Tomato (Lycopersicon esculentum) belongs to the Solanum genus in the family of Solanaceae, originated in Central and South America, and is a globally important cash crop. Tomatoes contain sugar, organic acids, lycopene, and vitamins A, C, E and other nutrients, has an important role in human health, can reduce the risk of cancer and heart disease (Boffetta et at., 2010).

For a long time, tomato breeding has mainly focused on yield, fruit size, hardness, disease resistance, etc., without paying attention to tomato flavor quality, thus leading to the decline of tomato flavor quality in the market (Baxter et al., 2005). Tomato flavor quality is composed of more than 400 volatile and non-volatile components, and the main components that determine the taste quality are soluble sugars, organic acids, and amino acids, of which soluble sugars are mainly glucose, fructose, and sucrose, and organic acids are mainly citric acid and malic acid (Denise et al., 2006). Soluble sugars and organic acids make up 60% of the dry weight of tomato and are the main substances that make up the soluble solids of tomato (Bastias et al., 2011). Soluble solids are an important trait of tomato that determines the flavor and processing quality of tomato. Although tomato soluble solids are composed of a variety of components, they can usually be expressed in terms of soluble sugar content (Beckles, 2012).

1 QTL Loci Associated with Soluble Solids in Tomatoes

Tomato wild seed is an important germplasm resource used to improve cultivated tomato and introduce loci controlling important agronomic traits into cultivated tomato. A large number of QTLs associated with soluble solids have been identified by studying hybrid populations of cultivated and wild species, and in order to further investigate the trait differences between cultivated and wild tomatoes, Eshed and Zamir (1995) constructed an osmotic line with 76 lines covering the entire genome of the Lycopersicon pennellii, using the Lycopersicon pennellii LA0716 and the cultivated tomato M82. A population of osmotic lines covering the entire genome of Pennerley tomato was constructed using Lycopersicon pennellii LA0716 and cultivated tomato M82, from which the osmotic material IL9-2-5 with high soluble solids was obtained. Subsequently, the near-isogenic lines were used to localize to a principal QTL locus Lin5, encoding an exoplasmic Sucrose Converting Enzyme gene in Brix9-2-5, which was specifically expressed in the fruits and flowers (Fridman and Zamir, 2003). Further studies revealed that polymorphisms in the bases near the catalytic site of Lin5 converting enzyme affected the converting enzyme activity and consequently the accumulation of sugars in tomato fruits, and that Brix9-2-5 increased glucose content by 28% and fructose content by 18% in tomato fruits without affecting yield (Baxter et al., 2005).

As early as 1988, a comparative study of changes in carbohydrates and related enzymes during fruit development of the Lycopersicon chmielewskii LA1028 and the cultivated tomato UC82B revealed that LA1028 fruits accumulated sucrose while UC82B fruits accumulated fructose and glucose (Serge et al., 1988). The study of isolated progeny of LA1028 and UC82B revealed that sucrose accumulation was mainly associated with low activity of acid sucrose converting enzyme (Serge et al., 1991). Gene localization using RFLP and isozyme markers identified the primary QTL locus Sucr on chromosome 3 controlling sucrose accumulation, which was co-segregated with the TG102 marker encoding a leaf vesicle-type sucrose converting enzyme capable of increasing sucrose, total sugar, and soluble solids content in ripening fruits (Chetelat et al., 1995b). In addition, AgpL1^H, a gene encoding a large subunit of adenosine diphosphate glucose pyrophosphorylase, was identified in Solanum habrochaites, Lycopersicon hirsutum LA1777, and was able to increase the starch content of immature fruits and soluble solids content of ripe fruits (Petreikov et al., 2006). Another QTL locus, Fgr^H, was identified in Solanum habrochaites, Lycopersicon hirsutum 4 of hirsute tomato, which increased the fructose content and decreased the glucose content of ripe fruits but did not change the total hexose content of fruits (Levin et al., 2000).

