Abstract
Citric acid metabolism is considered to be the central cellular process of metabolite conversions. ATP-citrate lyase (ACL) and cytosolic aconitase (cyt-ACO) are the two citrate-degrading enzymes that decide the carbon flux towards different metabolite biosynthesis pathways. However, the correlation of their activities with metabolite concentrations in citrus fruits is still unclear. Here, the concentrations of soluble sugars, organic acids, acetyl-CoA, flavonoids, carotenoids, and γ-aminobutyric acid, as well as the activities of ACL, cyt-ACO, acetyl-CoA C-acetyltransferase, and acetyl-CoA carboxylase, were compared among the fruits of six citrus cultivars during fruit development and ripening. The results showed that the correlation between citrate concentration and cyt-ACO or ACL activity varied greatly among cultivars, while the activities of cyt-ACO and ACL had a significantly negative correlation (r = −0.4431). Moreover, ACL overexpression and RNA interference in the Citrus callus indicated that increasing and decreasing the ACL activity could reduce and induce cyt-ACO activity, respectively. In addition, significant correlation was only observed between the ACL activity and the concentration of acetyl-CoA (r = 0.4333). Taken together, the present study suggested that ACL and cyt-ACO synergistically control the citrate fate for the biosynthesis of other metabolites, but they are not the key determinants for the accumulation of citrate, as well as other metabolites in citrus fruits.
Highlights
Citrus fruit is a part of the human essential diet and is popularly consumed for its delicious taste and health promotion [1]
This study compared the activities of citrate-degrading enzymes, as well as the concentrations of sugars, organic acids, flavonoids, carotenoids, and GABA, during fruit development of six citrus cultivars due to the significance of the cytosolic citrate utilization in metabolite conversions, and found that the balance of ATP-citrate lyase (ACL) and cytosolic aconitase (cyt-ACO) activities plays a key role in deciding the citrate fate for metabolite conversions, but are not the determinants for the accumulation of related metabolites
At the T4 stage, the sucrose concentration was more than 60 mg·g−1 (FW) in ‘Newhall’ navel orange, ‘Guoqing No
Summary
Citrus fruit is a part of the human essential diet and is popularly consumed for its delicious taste and health promotion [1]. The edible part of citrus fruit contains low protein and fat, but abundant carbohydrates (sucrose, glucose, and fructose), organic acids (citric acid and malic acid), and some secondary metabolites, such as bioactive compounds (vitamins, carotenoids, flavonoids, limonoids) and aroma volatiles [2]. The combination of these metabolites in the cell vacuole or other organelles determines the fruit flesh quality [3,4,5,6]. OAA and acetyl-CoA can reenter mitochondrion for citrate biosynthesis used for gluconeogenesis or amino[13]. Carboxylase (ACCase, EC 6.4.1.2) [13,15,16,17]
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