Ali, Hazem Abd El-Rahman Obiadalla: Understanding of Carbon Partitioning in Tomato Fruit

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Kapitel 2. Review of literature

2.1 Carbon Metabolism in Photosynthetic Tissue

Life on our planet obtains its substance and energy through the process of photosynthesis- by which photosynthetic organisms use the electromagnetic energy of sunlight to synthesize carbohydrates and other cellular constituents from carbon dioxide and water. Photosynthesis can be broadly divided into two phases: a light phase in which the electromagnetic energy of sunlight is trapped and converted to ATP and NADH, and a synthetic phase in which the ATP and NADH thus generated are used in part for biosynthetic reduction of assimilated carbon dioxide (Calvin-Benson cycle; for review see Leegood, 1996). The overall reactions of the Calvin-Benson cycle can be described as the fixation of three molecules of carbon dioxide into triose phosphate (triose-P) with the incorporation of one Pi derived from hydrolysis of ATP. Light functions to regulate not only the source of reductant but also the synthetic and carbon reductive phases of photosynthesis and related biochemical processes of chloroplasts.

In most plants the major products of photosynthesis are starch (formed in the chloroplasts) and sucrose (formed in the cytosol). Both of these products are synthesized from photosynthetically generated dihydroxyaceton phosphate (DHAP). In the first case, DHAP is converted into hexose phosphates (hexose-P) by the concerted action of aldolase and chloroplastic fructose-1,6-bisphosphatase (FBPase), these hexose-P are in turn converted to starch following the reactions of chloroplastic isoforms of phosphoglucoisomerase (PGI) and phosphoglucomutase (PGM) and those of ADP-glucose pyrophosphorylase (AGPase) and the starch polymerising enzymes starch synthase and starch branching enzyme. In sucrose synthesis DHAP, or a derivative thereof is transported to the cytosol where it is converted firstly to fructose 6-phosphate (Fru-6-P) through operation of cytosolic isoforms of aldolase and FBPase and then to sucrose via the route defined below. When the rate of photosynthesis exceeds the rate of sucrose export from the source tissue, sucrose initially accumulates in the vacuole, where it has little effect on the rate of triose-P export from the chloroplast. At saturation of the vacuolar sucrose capacity sucrose synthesis is inhibited and instead photosynthate is converted to starch, which is transiently stored in the chloroplast. The relative rates of sucrose and starch production in photosynthetically active tissues are maintained by tight and complex regulation patterns known as feedforward and feedback control mechanisms.


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2.1.1 Feedforward control of photosynthesis

At the start of the photoperiod the rate of photosynthesis increases. This results in an increased cytosolic DHAP concentration due to a greater rate of export from the chloroplast via the triose-P translocator in exchange for Pi (Heldt and Flügge, 1987), and therefore the cytosolic 3-PGA/Pi ratio rises (Gerhardt et al., 1987; Neuhause and Stitt, 1989; Stitt et al., 1984b,c) (Fig. 1). These changes bring about an increase in the cytosolic concentration of the substrate of the cytosolic FBPase, Fru-1,6-P2, which is nearly in equilibrium with triose-P. Simultaneous with the increase in the levels of Fru-1,6-P2 is a rapid drop in the concentration of Fru-2,6-P2 (a potent inhibitor of cytosolic FBPase) which relieves inhibition of the enzyme and thus increases flux through the reaction it catalyes. A consequence of the increased flux through FBPase is an increase in the cytosolic concentration of glucose 6-phosphate (Glc-6-P). This leads to an increased Glc-6-P/Pi ratio and causes potent allosteric activation of, one of the routes of sucrose synthesis that catalysed by, sucrose phosphate synthase (SPS) (Huber and Huber, 1992) resulting in an increased rate of sucrose synthesis. Furthermore the elevated Pi in the plasted results in an inhibition of the reaction catalysed by AGPase (Preiss, 1988) and thus restricts the partitioning of photoassimilate towards starch.