2 Soluble Solids and Fruit Size

Eshed and Zamir (1995) identified 23 QTLs related to soluble solids in a Lycopersicon pennellii osmotic line population, and further studies revealed that soluble solids content was negatively correlated with tomato yield. Bernacchi et al. (1998) identified five QTLs related to soluble solids in a Solanum habrochaites, Lycopersicon hirsutum osmotic line population, of which ssc3.1, ssc3.2, ssc5.1, and ssc9.1 were able to increase soluble solids content but were negatively correlated with yield. ssc3.1, ssc3.2, ssc5.1 and ssc9.1 were able to increase soluble solids content but were negatively correlated with yield. Previous studies have found that the genetic distance between the loci regulating soluble solids content and fruit weight in tomato fruit is very close, and that soluble solids content is negatively correlated with fruit weight (Tanksley, 2004).

For a long time, tomato breeding has mainly focused on how to increase the yield and extend the shelf life of tomato fruits, while ignoring the changes in tomato flavor substances, so that the quality as well as the content of soluble solids, an important ingredient in cultivated tomato fruits, showed a downward trend during long-term domestication. Wild species have high fruit soluble solids but very low single fruit weight and yield, while common cultivated tomatoes are the opposite of wild species. One suggestion is that sequence polymorphisms at the fw2.2 locus affect fruit size in both wild and cultivated tomatoes by altering pre-pollination ovary cell division (Serge et al., 1991). There is also an explanation that the single fruit weight of common tomato is much larger than that of the wild species, due to a dilution effect resulting in lower soluble solids content (Bertin et al., 2009). Hexose has a higher osmotic pressure compared to sucrose, which leads to more water entering the cells, which in turn leads to larger cell size and larger fruits (Stommel, 1992). It has also been suggested that the difference in the relative proportions of hexose and sucrose in wild and cultivated tomato species affects cell division, which in turn leads to large differences in fruit size (Masa-aki et al., 2005).

To date, domestic and foreign researchers have identified at least 28 QTL loci controlling tomato fruit size, but only fw2.2, fw3.2, and fw11.3 have been finely localized and cloned (Frary et al., 2000; Chakrabarti et al., 2013; Mu et al., 2017). fw2.2 and fw3.2 increase cell number by controlling cell division to increase the number of pericarp cell layers early in fruit development, but fw3.2 decreases the number of flowers on the flocculus, while fw2.2 does not (Chakrabarti et al., 2013). fas and locule number (lc) both increase the number of chambers by controlling pericarp or fruit development at anthesis, which affects the fruit size (Bin et al., 2008; van der Knaap et al., 2014).

3 Types of Sugar Accumulation in Tomato Fruits

The value of soluble solids of tomato wild species is generally 10%~15%, while the value of soluble solids of common cultivated tomato is generally 4%~6% (Beckles et al., 2011). The fruit sugar content of wild species of tomato is two to three times higher than that of common tomato, making it an important resource for breeding high-sugar tomatoes and studying the mechanisms of fruit sugar metabolism. According to the type of fruit sugar accumulation during ripening, tomato can be categorized into two types, one accumulating hexose and the other accumulating sucrose. The former tomato fruits mainly accumulate fructose and glucose, with very low sucrose content; the latter fruits accumulate large amounts of sucrose. S. chmielewskii (Chetelat et al., 1995a), S. hirsutum and S. hirsutum f. glabratum (Marina et al., 2009), S. peruvianum (Stommel, 1992), S. neorickii (Nicolas et al., 2005) accumulate sucrose, while red (pink) fruited tomatoes accumulate hexose, mainly S. cheesmanii (Balibrea et al., 2006), S. esculentum, S. pimpinellifolium (Husain et al., 2001), and others.

Roger et al. (1993) showed that the type of sugar accumulated in tomato fruits was mainly determined by the activity of sucrose converting enzyme during fruit ripening, and Miron et al. (1991) found that the cause of sucrose accumulation was related to high sucrose synthase and phosphosynthetic enzyme activities in addition to low sucrose converting enzyme activity. In contrast, hexose-accumulating Lycopersicon pennellii fruits ripened with increased sucrose converting enzyme activity in the mesocolumnar ectodomain, which in turn expanded the intra- and extracellular sucrose gradient (Husain et al., 2001).

