Sink Metabolism in Tomato Fruit

Abstract

Carbohydrate composition and key enzymes involved in carbohydrate metabolism were assayed throughout development of Lycopersicon esculentum and L. chmielewskll fruit. Translocation and assimilation of asymmetric sucrose and total soluble solids content was also determined in both species. The data showed that L. chmielewskii accumulated less starch than L. esculentum, and this was related to a lower level of ADPglucose pyrophosphorylase and a higher level of phosphorylase in L. chmielewskii. L. chmielewskii accumulated sucrose throughout fruit development rather than glucose and fructose which were accumulated by L. esculentum. A low level of invertase and nondetectable levels of sucrose synthase were associated with the high level of sucrose in L. chmielewskii. Translocation and assimilation of asymmetrically labeled sucrose indicated that sucrose accumulated in L. chmielewskii fruit was imported and stored directly in the fruit without intervening metabolism along the translocation path. In contrast, the relatively low level of radioactive sucrose found in L. esculentum fruit appeared to arise from hydrolysis and resynthesis of sucrose. The possible relationship between the level of soluble solids and differences in carbohydrate metabolism in sink tissue of the two species is discussed.The harvestable yield of tomato appears to be regulated by the net assimilation rate ofthe crop, the rate ofimport into individual fruit, and sink activity (9). High sink demand can significantly increase the quality of the tomato fruit by high accumulation of soluble solids, an important factor for processing tomatoes. Sugars are the major components of the soluble solids content in tomato comprising approximately 65% of the soluble solids.As reported by Gifford and Evans (6), the processes localized in sink tissue largely determine the distribution of photoassimilate between competing sinks. According to Walker and Ho (16), sink strength of a tomato fruit is principally affected by the sink activity of the fruit. The major mechanisms involved in sink activity are: (a) unloading ofsucrose by the phloem, (b) hydrolysis and uptake ofsugars, (c) biosynthesis and storage ofcarbohydrate (10). It has been suggested (16) that invertase activity may play a major role in regulating the rate of carbon translocation in tomato fruit.

Differences in regulation of carbohydrate metabolism during early fruit development between domesticated tomato and two wild relatives

“…We have initiated a comparative study of developmental changes in the initial steps in carbohydrate metabolism during early fruit development in the S. lycopersicum cv Money Maker and the wild accessions S. peruvianum LA0385 and S. habrochaites LA1777. DiVerences between accessions have been described before but concern only the activity of a limited number of enzymes, e.g., invertase and sucrose synthase (Yelle et al 1988;Miron and SchaVer 1991;Sun et al 1992;Stommel 1992;Balibrea et al 2003). We describe not only the enzymes involved in sucrose breakdown but the next steps in carbohydrate metabolism as well.…”

Divergent Fructokinase Genes Are Differentially Expressed in Tomato

“…In tomato (Lycopersicon esculentum) starch accumulates during the early stage of fruit development and then decreases during maturation (Yelle et al, 1988). SUC synthase has been proposed to be the primary enzyme in the metabolism of imported Suc during the early developmental stages, and localization of its mRNA suggests a relationship between SUC synthase and starch granule accumulation (Wang et al, 1993(Wang et al, , 1994.…”

SlARF4, an Auxin Response Factor Involved in the Control of Sugar Metabolism during Tomato Fruit Development

“…Starch biosynthesis is known to involve a series of enzyme-catalyzed processes (Smith, 1999;Liang et al, 2001;James et al, 2003) belonging to three separate enzyme families (Fig. 5), AGPase, STS, and SBE (Yelle et al, 1988;Schaffer and Petreikov, 1997). Building on the annotated tomato genome sequence, genome-wide in silico screening allowed for the identification of all members of the three enzyme families involved in starch synthesis in tomato.…”