Watermelon metabolomics

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 RKNsusceptible 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.

Originally from Africa, watermelon is a staple crop in South Carolina and rich source of important phytochemicals that promote human health. As a result of many years of domestication and selection for desired fruit quality, modern watermelon cultivars are susceptible to biotic and abiotic stress. The present review discusses how genetic selection and breeding combined with geospatial technologies (precision agriculture) may help enhance watermelon varieties for resistance to biotic and abiotic stress. Gene loci identified and selected in undomesticated watermelon accessions are responsible for resistance to diseases, pests and abiotic stress. Vegetable breeding programs use traditional breeding methodologies and genomic tools to introduce gene loci conferring biotic or abiotic resistance into the genome background of elite watermelon cultivars. This continuous approach of collecting, evaluating and identifying useful genetic material is valuable for enhancing genetic diversity and tolerance and combined with precision agriculture could increase food security in the Southeast.

Nuclear magnetic resonance (NMR) was used to study the metabolic profiles of different watermelon lines and to highlight metabolic variation between several Citrullus lanatus var. citroides (CLC) accessions and Charleston Gray. Three days old plant roots were collected and analyzed using one-dimensional (1D) (1) H NMR spectroscopy. AMIX and MatLab software were used to highlight the significant changing metabolites between the PI 189225 and Charleston Gray. The significantly changing metabolites present in higher concentration in PI 189225 were L-Asparagine, L-Valine, L-Glutamine, O-phosphocholine, L-Isoleucine, L-Arginine, L-Glutamate, Ethanolamine and L-Choline. In Charleston Gray, two metabolites were present in significantly higher quantities (Malate and Glucose) when compared to PI 189225. Not all significant changing metabolites could be identified using Chenomx database, thus they were labeled as “unknowns”. Further studies will be performed to identify the “unknown” metabolites. In summary, this study is the first (to our knowledge) to describe the methodology of metabolite extraction from watermelon roots. It uncovers the full roots’ metabolomic profile of the cultivated watermelon and of the wild watermelon, while also highlighting the metabolic differences between the two.