Metabolic Profiling of Organic Acids Reveals the Involvement of HuIPMS2 in Citramalic Acid Synthesis in Pitaya

Jiaxuan Chen,Fangfang Xie,Yonghua Qin,Rong Zhang,Zhike Zhang,Jianye Chen,Jietang Zhao,Yuanju Yuan,Guibing Hu

doi:10.3390/horticulturae8020167

Abstract

Pitayas are rich in organic acids, especially citramalic acid, which is significantly higher than the plants. However, the mechanism of citramalic acid biosynthesis remains to be fully elucidated. In this study, organic acid compositions and contents, as well as expression patterns of key genes related to organic acid metabolism were analyzed during fruit maturation of four different pitaya cultivars i.e., ‘Guanhuabai’ (GHB), ‘Guanhuahong’ (GHH), ‘Wucihuanglong’ (WCHL), and ‘Youcihuanglong’ (YCHL). The total organic acid contents increased first and then declined during fruit maturation. The main organic acids were citramalic acid during the early stages of GHB, GHH, and WCHL pitayas, and dominated by malic acid as fruit maturation. In comparison, citric acid and malic acid were main organic acid for ‘YCHL’ pitaya. Citramalate synthase (IPMS) was involved in the synthesis of citramalic acid, and three types of HuIPMS i.e., HuIPMS1, HuIPMS2, and HuIPMS3, were obtained in our study. Highest expression levels of HuIPMS1 were detected in sepals, while HuIPMS2 and HuIPMS3 exhibited preferential expression in tender stems and ovaries. The expression levels of HuIPMS2 and HuIPMS3 were positively correlated with the content of citramalic acid in the four pitaya cultivars. HuIPMS2 was a chloroplast-localized protein, while HuIPMS3 presented a cytoplasmic-like and nuclear subcellular localization. These findings provide an important basis for further understanding of the molecular mechanism that leads to citramalic acid metabolism during pitaya fruit maturation.

Highlights

Organic acids are important components of fruit flavor and nutritional quality which plays key roles in digestion and appetite
The present study provides insights of the key candidate genes involved in organic acid metabolism which can be used for fruit quality improvement in pitaya
Organic acid compositions and contents, as well as expression patterns of key genes related to organic acid metabolism were analyzed during fruit maturation of ‘Guanhuabai’ (GHB), ‘Guanhuahong’ (GHH), ‘Wucihuanglong’ (WCHL), and ‘Youcihuanglong’ (YCHL) pitayas

Summary

Introduction

Organic acids are important components of fruit flavor and nutritional quality which plays key roles in digestion and appetite. The types and ratios of organic acid components are responsible for fruit sourness and taste, and their differences contribute to unique flavors. Organic acids participate in the tricarboxylic acid cycle (TCA) and amino acid metabolism, and are further converted into sugars and esters [1]. The content of organic acids is usually higher in the early stages of fruit development, and is consumed as a respiratory substrate during fruit ripening [2]. Citric acid and malic acid are the main types of organic acids in most fruits [3,4]. The organic acid metabolism of fruits is relatively complex, involving a multi-enzyme system such as citrate synthase (CS, EC 4.1.3.7), aconitase (ACO, EC 4.2.1.3), NAD-isocitrate dehydrogenase (NAD-IDH, EC 1.1.1.41), NAD-malate

Methods

Discussion

Conclusion