4 Key Enzymes and Transcription Factors Affecting Sugar and Starch Accumulation

Sugar accumulation is the key to fruit quality formation, and the level of sucrose, fructose and glucose content has a great influence on fruit quality (Zhang and Li, 2002). Sucrose metabolism is an important link in sugar accumulation and is the main substance for the operation of photosynthetic products in tomato.

4.1 Key enzymes and transcription factors affecting sugar accumulation

The enzymes closely related to sucrose metabolism are sucrose synthase (SS), sucrose phosphate synthase (SPS) and sucrose invertase. Sucrose synthase (SS), sucrose phosphate synthase (SPS) and sucrose invertase are the key enzymes for sugar metabolism in tomato fruit (Zhang, 2008). Sucrose synthase is responsible for catalyzing sucrose catabolism and synthesis, and plays an important regulatory role in the synthesis of starch and cellulose, and the formation of fruit quality (Fang, 2017). Sucrose phosphate synthase catalyzes the synthesis of sucrose 6-phosphate using glucose uridine diphosphate as the donor and fructose 6-phosphate as the acceptor; sucrose 6-phosphate is dephosphorylated and hydrolyzed to produce sucrose, and the reaction process is not reversible (Huang, 2012). Sucrose converting enzyme irreversibly catalyzes the decomposition of sucrose into glucose and fructose, and plays an important role in various processes such as fruit sugar metabolism and regulation, growth and development (Zhang et al., 2012). According to the optimal pH, sucrose invertase is divided into acid invertase (AI) and neutral invertase (NI), of which cell wall invertase (CWIN), vacuolar invertase (VIN) are acid invertases. cytoplasmic invertase (CIN) is a neutral sucrose converting enzyme (Zhao, 2016). Sucrose conversion in fruits is dominated by acidic sucrase, and neutral sucrase activity accounts for less than 5% of the total sucrase activity, which is high during fruit ripening in common tomato and mainly inhibits sucrose accumulation (Husain et al., 2001).

It was found that differences in sugar accumulation during wild species tomato fruit development were mainly determined by sucrose converting enzyme activity and starch accumulation, and the magnitude and direction of the changes in both varied markedly among different wild species (Beckles, 2012). In addition, the tomato sucrose transporter (SUT) protein can alter the sucrose unloading capacity of the phloem and repress the expression of the SUT2 gene, which in turn reduces glucose, fructose, and sucrose content in tomato fruit (Hackel et al., 2006). Transcription factors SlARF4 and SlARF10 regulate sugar accumulation by affecting fruit photosynthesis and starch accumulation (Yuan et al., 2018). Transcription factors SlbZIP1 and SlbZIP2 affect tomato fruit sugar content through sucrose-induced translational silencing (Sagor et al., 2016). Deficiency of phytochrome choline (PΦB) synthesis inhibits sugar accumulation in tomato fruits by transcriptionally suppressing cell wall sucrose convertase, sucrose transporter protein, and AGPase activities (Bianchetti et al., 2017).

