Light versus Dark Carbon Metabolism in Cherry Tomato Fruits

Abstract

Previous studies (6, 7) have shown that high PEP' carboxylase activities are found in green tomato fruit tissues; this finding was subsequently confirmed (12). More recently, we demonstrated that the incubation of tomato tissues with "4C-bicarbonate in the light results in the incorporation of radioactivity into malate and RPPC compounds (8). The external pericarp of thE fruit, the flesh, contains a relatively high RuDP carboxylase activity and exhibits a large incorporation of "4CO2 into RPPC compounds. There is also a pronounced labeling of malate, as a result of the very high PEP carboxylase activity of the flesh. In the internal pulp, however, "4C incorporation into RPPC compounds is very low, consistent with the very low RuDP carboxylase activity demonstrated in this tissue (8).In the present report we attempt to correlate malate metabolism and photosynthetic activities in tomato tissues. Precursors differentially labeled with 14C were vacuum-infiltrated into tissues in darkness and the appearance of radioactivity in other metabolic intermediates in the light, especially RPPC compounds, sugars, and starch, was followed. A The results show that only a small rapidly turning over malate pool seems very active. It can originate from the fixation of respiratory CO2 and can be the precursor of RPPC compounds. MATERIALS AND METHODSFruits of the cherry tomato (Lycopersicum esculentum var. cerasiforme Dun A. Gray) were picked at stage a (8). The experiments reported here were performed during the summer when fruits exhibit high photosynthetic activities (8).The extraction and determination of malate in tissues were performed as previously described (3,8).Vacuum-infiltration of Labeled Precursors. Fruit tissues were mechanically separated into the firm outer flesh and the jellylike internal pulp. Four hundred mg of flesh were cut into 0.5-mm cubes and 800 mg of pulp were gently dispersed. '4C-4-or '4C-3-malate or '4C-2-pyruvate, 100 to 200 uCi of each (specific radioactivity 2-10 mCi/mmol), were infiltrated into tissues in darkness with two successive 1-min periods of vacuum. The infiltrated tissues were then rinsed, drained, and finally placed in Petri dishes whose center wells contained a small amount of 40% KOH to trap any CO2 liberated. The incubation vessels were then sealed with adhesive tape and were placed either in white light (40 w/m2) or in darkness at 20 C. The "4CO2 liberated and subsequently trapped by the base was determined by counting small aliquots of KOH in a scintillation counter. The extraction and analysis of labeled metabolic intermediates were performed as previously described (8).

Overexpression of the phosphoenolpyruvate carboxykinase gene (<i>SlPEPCK</i>) promotes soluble sugar accumulation in fruit and post-germination growth of tomato (<i>Solanum lycopersicum</i> L.)

“…These results indicate that the modified PEPCK level directly affects the sugar and organic acid contents, likely through gluconeogenesis. While it has been reported that the predominant metabolic flow in the flesh of tomato and grape is glycolysis (Carrari et al 2006;Famiani et al 2014), early labelling studies demonstrated that gluconeogenesis occurs during tomato fruit ripening (Farineau and Laval-Martin 1977;Halinska and Frenkel 1991). Our present results indicate that gluconeogenesis is involved in sugar accumulation in fruit, and PEPCK plays a regulatory role in this process.…”

Salinity Stress Affects Assimilate Metabolism at the Gene-expression Level during Fruit Development and Improves Fruit Quality in Tomato (Solanum lycopersicum L.)

“…Bahrami et al (2001) isolated the gene from tomato and showed that its expression increases during ripening. Although previous labeling studies have demonstrated that gluconeogenesis occurs in ripening tomato fruit (Farineau and Laval-Martin, 1977;Halinska and Frenkel, 1991), and cold-inducible PEPCK was reported in rapeseed (Sáez-Vásquez et al, 1995), its regulation in response to abiotic stress is not fully understood in plants. Our data suggest that salinity stress enhances gluconeogenesis, as well as metabolic flow in ripe fruit, resulting in high-Brix fruit.…”

Alteration of the Interconversion of Pyruvate and Malate in the Plastid or Cytosol of Ripening Tomato Fruit Invokes Diverse Consequences on Sugar But Similar Effects on Cellular Organic Acid, Metabolism, and Transitory Starch Accumulation

“…Evidence from radiolabeling work in tomato fruit (Farineau, 1977;Halinska and Frenkel, 1991) suggested that gluconeogenesis does occur during ripening stages, when sugars are accumulating rapidly. Here, we observed that when cytosolic PEPCK was reduced (PEPCK lines), this led to reduced levels of the major sugars, Glc and Fru, during fruit ripening, accompanied by an accumulation of malate, which provides further evidence for gluconeogenesis from organic acids (Halinska and Frenkel, 1991) and the important role of PEPCK enzymes in this process.…”

“…This has led to the suggestion that PEPCK is involved in this dissimilation and that part of this dissimilated malate may be used in gluconeogenesis (Leegood and Walker, 2003;Famiani et al, 2005Famiani et al, , 2009. In contrast, NADP-ME isoforms function in chloroplasts and the cytosol (Drincovich et al, 2001;Gerrard Wheeler et al, 2005) and catalyze the reversible conversion of malate and pyruvate (Farineau, 1977;Drincovich et al, 2001). In tomato, NADP-ME has been implicated in respiration during ripening, providing pyruvate and/or NADPH as a substrate for respiration (Farineau, 1977;Edwards and Andreo, 1992).…”

“…In contrast, NADP-ME isoforms function in chloroplasts and the cytosol (Drincovich et al, 2001;Gerrard Wheeler et al, 2005) and catalyze the reversible conversion of malate and pyruvate (Farineau, 1977;Drincovich et al, 2001). In tomato, NADP-ME has been implicated in respiration during ripening, providing pyruvate and/or NADPH as a substrate for respiration (Farineau, 1977;Edwards and Andreo, 1992).…”