and environmental preservation to create a sustainable future that combines nature's wisdom with human innovation.

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.

Callose metabolism and the regulation of cell walls and plasmodesmata during plant mutualistic and pathogenic interactions

Cell walls are essential for plant growth and development, providing support and protection from external environments. Callose is a glucan that accumulates in specialized cell wall microdomains including around intercellular pores called plasmodesmata. Despite representing a small percentage of the cell wall (~0.3% in the model plant Arabidopsis thaliana), callose accumulation regulates important biological processes such as phloem and pollen development, cell division, organ formation, responses to pathogenic invasion and to changes in nutrients and toxic metals in the soil. Callose accumulation modifies cell wall properties and restricts plasmodesmata aperture, affecting the transport of signaling proteins and RNA molecules that regulate plant developmental and environmental responses. Although the importance of callose, at and outside plasmodesmata cell walls, is widely recognized, the underlying mechanisms controlling changes in its synthesis and degradation are still unresolved. In this review, we explore the most recent literature addressing callose metabolism with a focus on the molecular factors affecting callose accumulation in response to mutualistic symbionts and pathogenic elicitors. We discuss commonalities in the signaling pathways, identify research gaps and highlight opportunities to target callose in the improvement of plant responses to beneficial versus pathogenic microbes.

Activation of apoplastic sugar at the transition stage may be essential for axillary bud outgrowth in the grasses

Shoot branches develop from buds in leaf axils. Once formed from axillary meristems, the buds enter a transition stage before growing into branches. The buds may transition into dormancy if internal and environmental factors limit sucrose supply to the buds. A fundamental question is why sucrose can be limiting at the transition stage for bud outgrowth, whereas new buds continue to be formed. Sucrose is transported to sink tissues through symplastic or apoplastic pathways and a shift from symplastic to apoplastic pathway is common during seed and fruit development. In addition, symplastic connected tissues are stronger sinks than symplastically isolated tissues that rely on sugars effluxed to the apoplast. Recent studies in sorghum, sugarcane, and maize indicate activation of apoplastic sugar in buds that transition to outgrowth but not to dormancy, although the mode of sugar transport during bud formation is still unclear. Since the apoplastic pathway in sorghum buds was specifically activated during bud outgrowth, we posit that sugar for axillary bud formation is most likely supplied through the symplastic pathway. This suggests a key developmental change at the transition stage, which alters the sugar transport pathway of newly-formed buds from symplastic to apoplastic, making the buds a less strong sink for sugars. We suggest therefore that bud outgrowth that relies on overflow of excess sucrose to the apoplast will be more sensitive to internal and environmental factors that enhance the growth of sink tissues and sucrose demand in the parent shoot; whereas bud formation that relies on symplastic sucrose will be less affected by these factors.

Sphingolipids at Plasmodesmata: Structural Components and Functional Modulators

Plasmodesmata (PD) are plant-specific channels connecting adjacent cells to mediate intercellular communication of molecules essential for plant development and defense. The typical PD are organized by the close apposition of the plasma membrane (PM), the desmotubule derived from the endoplasmic reticulum (ER), and spoke-like elements linking the two membranes. The plasmodesmal PM (PD-PM) is characterized by the formation of unique microdomains enriched with sphingolipids, sterols, and specific proteins, identified by lipidomics and proteomics. These components modulate PD to adapt to the dynamic changes of developmental processes and environmental stimuli. In this review, we focus on highlighting the functions of sphingolipid species in plasmodesmata, including membrane microdomain organization, architecture transformation, callose deposition and permeability control, and signaling regulation. We also briefly discuss the difference between sphingolipids and sterols, and we propose potential unresolved questions that are of help for further understanding the correspondence between plasmodesmal structure and function.