4.2 Key enzymes and transcription factors affecting starch accumulation

Starch metabolism has a direct effect on tomato fruit soluble solids and yield. ADP-glucose pyrophosphorylase mainly controls starch synthesis in tomato fruits during fruit development and affects the composition and relative content of sugars in fruits (Petreikov et al., 2006). Tomatoes that up-regulated the expression of AGPase increased fruit starch and soluble solids content, whereas lowering its expression resulted in reduced starch synthesis, delayed flowering, and lower yields (Beckles, 2012). Salt stress increased the expression level of AGPase, which increased the accumulation of green fruit starch and ultimately converted it to hexose during fruit ripening thereby affecting the soluble sugar content at red ripening stage (Yin et al., 2010). Hou et al. (2019) investigated the effect of NADPH-thioredoxin reductase C NADPH-thioredoxin reductase C on the soluble sugar content of red ripening fruits. reductase C (NTRC) down-regulated expression in transgenic tomatoes found that NTRC down-regulated expression resulted in reduced AGPase expression, smaller tomato fruits and reduced starch accumulation, and affected fruit soluble sugar content at maturity. Many tomato materials with high soluble solids accumulated more starch at the green fruit stage, and AGPase is a key enzyme in starch synthesis (Baxter et al., 2005). Solanum habrochaites, Lycopersicon hirsutum LA1777 maintains high levels of AGPase expression during fruit development, resulting in increased starch content in the fruit (Petreikov et al., 2006).

5 Prospect

Utilizing transgenic tomato materials with overexpression, knockout or down-regulated expression of key enzyme genes for sugar and starch metabolism in tomato fruits to study the roles of SS, SPS, AI, NI and AGPase in the process of fruit sugar metabolism, as well as the sugar signaling and the perception of sugar signaling by sucrose-converting enzyme and hexokinase, etc. will be of great significance in unraveling the molecular mechanisms controlling sugar accumulation in fruits. Continuing to utilize wild species tomato resources to mine QTL loci controlling high soluble solids will still be of practical significance.

In recent years, the tomato market has been demanding higher and higher quality, so the breeding of high-quality high sugar, large-fruit, and storage-tolerant tomatoes is one of the important directions in tomato breeding. Research on the mechanisms of sugar and starch accumulation in large-fruited, high-sugar tomatoes and screening of related candidate genes are of great value for practical breeding. In the future, it is necessary to consider the balance between hexose and sucrose accumulation during the ripening period of tomato fruits, and the screening of materials with a large amount of starch accumulation before fruit ripening and a high level of hexose and sucrose accumulation after ripening is of great value for the cultivation of high-sugar, large-fruited tomatoes.

Authors’ contributions

FY and LC were the main writers of the review; LYT completed the collection of relevant literature; ZYY and ZLY participated in analyzing and organizing the literature; YXD was in charge of the idea and revision of the paper; ZH and ZWM were in charge of the design guidance. All authors read and approved the final manuscript.

Acknowledgments

This study was funded by the National Key Research and Development Program of China (2017YFD0101902; 2016YFD0101703).

References

Balibrea M.E., Martínez-Andújar C., Cuartero J., Bolarín M.C., and Pérez-Alfocea F., 2006, The high fruit soluble sugar content in wild lycopersicon species and their hybrids with cultivars depends on sucrose import during ripening rather than on sucrose metabolism, Functional Plant Biology, 33(3): 279-288.

https://doi.org/10.1071/FP05134

Bastias A., Lopez-Climent M., Valcarcel M., Rosello S., Gomez-Cadenas A., and Casaretto J.A., 2011, Modulation of organic acids and sugar content in tomato fruits by an abscisic acid-regulated transcription factor, Physiologia Plantarum, 141(3): 215-226.

https://doi.org/10.1111/j.1399-3054.2010.01435.x

Beckles D.M., 2012, Factors affecting the postharvest soluble solids and sugar content of tomato (Solanum lycopersicum L.) fruit, Postharvest Biology and Technology, 63(1):129-140.

https://doi.org/10.1016/j.postharvbio.2011.05.016

Beckles D.M., Hong N., Stamova L., and Luengwilai K., 2011, Biochemical factors contributing to tomato fruit sugar content: a review, Fruits, 67(1): 49-64.

https://doi.org/10.1051/fruits/2011066

Bernacchi D., Beck-Bunn T., Eshed Y., Lopez J., Petiard V., Uhlig J., Zamir D., and Tanksley S., 1998, Advanced backcross QTL analysis in tomato. I. Identification of QTLs for traits of agronomic importance from Lycopersicon hirsutum, Theoretical and Applied Genetics, 97(3): 381-397.

