Why do plants make sucrose

Invertase sucrase is the only enzyme that will touch it and this is unlikely to be present in the phloem sieve tubes.

Glucose could be broken down by the enzymes that are present in all cells as part of glycolysis etc. Maltose is also a reducing sugar. There is a paper you could read:- Arnold,W.

The selection of sucrose as a translocate of higher plants. J Theor Biol This has to do with the fact that animal cells do not have the same transport mechanisms and enzyme distribution as plants do:. In plants sucrose is converted back to glucose and fructose by an enzyme called sucrase. Animals produce less sucrase , the presence of these enzymes is limited to certain tissues. Animals have specific mechanisms to transport glucose to the target tissues.

However, they have no sucrase transporters. Some cells convert sucrose into glucose and fructose. Glucose can go into glycolysis in almost all tissues.

Fructolysis is limited to the liver, so most cels don't have the enzymes to deal with fructose. Why is it that during transportation of carbohydrates in plants it is in the form of Sucrose but in animals it is in the form of Glucose?

Biology Molecular Biology Basics Carbohydrates. Sep 11, Sink strength can be defined as the ability of an organ to import photoassimilates. Sink strength mainly relies on two parameters, sink organ size as a physical constraint and activity as a physiological constraint Ho, Although imported photoassimilate can be used for respiration, sink-strength estimations are mainly based on net weight gain Ho, Phloem loading is thought to be highly important for defining sink strength and the breakdown of Suc in sink organs may also contribute to sink strength.

Work with isoform-specific antibodies has revealed that specific SuSy isoforms are more abundant in the phloem. SuSy proteins have also been detected in citrus Citrus paradisi and maize phloem companion cells using immunohistological analysis Nolte and Koch, However, SuSy appear to localize to the phloem not only in the Suc unloading zones, but also in loading zones in mature leaves of citrus, maize, rice, Arabidopsis, and poplar Nolte and Koch, ; Regmi et al.

In the phloem, SuSy may play a role in the maintenance of equilibrium between Suc and its breakdown products, supplying hexoses for energy production in companion cells and substrates for complex carbohydrates, like callose Nolte and Koch, There is plenty of evidence that SuSy, and not INV, is the primary active enzyme in actively growing sink organs of different species, such as potato tubers, cassava Manihot esculenta roots, lima bean Phaseolus lunatus seeds, and tomato fruits Morrell and Rees, ; Sung et al.

Numerous studies involving plants that have altered SuSy activity and exhibit altered growth rates and weight gain in their sink organs support the putative role of SuSy in sink strength. Pea Pisum sativum SuSy mutants rug4 also exhibited significantly reduced SuSy activity in their embryos, reduced seed weight and a wrinkled-seed phenotype Craig et al.

Transgenic potato plants with reduced SuSy activity only in tubers exhibited reduced tuber dry weight Zrenner et al. SuSy activity was also suggested as an indicator for high rice grain yield in rice breeding programs Counce and Gravois, A recent study found that the Suc-cleavage activity of a castor bean SuSy, RcSUS1, is inhibited by trehalose 6-phosphate, suggesting another mechanism by which Suc flux can be controlled in heterotrophic tissues Fedosejevs et al.

Other studies have produced somewhat contradictory evidence for the role of SuSy in sink strength. In another transgenic tomato with reduced SUS expression only in fruits, there were no reported effects on fruit development or the accumulation of starch and sugar in young green fruit, challenging the suggested importance of SuSy for sink strength Chengappa et al.

In Arabidopsis, AtSUS2 and AtSUS3 mutants had altered metabolism and accumulated less transient starch during seed development, with no effect on agronomic traits like seed weight and oil content Angeles-Nunez and Tiessen, Interestingly, even a double mutant sus2 and sus3 and a quadruple mutant sus1 , sus2 , sus3 , and sus4 did not show any seed-related phenotypes Bieniawska et al.

Overall, the data suggest that SuSy might be important for sink strength, especially in starch-accumulating organs, although that role is probably not conserved in all plants. For many decades, researchers have thought that SuSy may play important roles in the conversion of Suc into starch. The first genetic evidence for this came from the characterization of the maize sh mutant.

Similarly, a pea SuSy mutant rug4 also showed reduced seed starch content Craig et al. Other studies involving transgenic plants with suppressed SUS expression have demonstrated reduced starch accumulation in potato tubers Zrenner et al. Suc can be converted into starch via different pathways, which also differ between chloroplasts and heterotrophic tissues For a comprehensive review of starch synthesis, see Bahaji et al. Adenosine diphosphate glucose is the main molecule converted into starch by the starch synthases in plastids.

In chloroplasts, the main starch-synthesis pathway starts with two molecules of triose-P produced by photosynthesis, which yield F1,6BP. This chloroplast starch synthesis pathway does not require Suc cleavage and, therefore, cytosolic SuSy is not expected to play an important role in leaf starch synthesis.

