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The phloem of Cpd1/+ mutant leaves contains ectopic callose and lignin deposition. (A, C, E, G, I, K, and L) Images of wild-type leaf sections; (B, D, F, H, J, M, and N) images of Cpd1/+ mutant leaf sections. (A, B, K-N) Bright-field images; (C and D) UV autofluorescence, (E-J) aniline blue staining of callose under UV illumination. Callose deposition in cross-sections of Cpd1/+ mutant minor (B, D, F) and lateral veins (J) observed under bright-field (B) and UV light (without and with aniline blue staining, D and F). The inset in (F) shows a close-up of the phloem region containing ectopic callose. Asterisks signify CCs (inset F). Paradermal longitudinal sections of wild-type (G) and Cpd1/+ mutant (H) veins show that callose deposits along the length of the sieve tube are only observed in the mutant (arrowheads). In addition to callose staining by aniline blue (white arrowhead), non-aniline-bluepositive staining material can also be observed in the mutant Cpd1/+ vein (black arrows in J). Ectopic lignin deposition in the Cpd1/+ mutant phloem was revealed by Maule staining in both minor and lateral veins in chlorotic tissue (M and N). Lignin was only observed in the xylem cell walls of wild-type tissue (K and L). Scale bars in A-F=25 µm; in G, H, K, M=100 µm; in I, J, L, N=50 µm. (This figure is available in colour at JXB online.)
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Plants synthesize carbohydrates in photosynthetic tissues, with the majority of plants transporting sucrose to non-photosynthetic tissues to sustain growth and development. While the anatomical, biochemical, and physiological processes regulating sucrose long-distance transport are well characterized, little is known concerning the genes controllin...
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... In this study, there was a significant accumulation of callose in the stem of S. trilobata compared to native species under low treatment (Fig. 3C). Studies have shown that the rapid accumulation of callose leads to blockage in the phloem, affecting the transport of organic substances and and consequently leading to increased carbohydrate accumulation (Julius et al. 2018;Maeda et al. 2006;Zhang et al. 2013). Under the blockage effect, more carbohydrates, including glucose, fructose, sucrose, and soluble sugars, accumulated in the stem of S. trilobata (Fig. 5A-D), and positively regulated the expression of anthocyanin synthesis genes (Fig. 6), thereby promoting the production of anthocyanins (Fig. 7B). ...
Sphagneticola trilobata, originally from the tropical regions of the Americas, has successfully invaded the subtropical regions of southern China and displays a tendency to spread towards colder northern regions. The accumulation of anthocyanins in stems under low temperature conditions exhibits strong cold tolerance, and therefore may be one mechanism supporting the northward spread of the species. However, the role and synthesis mechanism of anthocyanins in the stems of S. trilobata when confronted with low temperature stress are still unclear. Field experiments have shown that compared to in summer, the stems of S. trilobata significantly accumulated anthocyanins to cope with winter. Further short-term low-temperature treatments (0 °C) were conducted on red and green stems of S. trilobata, and the results showed that the red stems exhibited lower levels of reactive oxygen species, membrane damage, and chlorophyll fluorescence changes compared to the green stems. In an indoor low-temperature control experiment, it was observed that S. trilobata exhibited significant accumulation of callus in the periderm of its stems compared to S. calendulacea, which subsequently resulted in increased levels of sucrose, glucose, and fructose contents. Furthermore, there was a significantly induced higher levels of abscisic acid and cytokinin in S. trilobata stems under low temperatures. Under the joint regulation of these carbohydrates and hormones, the key structural genes associated with anthocyanins synthesis pathway in S. trilobata stems were more strongly induced compared to S. calendulacea. The upregulation of CHS, CHI, F3H, and DFR gene expression levels in S. trilobata was higher than that of native species, which directly leads to the accumulation of more anthocyanins in the epidermis of the stem of S. trilobata, thereby reducing the degree of oxidative stress and maintaining normal growth under low temperature. In summary, anthocyanins play an important light filtering role in the response of S. trilobatas stem to low temperature stress, which is one of the important mechanisms for its successful invasion into southern China. In the context of global climate change, we need to increase our vigilance against further invasion of S. trilobata into colder inland regions such as temperate regions. This research holds significant theoretical and practical implications for the prevention and control of S. trilobata invasion.