2.1.2 Feedback control of photosynthesis

During the day, the rate of sucrose synthesis increases with the rate of photosynthesis. If the rate of sucrose production exceeds its rate of export from the cell, sucrose will accumulate. However, in response to feedback signals, probably related to the absolute level of sucrose, the rate of synthesis is decreased via inhibition of SPS (Stitt, 1990) (Fig. 2). This inhibition leads to increased cytosolic levels of hexose-P, which result in a large increase in the Fru-2,6-P2 level leading to inhibition of cytosolic FBPase. The inhibition of cytosolic FBPase results in increased cytosolic levels of triose-P, which prevent export of chloroplastic triose-P. Consequently, more carbon is retained in the chloroplast and enters the pathway of starch synthesis. Studies on mutants of Clakia xantiana, which have reduced levels of cytosolic PGI, support this theory (Neuhause et al., 1989). These plants have a higher Fru-2,6-P2 concentration than wild type (due to an increase in Fru-6-P concentration) and all of the above effects on metabolite concentrations were observed (Krukeberg et al., 1989; Neuhause et al., 1989). The reduced rate of sucrose synthesis additionally prevents Pi cycling, which has consequently been shown to result in an accumulation of 3-PGA in isolated chloroplasts (Heldt et al., 1977). This is probably due to the fact that phosphoglycerate kinase (PGK) is particularly sensitive to the falling concentrations of ATP that occur during these conditions.


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The elevated chloroplastic 3-PGA/Pi ratio stimulates starch synthesis by the allosteric activation of AGPase (Preiss, 1988).

Figure 1: The role of Fru-2,6-P2 in feedforward control of sucrose synthesis.

+ represents allosteric activation. Reactions shown are catalysed by the following enzymes (note in some instances multiple reactions are represented by a single arrow): 1, Rubisco; 2, chloroplastic PGK and chloroplastic TPI; 3, chloroplastic Fru-1-6-P2 aldolase; 4, chloroplastic FBPase; 5, transketolase, sedoheptolase-1,7-bisphosphatase aldolase, sedoheptolase-1,7-bisphosphatase, phosphopentoepimerase, phosphoriboisomerase and phosphoribulokinase; 6, triose phosphate transporter; 7, cytoslic PGK and cytosolic TPI; 8, cytosolic Fru-1-6-P2 aldolase; 9, cytosolic FBPase; 10, cytosolic PGI ; 11, cytosolic PGM , 12, UGPase, 13, SPS, 14, sucrose phosphatase.


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Figure 2: The role of Fru-2,6-P2 in feedback control of sucrose synthesis.

+ represents allosteric activation, - represents allosteric inhibition. Reactions shown are catalysed by the following enzymes (note in some instances multiple reactions are represented by a single arrow): 1, Rubisco; 2, chloroplastic PGK and chloroplastic TPI; 3, chloroplastic Fru-1-6-P2 aldolase; 4, chloroplastic FBPase; 5, transketolase; sedoheptolase-1,7-bisphosphatase aldolase; sedoheptolase-1,7-bisphosphatase; phosphopentoepimerase, phosphoriboisomerase and phosphoribulokinase; 6, triose phosphate transporter; 7, cytoslic PGK and cytosolic TPI 8, cytosolic Fru-1-6-P2 aldolase; 9, cytosolic FBPase; 10, cytosolic PGI; 11, cytosolic PGM; 12, UGPase; 13, SPS; 14, sucrose phosphatase; 15, choroplastic PGI and chloroplstic PGM; 16, AGPase; 17, starch synthase and branching enzyme.

In summary, the rate of carbon export from the chloroplast and therefore ultimately the rate of sucrose synthesis depends on a balance between feedforward mechanisms that decrease Fru-2,6-P2 (and activate SPS) and feedback mechanisms that increase Fru-2,6-P2 (and inhibit SPS). Similarly the accumulation of starch is a function of the relative activities of the enzymes which synthesize and degrade it. In leaves, starch is accumulated during the day and is nocturnally degraded to provide carbohydrate required for various anabolic reactions (Beck and Zieger, 1989; Trethewey and Smith, 2000). The metabolism of transitory starch is dynamic and regulation of this process results in alternating periods of net synthesis and degradation (Stitt and Heldt, 1981). Degradation of transitory starch is initiated by á-amylases at the surface of starch granule (Beck and Zieger, 1989; Trethewey and Smith, 2000), and involves the co-operative attack of phosphorolytic and hydrolytic activities (Steup


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et al., 1983). The final starch degradation products glucose or triose-P are exported into the cytosol (Trethewey and ap Rees, 1994a,b) where they are metabolised to sucrose.

