Journal of Experimental Botany, Vol. 53, No. 377, pp. 2099-2106,
October 1, 2002
© 2002 Oxford University Press
Increasing antioxidant levels in tomatoes through modification of the flavonoid biosynthetic pathway
Received 12 April 2002; Accepted 29 May 2002
1 Unilever R&D Colworth, Sharnbrook, Bedford MK44 1LQ, UK
2 Plant Research International, PO Box 16, 6700 AA Wageningen, The Netherlands
Abbreviations: cDNA, complementary DNA; CHS, chalcone synthase; CHI, chalcone isomerase; F3H, flavanone 3-hydroxylase; FLS, flavonol synthase; PAL,-phenylalanine ammonia lyase; FW, fresh weight; DW, dry weight; dpa, days post-anthesis; PCR, polymerase chain reaction.
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Abstract |
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 Top Abstract IntroductionFlavonoid biosynthesis in...Enhancing flavonol accumulation...Enhancing flavonol accumulation...Enhancing flavonol accumulation...ConclusionsReferences |
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Flavonoids are a diverse group of phenolic secondary metabolites that occur naturally in plants and therefore form an integral component of the human diet. Many of the compounds belonging to this group are potent antioxidants in vitro and epidemiological studies suggest a direct correlation between high flavonoid intake and decreased risk of cardiovascular disease, cancer and other age-related diseases. Enhancing flavonoid biosynthesis in chosen crops may provide new raw materials that have the potential to be used in foods designed for specific benefits to human health. Using genetic modification, it was possible to generate several tomato lines with significantly altered flavonoid content and to probe the role and importance of several key enzymatic steps in the tomato flavonoid biosynthetic pathway. Most notably an up to 78-fold increase in total fruit flavonols was achieved through ectopic expression of a single biosynthetic enzyme, chalcone isomerase. In addition, chalcone synthase and flavonol synthase transgenes were found to act synergistically to up-regulate flavonol biosynthesis significantly in tomato flesh tissues.
Key words: Key words: Chalcone isomerase, chalcone synthase, flavonol synthase, flavonols, tomato, transcription factors.
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Introduction |
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TopAbstractIntroductionFlavonoid biosynthesis in...Enhancing flavonol accumulation...Enhancing flavonol accumulation...Enhancing flavonol accumulation...ConclusionsReferences |
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The flavonoids form a large family of low molecular weight polyphenolic compounds, which occur naturally in plant tissues and include the flavonols, flavones, flavanones, catechins, anthocyanins, isoflavonoids, dihydroflavonols, and stilbenes (Haslam, 1998). To date, more than 4000 flavonoids have been described. Notably, most are conjugated to sugar molecules and are commonly located in the upper epidermal layers of leaves (Stewart et al., 2000). However, they also occur naturally in fruits, vegetables, nuts, seeds, and flowers and therefore form an integral part of the human diet. In plants, the flavonoids are thought to have many functions including protection against UV-B radiation, defence against pathogen attack, attractants to pollinating insects, and as signal compounds for the initiation of symbiotic relationships (Parr and Bolwell, 2000).
As a dietary component, the flavonoids are thought to have health-promoting properties, probably due to their high antioxidant capacity (Duthie and Crozier, 2000; Pietta, 2000). This function/activity is supported by their ability, in vitro, to induce human protective enzyme systems (Nijveldt et al., 2001) and by a number of epidemiological studies which suggest a protective effect against cardiovascular disease in particular (Hertog et al., 1997), but also against cancer (Knekt et al., 1997) and other age-related diseases such as dementia (Commenges et al., 2000).
However, more evidence for their beneficial properties is needed from in vivo supplementation studies, and a few encouraging reports have been published recently. For example, Duarte et al. (2001) reported that quercetin has the ability to lower blood pressure in spontaneously hypertensive rats. Another study in humans concluded that alcohol-free red wine extract and one of its components, quercetin, can inhibit LDL oxidation after in vivo supplementation (Chopra et al., 2000).