The main genetic evidence supporting this claim is that the plastidic PGI, PGM and AGPase mutants, and plants with reduced expression of these genes are either starchless or contain very low levels of starch in their leaves Bahaji et al. However, there is also growing line of evidence suggesting that SuSy might play some role in leaf starch synthesis.

Based on this and many other studies, Bahaji et al. According to this model, the rate of starch accumulation is determined by the rate of cytosolic SuSy activity that yields ADP-G, cytosolic ADP-G transport to the chloroplast, starch synthesis, starch breakdown and the efficiency of the recycling of the products of the breakdown of starch.

There is much more evidence linking SuSy to starch synthesis in non-photosynthetic tissues or sink tissues. For example, a reduction in SuSy activity reduced starch content in potato tubers, carrot taproots and maize endosperm Chourey and Nelson, ; Zrenner et al. In addition, five maize starch-deficient endosperm mutants were screened for metabolic enzyme activity and all showed reduced SuSy activity Doehlert and Kuo, All these observations support the role of SuSy in starch accumulation.

There is a lot of evidence that SUS are highly expressed in vascular tissues. SUS promoters are expressed in the vasculature of many plant species, mostly in the phloem Yang and Russell, ; Martin et al. SuSy protein has also been immunolocalized to the phloem companion cells in citrus and maize Nolte and Koch, and in leaves of 9-day-old barley seedlings Guerin and Carbonero, SuSy is also the main enzyme metabolizing Suc in the phloem of Ricinus communis Geigenberger et al.

Only a few studies have used mutant and transgenic plants to elucidate the roles of SuSy in phloem. In cucumber Cucumis sativa , transgenic plants with suppressed CsSUS3 , which is mainly expressed in root phloem companion cells, were found to be more sensitive to hypoxic stress caused by flooding Wang et al.

In contrast, the Arabidopsis double mutant of the two phloem-specific SUS sus5 sus6 exhibited no specific phenotype, even under hypoxic stress Bieniawska et al.

However, the double mutant had less callose in its sieve plates and in response to wounding, as compared with WT or quadruple-mutant sus1 , sus2 , sus3 , and sus4 plants, suggesting that AtSUS5 and AtSUS6 are essential for callose synthesis Barratt et al.

There is also sufficient evidence for the localization of SuSy to xylem tissues. Sucrose synthase activity was found in immature metaxylem and the central vessel in the elongation zone of wheat seedlings following hypoxia Albrecht and Mustroph, Relatively high mRNA levels and activity were also reported in carrot tap root xylem Sturm et al. High SuSy activity and protein levels were reported in differentiating xylem of Robinia pseudoacacia during the spring Hauch and Magel, Sucrose synthase activity in developing xylem vessels or fibers may be particularly important for the cellulose synthesis needed for the construction of thick secondary cell walls.

Work with a cell culture of Zinnia elegans revealed that SuSy is highly enriched in differentiating tracheary elements near the plasma membrane, where secondary cell-wall thickening occurs Salnikov et al. Overexpression of SUS in several plant species increased the thickness of xylem cell walls Coleman et al. There is sufficient supporting evidence for these proposed roles from the SuSy subcellular localization to cell walls and adjacent to plasma membranes. Immunolocalization of the cotton fiber SuSy revealed an arrangement pattern similar to that of cellulose microfibril deposition Amor et al.

It was later found that a cellulose synthase rosette-like structure, isolated from azuki beans, lacks cellulose-synthesis activity in the absence of another particle referred to as the catalytic unit. Another immunolocalization study also demonstrated that cotton fiber SuSy is co-localized with callose, suggesting a dual role for SuSy in cellulose and callose synthesis Salnikov et al.

Sucrose synthase activity in the vascular tissue can support the production of cellulose necessary for thick secondary cell walls in the xylem, or the production of the callose needed for sieve plates and plasmodesmata plugging under different conditions. Evidence for a role of SuSy in callose deposition was found in an Arabidopsis double mutant of phloem-specific SUS sus5 sus6. That double mutant had less callose in its phloem plasmodesmata and in response to leaf wounding, as compared with WT or quadruple mutant sus1 , sus2 , sus3 , and sus4 plants Barratt et al.

The role of SuSy in the synthesis of cellulose and callose has been thoroughly investigated in cotton, with cotton fibers serving as a model for these processes. The development of cotton fibers starts with the initiation and elongation of the epidermal cells, followed by secondary growth and maturation marked by massive cellulose production. A fiberless cotton mutant lacking SuSy protein and activity at anthesis was identified, indicating that SuSy might be crucial for fiber initiation Ruan and Chourey, It was later shown that transgenic cotton plants with SUS suppression exhibit reduced fiber initiation and elongation Ruan et al.