https://doi.org/10.1007/s001220050908

Bertin N., Causse M., Brunel B., Tricon D., and Genard M., 2009, Identification of growth processes involved in QTLs for tomato fruit size and composition, J. Exp. Bot., 60(1): 237-248.

https://doi.org/10.1093/jxb/ern281

Bianchetti R.E., Cruz A.B., Oliveira B.S., Demarco D., Purgatto E., Peres L.E.P., Rossi M., and Freschi L., 2017, Phytochromobilin deficiency impairs sugar metabolism through the regulation of cytokinin and auxin signaling in tomato fruits, Scientific Reports, 7(1): 7822.

https://doi.org/10.1038/s41598-017-08448-2

Bin C., Luz S.B., and Steven D.T., 2008, Regulatory change in YABBY-like transcription factor led to evolution of extreme fruit size during tomato domestication, Nature Genetics, 40(6): 800-804.

https://doi.org/10.1038/ng.144

Boffetta P., Couto E., Wichmann J., Ferrari P., Trichopoulos D., Bueno-de-Mesquita H.B., van Duijnhoven F.J.B., Buchner F.L., Key T., Boeing H., Nöthlings U., Linseisen J., Gonzalez C.A., Overvad K., Nielsen Michael R.S., Tjønneland A., Olsen A., Clavel-Chapelon F., Boutron-Ruault M.C., Morois S., Lagiou P., Naska A., Benetou V., Kaaks R., Rohrmann S., Panico S., Sieri S., Vineis P., and Palli D., 2010, Fruit and vegetable intake and overall cancer risk in the European Prospective Investigation into Cancer and Nutrition (EPIC), Journal of the National Cancer Institute, 102(8): 529-537.

https://doi.org/10.1093/jnci/djq072

Baxter C.J., Carrari F., Bauke A., Overy S., Hill S.A., Quick P.W., Fernie A.R., and Sweetlove L.J., 2005, Fruit carbohydrate metabolism in an introgression line of tomato with increased fruit soluble solids, Plant & Cell Physiology, 46(3): 425-437.

https://doi.org/10.1093/pcp/pci040

Chetelat R.T., Deverna J.W., and Bennett A.B., 1995a, Effects of the Lycopersicon chmielewskii sucrose accumulator gene (sucr) on fruit yield and quality parameters following introgression into tomato, Theor. Appl. Genet., 91(2): 334-339.

https://doi.org/10.1007/BF00220896

Chetelat R.T., Deverna J.W., and Bennett A.B., 1995b, Introgression into tomato (Lycopersicon esculentum) of the L.chmielewskii sucrose accumulator gene (sucr) controlling fruit sugar composition, Theor. Appl. Genet., 91(2): 327-333.

https://doi.org/10.1007/BF00220895

Denise M.T., Michelle Z., Eric A.S., Mark G. T., Peter B., Matias K., and Harry J.K., 2006, Identification of loci affecting flavour volatile emissions in tomato fruits, J. Exp. Bot., 57(4): 887-896.

https://doi.org/10.1093/jxb/erj074

Eshed Y., and Zamir D., 1995, An introgression line population of Lycopersicon pennellii in the cultivated tomato enables the identification and fine mapping of yield-associated QTL, Genetics, 141(3): 1147-1162.

https://doi.org/10.1093/genetics/141.3.1147

Fang J.G, Zhu X.D, Jia H.F, and Wang C., 2017, Research advances on physiological function of plant sucrose synthase, Nanjing Nongye Daxue Xuebao (Journal of Nanjing Agricultural University), 40(5): 759-768.