2.2 Carbohydrate Allocation from photosynthetic “source“ to heterotrophic “sink Tissues“

In photosynthetic tissues sucrose is predominantly exported from cells, most probably by facilitated diffusion and subsequently taken up by the phloem complex by a specific sucrose/H+ co transport mechanism (Riesmeier et al., 1994; Frommer and Sonnewald, 1995). Once in the phloem complex sucrose is transported to cells in heterotrophic “sink“ tissues. At least two distinct classes of sink tissues can be differentiated: (i) “utilisation sinks“, highly metabolically active, rapidly growing tissues like meristems and immature leaves, and (ii) “storage sinks“, such as tubers, seeds, roots or fruits which deposit imported carbohydrates as storage compounds (e.g. starch, sucrose, lipid or protein) (Sonnewald and Willmitzer, 1992). However, the route of carbon transport to these and the mechanisms by which the different types of sink obtain carbon in the form of sucrose is the same so they will be considered together for the purposes of this report. Sucrose obtained through translocation, by sink tissues, can enter a cell directly via the symplasm (see Fig., 3A) or the apoplasm (whereby it is transported by specific sucrose or, following cleavage to its component hexoses, monosaccharide transporters (see Fig., 3B). In many plants the nature of the predominantly used route of sucrose unloading is heatedly debated. Several studies using asymmetrically labelled sucrose suggest that carbon obtained by heterotrophic cells moves primarily through the symplastic route and is not cleaved to glucose and fructose during transport. It seems likely that cells of many species receive most of their sucrose by such as route (Patrick, 1990; Tegeder et al., 1999; Lalonde et al., 1999). However, in certain tissues it is clear that sucrose must be supplied through the apoplasm. This is certainly the case in developing seeds in which protoplasmic concentrations between maternal and embryonic tissue simply do not exist. In potato tuber recent studies using a combination of confocal microscopy, autoradiography and biochemical analyses have provided definitive evidence that unloading in the potato tuber is predominantly apoplastic during stolon elongation and becomes primarily symplastic during initial phases of tuberisation (Viola et al., 2001). This is in direct contrast to the situation observed in the developing tomato fruit in which sucrose unloading is predominantly symplastic during early, starch accumulating, stages of development (Damon et al., 1988; Ruan and Patrick, 1995) and apoplastic during later, hexose accumulating stages (Patrick, 1990; Ruan and Patrick, 1995). The amount of


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sucrose unloaded into tomato fruits differs with the age (Walker and Ho, 1977) and developmental stage of fruit. Being high during early periods of high growth and maintained albeit it at a much reduced level during the later phases of slow growth (Walker and Ho, 1977). It has been suggested that sucrose unloading may be controlled, at least in part, by the activity of the sugar transporters which may in turn be influenced by the activity of the enzymes of sucrose cleavage within the sink tissues (D‘Aoust et al., 1999; N‘tchobo, 1998). Definitive proof in support of this suggestion is however still lacking and it is important to note that although the transport mechanism of the much studied potato sucrose transporter SUT1 has been characterised by expression in Xenopus oocytes (Boorer et al., 1996). Its precise role in planta has yet to be fully elucidated. Since this is one of the best characterized transporters it therefore follows that much work is required before the factors controlling the intracellular movement of sugars can be fully resolved.

2.3 Mobilisation of Sucrose in sink tissues

Sucrose delivered to the sink tissue can be cleaved in one of three ways (i) in the apoplast, as described above, by the action of an acid invertase or in the cytosol by either (ii) alkaline invertase or (iii) sucrose synthase (SuSy). As indicated in Fig., 3A and B the primary route of sucrose cleavage mirrors the mechanism of unloading with invertase activities being high in during the early stages of tuber inititation whilst SuSy predominates in the developing tuber (Appeldoorn et al., 1999), whereas the opposite is true for the developing tomato fruit (Damon et al., 1988; Robinson et al., 1988; DemnitzKing et al., 1997). The products of sucrose cleavage enter into metabolism by the concerted action of fructokinase (FK) and UDP-glucose pyrophosphorylase (UGPase) (Zrenner et al., 1993) or FK and hexokinase (HK) (Smith et al., 1993; Veramendi et al., 1999) in the case of the SuSy and invertase pathways, respectively. Hexose phosphates produced by these pathways are then equilibrated by the action of cytosolic isoforms of PGI and PGM. Hexose phosphates are then partitioned between starch synthesis within the amyloplast and glycolytic pathway of the cytosol (and plastid).


