Abstract
Citrullus amarus ( CA ) (previously known as Citrullus lanatus var. citroides ) accessions collected in southern Africa are known to have resistance to root-knot nematodes (RKN) and are suitable rootstocks for grafted watermelon. The objective of this study was to conduct a comparative metabolomics analysis and identify unique metabolites in roots of CA accessions versus roots of watermelon cultivars ( Citrullus lanatus (Thunb.) Matsum. and Nakai var. lanatus; CL ). Nuclear magnetic resonance (NMR) technology and principal component analysis (PCA) were used to analyze and compare metabolic profiles of seven CA accessions resistant to RKN along with two RKN-susceptible watermelon cultivars (Charleston Gray and Crimson Sweet). Calculation of the Mahalanobis distance revealed that the CA United States Plant Introduction (PI) 189225 (Line number 1832) and PI 482324 (1849) have the most distinct metabolic profiles compared with the watermelon cultivars Charleston Gray and Crimson Sweet, respectively. Several amino acids identified in the CA accessions were reported in previous studies to have a nematicidal effect. The results in this study indicate that roots of watermelon accessions collected in the wild are rich in metabolic compounds. These metabolic compounds may have been diminished in watermelon cultivars as a consequence of many years of cultivation and selection for desirable fruit qualities.
Highlights
The first step in this study was to compare the metabolic profiles of roots of 3 day old seedlings of Citrullus amarus (CA) Plant Introduction (PI) versus those of the watermelon cultivars Charleston Gray
The significant metabolite fold change analysis for the seven CA PIs versus Charleston Gray indicated that choline, malate, glucose, raffinose, and UNK 1 content were significantly higher in concentration in the CA PIs
When considering the many years of domestication and narrow genetic base shared by watermelon cultivars, possibly due to the “Founder effect,” major differences between the wild type watermelon and watermelon cultivars were expected
Summary
Seeds of seven homozygous resistant germplasm lines, each derived (through self-pollination and single seed descent through four-five generations) from a PI resistant to RKN (CA) and watermelon cultivars (CL; Crimson Sweet and Charleston Gray) susceptible to RKN (Thies and Levi, 2003, 2007) were grown in a greenhouse at the USDA, ARS, U.S Vegetable Laboratory in Charleston, SC. Roots of six plants with similar size (fresh and dry weight) were selected to represent each of the nine genotypes and each of these six root samples was used separately for metabolite extraction and analysis. The first increment of a presat-noesy experiment was acquired using the standard Bruker noesypr1d pulse sequence This resulted in a one-dimensional 1H spectrum (metabolic profile) for each sample (9 genotypes × 6 replicates). After the 1H NMR data was collected on each of the 54 individual samples in this study, the first step was to generate bucket tables using AMIX software (version 3.9.7, BrukerBioSpin). Principal component analysis (PCA) was performed on the bucket tables generated from AMIX using MetaboAnalyst3.0 (Xia and Wishart, 2016). Metabolic data interpretation was facilitated with the Metabolic Pathway Analysis (MetPA) component of MetaboAnalyst3.0 (Xia and Wishart, 2016)
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