Based on these types of studies, there is a growing interest in the development of food crops with enhanced levels of flavonoids. Tomato is an excellent candidate: it is a major crop, and the peel of its fruit already contains modest levels of flavonols, a sub-class of flavonoids thought to be particularly beneficial to human health. Therefore, the aim was to up-regulate flavonoid biosynthesis in tomato in order to augment the fruit’s antioxidant capacity and widen its range of health benefit properties. Other groups have reported modification of the flavonoid pathway in other species, however, this research has been mainly directed towards alteration of anthocyanin pigmentation (Dixon and Steele, 1999; Forkmann and Martens, 2001; Mol et al., 1998). In tomato, flavonol biosynthesis was characterized at the molecular, biochemical and tissue-specific levels using a commercial processing tomato variety, FM6203. This knowledge was used to adopt a transgenic approach generating tomato transformants in which sequences encoding key biosynthetic enzymes, either alone or in combination, or encoding regulatory elements are ectopically expressed. These approaches resulted in a number of modified tomato lines with increased levels of flavonols. A review of this work is presented in this paper.
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Flavonoid biosynthesis in tomatoes |
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TopAbstractIntroductionFlavonoid biosynthesis in...Enhancing flavonol accumulation...Enhancing flavonol accumulation...Enhancing flavonol accumulation...ConclusionsReferences |
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The flavonoid biosynthetic pathway (Fig. 1) and its regulation have been well studied in plants (Weisshaar and Jenkins, 1998) and many enzymes required for the production of different flavonoid classes have been characterized (Winkel-Shirley, 2001; Holton and Cornish, 1995; Mol et al., 1998).
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Previous studies have indicated that flavonoids accumulate in tomato fruit in a tissue-specific manner (Krause and Galensa, 1992). The analyses of tomato fruit of the commercial variety FM6203, at different ripening stages, showed that the accumulation of flavonoids in tomato fruit is indeed regulated in a tissue-specific, but also in a development-dependent, manner. The main flavonoid found is naringenin chalcone. Naringenin chalcone accumulates almost exclusively in peel tissue and is simultaneously formed with colouring of the fruit, peaking at up to
1% on a dry weight basis in turning peel tissue (Muir et al., 2001). In addition, the flavonols quercetin-rutinoside (rutin) and, to a lesser extent, kaempferol-rutinoside, also accumulate almost exclusively in ripening tomato peel. Furthermore, analysis of flavonol accumulation during fruit ripening shows that these flavonols are also simultaneously formed with colouring of the fruit, peaking at up to 0.1% on a dry weight basis in overripe (45 dpa) peel tissue (Muir et al., 2001). This tissue-specific accumulation is in agreement with other studies which have shown between 95–98% of the naringenin chalcone and 98% of the flavonols (quercetin- and kaempferol-glycosides) to be located in peel tissue (Krause and Galensa, 1992; Stewart et al., 2000). Northern analysis showed that, in peel tissue from FM6203 fruit, chs, f3h and fls transcripts were abundantly expressed whereas chi transcripts were below the levels of detection. Similarly, in tomato flesh tissues all three transcripts chs, chi and fls were below the level of detection (Muir et al., 2001; S Muir, unpublished observations). These observations appear to correlate with the biochemical phenotype, i.e. accumulation of naringenin chalcone in peel tissue and negligible levels of flavonoid accumulation in flesh (pericarp and columella) tissues.
Given the low levels of flavonols detected in FM6203, a number of tomato varieties were analysed in order to investigate the extent of natural variation within existing germplasm. The analyses of fruit from 28 varieties, grown simultaneously and under the same conditions indicated a variation of up to 10-fold in flavonol levels. Quantification of quercetin present in hydrolysed peel extracts showed that levels were in a range between 6.3–64.9 µg g–1 FW, however, the higher values only represent an up to 2.5-fold increase relative to the commercial variety FM6203 (data not shown). Interestingly, Stewart et al. (2000) also recently reported their analyses of a number of different tomato varieties and showed a similar degree of variation (1.3–22.2 µg g–1 FW) in total fruit flavonols. These findings suggest that significant increases in fruit flavonol levels, above those found in FM6203, may not be obtained from utilizing existing tomato cultivars.