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Figure 3: The predominant route of sucrose unloading and subsequent mobilization.

(A) Symplasmic unloading. (B) Apoplasmic unloading. The numbers denote the following enzymes: 1, Sucrose transporter; 2 and 3, Hexose transporter(s); 4, Invertase; 5, SuSy; 6, UGPase; 7, HK; 8, FK; 9, PGM and 10, PGI. The thickness of the arrow indicates the predominant flux.


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2.4 Uptake of carbon into amyloplasts

The form in which carbon crosses the amyloplast membrane and enters into starch biosynthesis has been the subject of considerable debate. Categorical evidence that carbon enters the amyloplasts of a wide range of species, including the Solanaceous species tobacco and potato, in the form of hexose monophosphates (or nucleosides) rather than triose phosphates was provided by determination of the degree of randomisation of radiolabel in glucose units isolated from starch following incubation of various tissues with glucose labelled at the C1 or C6 position (Keeling et al., 1988; Viola et al., 1991; Hatzfeld and Stitt, 1990; Fernie et al., 2001). These data are in agreement with the observation that many heterotrophic tissues lack plastidial FBPase activity (Entwistle and ap Rees, 1990) and the failure to find expression of plastidial FBPase in potato tubers (Kossmann et al., 1992).

Although it is clear that triose phosphates are not the substrate taken up by amyloplasts to support starch synthesis there has been considerable debate as to whether Glc-1-P (Naeem et al., 1997; Tetlow et al., 1994; Tyson and ap Rees, 1988) or Glc-6-P (Schott et al., 1995; Wischmann et al., 1999) is the preffered substrate for uptake. Recently, particularly in cereals, the uptake of cytosolically produced ADP-glucose has also been much discussed (Pozeuta-Romero et al., 1991a,b; ap Rees, 1995). The results of many recent transgenic and immunolocalisation experiments have indicated that the substrate for uptake is most probably species specific. Clear evidence for the predominant route of carbon uptake in the tuber being in the form of both transgenic experiments (Tauberger et al., 2000) and the recent cloning of a Glc-6-P transporter (Kammerer et al., 1998). Whilst a wealth of experimental evidence indicates that in barley, wheat, oat and possibly maize the predominant form of uptake is as ADP-glucose (Denyer et al, 1996; Thorbjornsen et al., 1996b; Shannon et al., 1998). In tomato the form in which carbon crosses the amyloplast membrane is contentious. Studies comparing the ratio of ADP-glucose to UDP-glucose (Beckles et al., 2001a) and comparing the activity of AGPase that is confined to the plastid with that of other enzymes known to be confined to the plastid (Beckles et al., 2001b) suggest the absence of a cytosolic AGPase in this species. However, these are in contradiction to earlier immunolocalisation studies using antisera raised against AGPase that suggested the presence of an extra-plastidiary isoform of the enzyme in tomato fruit (Chen et al., 1998).


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2.5 The synthesis of starch

Following uptake of carbon into the amyloplast, starch synthesis proceeds variously via (i) plastial PGM and plastidial AGPase, (ii) only via plastidial ADP-glucose or (iii) via no intermediate steps prior to the polymerising reactions of starch synthases and branching enzymes (Smith et al., 1997). The involvement of plastidial enzymes upstream of starch synthase being determined by the route of carbon import (see Fig., 2). The first reaction of heterotrophic plastidial starch metabolism within both the potato tuber (Tauberger et al., 2000), the pea embryo (Hill and Smith, 1991) and most probably the tomato fruit also is the interconversion of Glc-6-P and Glc-1-P catalysed by plastidial PGM. Compelling evidence for the involvement of this enzyme in pea starch synthesis was provided by studies on the rug3 mutant which revealed that this locus encodes a plastidial PGM and that mutation at this locus results in a severe depletion of starch levels in pea embryos (Harrisson et al., 1998). The next reaction on the path to starch synthesis, that catalysed by plastidial AGPase has received much attention for a number of years. This reaction is often considered to be the first committed step of starch synthesis it utilizes ATP and produces pyrophosphate (PPi), which is then hydrolysed by a specific pyrophosphatase to yield 2Pi. The hydrolysis of PPi serves to remove the AGPase reaction away from equilibrium. As discussed above, in many species including pea embryos, soybean cell suspension cultures and cauliflower buds AGPase appears to be located exclusively in the plastid (Macdonald and ap Rees, 1983; Journet and Douce, 1985; Smith, 1988) and this isoform thus plays an importance role in mediating the flux of carbon to starch. In keeping with this statement the removal or severe reduction of the AGPase activity in Arabidopsis or potato resulted in a dramatic reduction in the starch level in all tissues (Lin et al., 1988a,b; Müller-Röber et al., 1992).