In the secondary growth phase of cotton fibers, cellulose synthesis can increase fold relative to the elongation phase Delmer, and this process probably involves SusC and SusA Brill et al. It would be very interesting to see whether the proposed roles of the cell wall SuSy in cellulose and callose synthesis could be observed in transgenic cotton plants with SusC suppression or overexpression. Different reports also support the roles of SuSy in cellulose synthesis in other plant species.

Although overexpression of cotton SUS in tobacco plants did not affect cellulose content Coleman et al. Similarly, overexpression of poplar xylem SUS in tobacco plants also resulted in increased cellulose content and xylem cell-wall thickness Wei et al.

Oxygen deficiency hypoxia and a complete absence of oxygen anoxia are forms of serious abiotic stress that often cause reduced plant growth and productivity. Low-oxygen stress in plants is often caused by flooding, but may also occur naturally in dense, bulky and inner organs and tissues or in very rapidly growing tissues in which oxygen consumption is high. Oxygen is the final acceptor in the mitochondrial electron transport chain and the absence of oxygen blocks electron transfer and cellular ATP production.

One of the enzymes thought to be involved in plant responses to hypoxia is SuSy. Transcript levels of some SUS genes have been found to increase in response to low levels of oxygen in potato Biemelt et al. Oxygen deficiency has also been shown to increase SuSy protein levels in Arabidopsis roots Bieniawska et al. Increased SuSy activity under low-oxygen conditions has been noted in many plants and is often seen in combination with reduced INV activity in rice seedlings Guglielminetti et al.

The possible role of SuSy in metabolism under reduced-oxygen conditions is further supported by the findings of studies with SUS mutants and transgenic plants. Potato tubers of a SUS antisense transgenic line were more sensitive to hypoxia than control plants Biemelt et al. A study of potato tubers of transgenic plants overexpressing INV or Suc pyrophosphorylase, which allows a way to bypass the degradation of Suc by SuSy, revealed a steeper reduction in oxygen levels inside the tubers, reduced starch synthesis and a lower ATP to ADP ratio, underscoring the importance of SuSy under low-oxygen conditions Bologa et al.

In Arabidopsis, a double-knockout mutant sus1 and sus4 was found to be more sensitive to flooding than the control Bieniawska et al. In cucumber, antisense suppression of CsSUS3 led to increased sensitivity to hypoxic stress Wang et al. SuSy activity has also been found to be correlated with rice coleoptile length under submerged conditions, further indicating the advantage of Suc metabolism that involves SuSy under anoxic conditions Fukuda et al.

Overall, the data show that some SuSy isoforms may indeed play a vital role in metabolism under low-oxygen conditions. Sucrose synthase may play another, less studied role in the development of shoot apical meristem SAM. RNAseq data obtained by Park et al.

The SlSUS4 transcript was shown to be present asymmetrically and localized to the primordia from very early stages of development using in situ hybridization with an SlSUS4 antisense probe Pien et al. In situ hybridization also revealed the presence of SUS transcript in young maize leaf primordia, suggesting a role for SuSy in early leaf development Hanggi and Fleming, Other work involving transgenic plants that overexpress SUS genes has revealed altered growth rates that may suggest some possible effects of these genes on SAM function.

Overexpression of potato SUS in cotton plants led to increased vegetative growth Xu et al. Similarly, overexpression of aspen Popolus tremuloides SUS in Arabidopsis resulted in an increased growth rate and increased plant biomass, and also induced early flowering Xu and Joshi, Overexpression of cotton SUS in tobacco also led to an increased growth rate and taller plants Coleman et al.

Although the mechanism by which SUS overexpression speeds up the growth rate is not clear, it is tempting to speculate that increased SuSy activity in the meristem may facilitate increased cell proliferation. The transgenic plants overexpressing AtSUS1 showed increased chlorophyll levels, as well as increased photosynthesis, TSS total soluble sugars , starch, Suc and Fru, as well as increased enzymatic activity of SPS and SPP in leaves, indicating increased sugar production in the transgenic plants.

In addition to serving as energy resources and structural components, sugars such as Suc, Glc, and Fru may also act as signaling molecules to regulate developmental processes and responses to environmental changes Sheen et al. These sugars have also been shown to rapidly affect the expression of many genes, even at concentrations as low as 1 mM Kunz et al.

The role of sugars as signaling molecules in the SAM is a subject of lively debate and it is not always easy to differentiate between their signaling function and their metabolic role.

In work with Arabidopsis seedlings conducted by Pfeiffer et al. Those authors also found that a non-metabolizable Suc analog, palatinose, has no effect on WUS expression in the dark, possibly indicating that Suc per se does not act as a signaling molecule in the SAM during seedling establishment.

