Invertases (P-fructosidase, EC 3.2.1.26) are a group of related enzymes that hydrolyze Suc to Glc and Fru. Acid invertase has a pH optimum for activity between 3 and 5, whereas neutra1 invertase has a pH optimum of about 7. Acid invertases are divided into vacuolar (or soluble) and extracellular (or insoluble) forms, with acid (Unger et al., 1992) and basic (Laurière et al., 1988) pls, respectively. Neutra1 (or alkaline) invertase is presumed to be localized in the cytosol (Ricardo, 1974). Plant genes for acid invertase have been cloned from tomato (Lycopersicon esculentum Mill.; Klann et al., 1992) and severa1 other plants, a11 sharing homology to the active site of yeast invertase (Unger et al., 1994). In plants acid invertase is thought to be involved primarily in SUC metabolism for energy in growing tissues (ap Rees, 1974), and increasing extracellular invertase activity results in severely stunted growth and inhibition of photosynthesis (von Schaewen et al., 1990).In tomato acid invertase activity is present during fruit development, and hexose sugars stored in the vacuole are
A wild tomato species, Lycopersicon chmielewskii, accumulates high levels of soluble sugar in mature fruit and, unlike the domesticated tomato species, Lycopersicon esculentum, accumulates sucrose rather than glucose and fructose. Cenetic and biochemical analyses of progeny resulting from a cross of L. chmielewskii with L. esculentum have previously indicated that the trait of sucrose accumulation is controlled by a single recessive gene and is associated with low levels of acid invertase protein in the developing fruit. Analysis of progeny from the BC& generation from the L. esculentum X L. chmielewskii cross revealed that sucrose-accumulating fruit accumulate sugar in two phases corresponding to fruit expansion and fruit maturation and that the majority of the sucrose was stored in the latter phase after the fruit had reached maximum size. l h e only significant enzymic difference between the sucrose-accumulating and hexose-accumulating fruit was the lack of acid invertase activity in sucrose-accumulating fruit. Sucrose phosphate synthase activity did not increase in the sucroseaccumulating fruit during late development when the rate of sucrose accumulation increased. l h e lack of acid invertase activity in sucrose-accumulating fruit was correlated with inheritance of the L. chmielewskii acid invertase gene and the absence of acid invertase mRNA in developing fruit. This suggests that the L. chmielewskii invertase gene is transcriptionally silent in fruit and that this is the basis for sucrose accumulation in progeny derived from the interspecific cross of L. esculeotum and 1. chmielewskii.Carbohydrate content and composition are important determinants of tomato fruit quality both in terms of flavor, which is determined largely by the concentration of sugars and acids (Stevens et al., 1977), and as a major component of soluble solids, which contribute to tomato processing quality (Davies and Hobson, 1981). The tomato is a particularly attractive species in which to study metabolism related to soluble carbohydrate accumulation because of the natural genetic variation and well-developed genetic and physiological information in Lycopersicon esculentum and related species.We have previously studied a wild tomato relative, Lycopersicon chmielewskii, from the subgenus Eriopersicon, that produces small green fruit. L. chmielewskii has approximately twice the soluble sugar concentration in mature fruit compared with that of the domestic tomato, L. esculentum (Rick, 1974; Yelle et al., 1988). In contrast to L. esculentum, L.chmielewskii, as well as Lycopersicon peruvianum and Lycoper-
Fruit of the domestic tomato (Lycopersicon esculentum Mill.) accumulate soluble sugars primarily in the form of the hexoses, glucose and fructose. In contrast, the predominant sugar in fruit of the wild tomato relative, L. chmielewskii, is sucrose. In the present study, the inheritance and linkage relations of sucrose accumulation were examined in interspecific L. esculentum x L. chmielewskii populations. In backcrosses to either the wild or domestic tomato, segregation for sucrose accumulation permitted qualitative analysis of the trait and indicated monogenic recessive control, although deviations from Mendelian inheritance were observed in some populations. This major gene, designated sucr, was mapped in F2, F3, and BC1F2 populations using a set of 95 informative RFLP and isozyme markers covering the tomato genome. A map location near the centromere of chromosome 3 was established, with tight linkage to the genomic clone TG102. Association of sucrose accumulation with yellow fruit, encoded by an allele of the r gene, permitted alignment with the classical map, thereby confirming the map location of sucr. A linkage map of the region surrounding sucr was obtained by monitoring recombination between flanking markers in the back‐crosses to tomato. A cDNA clone of tomato fruit acid invertase, TIV1, was mapped to TG102 and sucr, with no recombination between the two RFLP markers observed in over 1700 meiotic products. Despite the tight linkage, TG102 and TIV1 hybridize to distinct restriction fragments, hence do not represent the same gene. The genetic data strongly suggest that sucr is an allele of the invertase gene and thus support previous biochemical studies that demonstrated low invertase activity in sucrose‐accumulating fruit. L. hisutum, another low‐invertase, sucrose‐accumulating species, was hybridized with L. chmielewskii and the resulting F1 plants accumulated sucrose, indicating that genetic control of soluble sugar composition is conserved in these two species.
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