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Enhancing flavonol accumulation in tomato peel |
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TopAbstractIntroductionFlavonoid biosynthesis in...Enhancing flavonol accumulation...Enhancing flavonol accumulation...Enhancing flavonol accumulation...ConclusionsReferences |
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At a biochemical level it has been shown that peel tissue accumulates naringenin chalcone to a high level throughout fruit ripening, but peaks at up to
1% DW in turning fruit (31 dpa). It has also been reported that, although chs, f3h and fls transcripts are expressed abundantly throughout ripening, the level of chi transcript is below the level of detection (Muir et al., 2001; S Muir, unpublished observations). Notably, the enzyme chalcone isomerase utilizes naringenin chalcone as a substrate to produce dihydroflavonols (Fig. 1) and, therefore, the biochemical and molecular analyses suggest that chi expression might be a rate-limiting step in flavonol biosynthesis in tomato peel. Clearly, one approach to overcoming a potential rate-limiting step is ectopically to express a sequence encoding the particular enzymic activity. Therefore, tomato was transformed with a sequence from P. hybrida encoding CHI, under the control of the strong constitutive double CaMV35S promoter and the resulting tomato plants analysed for flavonoid accumulation in red-ripe (40 dpa) fruits (Muir et al., 2001). Analysis of the primary transformants, which harbour the CHI transgene, revealed dramatic increases in fruit peel flavonol levels compared with control plants (up to 78-fold increase in individual fruits) (Fig. 2). This rise in total flavonol accumulation mainly comprised increases in the accumulation of rutin (quercetin 3-O-rutinoside), isoquercitrin (quercetin-3-O-glucoside) and kaempferol-3-O-rutinoside in the peel tissues. Furthermore, there was a direct relationship between quercetin and kaempferol levels in hydrolysed extracts from transformed fruit, with those plants displaying a high quercetin phenotype also possessing high levels of kaempferol (the ratio of quercetin:kaempferol being approximately 10:1). The analyses also showed that, in contrast to control fruit, the level of naringenin chalcone accumulation was severely depleted in the high flavonol fruit. Given that naringenin chalcone is a substrate for CHI, this finding suggests that ectopic expression of the P. hybrida CHI results in an increased level of CHI activity, which in turn utilizes the naringenin chalcone pool. In agreement, Northern analysis of transgenic fruit peel revealed low but detectable levels of CHI transcript at all stages of fruit development. By contrast, chi transcripts were below the level of detection in control fruit. It is also noteworthy that flavonol accumulation was essentially linear at the post-‘mature-green’ stage (7 dpa) of fruit ripening, correlating with the kinetics of naringenin chalcone accumulation detected in non-transformed fruit. These results may suggest that during fruit development the availability of naringenin chalcone is limiting in green fruit from CHI transformants rather than CHI activity as CHI transgene transcripts are present throughout fruit ripening.
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The exact mechanism by which the increase in CHI messenger RNA (mRNA) leads to higher levels of flavonols in peel is unclear. However, it has been shown that ectopic expression of CHS, F3H, FLS singly and CHS-FLS, CHS-F3H or F3H-FLS in combination does not significantly increase the level of flavonols in peel tissue (S Muir, S Colliver, unpublished observations). As such, one might predict that the endogenous CHI activity constitutes the sole rate-limiting step in peel in post-mature-green tomato fruit. However, it is interesting to note, from a mass balance perspective, that the magnitude of rutin accumulation is significantly greater than the initial level of naringenin chalcone detected. This might suggest that, in fact, ectopic expression of CHI utilizes the naringenin chalcone pool and that depletion of this substrate pool removes a point of negative feedback in flavonol biosynthesis resulting in increased flux into the pathway.
Alternatively, the increase in the level of CHI mRNA could lead to changes in the activity of other enzymes in the pathway, for example, by other feedback/forward regulation mechanisms, or by stabilization of biosynthetic complexes containing CHI (Burbulis and Winkel-Shirley, 1999). It is interesting to note that ectopic expression of CHI did not exclusively result in the accumulation of rutin, the quercetin glycoside found in wild-type tomato peel. In fact, the significant accumulation of isoquercitrin, the immediate precursor to rutin, perhaps raises the possibility that by increasing flux through the flavonol pathway the enzyme that converts isoquercitrin into rutin, rhamnosyl transferase, has become limiting. This accumulation of isoquercitrin is very interesting from a nutritional point of view since this compound is thought to be a more bioavailable glycoside of quercetin (Olthof et al, 2000).
Analyses of the vegetative phenotype showed that there were no gross differences in appearance between the high flavonol tomato lines and control lines (Fig. 3). In addition, it has been shown that this high flavonol phenotype is stable and segregates with the CHI transgene through at least four generations. Fruit flavonol levels in the best lines were similar to those found in yellow onions, a crop with naturally high levels of flavonols (Ewald et al., 1999). Furthermore, the effect of processing on these high flavonol fruit has been tested and it was found that most of the flavonols were retained in processed paste, which was indistinguishable from control paste both in taste and flavour (data not shown).