Other studies have found correlations between Suc treatments or levels and flowering, suggesting that Suc may play a signaling role in the development of SAM into flowers reviewed by Cho et al. The Suc signal for flowering may be mediated by trehalose 6-phosphate T6P.

Another reason to believe that Suc and SuSy may play some regulatory function rather than just a metabolic one comes from tomato plants in which the expression of three SUS genes was suppressed Goren et al. These plants exhibited abnormal leaf development and irregular auxin patterning, suggesting that altered sugar signaling in the SAM or primordia, rather than lower sugar metabolism, is likely to be the cause of these developmental changes.

Sucrose synthase may also play other important roles, in addition to its role in Suc cleavage. The localization of SuSy to mitochondria and its possible interaction with a high voltage-dependent anion channel suggest that these SuSy may play a role in regulating solute fluxes between the mitochondria and the cytosol Subbaiah et al.

Plant SuSy have also been found to play a role in mutualism with symbiotic organisms like Rhizobium bacteria. Those plants were incapable of effective nitrogen fixation, even though the nodules appeared normal Gordon et al. Although nitrogenase protein levels were normal, there was no nitrogenase activity.

It was suggested that Susy activity might be essential for nitrogen fixation in root nodules, due to the low-oxygen environment in the nodules Gordon et al. Sucrose synthase may also play a role in metabolism under heat stress. A recent study found that a SUS3 allele that is highly expressed during seed ripening may confer resistance to the chalky grain phenotype of brown rice caused by heat stress Takehara et al.

The expression of the SUS3 gene was found to be higher in the resistant line under heat stress. Interestingly, transgenic plants of a commercial rice cultivar expressing SUS3 showed a decreased percentage of chalky grains under heat stress only when both the promoter and the cDNA of the heat-tolerant allele were introduced, indicating not only the importance of the SUS3 protein, but also the response rate to heat stress in terms of gene expression Takehara et al.

Another potential heat-tolerant SuSy was purified from a heat-tolerant line of wheat WH In strawberry Fragaria ananassa , SuSy may play an important role in fruit ripening. Strawberry fruits with RNAi suppression of FaSUS1 by virus-induced gene-silencing exhibited delayed fruit ripening, maintained their firmness and exhibited delayed anthocyanin accumulation Zhao et al.

To summarize, plant SuSy activity has been shown to play important roles in plant sugar metabolism, primarily in sink tissues. Plant SuSy proteins are found primarily in the cytosol or adjacent to the plasma membrane, although some SuSy isoforms are found in cell walls, mitochondria or vacuoles, or are bound to actin. Plant SuSy enzymes have been shown to be involved in several different metabolic pathways, such as those for starch, callose and cellulose synthesis, and to play developmental and possibly signaling roles in sink carbohydrate flux, vascular tissues and meristem functioning.

One possibility is suggested by their differential subcellular locations. The SuSy in plasma membranes and cell walls and their production of UDP-G may be important for directing carbon toward cellulose or callose synthesis; whereas INV may direct carbon to other metabolic pathways. SuSy activity is feedback-inhibited by its product, Fru, and its activity is also reversible.

These features of SuSy may help to control the amount of Suc consumed in different organs, for example, in stems and petioles. Only controlled amounts of the transported Suc must be cleaved and metabolized to support the maintenance and development of vascular and other supporting tissues.

It is likely that feedback inhibition of SuSy activity and substrate inhibition of fructokinase by Fru Schaffer and Petreikov, ; Kanayama et al. That is, in case of excess cleavage of Suc by SuSy, the increased fructose Fru inhibits fructokinase activity so that fructose accumulates and that accumulated Fru inhibits further cleavage of Suc by SuSy.

Although plant SuSy proteins have been the subject of intensive study, we are still faced with major gaps in our understanding of the functions of these enzymes.

OS and DG jointly wrote the manuscript, and read and approved the final manuscript. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

National Center for Biotechnology Information , U. Journal List Front Plant Sci v. Front Plant Sci. Published online Feb 8. Author information Article notes Copyright and License information Disclaimer. Received Nov 8; Accepted Jan The use, distribution or reproduction in other forums is permitted, provided the original author s and the copyright owner s are credited and that the original publication in this journal is cited, in accordance with accepted academic practice.

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Abstract Sucrose is the end product of photosynthesis and the primary sugar transported in the phloem of most plants. Keywords: sucrose metabolism, sugar signaling, plant development, cellulose synthesis, callose synthesis, starch synthesis, meristem. Introduction Photosynthesis carried out by plants, algae and cyanobacteria is the major source of fixed carbon for all life on earth. Open in a separate window. Structure and Enzymatic Activity of SuSy Sucrose synthase belongs to the glycosyltransferase-4 subfamily of glycosyltransferases, a large family that includes a wide variety of glycosyltransferases, including SPS, trehalose synthase, and trehalose phosphorylase.