FaGAPC2/FaPKc2.2 and FaPEPCK reveal differential citric acid metabolism regulation in late development of strawberry fruit

Abstract

Citric acid is the primary organic acid that affects the taste of strawberry fruit. Glycolysis supplies key substrates for the tricarboxylic acid cycle (TCA cycle). However, little is known about the regulatory mechanisms of glycolytic genes on citric acid metabolism in strawberry fruits. In this study, the citric acid content of strawberry fruit displayed a trend of rising and decreasing from the initial red stage to the full red stage and then dark red stage. Thus, a difference in citric acid metabolic regulation was suspected during strawberry fruit development. In addition, overexpression of either cytoplasm glyceraldehyde-3-phosphate dehydrogenase (FxaC_14g13400, namely FaGAPC2) or pyruvate kinase (FxaC_15g00080, namely FaPKc2.2) inhibited strawberry fruit ripening and the accumulation of citric acid, leading to a range of maturity stages from partial red to full red stage. The combined transcriptome and metabolome analysis revealed that overexpression of FaGAPC2 and FaPKc2.2 significantly suppressed the expression of phosphoenolpyruvate carboxykinase (FxaC_1g21491, namely FaPEPCK) but enhanced the content of glutamine and aspartic acid. Meanwhile, the activities of PEPCK and glutamate decarboxylase (GAD) were inhibited, but the activities of glutamine synthase (GS) were increased in FaGAPC2/FaPKc2.2-overexpressed fruit. Further, functional verification demonstrated that overexpression of FaPEPCK can promote strawberry fruit ripening, resulting in a range of maturity stage from full red to dark red stage, while the citric acid synthase (CS) activities and citric acid content were significantly decreased. Overall, this study revealed that FaGAPC2/FaPKc2.2 and FaPEPCK perform an important role in reducing citric acid content in strawberry fruit, and FaGAPC2/FaPKc2.2 mainly by promoting the GS degradation pathway and FaPEPCK mainly by inhibiting the CS synthesis pathway.

Exploration of a European-centered strawberry diversity panel provides markers and candidate genes for the control of fruit quality traits

“…TCA is the central metabolic cycle that uses substrates from the glycolysis to produce energy. It fulfills major roles in the fruit, among which the metabolism of citric acid 53 . While aconitase has been shown to contribute to the regulation of acidity in the citrate-accumulating lemon 54 , we did not detect any TA QTL corresponding to the 6C Brix QTL.…”

“…The CG underlying the 1A TA QTL encodes pyruvate kinase (PK) a crucial enzyme for gluconeogenesis which has already been demonstrated to regulate citric acid metabolism during strawberry fruit ripening 53 . Two additional CG for the TA QTL located on 6A (PVE 35.3%) and 6C (PVE 6.8%) encode subunits of the V-type proton ATPase ( VMA-G and VMA-C ), respectively.…”

Exploration of a European-centered strawberry diversity panel provides markers and candidate genes for the control of fruit quality traits

“…TCA is the central metabolic cycle that uses substrates from the glycolysis to produce energy. It fulfills major roles in the fruit, among which is the metabolism of citric acid [ 56 ]. While ACO has been shown to contribute to the regulation of acidity in the citrate-accumulating lemon [ 57 ], we did not detect any TA QTL corresponding to the 6C Brix QTL.…”

“…The CG underlying the 1A TA QTL encodes pyruvate kinase (PK), a crucial enzyme for gluconeogenesis, which has already been demonstrated to regulate citric acid metabolism during strawberry fruit ripening [ 56 ]. Two additional CG for the TA QTL located on 6A (PVE 35.3%) and 6C (PVE 6.8%) encode subunits of the V-type proton ATPase ( VMA-G and VMA-C ), respectively.…”