... Crop yield depends on the potential of the sink (to store assimilates) and the capacity of the source (to output photosynthetic products) [1][2][3]. Mature leaves with net photosynthetic output capacity and fast-growing fruits that store assimilates are the most typical "source" and "sink" organs in fruiting crops. An inflated source/sink ratio may result in a waste of carbon assimilation, and a slow output of photosynthetic products from leaves will feed back to inhibit source assimilation activity [4,5]. ...
The optimization of the sink-source relationship is of great importance for crop yield regulation. Cucumber is a typical raffinose family oligosaccharide (RFO)-transporting crop. DNA methylation is a common epigenetic modification in plants, but its role in sink-source regulation has not been demonstrated in RFO-translocating species. Here, whole-genome bisulfite sequencing (WGBS-seq) was conducted to compare the nonfruiting-node leaves (NFNLs) and leaves of fruit setting (FNLs) at the 12th node by removing all female flowers in other nodes of the two treatments. We found considerable differentially methylated genes enriched in photosynthesis and carbohydrate metabolic processes. Comparative transcriptome analysis between FNLs and NFNLs indicated that many differentially expressed genes (DEGs) with differentially methylated regions were involved in auxin, ethylene and brassinolide metabolism; sucrose metabolism; and RFO synthesis pathways related to sink-source regulation. Moreover, DNA methylation levels of six sink-source-related genes in the pathways mentioned above decreased in leaves after 5-aza-dC-2’-deoxycytidine (5-Aza-dC, a DNA methyltransferase inhibitor) treatment on FNLs, and stachyose synthase (CsSTS) gene expression, enzyme activity and stachyose content in RFO synthesis pathway were upregulated, thereby increasing fruit length and dry weight. Taken together, our findings proposed an up-to-date inference for the potential role of DNA methylation in the sink-source relationship, which will provide important references for further exploring the molecular mechanism of DNA methylation in improving the yield of RFO transport plants.
... Considering that the transcriptional regulation of genes encoding sucrose transporters could not completely explain the differential characteristics observed in plants with altered levels of AtHB5 expression, we wondered whether the differences observed in sugar accumulation could be the consequence of callose deposition in the phloem. Such deposits usually obstruct the proper transport from source to sink tissues (van Bel, 2003;Julius et al., 2018). Leaves of mutant, overexpressor, and WT plants were stained with aniline blue to detect callose. ...
... This accumulation was due to inhibited sucrose export that could be explained by ectopic callose deposits in the phloem of mutant leaves. As a consequence, cpd1 mutant plants showed reduced height and yield (Julius et al., 2018), similar to the phenotype observed in AT5 plants (Fig. 5). Sugars are transported between intracellular compartments and different cells by SUC/SUT and SWEET transporters. ...
Carbohydrates are transported from source to sink tissues. The efficiency of such transport determines plant growth and development. The process is finely regulated, and transcription factors are crucial in such modulation. AtHB5 is a homeodomain-leucine zipper I transcription factor, repressed during stem maturation. However, its function in this developmental event was unknown. Here, we investigated the expression pattern and role of AtHB5. AtHB5 localized in conductive tissues: roots, hypocotyls, stems, petioles, pedicels, and central leaf veins. Mutant plants exhibited wider and more lignified stems than controls, whereas overexpressors showed the opposite phenotype. Cross-sections of athb5 mutant stems showed enlarged vascular bundle, xylem, phloem, and petiole areas, whereas AtHB5 overexpressors exhibited callose deposits. Several genes involved in starch biosynthesis and degradation had altered transcript levels in athb5 mutants and AtHB5 overexpressors. Rosette and stem biomasses were enhanced in athb5 mutants, positively impacting seed yield, protein, and lipid content. Moreover, these effects were more evident in debranched plants. Finally, the transport to roots significantly slowed down in AtHB5 overexpressors.
Altogether, the results indicated that AtHB5 is a negative modulator of carbon partitioning and sucrose transport from source to sink tissues, and its overexpression diminished plant biomass and seed yield.
... CARBOHYDRATE PARTITIONING DEFECTIVE1 (CPD1) and Brittle stalk2-like3 (Bk2L3) affect the structure of the vasculature system. Cpd1 is a semi-dominant mutant caused by an unknown mutation that increases callose deposition in the phloem tissue (Julius et al., 2018). Bk2L3 encodes a COBRA-like protein, which is involved in cell wall development. ...
