Abstract
Organic acids are key components that determine the taste and flavor of fruits and play a vital role in maintaining fruit quality and nutritive value. In this study, the fruits of two cultivars of passion fruit Yellow (Passiflora edulis f. flavicarpa) and purple (Passiflora edulis f. edulis) were harvested at five different developmental stages (i.e., fruitlet, green, veraison, near-mature and mature stage) from an orchard located in subtropical region of Fujian Province, China. The contents of six organic acids were quantified using ultra-performance liquid chromatography (UPLC), activities of citric acid related enzymes were determined, and expression levels of genes involved in citric acid metabolism were measured by quantitative real-time PCR (qRT-PCR). The results revealed that citric acid was the predominant organic acid in both cultivars during fruit development. The highest citric acid contents were observed in both cultivars at green stage, which were reduced with fruit maturity. Correlation analysis showed that citrate synthase (CS), cytosolic aconitase (Cyt-ACO) and cytosolic isocitrate dehydrogenase (Cyt-IDH) may be involved in regulating citric acid biosynthesis. Meanwhile, the PeCS2, PeACO4, PeACO5 and PeIDH1 genes may play an important role in regulating the accumulation of citric acid. This study provides new insights for future elucidation of key mechanisms regulating organic acid biosynthesis in passion fruit.
Highlights
The passion fruit is native to tropical America and has more than 500 species of which at least 50 or more are edible [1]
Citrate can be degraded to α-ketoglutarate (α-KG) through the activities of aconitase (ACO) and NADP-dependent isocitrate dehydrogenase (NADPIDH) in the cytosol [16]. These metabolic pathways indicate that citrate synthase (CS) and phosphoenolpyruvate carboxylase (PEPC) mainly catalyze the synthesis of citric acid, while ACO and IDH promote the decomposition of citric acid [17]
The results indicated that citric acid was the most abundant organic acid followed by lactic acid
Summary
The passion fruit is native to tropical America and has more than 500 species of which at least 50 or more are edible [1]. The final organic acid concentration in ripened fruits depends on the balance of organic acid synthesis, membrane transport and degradation or utilization [13,14] In this process, acid metabolism-related enzymes including citrate synthase (CS), phosphoenolpyruvate carboxylase (PEPC), aconitase (ACO), and isocitrate dehydrogenase (IDH) may potentially play a role in fruit organic acid biosynthesis and degradation [15]. Citrate can be degraded to α-ketoglutarate (α-KG) through the activities of aconitase (ACO) and NADP-dependent isocitrate dehydrogenase (NADPIDH) in the cytosol [16] These metabolic pathways indicate that CS and PEPC mainly catalyze the synthesis of citric acid, while ACO and IDH promote the decomposition of citric acid [17]. Changes of organic acid contents in two cultivars of passion fruit at five developmental stages were studied, and the activity of crucial enzymes involved in organic acid metabolism and the expression of the related genes was analyzed. Our findings will provide useful information for improving the organoleptic quality of passion fruit via reducing the contents of citric acid
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