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These studies show that ectopic expression of one gene, P. hybrida CHI, is sufficient to increase flavonol accumulation in tomato peel. However, despite the fact that CHI transcripts were detected at relatively high levels in leaf samples, and in green, breaker and turning flesh (pericarp) from high flavonol plants, no increases in flavonol levels were observed in these tissues. This indicates that, at least in tomato, flavonoid biosynthesis is subject to tissue-specific regulation and that to achieve significant increases in flavonol accumulation in tomato flesh a different approach is required.
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Enhancing flavonol accumulation in tomato flesh tissue |
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TopAbstractIntroductionFlavonoid biosynthesis in...Enhancing flavonol accumulation...Enhancing flavonol accumulation...Enhancing flavonol accumulation...ConclusionsReferences |
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In flesh (pericarp and columella) tissue, the level of expression of endogenous flavonoid genes (pal, chs, chi, f3h, fls) was shown to be below detection limits, even when using TAQMAN analysis (A Bovy, unpublished observations). These findings are in agreement with the biochemical analyses, which show that tomato flesh tissue accumulates only trace amounts of flavonols. Further, the analyses indicate that to achieve up-regulation of flesh flavonols would require a different approach to that used successfully in peel tissue. One such approach might be to up-regulate the endogenous pathway by ectopic expression of appropriate regulatory elements. Several groups have successfully demonstrated this principle by using transcription factors such as Lc and C1, to increase anthocyanin levels in plants (Bradley et al., 1998; de Majnik et al., 2000; Goldsbrough et al., 1996). However, none of these groups has reported increases in flavonol levels. By contrast, an up to 60-fold increase in kaempferol glycosides has recently been achieved in tomato flesh tissue by simultaneous ectopic expression of the two maize transcription factors Lc and C1. This resulted in an overall increase in total fruit flavonols of up to 20-fold (Bovy et al., 2002).
In an alternative approach, P. hybrida sequences encoding each of the key biosynthetic enzymes leading to flavonols, chalcone synthase (CHS), chalcone isomerase (CHI), flavanone-3-hydroxylase (F3H), and flavonol synthase (FLS) were ectopically expressed simultaneously. HPLC analyses of primary transformants containing all four transgenes showed that a number (75%) of these tomato lines accumulate very high levels of quercetin glycosides in the peel and, more modest, but significantly increased levels of kaempferol- and naringenin-glycosides in columella tissue (Colliver et al., 2002) (Fig. 4). The high quercetin phenotype in the peel was expected because of the presence of the CHI transgene, and it is noteworthy that the levels detected were similar to those found in CHI-only transformants. However, the role of each of the individual transgenes in determining the accumulation of kaempferol- and naringenin-glycosides in columella tissue remained unclear.
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Interestingly, transcript analysis of Lc+C1 transformed fruit showed that chs and f3h gene expression is increased
160-fold and 174-fold, respectively. In addition, fls gene expression was also found to be slightly increased (10-fold) (A Bovy et al., unpublished data). This observation might suggest that chs and f3h are the key genes required for flavonol accumulation in tomato flesh. To test this hypothesis, tomato was transformed with a construct containing the P. hybrida CHS and F3H transgenes. In addition, the P. hybrida CHI transgene was included for enhanced flavonol accumulation in peel tissue. Surprisingly, it was found that ectopic expression of CHS and F3H in conjunction with CHI led to the expected increase in peel flavonols, but was not sufficient to up-regulate flavonol accumulation in flesh (pericarp and columella) tissues (Colliver et al., 2002). As described above, it was shown that concomitant ectopic expression of CHS, CHI, F3H, and FLS in tomato fruit does result in increased levels of flavonol accumulation in both peel and flesh tissues. Therefore, comparing these findings with those from CHS–CHI–F3H transformants suggested that FLS might play an important role in flavonol synthesis in flesh tissues after all. These studies emphasize the complex nature of flavonoid regulation in tomato fruit, at least at the biosynthetic gene level, and the potential hazards of trying to correlate molecular (transcript) and biochemical phenotypes. In an attempt to elucidate the key biosynthetic enzymes that determine accumulation of flavonols throughout the tomato fruit tissue, each of the key enzymes have been ectopically expressed both individually and in selected combinations.