... We confirmed this hypothesis by transcriptome analysis in this study, which demonstrated that genes related to sucrose hydrolysis are not significantly upregulated, suggesting that the daytime increase in glucose and fructose levels does not come from hydrolysis of the photosynthetic product sucrose, but instead from the metabolism of preexisting carbon. By contrast, in mutants defective in sucrose transport such as cpd1, the contents of sucrose, glucose and fructose are significantly higher than those in the wild type, both during the day and at night (Julius et al., 2018). We speculate that this is due to the excessive accumulation of sucrose, which leads to an increased conversion into hexose. ...
Carbohydrate partitioning is essential for plant growth and development, and its hindrance will result in excess accumulation of carbohydrates in source tissues. Most of the related mutants in maize (Zea mays L.) display impaired whole‐plant sucrose transport, but other mechanisms affecting carbohydrate partitioning have seldom been reported. Here, we characterized chlorotic leaf3 (chl3), a recessive mutation causing leaf chlorosis with starch accumulation excessively in bundle sheath chloroplasts, suggesting that chl3 is defective in carbohydrate partitioning. Positional cloning revealed that the chl3 phenotype results from a frameshift mutation in ZmPHOH, which encodes starch phosphorylase 2. Two mutants in ZmPHOH exhibited the same phenotype as chl3, and both alleles failed to complement the chl3 mutant phenotype in an allelism test. Inactivation of ZmPHOH in chl3 leaves reduced the efficiency of transitory starch conversion, resulting in increased leaf starch contents and altered carbohydrate metabolism patterns. RNA‐seq revealed the transcriptional downregulation of genes related to photosynthesis and carbohydrate metabolism in chl3 leaves compared to the wild type. Our results demonstrate that transitory starch remobilization is very important for cellular carbohydrate partitioning in maize, in which ZmPHOH plays an indispensable role.
... Photosynthates are mostly transported from the carbon source to sink in the form of sucrose (Vu 2005, Gupta et al. 2017, Ohara and Satake 2017, Mizuno et al. 2018. Plant efficiency relies upon the efficient transfer of photosynthates from carbon source to sink (Julius et al. 2018). Besides, waterlogging causes an imbalance in the plant source-sink interaction. ...
Photosynthesis is a process highly sensitive to various abiotic and biotic stresses in plants. Among them, the major abiotic stress, waterlogging, affects the crop's growth and productivity. Under waterlogging, the photosynthetic apparatus of plants was destroyed. Waterlogging reduced chlorophyll content and the net photosynthetic rate. Therefore, this updated review summarized the effect of waterlogging on chloroplast ultrastructure, photosynthetic characteristics, and chlorophyll fluorescence attributes of plant species. By studying various research papers, we found that intercellular concentration of available carbon dioxide in mesophyll cells, assimilation of carbon, and the net photosynthetic ratio declined under waterlogging. The chlorophyll fluorescence efficiency of plants decreased under waterlogging. Thus, the study of photosynthesis in plants under waterlogging should be done with respect to changing climate. Moreover, the recognition of photosynthetic characteristics present in tolerant species will be beneficial for designing the waterlogging-tolerant crop plant in changing environments.
... The number and frequency of PD can affect the rate and mode of phloem loading (van Bel, 1993). In sucrosetranslocating plants, mutants defective in carbohydrate partitioning had abnormal vein structure and inhibited sucrose export (Julius et al., 2018;Tran et al., 2019). For example, the maize sucrose export defective1 mutant showed aberrant PD structure, which led to symplasmic interruption and lack of phloem loading capability (Russin et al., 1996). ...
