Putative model of CsNADP-ME2 function in cucumber fruit. (IMAGE)

Nanjing Agricultural University The Academy of Science

Putative model of CsNADP-ME2 function in cucumber fruit.

Caption

Putative model of CsNADP-ME2 function in cucumber fruit. Based on gene expression, tissue-specific localization, activity assay of enzymes, and related physiological speculations, the role and active sites of NADP-ME (one of the decarboxylases), the TCA cycle and glycolysis in cucumber fruit are marked. Solid black arrows indicate metabolic flux. Dotted black arrows indicate that the process consists of multiple catalytic reactions. In CsNADP-ME2-OE plants (left) with upregulation of CsNADP-ME2 expression and enzyme activity (up arrow) when compared with WT plants, the content of substrate malate decreased (down arrows), then the increased transcript levels and maintained enzyme activities of PK (the primary regulation) and PFK (the secondary regulation) (up arrows) could result in a continuous bottom-up feedback-promotion of glycolysis. On the other hand, as the more intermediate products of the TCA cycle were likely used to produce energy by its own reaction, the availability of carbon skeleton for amino acid metabolism was suppressed. Compared with wild-type plants, in RNAi plants (right) characterized by the downregulation of CsNADP-ME2 expression and enzyme activity (down arrow), the substrate malate accumulated significantly (up arrows). Accumulated malate could feedback-inhibit the expressions and activities of PK and PFK (down arrows). In addition, downregulation of CsNADP-ME2 in the fruit of RNAi lines might slow down the rate of TCA cycle activity due to the accumulation of the substrate malate. Some intermediates in the TCA cycle can be more effectively used as a precursor for the production of other derivatives/amino acids, thus promoting nitrogen assimilation, such as amino acid synthesis, in the exocarp. F-1,6-BP, fructose-1,6-bisphosphate; F-6-P, fructose-6-phosphate; 2OG, 2-oxoglutarate; Mal, malate; OAA, oxaloacetate; PFK, ATP-dependent phosphofructokinase; PK, pyruvate kinase; PYR, pyruvic acid; Suc, sucrose.

Credit

Horticulture Research

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