Considerable research effort has been dedicated to improving tomato fruit solids content because of the influence of solids on fruit quality (Stevens 1986). Sugars comprise 55 to 65% of the total soluble solids fraction and approximately 50% of the total solids in tomatoes. Considerable variability for soluble solids concentration is present within the cultivated tomato and its wild relatives. Soluble solids concentration of commercial hybrid cultivars generally ranges from 4.5 to 6.0% and can approach 15% in fruit of wild tomato species (Hewitt and Garvey 1987). Transient starch accumulation which occurs prior to fruit maturation contributes to sink strength and solids accumulation in developing fruit by maintaining a concentration gradient for sucrose between the leaf and the fruit (Dinar and Stevens 1981). Schaffer etal. (2000) demonstrated a relationship between increased levels of starch in immature S. habrochaites fruit and ADPglu-cose pyrophosphorylase activity. Increased starch levels in fruit of plants descended from S. lycopersicum x S. habrochaites crosses were attributed to a S. habrochaites derived introgression coding for the large subunit of ADPglucose pyrophosphorylase (AGPaseL1).
Fruit of the cultivated tomato and those of red-fruited wild tomato species accumulate the reducing sugars glucose and fructose as the principal storage sugars during fruit development. Little or no sucrose is detectable in the mature fruit. In contrast, fruit of the green-fruited wild tomato species accumulate significant quantities of sucrose in addition to reducing sugars (Davies 1966b). Biochemical factors associated with sucrose accumulation in S. chmielewskii (Yelle et al. 1991), S. habrochaites (Miron and Schaffer 1991), and S. peruvianum (Stommel 1992) have been described. Inheritance studies demonstrated that sucrose accumulation is controlled by a single recessive gene, sucr, in the green-fruited species S. habrochaites (Stommel and Haynes 1993) and S. chmielewskii (Chetelat et al. 1993). This locus maps to the pericen-tromeric region of chromosome 3. Transgenic tomato plants expressing a constitutive antisense invertase transgene had increased sucrose and decreased hex-ose storage concentrations and reduced levels of acid invertase in ripe fruit (Klann et al. 1996). Accumulated evidence indicates that sucr encodes an inactive invertase allele.
In typical ripe fruit of S. lycopersicum, slightly higher amounts of fructose than glucose result in glu-cose:fructose (G:F) ratios of 0.8 to 1.0 (Davies 1966b). In hexose-accumulating fruit derived from interspecific crosses with S. habrochaites, glucose concentrations are commonly low relative to those of fructose and result in much lower G:F ratios than those typically noted in S. lycopersicum. Observed segregation in S. lycopersicum x S. habrochaites populations indicated that G:F ratios were controlled by at least two genes (Stommel and Haynes 1993). More recent investigations demonstrated that a major locus (FGR) located on chromosome 4 influences G:F ratios in tomato fruit and that an additional genetic factor may be involved in determining the ratio of hexose sugars (Schaffer et al. 1999; Levin et al. 2000). FGR increases levels of fructose, relative to glucose, and exhibits an allelic dosage effect. An additional locus (FK2)located on chromosome 6 is epistatic to FGR and may decrease G:F (Levin et al. 2000). FK2 is subject to marked genotype x environment interaction.
Numerous studies have identified QTLs intro-gressed into tomato from wild species that influenced fruit soluble solids content. Many of these QTLs had a positive impact on solids content, but negatively influenced fruit yield (Eshed and Zamir 1994b; Tanksley et al. 1996; Chen et al. 1999; Yates et al. 2004). Additional studies identified chromosomal segments from S. chmielewskii and S. galapagense that had a positive influence on fruit soluble solids while maintaining acceptable fruit size, pH, and yield (Triano and St. Clair 1995; Yousef and Juvik 2001). Collectively, these and other QTL studies underscore the contribution that multiple, non-allelic loci have on controlling soluble solids in tomato. Recent efforts to characterize a QTL that increases the hexose sugar component of soluble solids revealed a likely regulatory level for an apoplas-tic invertase gene (LIN5) and highlight the importance of intragenic recombination in genetic variability in soluble solids and other quantitatively inherited traits (Fridman et al. 2000).
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