Sugars are necessary for plant growth and fruit development. Cucumber (Cucumis sativus L.) transports sugars, mainly raffinose family oligosaccharides (RFOs), in the vascular bundle. As the dominant sugars in cucumber fruit, glucose and fructose are derived from sucrose, which is the product of RFO hydrolysis by α-galactosidases. Here, we characterized the cucumber alkaline α-galactosidase 2 (CsAGA2) gene and found that CsAGA2 has undergone human selection during cucumber domestication. Further experiments showed that the expression of CsAGA2 increases gradually during fruit development, especially in fruit vasculature. In CsAGA2-RNA interference (RNAi) lines, fruit growth was delayed because of lower hexose production in the peduncle and fruit main vascular bundle (MVB). In contrast, CsAGA2-overexpressing (OE) plants displayed bigger fruits. Functional enrichment analysis of transcriptional data indicated that genes related to sugar metabolism, cell wall metabolism, and hormone signaling were significantly downregulated in the peduncle and fruit MVBs of CsAGA2-RNAi plants. Moreover, downregulation of CsAGA2 also caused negative feedback regulation on source leaves, which was shown by reduced photosynthetic efficiency, fewer plasmodesmata at the surface between mesophyll cell and intermediary cell or between intermediary cell and sieve element, and downregulated gene expression and enzyme activities related to phloem loading, as well as decreased sugar production and exportation from leaves and petioles. The opposite trend was observed in CsAGA2-OE lines. Overall, we conclude that CsAGA2 is essential for cucumber fruit set and development through mediation of sugar communication between sink strength and source activity.
... However, in several cases, the differences could be due to indirect effects (e.g., reduction in PD number) rather than PD conductance. For example, maize mutants in Carbohydrate partitioning defective33 showed decreased sucrose export yet had lower PD numbers at the SE-CC interface (Julius et al., 2018;Tran et al., 2019). The development of direct assays to monitor transport would help boosting our understanding of the mechanisms and regulation of PD conductance. ...
During multicellularization, plants evolved unique cell-cell connections, the plasmodesmata (PD). PD of angiosperms are complex cellular domains, embedded in the cell wall and consisting of multiple membranes and a large number of proteins. From the beginning, it had been assumed that PD provide passage for a wide range of molecules, from ions to metabolites and hormones, to RNAs and even proteins. In the context of assimilate allocation, it has been hypothesized that sucrose produced in mesophyll cells is transported via PD from cell to cell down a concentration gradient towards the phloem. Entry into the sieve element companion cell complex (SECCC) is then mediated on three potential routes, depending on the species and conditions, – either via diffusion across PD, after conversion to raffinose via PD using a polymer trap mechanism, or via a set of transporters which secrete sucrose from one cell and secondary active uptake into the SECCC. Multiple loading mechanisms can likely coexist. We here review the current knowledge regarding photoassimilate transport across PD between cells as a prerequisite for translocation from leaves to recipient organs, in particular roots and developing seeds. We summarize the state-of-the-art in protein composition, structure, transport mechanism and regulation of PD to apprehend their functions in carbohydrate allocation. Since many aspects of PD biology remain elusive, we highlight areas that require new approaches and technologies to advance our understanding of these enigmatic and important cell-cell connections.
... Processes involved in the translocation have been well-discussed, but little is known about the genes involved in sugar translocation and accumulation. The carbohydrate partitioning defective1 (Cpd1) gene plays a role in the early production of phloem and controls the distribution of carbohydrates throughout the plant (Julius et al., 2018). In the case of low expression of cpd1, plants are unable to transport sugars from the source to sink tissues, which results in the deposition of sugars in leaves and reduction in sink tissues (stems and roots) causing stunted growth, decreased sturdiness of the stem, and delayed silking and anthesis, which affect kernel size and weight (Julius et al., 2018). ...
... The carbohydrate partitioning defective1 (Cpd1) gene plays a role in the early production of phloem and controls the distribution of carbohydrates throughout the plant (Julius et al., 2018). In the case of low expression of cpd1, plants are unable to transport sugars from the source to sink tissues, which results in the deposition of sugars in leaves and reduction in sink tissues (stems and roots) causing stunted growth, decreased sturdiness of the stem, and delayed silking and anthesis, which affect kernel size and weight (Julius et al., 2018). Agpsemzm and Agpllzm enzymes play roles in the regulation of starch levels in different tissues (Julius et al., 2018). ...
... In the case of low expression of cpd1, plants are unable to transport sugars from the source to sink tissues, which results in the deposition of sugars in leaves and reduction in sink tissues (stems and roots) causing stunted growth, decreased sturdiness of the stem, and delayed silking and anthesis, which affect kernel size and weight (Julius et al., 2018). Agpsemzm and Agpllzm enzymes play roles in the regulation of starch levels in different tissues (Julius et al., 2018). ...
