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Yosef Burger and Arthur A. Schaffer

The relationship between sugar accumulation and sucrose metabolism enzyme activities was studied among seven genotypes of Cucumis melo L., covering the broad genetic range of sucrose accumulation found in the species. The primary determinant correlated with sucrose levels was the genetic variation for developmental loss of soluble acid invertase (AI) activity. Sucrose accumulation in the developing fruit began only when AI activity declined to less than an experimentally determined threshold value, and continued until removal of the fruit from the plant. In addition, the activities of sucrose phosphate synthase (SPS), sucrose synthase (SuSy), and neutral invertase (NI) were all positively correlated with sucrose accumulation among the genotypes. The low-sucrose-accumulating genotypes were characterized by low activities of each of the three enzymes, irrespective of their invertase activities. Final sucrose content was best predicted for each genotype by the number of days the fruit remained attached to the plant while characterized by “sucrose accumulation metabolism,” which was characterized primarily by AI activity less than threshold values, together with SPS, SuSy, and NI activities higher than threshold levels.

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Marina Petreikov, Lena Yeselson, Shmuel Shen, Ilan Levin, Arthur A. Schaffer, Ari Efrati and Moshe Bar

Soluble sugar accumulation is a major determinant of tomato (Solanum lycopersicum) fruit quality. One strategy of increasing sugar levels in the mature fruit is via the increase of the transient starch pool in the immature fruit, which is subsequently degraded to contribute to its soluble sugar levels. ADP-glucose pyrophosphorylase [AGPase (E.C. 2.7.7.27)] is a limiting enzyme in starch synthesis and we therefore developed introgression lines of cultivated tomato harboring the wild species (Solanum habrochaites) allele for the regulatory large subunit (L1H) of this heterotetrameric enzyme. Comparison of numerous near-isogenic lines of tomato segregating for the L1 allele, during multiple seasons, showed that the wild species allele led to an increase in fruit total soluble solids concentration (TSS) without a concomitant decrease in fruit size. Rather, in practically all lines studied, fruit size increased together with TSS, leading to an even larger increase in TSS × yield. A comparative developmental study of fruit carbohydrates, starch, and sugars between genotypes showed that the wild species allele led to increases in fruit size, carbohydrate concentration, and carbohydrate content of the whole fruit unit. This was related to a large increase in the transient starch reservoir that, upon degradation, accounted for the subsequent increase in soluble sugars. These results are evidence that modifying fruit sink carbohydrate metabolism via a single rate-limiting enzymatic step can increase the net import of photoassimilate into the fruit.

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Yosef Burger, Uzi Saar, Nurit Katzir, Harry S. Paris, Yelena Yeselson, Ilan Levin and Arthur A. Schaffer

Fruit sweetness is the major determinant of fruit quality in melons (Cucumis melo L.) and reflects the concentration of the three major soluble sugars, sucrose, glucose, and fructose, present in the fruit flesh. Of these three sugars, sucrose is the prime factor accounting for both the genetic and the environmental variability observed in sugar content of C. melo fruit. Faqqous (subsp. melo var. flexuosus), a cultivar having a low sucrose and total sugar content, was crossed with Noy Yizre'el (subsp. melo var. reticulatus), a cultivar having a high sucrose and total sugar content. F1 plants had a sucrose content averaging slightly higher than that of the low-sucrose parent, indicating that low sucrose content is nearly completely dominant. Segregation in the F2 and backcross progenies indicated that high sucrose accumulation in melon fruit flesh is conferred by a single recessive gene herein designated suc. When the high-sucrose parent was crossed with the moderate-sucrose landrace known as Persia 202 (subsp. melo var. reticulatus), the segregation in the filial and backcross progenies suggested that additional genetic factors affect the amount of sucrose accumulation.

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Yosef Burger, Uzi Sa'ar, Asaph Distelfeld, Nurit Katzir, Yelena Yeselson, Shmuel Shen and Arthur A. Schaffer

The sweet cultivars of Cucumis melo are characterized by high sucrose levels, together with low acid levels in the mature fruit flesh. The trait of high sugar accumulation in C. melo fruit is determined by a single recessive gene, suc. High acid content, conferred by a single dominant gene, So, is found only in C. melo varieties that do not accumulate high levels of sugar and are used for nondessert purposes. We combined the genetic traits of high acid content (low pH) and high sugar levels by crossing the nonsweet, high acid C. melo var. flexuosus, `Faqqous' (So/So, Suc/Suc), with high sugar, low acid C. melo genotypes (so/so, suc/suc) and generating the recombinant genotype So/—, suc/suc. Segregating F2 populations derived from the cross between `Faqqous' and a standard high sugar, low acid line showed that the traits of high sugar and low pH were inherited independently of each other. The accumulation of acid and sugar in the developing fruit of a recombinant high acid, high sugar breeding line, A6, were also temporally independent, with acid accumulation preceding the rise in sucrose levels. The low pH of A6 was correlated with the developmental increase in titratable acidity and particularly of citric acid levels. The combination of increased acidity and high sugar provides the melons with a unique taste due to a sugar to acid ratio not present in sweet C. melo cultivars. These results are discussed in terms of the evolution under domestication of C. melo.

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Arthur A. Schaffer, Marina Petreikov, Daphne Miron, Miriam Fogelman, Moshe Spiegelman, Zecharia Bnei-Moshe, Shmuel Shen, David Granot, Rivka Hadas, Nir Dai, Moshe Bar, Michael Friedman, Meir Pilowsky, Nehama Gilboa and Leah Chen

The carbohydrate economy of developing tomato fruit is determined by wholeplant source–sink relationships. However, the fate of the imported photoassimilate partitioned to the fruit sink is controlled by the carbohydrate metabolism of the fruit tissue. Within the Lycopersicon spp. there exists a broad range of genetic variability for fruit carbohydrate metabolism, such as sucrose accumulation and modified ratios of fructose to glucose in the mature fruit and increased starch synthesis in the immature fruit. Metabolic pathways of carbohydrate metabolism in tomatoes, as well as natural genetic variation in the metabolic pathways, will be described. The impact of sink carbohydrate metabolism on fruit non-structural carbohydrate economy will be discussed.

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Arthur A. Schaffer, Daphne Miron, Marina Petreikov, Miriam Fogelman, Moshe Spiegelman, Zecharia Bnei-Moshe, Shmuel Shen, David Granot, Rivka Hadas, Nir Dai, Moshe Bar, Ilan Levin, Michael Friedman, Meir Pilowsky, Nehama Gilboa and Leah Chen