Plant AGPases are multisubunit proteins and expression studies in which the potato tuber enzyme was expressed in E.coli revealed that maximal activity can only be achieved on expression of both the large and small subunit (Iglesias et al., 1993). Moreover they are allosterically regulated, being activated by 3-PGA and inhibited by Pi (Preiss, 1988), and there is clear evidence that changes in these metabolites are involved in the regulation of starch synthesis within leaves allowing the co-ordination of carbon assimilation, sucrose synthesis and starch synthesis (Stitt, 1997). The AGPase from potato tuber resembles that found in leaves with respect to its kinetic properties (Sowokinos and Preiss, 1982; Ballicora et al., 1995). There is also increasing evidence of a strong correlation between the 3-PGA and ADP-glucose levels and the rate of starch synthesis within potato tubers under a wide range of conditions (Geigenberger et al., 1997; 1998). Whilst there have been few direct studies of


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the allosteric properties of AGPase from tomato it is likely that these will be similar to those found in potato.

Whilst the involvement of the above enzymes in starch biosynthesis are strictly species dependent, the starch polymerising activities are ever present and responsible for the formation of the two different macromolecular forms of starch, amylose and amylopectin. Starch synthases catalyse the transfer of the glucosyl moiety from ADP-glucose to the non-linear end of an alpha-1,4 glucan. The various starch synthases are able to extend 1,4-glucans in both amylose and amylopectin. At least four different classes of starch synthases exist, designated as GBSS (granule-bound starch synthase), SSI, SSII and SSIII which vary greatly in molecular weight, need for primers, substrate affinities and antigenic properties. It seems likely that most plant species contain the four different classes of starch synthase, however, the extent to which they contribute in vivo probably differs considerably between species (Denyer et al., 2001). Starch branching enzymes are responsible for the formation of alpha-1,6 branch points within amylopectin. Although there are more than two isoforms present in most plant species, all isoforms can be separated into two classes - most simply designated as A and B forms (Burton et al., 1995). The precise mechanism by which this is achieved is unknown, however it is thought to involve cleavage of a linear alpha-1,4 linked glucose chain and reattatchment of the chain to form an alpha-1,6 linkage (Kossmann and Lloyd, 2000). The combined action of starch synthases and branching enzymes play an important role in determining the structure of starch which will be described in detail below. Other enzymes of starch synthesis and degradation are less well understood. Disproportionating enzyme (D-enzyme) is able to synthesise alpha-1,4-glucans from maltose and has been suggested to be a candidate as a source of the malto-oligosaccharide primers required for starch synthesis. However several lines of evidence suggest this is unlikely to play a major role in starch synthesis in vivo. The maltose present in plant tissues is almost exclusively derived from starch (Kossmann and Lloyd, 2000) and transgenic plants exhibiting reduced D-enzyme expression had no effect on starch content (Takaha et al., 1998). Furthermore, recent studies on an Arabidopsis mutant deficient in D-enzyme reveal a minor decrease in starch under certain conditions, however, they indicate that this enzyme primarily plays a role in the removal of malto-oligosaccharides during starch degradation (Critchley et al., 2001).

Recent studies of 14-3-3 proteins within starch granules of Arabidopsis chloroplasts (Sehnke et al., 2001) and of an AGPase from barley leaves (Rodriguez-Lopez et al., 2000) indicate that enzymes other than those classically considered to constitute the starch synthetic pathway


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may also contribute to this process. However, there is no evidence as yet for a physiological role for either of these proteins within plant systems.