In flesh (pericarp and columella) tissues, analysis following expression of single gene constructs showed that none of the four biosynthetic genes were able, individually, to up-regulate flavonol biosynthesis in tomato flesh. However, notably, ectopic expression of CHS did indicate that the transgene was capable of directing flux towards flavonoid biosynthesis resulting in an increase in the accumulation of naringenin-glycosides, particularly naringenin-7-glucoside, in columella tissue (Colliver et al., 2002). This finding might suggest that additional enzyme(s) are required to ‘pull’ carbon flux through to flavonol end-products.
In addition to the ‘single gene’ transformants, a number of ‘two-gene’ combinations were generated by crossing of parent plants harbouring single gene constructs. HPLC analyses of fruit from these transformed lines revealed that the genes that appear to be critical in leading to flavonol biosynthesis in tomato flesh (pericarp and columella) tissue are CHS and FLS (Colliver et al., 2002). As described previously, ectopic expression of CHS resulted in modified tomatoes accumulating increased levels of naringenin-glycosides but with no increase in flavonols. By contrast, analysis of tomatoes harbouring the FLS transgene showed that no significant difference in biochemical phenotype was detectable when compared to control fruit (results not shown). The analyses have shown that concomitant expression of both CHS and FLS has a synergistic effect resulting in a significant accumulation of both naringenin- and kaempferol-glycosides in tomato flesh (Fig. 5). This finding was unexpected since transcript analysis of flesh from tomatoes transformed with Lc and C1 had shown up-regulation of chs and f3h expression, whilst for fls only a very small increase was observed. However, these studies clearly indicate that in flesh tissue CHS is a key gene in ‘pushing’ carbon into the pathway (naringenin chalcone) and that FLS, but not CHI or F3H, is required to ‘PULL’ carbon flux through from naringenin chalcone to the flavonol end-product. Given the organization of the biosynthetic pathway (Fig. 1), these findings, perhaps, suggest that CHS and FLS play a key role in stabilizing a metabolic complex comprising CHS, CHI, F3H, and FLS in tomato flesh tissue.
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Enhancing flavonol accumulation throughout tomato fruit |
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TopAbstractIntroductionFlavonoid biosynthesis in...Enhancing flavonol accumulation...Enhancing flavonol accumulation...Enhancing flavonol accumulation...ConclusionsReferences |
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The investigations to date indicate that chi gene activity appears to be key to flavonol accumulation in tomato peel, whilst chs and fls activities are required for the production of flavonols in flesh tissue. Therefore, it was reasoned that, to achieve increased flavonol accumulation throughout the tomato fruit, ectopic expression of three genes encoding the biosynthetic enzymes CHS, CHI and FLS would be sufficient. Indeed a cross harbouring these three genes accumulates increased levels of quercetin-glycosides in peel and kaempferol-glycosides in flesh (S Colliver, unpublished results). It is also noteworthy that a similar phenotype can be achieved by crossing tomatoes containing Lc and C1 transgenes with tomatoes containing the CHI transgene (S Muir, unpublished results).
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Conclusions |
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TopAbstractIntroductionFlavonoid biosynthesis in...Enhancing flavonol accumulation...Enhancing flavonol accumulation...Enhancing flavonol accumulation...ConclusionsReferences |
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These studies demonstrate different routes to up-regulate flavonoid biosynthesis in tomatoes, through the ectopic expression of either a select number of key biosynthetic genes or key regulatory elements, or a combination of both. In peel tissue, chalcone isomerase gene activity appears to be critical and expression of a sequence encoding the P. hybrida chalcone isomerase leads to a large increase in the level of quercetin-glycoside accumulation. It was further demonstrated that concomitant expression of the sequences encoding chalcone synthase and flavonol synthase from P. hybrida is sufficient to achieve accumulation of kaempferol-glycosides in tomato flesh. In addition, these studies have shown that ectopic expression of three genes encoding the biosynthetic enzymes CHS, CHI and FLS is required to achieve increased flavonol accumulation throughout the tomato fruit. Alternatively, ectopic expression of the regulatory genes Lc and C1, together with the biosynthetic gene CHI results in a similar phenotype. Finally, as quercetin is a more potent antioxidant than kaempferol, it may be desirable to enhance the accumulation of quercetin- glycosides in tomato flesh, rather than kaempferol-glycosides. This could be achieved by overexpression of flavanone 3'-hydroxylase, the enzyme that converts dihydrokaempferol to dihydroquercetin. Dihydroquer cetin would then be converted to quercetin by the action of flavonol synthase.
New tomato varieties enriched in flavonols potentially offer opportunities for tomato-based products with heart health benefits.
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