Frary A., Nesbitt T.C., Grandillo S., Knaap E., Cong B., Liu J., Meller J., Elber R., Alpert K.B., and Tanksley S.D., 2000, fw2.2: a quantitative trait locus key to the evolution of tomato fruit size, Science, 289(5476): 85-88.

https://doi.org/10.1126/science.289.5476.85

Fridman E., and Zamir D., 2003, Functional divergence of a syntenic invertase gene family in tomato, potato, and Arabidopsis, Plant Physiol., 131(2): 603-609.

https://doi.org/10.1104/pp.014431

Hackel A., Schauer N., Carrari F., Fernie A.R., Grimm B., and Kuhn C., 2006, Sucrose transporter LeSUT1 and LeSUT2 inhibition affects tomato fruit development in different ways, The Plant Journal, 45(2): 180-192.

https://doi.org/10.1111/j.1365-313X.2005.02572.x

Hou L.Y., Ehrlich M., Thormahlen I., Lehmann M., Krahnert I., Obata T., Cejudo F.J., Fernie A.R., and Geigenberger P., 2019, NTRC plays a crucial role in starch metabolism, redox balance, and tomato fruit growth, Plant Physiol., 181(3): 976-992.

https://doi.org/10.1104/pp.19.00911

Huang D.L., Li S.X., Liao Q., Qin C.X., Lin L., Fang F.X., and Li Y.R., 2012, Research progress of sucrose phosphate synthase in plants, Zhonguo Shengwu Gongcheng Zazhi (China Biotechnology), 32(6): 109-119.

Husain S.E., James C., Shields R., and Foyer C.H., 2001, Manipulation of fruit sugar composition but not content in Lycopersicon esculentum fruit by introgression of an acid, New Phytologist, 150(1): 65-62.

https://doi.org/10.1046/j.1469-8137.2001.00070.x

Levin I., Gilboa N., Yeselson E., Shen S., and Schaffer A.A., 2000, Fgr, a major locus that modulates the fructose to glucose ratio in mature tomato fruits, Theor. Appl. Genet., 100(2): 256-262.

https://doi.org/10.1007/s001220050034

Chakrabarti M., Zhang N., Sauvage C., Muños S., Blanca J., Cañizares J., Diez M.J., Schneider R., Mazourek M., McClead J., Causse M., and van der Knaap E., 2013, A cytochrome P450 regulates a domestication trait in cultivated tomato, Proc. Nat. Acad. Sci. USA, 110(42): 17125-17130.

https://doi.org/10.1073/pnas.1307313110

Marina P., Lena Y., Shmuel S., Ilan L., Arthur A.S., Ari E., and Moshe B., 2009, Carbohydrate balance and accumulation during development of near-isogenic tomato lines differing in the AGPase-L1 allele, American Society for Horticultural Science, 134(1): 134-140.

https://doi.org/10.21273/JASHS.134.1.134

Masa-aki O., Robert L.F., Robert B.G., Kenzo N., and John J.H., 2005, Control of seed mass by APETALA2, Proc. Nat. Acad. Sci. USA, 102(8): 3123-3128.

https://doi.org/10.1073/pnas.0409858102

Miron D., and Schaffer A.A., 1991, Sucrose phosphate synthase, sucrose synthase, and invertase activities in developing fruit of Lycopersicon esculentum Mill, and the sucrose accumulating Lycopersicon hirsutum Humb, and Bonpl, Plant Physiol. 95(2): 623-627.

https://doi.org/10.1104/pp.95.2.623

Mu Q., Huang Z.J., Chakrabarti M., Illa-Berenguer E., Liu X.X., Wang Y.P., Ramos A., and van der K.E., 2017, Fruit weight is controlled by Cell Size Regulator encoding a novel protein that is expressed in maturing tomato fruits, PLoS Genetics, 13(8): e1006930.

https://doi.org/10.1371/journal.pgen.1006930

Petreikov M., Shen S., Yeselson Y., Levin I., Bar M., and Schaffer A.A., 2006, Temporally extended gene expression of the ADP-Glc pyrophosphorylase large subunit (AgpL1) leads to increased enzyme activity in developing tomato fruit, Planta, 224(6): 1465-1479.

https://doi.org/10.1007/s00425-006-0316-y

Roger T.C., Ellen K., Joseph W.D., Serge Y., and Alan B.B., 1993, Inheritance and genetic mapping of fruit sucrose accumulation in Lycopersicon chmielewskii, The Plant Journal, 4(4): 643-650.

