Metabolic Profiling of Pitaya (Hylocereus polyrhizus) during Fruit Development and Maturation

Abstract

Pitaya (Hylocereus polyrhizus) has attracted much interest from consumers as it is a novelty fruit with high nutrient content and a tolerance to drought stress. As a group of attractive pigment- and health-promoting natural compounds, betalains represent a visual feature for pitaya fruit quality. However, little information on the correlation between betalains and relevant metabolites exists so far. Currently, color (Commission International del'Eclairage, CIE) parameters, betalain contents, and untargeted metabolic profiling (gas chromatography-time-of-flight-mass spectrometry, GC⁻MS and liquid chromatography tandem mass spectrometry, LC⁻MS) have been examined on 'Zihonglong' fruits at nine different developmental stages, and the variation character of the metabolite contents was simultaneously investigated between peel and pulp. Furthermore, principal component analysis (PCA) and partial least-squares discriminant analysis (PLS-DA) were used to explore metabolite profiles from the fruit samples. Our results demonstrated that the decrease of amino acid, accompanied by the increase of sugars and organic acid, might contribute to the formation of betalains. Notably, as one of four potential biomarker metabolites, citramalic acid might be related to betalain formation.

Keywords: betalains; biomarker metabolites; fruit ripening; metabolic profiling; pitaya (Hylocereus polyrhizus; ‘Zihonglong’).

Figure 1

Betalain biosynthetic pathway (drawn according to [4]); TYR, tyrosinase; CYP76AD1, cytochrome P450; DODA, DOPA 4, 5-dioxygenase; BvMYB1/Y; *S, spontaneous steps; GT, glycosyl transferase; AT, acyltransferase; DOPA, 5, 6-dihydroxy-phenylalanine.

Figure 2

Experimental design of pitaya ‘Zihonglong’ (Hylocereus polyrhizus) harvested at nine stages of development.

Figure 3

Betacyanin and betaxanthin contents during different fruit-developmental stages.

Figure 4

Principal component analysis (PCA) score plots and metabolite hot map derived from nontargeted metabolome profiling. (A) Metabolite PCA from the GC–MS of fruit peel and pulp; metabolite PCA from the LC−MS of fruit (B) peel and (C) pulp (bright-red dots, 10 DPA; blue dots, 16 DPA; red-brown dots, 19 DPA; gray dots, 21 DPA; green dots, 23 DPA; pink dots, 25 DPA; dark-red dots, 26 DPA; bright-blue dots, 27 DPA; dark-blue dots, 29 DPA); (D) metabolite heat map from GC–MS and LC–MS/MS.

Figure 5

Developmental concentration of amino acids (A) and secondary metabolites (B) in pitaya fruit.

Figure 6

Developmental concentration of major sugars and organic acid in pitaya fruit. Note: The unit of sugar and acid content in the figure is mg/g DW.