Tissue-Specific Accumulation of Sulfur Compounds and Saponins in Different Parts of Garlic Cloves from Purple and White Ecotypes

Abstract

This study set out to determine the distribution of sulfur compounds and saponin metabolites in different parts of garlic cloves. Three fractions from purple and white garlic ecotypes were obtained: the tunic (SS), internal (IS) and external (ES) parts of the clove. Liquid Chromatography coupled to High Resolution Mass spectrometry (LC-HRMS), together with bioinformatics including Principal Component Analysis (PCA), Hierarchical Clustering (HCL) and correlation network analyses were carried out. Results showed that the distribution of these metabolites in the different parts of garlic bulbs was different for the purple and the white ecotypes, with the main difference being a slightly higher number of sulfur compounds in purple garlic. The SS fraction in purple garlic had a higher content of sulfur metabolites, while the ES in white garlic was more enriched by these compounds. The correlation network indicated that diallyl disulfide was the most relevant metabolite with regards to sulfur compound metabolism in garlic. The total number of saponins was almost 40-fold higher in purple garlic than in the white variety, with ES having the highest content. Interestingly, five saponins including desgalactotigonin-rhamnose, proto-desgalactotigonin, proto-desgalactotigonin-rhamnose, voghieroside D1, sativoside B1-rhamnose and sativoside R1 were exclusive to the purple variety. Data obtained from saponin analyses revealed a very different network between white and purple garlic, thus suggesting a very robust and tight coregulation of saponin metabolism in garlic. Findings in this study point to the possibility of using tunics from purple garlic in the food and medical industries, since it contains many functional compounds which can be exploited as ingredients.

Keywords: LC-HRMS; correlation networks; purple and white garlic; saponins; sulfur compounds.

Figure 1

Heatmap visualization of sulfur compounds (A) and saponins (B) detected in three tissues (SS, ES and IS) of the white and purple ecotypes. Data represent, for each metabolite, the fold over the internal standard (IS) intensity.

Figure 2

Hierarchical clustering (HCL) visualization, applied to rows and columns, of sulfur compounds (A) and saponins (B) detected in three tissues (SS, ES and IS) of the white and purple ecotypes of garlic. Data represent, for each metabolite, the fold over the internal standard (IS) intensity.

Figure 3

Principal component analysis (PCA), according to the metabolites, of sulfur compounds (A) and saponins; (B) detected in three tissues (SS, ES and IS) of the white and purple ecotypes of garlic. Data represent, for each metabolite, the fold over the internal standard (IS) intensity.

Figure 4

Correlation networks of sulfur compounds (A) and saponins (B) detected in three tissues (SS, ES and IS) of the white and purple ecotypes of garlic. Pearson correlation (ρ) coefficients were calculated for each metabolite pair. Node colors identify sulfur compounds (light orange, left) and saponins (light blue), while node size was proportional to node strength (ns). Red and blue edges refer to, respectively, positive and negative correlations. Edge thickness was according to the corresponding |ρ|. Only ρ >|0.50| were shown.