Abstract
BackgroundDates are tropical fruits with appreciable nutritional value. Previous attempts at global metabolic characterization of the date metabolome were constrained by small sample size and limited geographical sampling. In this study, two independent large cohorts of mature dates exhibiting substantial diversity in origin, varieties and fruit processing conditions were measured by metabolomics techniques in order to identify major determinants of the fruit metabolome.ResultsMultivariate analysis revealed a first principal component (PC1) significantly associated with the dates’ countries of production. The availability of a smaller dataset featuring immature dates from different development stages served to build a model of the ripening process in dates, which helped reveal a strong ripening signature in PC1. Analysis revealed enrichment in the dry type of dates amongst fruits with early ripening profiles at one end of PC1 as oppose to an overrepresentation of the soft type of dates with late ripening profiles at the other end of PC1. Dry dates are typical to the North African region whilst soft dates are more popular in the Gulf region, which partly explains the observed association between PC1 and geography. Analysis of the loading values, expressing metabolite correlation levels with PC1, revealed enrichment patterns of a comprehensive range of metabolite classes along PC1. Three distinct metabolic phases corresponding to known stages of date ripening were observed: An early phase enriched in regulatory hormones, amines and polyamines, energy production, tannins, sucrose and anti-oxidant activity, a second phase with on-going phenylpropanoid secondary metabolism, gene expression and phospholipid metabolism and a late phase with marked sugar dehydration activity and degradation reactions leading to increased volatile synthesis.ConclusionsThese data indicate the importance of date ripening as a main driver of variation in the date metabolome responsible for their diverse nutritional and economical values. The biochemistry of the ripening process in dates is consistent with other fruits but natural dryness may prevent degenerative senescence in dates following ripening. Based on the finding that mature dates present varying extents of ripening, our survey of the date metabolome essentially revealed snapshots of interchanging metabolic states during ripening empowering an in-depth characterization of underlying biology.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-015-0672-5) contains supplementary material, which is available to authorized users.
Highlights
Dates are tropical fruits with appreciable nutritional value
Unlike dates from the second sample collection, date fruits from the first sample collection were measured by both MetaSysX and Metabolon, which led to two metabolomics datasets DS1-sysX and DS1-bolon, respectively
A parallel analysis based on calculating the average Euclidean distances ‘AVED’ between all metabolite measurements in a given sample ‘s’ and their corresponding counterparts in every other sample in the dataset revealed that the AVED between s and its biological duplicate has often the lowest value with both datasets (Additional file 1: Figure S3). This implies that even though the MetaSysX measurement was slightly noisier than the Metabolon measurement, as revealed by the relative standard deviation (RSD) values from above, with both platforms variation between the date samples was still higher than the intrinsic variation between individual fruits from the same sample
Summary
Dates are tropical fruits with appreciable nutritional value. Previous attempts at global metabolic characterization of the date metabolome were constrained by small sample size and limited geographical sampling. The development of the date fruit occurs in four stages known by their Arabic names as Kimri, Khalal, Rutab and Tamr [1, 5]. In the Kimri stage, the date fruit has a hard green texture and shows a rapid gain in size and moisture as well as elevated levels of acidic substances and astringent tannins [4]. Dates show the highest protein and free amino acid content at the Kimri green stage, which continues to decrease throughout the ripening process [6, 7]. Softening of the fruit begins at this stage and reaches its optimum level at the advanced Rutab stage The latter is characterized by increased aroma [10] and fruit browning [4]. The drying process can cause a reduction in the level of certain metabolites such as anthocyanins [11] and vitamin C [1] whilst promoting others including reducing sugars [1], unsaturated fatty acids [12] and Maillard substances [13]
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