2.6 Starch degradation

Starch is synthesised as a store for carbon. In leaves it is manufactured and degraded over a 24 hour period, being synthesised during the light period and degraded in the dark period; in storage organs, however, it can be stored for years, or even decades, prior to its mobilisation. Many enzymes have been isolated which can degrade starch yet, despite this, it is only recently becoming apparent which isoforms are actually important in this process. This is because many of the enzymes which can degrade starch (beta-amylase, starch phosphorylase, alpha-amylase) are present as multiple isoforms, some of which are present within the plastid, others being extra-plastidial.

Recently two papers have been published showing unequivocally that two different enzymes are involved in mobilising starch in leaves. The first was mentioned above as being an Arabidopsis mutant affected in D-enzyme activity (Critchley et al., 2001). The second was an isoform of beta-amylase in potato that was repressed using an antisense construct (Scheidig et al., 2002). Leaves from both of these plants did not degrade as much starch during the dark period as the controls, demonstrating a block in starch degradation.

In order to try and identify enzymes involved in starch degradation there have been several screens of mutant Arabidopsis populations to identify plants that do not degrade starch upon being shaded. Many mutants have been isolate and have been named sex mutants as they show a starch excess phenotype (Caspar et al., 1991). Two of these showing such a phenotype (sex1 and sex4) have been characterised in more detail. The sex1 mutant will be discussed below in a section on starch phosphorylation. The mutation in sex4 has not been identified, but the mutant plant has been shown to be deficient in a plastidial isoform of alpha-amylase (Zeeman et al., 1998). It is known, however, that the mutation does not lie in the gene coding for this alpha-amylase as that is situated on chromosome 1, while the mutation lies on chromosome 4 (Dr. Samuel Zeeman, University of Berne, personnel communication). More work needs to be performed, therefore, to identify the genetic lesion.

2.7 Starch phosphorylation

Phosphate residues have often been associated with starch granules. The nature of these residues is, however, dependant on the species. Starch from potato tuber, for example, contains large amounts of phosphate that is covalently bound either to the C3 or C6 positions


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of glucose residues. That from cereal endosperm contains almost no covalently bound phosphate, but large quantities of phospholipids which are associated with the granule. In this section I will concentrate solely on the covalently bound phosphate.

For many years it was speculated that covalently bound phosphate becomes incorporated into starch either through the action of starch phosphorylase, or AGPase (Kossman and Lloyd, 2000; Lloyd et al., 1999). The reason for this was that it no enzyme had been isolated which could phosphorylate starch directly. Recently however this has been accomplished. The cDNA coding for this protein was isolated using an early proteomic approach. It is known that many enzymes bind to starch granules, and that many of these are involved in starch metabolism. It was hypothesised that if one could identify the cDNA‘s coding for previously unidentified proteins which bind to starch granules they may well also be involved in starch metabolism. To achieve this, starch-granule-bound proteins were isolated, antibodies raised against them and these antibodies were used to screen a potato cDNA library. One of the clones isolated coded for a protein which is approximately 160 kDa in size. This protein was repressed in potato using an antisense construct, and two phenotypes were noted. The first was that the amount of covalently bound phosphate in the starch was greatly reduced, and that the plants were also inhibited in starch degradation in both leaves and tubers (Lorberth et al., 1998). It was not demonstrated for several years, however, that the protein could actually phosphorylase glucans, and therefore at the time it was called R1. To show that the R1 protein was indeed responsible for phosphorylating starch it was purified to homogeneity. It was then incubated with starch and various potential phosphate donor molecules. The amount of phosphorylation was measured after incubation and it was shown that the protein could indeed phosphorylate starch, and that it required ATP to do so. It was further shown that the mechanism of phosphorylation was a dikinase rather than a kinase. This means that the gamma-phosphate of ATP is released as inorganic phosphate, while the beta-phosphate is the one transferred to the glucan (Ritte et al., 2002). The protein is, therefore, a glucan water dikinase and has been renamed as GWD (Ritte et al., 2003).