https://doi.org/10.1046/j.1365-313X.1993.04040643.x

Sagor G.H.M., Berberich T., Tanaka S., Nishiyama M., Kanayama Y., Kojima S., Muramoto K., and Kusano T., 2016, A novel strategy to produce sweeter tomato fruits with high sugar contents by fruit-specific expression of a single bZIP transcription factor gene, Plant Biotechnology Journal, 14(4): 1116-1126.

https://doi.org/10.1111/pbi.12480

Serge Y., Roger T.C., Martin D., Joseph W.D., and Alan B.B., 1991, Sink metabolism in tomato fruit : IV, genetic and biochemical analysis of sucrose accumulation, Plant Physiol., 95(4): 1026-1035.

https://doi.org/10.1104/pp.95.4.1026

Serge Y., John D.H., Nina L.R., Susan D., and Alan B.B., 1988, Sink metabolism in tomato fruit: III, analysis of carbohydrate assimilation in a wild species, Plant Physiol., 87(3): 737-740.

https://doi.org/10.1104/pp.87.3.737

Stommel J.R., 1992, Enzymic Components of Sucrose Accumulation in the Wild Tomato Species Lycopersicon peruvianum, Plant Physiol., 99(1): 324-328.

https://doi.org/10.1104/pp.99.1.324

Tanksley S.D., 2004, The genetic, developmental, and molecular bases of fruit size and shape variation in tomato, The Plant Cell, 16: S181-S189.

https://doi.org/10.1105/tpc.018119

van der Knaap E., Chakrabarti M., Chu Y.H., Clevenger J.P., Illa-Berenguer E., Huang Z.J., Keyhaninejad N., Mu Q., Sun L., Wang Y.P., and Wu S., 2014, What lies beyond the eye: the molecular mechanisms regulating tomato fruit weight and shape, Frontiers in Plant Science, 5(5): 227.

https://doi.org/10.3389/fpls.2014.00227

Yin Y.G., Yoshie K., Atsuko S., Satoru K., Naoya F., Hiroshi E., Sumiko S., and Chiaki M., 2010, Salinity induces carbohydrate accumulation and sugar-regulated starch biosynthetic genes in tomato (Solanum lycopersicum L. cv. 'Micro-Tom') fruits in an ABA-and osmotic stress-independent manner, J. Exp. Bot., 61(2): 563-574.

https://doi.org/10.1093/jxb/erp333

Yuan Y.J, Mei L.H., Wu M.B., Wei W., Shan W., Gong Z.B., Zhang Q., Yang F.Q., Yan F., Zhang Q., Luo Y.Q., Xu X., Zhang W.F., Miao M.J., Lu W.J., Li Z.G., and Deng W., 2018, SlARF10, an auxin response factor, is involved in chlorophyll and sugar accumulation during tomato fruit development, J. Exp. Bot., 69(22): 5507-5518.

https://doi.org/10.1093/jxb/ery328

Zhang X.M., Wang W., Du L.Q., Xie J.H., Yao Y.L., and Sun G.M., 2012, Expression patterns, activities and carbohydrate-metabolizing regulation of sucrose phosphate synthase, sucrose synthase and neutral invertase in pineapple fruit during development and ripening, International Journal of Molecular Sciences, 13(8): 9460-9477.

https://doi.org/10.3390/ijms13089460

Zhang Y.P., Qiao Y.X., Yu J.Q., and Zhao Z.Z., 2008, Progress of researches of sugar accumulation mechanism of horticultural plant fruits, Zhongguo Nongye Kexue (Scientia Agricultura Sinica), 41(4): 1151-1157.

Zhao J.T., 2016, Advances in research on invertase in plant development and response to abiotic and biotic stresses, Redai Yarendai Zhiwu Xuebao (Journal of Tropical and Subtropical Botany), 24(3): 352-358.