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The evolution of citrate metabolism in acidic and acidless citrus genotypes during fruit development and ripening

Licciardello, C., Las Casas, G., Caruso, M., Caruso, P., Russo, M. P., Pietro Paolo, D., Russo, G., Reforgiato Recupero, G.
Acta horticulturae 2016 no.1135 pp. 53-60
ATP citrate synthase, Citrus, acetyl coenzyme A, aconitate hydratase, adenosinetriphosphatase, citrate (si)-synthase, citrates, citric acid, correlation, flowering, fruiting, gamma-aminobutyric acid, gene expression, genes, genotype, glutamate decarboxylase, metabolism, mitochondria, pH, protons, ripening, titratable acidity, tricarboxylic acid cycle
In the present work, citric acid accumulation and degradation was monitored during fruit development and ripening in two acidless varieties, Tarocco Ferreri (TF) and Vaniglia biondo (VB), in comparison with three acidic ones, namely Lanelate (L), Tarocco Meli (TM) and Mandalate [Fortune mandarin (2x) × Avana mandarin (4x)] (M), a tryploid hydrid. We evaluated titratable acidity (TA), pH and expression of genes involved in the critical steps of Krebs cycle, GABA shunt, acetyl-CoA metabolism and 'acid trap' from about 100 days post anthesis (DPA) until ripening. Based on the correlation between TA and pH, TF could not be considered a true ''acidless' as VB: even if the acidity levels of TF and VB at maturity reached lower values compared to acidic varieties (0.1 versus 1.6), the relation between TA and pH was negatively correlated in acidic varieties and TF, but weakly positively correlated in VB. The maximum of acidity in VB was identified at 160 DPA (0.28), while surprisingly TF anticipated the peak of acidity at 100 DPA, showing a higher value (2.35). Gene expression data showed marked differences between acidic and acidless varieties, as well as within acidic and within acidless ones. The main differences between acidic and acidless varieties were related to genes involved in Krebs cycle, as citrate synthase (CS), mitochondrial isoforms of aconitase (ACO) and NADP-isocitrate dehydrogenase (NADP-IDH), higher expressed in acidic varieties. We also observed that differences in gene expression within the acidic varieties are likely associated with differences in their citrate content. Specifically, CS, NADP-IDH and glutamate decarboxylase (GAD) are higher expressed in M, while ACO and ATP citrate lyase (ATP-CL) in TM, all compared to L. Differences between TF and VB were found in the expression of NADP-IDH, GAD, ATP-CL and H+ transporting ATPase isoform I, supporting the hypothesis of a different genetic basis of the acidless character in the two genotypes.