Other evidence has demonstrated that the GWD protein has a similar effect in other species. This comes from the identification of the starch accumulating sex1 mutant as being mutated in an Arabidopsis homolog of the GWD protein (Yu et al., 2001). It was also found that the phosphate content of the starch in the mutant Arabidopsis leaves was greatly reduced. This raises the question, however, as to why starch phosphorylation appears to affect starch degradation also. There is no clear answer to this. It may be that enzymes which degrade starch need covalently bound phosphate residues to act, or that they interact with the GWD


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protein in some way during the degradation process. The possibility of answering these questions in the future are, however, now greater because of the identification of the function of the GWD protein

2.8 Glycolysis

Hexose phosphates are not only precursor (and products) of starch synthesis (and degradation) but also important substrates for glycolysis. Respiratory carbon metabolism which is an essential provider of both the energy and the required precursors to support biosyntheisis in the heterotrophic cell couples the partial oxidation of glucose to pyruvate during gylcolysis to the complete oxidation of pyruvate to carbon dioxide during operation of the Krebs cycle. In plants, oxidation of carbohydrate via glycolysis provides the majority of substrate for the operation of the Krebs cycle - with the oxidative pentose phosphate pathway, protein and lipid only making minor contributions to respiration (ap Rees, 1980; Holtman et al., 1994). The glycolytic chain of reactions is often split into two parts (for a review see Hopkins, 1995), the first part comprising of the set of reactions by which substrates of glycolysis are converted to the common intermediate Fru-6-P and the second part comprising of steps following on from the conversion of Fru-6-P to Fru-1,6-P2 by the action of either ATP- or PPi-dependent PFK. The interconversion of Fru-6-P to Fru-1,6-P2 is often said to be the first committed step of glycolysis. In plants this step is very tightly regulated and is complicated by the presence of three enzymes involved in its interconversion the ATP-PFK which catalyse the production of Fru-1,6-P2 and FBPase and the PPi-PFK (which is freely reversible) which catalyse the production of Fru-6-P. Moreover, the interconversion of Fru-6-P to Fru-1,6-P2 in the cytosol is strongly influenced by the concentration of the signal metabolite Fru-2,6-P2 which potently inhibits the FBPase and activates PPi-PFK (Stitt, 1990). Following the phosphorylation of Fru-6-P, the resultant bisphosphate is cleaved to form the triose-P DHAP and G3P by the action of aldolase. The triose-P are readily equilibrated by triose phosphate isomerase (TPI), whilst G3P subsequently converted to 1,3-BPGA, 3-PGA, 2-PGA, PEP and pyruvate via the actions of glyceraldehyde 3phosphate dehydrogenase (G3P DH), (PGK), phosphoglycerate mutase, enolase and pyruvate kinase (PK) respectively.

PEP can alternatively be brought into the Krebs cycle by a different route in which it is carboxylated by the action of PEP carboxylase yielding oxaloacetate which is subsequently reduced to malate by the action of malate dehydrogenase which is then taken up into the mitochondrion. That said the major link between glycolysis and the Krebs cycle is provided by the uptake of pyruvate and its subsequent decarboxylation and oxidation and finally


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condensation of the resultant acetyl group with CoA to form acetyl CoA is all carried out by the large multienzyme complex known as pyruvate dehydrogenase (Bryce and Thornton, 1996; Hopkins, 1995). Acetyl CoA production by the pyruvate dehydrogenase complex thus fuels the operation of the Krebs cycle which in conjuncture with the respiratory electron transport chain provides for the majority of the energy requirements of the heterotrophic cell in addition to providing carbon skeletons for the biosynthesis of a wide range of primary and secondary metabolites and being an important source of reductant for the cell.

2.9 Fruit metabolism

Whilst the preceding chapters have largely considered organs as either photosynthetic or heterotrophic the situation in fruits is somewhat more complex. The tomato fruit is no exception to this generalisation with numerous studies investigating source sink interactions between leaves and fruits and the effect of crop yields of manipulating leaf photosynthetic activity by altering photon flux density, temperature, carbon dioxide concentration, nutrient and water supplies (for a review see Ho and Hewitt, 1986). However, there are many parts of the tomato plant other than the leaves that contain chlorophyll and capture light energy. Yet the photosynthetic contribution of these tissues to the maintenance and growth of the plant have received scant attention. The work described in this thesis is intended to investigate the impact of altering the activities of three enzymes associated with starch metabolism on the development and metabolism in the fruit. Whilst the enzymes involved in photosynthetic and heterotrophic starch synthesis are well known it is clear that their regulation and precise metabolic function within the fruit is not fully understood. The application of antisense technology approaches targeted at AGPase, FBPase and GWD and biochemical analysis of fruits, taken along a developmental axis, from the resultant transformant lines may well allow a better understanding both of the regulation of this important storage pathwaye in the fruit and of the relative importance of photosynthesis during early stages of fruit development.


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