High plant density is considered a proficient approach to increase maize production in countries with limited agricultural land; however, this creates a high risk of stem lodging and kernel abortion by reducing the ratio of biomass to the development of the stem and ear. Stem lodging and kernel abortion are major constraints in maize yield production for high plant density cropping; therefore, it is very important to overcome stem lodging and kernel abortion in maize. In this review, we discuss various morphophysiological and genetic characteristics of maize that may reduce the risk of stem lodging and kernel abortion, with a focus on carbohydrate metabolism and partitioning in maize. These characteristics illustrate a strong relationship between stem lodging resistance and kernel abortion. Previous studies have focused on targeting lignin and cellulose accumulation to improve lodging resistance. Nonetheless, a critical analysis of the literature showed that considering sugar metabolism and examining its effects on lodging resistance and kernel abortion in maize may provide considerable results to improve maize productivity. A constructive summary of management approaches that could be used to efficiently control the effects of stem lodging and kernel abortion is also included. The preferred management choice is based on the genotype of maize; nevertheless, various genetic and physiological approaches can control stem lodging and kernel abortion. However, plant growth regulators and nutrient application can also help reduce the risk for stem lodging and kernel abortion in maize.
... In a non-fruit context, ectopic callose deposition in the phloem in the Carbohydrate partitioning defective1 (Cpd1) mutant in maize lead to defective sucrose export and the accumulation of starch and soluble sugars in leaves [77]. Infection with Candidatus Liberibacter in citrus tree induces callose accumulation at PD connecting CC and SE, leading to reduced symplasmic transport and delayed sucrose export in comparison to uninfected leaves [78]. ...
Fruit consumption is fundamental to a balanced diet. The contemporary challenge of maintaining a steady food supply to meet the demands of a growing population is driving the development of strategies to improve the production and nutritional quality of fruit. Plasmodesmata, the structures that mediate symplasmic transport between plant cells, play an important role in phloem unloading and distribution of sugars and signalling molecules into developing organs. Targeted modifications to the structures and functioning of plasmodesmata have the potential to improve fruit development; however, knowledge on the mechanisms underpinning plasmodesmata regulation in this context is scarce. In this review, we have compiled current knowledge on plasmodesmata and their structural characterisation during the development of fruit organs. We discuss key questions on phloem unloading, including the pathway shift from symplasmic to apoplastic that takes place during the onset of ripening as potential targets for improving fruit quality.
... Studies have show that cadmium has no significant effect on new shoot growth and leaf fresh weight (Hatamian et al., 2020). Moreover, our metabolic results showed that carbohydrate increased in the leaf, which results in the enhancement of leaf growth (Julius et al., 2018). ...
... Transporting carbohydrates to nonphotosynthetic tissues is to maintain its growth and development, and inhibition of carbohydrate transport results in the hyperaccumulation of carbohydrates (e.g. sucrose, glucose, fructose, and starch) in the leaf and the inhibition of root growth (Julius et al., 2018). In this study, carbohydrates were down-regulated in the root and stem and up-regulated in the leaf of pumpkin seedlings under cadmium stress. ...
Changes in the types and contents of metabolites in plants can occur in response to environmental stress. In this study, pumpkin seeds were cultivated in a cadmium ion solution (cadmium sulfate) for 7 days, and growth parameters, antioxidant enzyme activities, and metabolites in the root, stem, and leaf were analyzed. The results showed that cadmium accumulation characteristics were in the order of root > stem > leaf. Cadmium restrained root growth and promoted superoxide dismutase, peroxidase, catalase activities in the root, but inhibited their activities in the leaf. Cadmium did not change the total biomass of pumpkin seedlings. Orthogonal partial least squares (OPLS) analyses were conducted to detect the relationships between fresh weight and metabolites. These analyses revealed that maltose had significantly positive relationships with the fresh weight of the root, stem, and leaf. Cadmium influenced glyoxylate and dicarboxylate metabolism, aminoacyl-tRNA biosynthesis, sulfur metabolism, butanoate metabolism, alanine, aspartate and glutamate metabolism, glutathione metabolism, glycine, serine and threonine metabolism in the root; glycolysis/gluconeogenesis in the stem; and biosynthesis of unsaturated fatty acids, galactose metabolism, cutin, suberine and wax biosynthesis in the leaf. It is important that cadmium inhibited root growth by inhibiting carbohydrate transport from the leaf to the root and promoted leaf growth by the accumulation of carbohydrates in the leaf. Furthermore, cadmium also restrained amino acid metabolism in the root of pumpkin seedlings. These results provide new information about how pumpkin seedlings respond to cadmium stress.
