Vascular pathogens are the causal agents of main diseases threatening the health and growth of olive crops worldwide. The use of endophytic microorganisms represents a challenging and promising strategy for management of vascular diseases in olive. Although current research has been focused on analyzing the structure and diversity of the endophytic microbial communities inhabiting the olive xylem, the characterization of this ecological niche has been overlooked and to date remain unexplored, despite that the characterization of the xylem sap composition is essential to unravel the nutritional requirements of xylem-limited microorganisms. In this study, branches from plantlets and adult olive trees of cultivars ‘Picual’ and ‘Arbequina' were selected to characterize the chemical composition of olive xylem sap extracted using a Scholander pressure chamber. Metabolome and ionome analyses of xylem sap were performed by proton nuclear magnetic resonance (NMR) spectroscopy-based and by inductively coupled plasma with optical emission spectroscopy (ICP-OES), respectively. Olive xylem sap metabolites included a higher relative percentage of sugars (54.35%), followed by alcohols (28.85%), amino acids (8.01%), organic acids (7.68%) and osmolytes (1.12%). Within each of these groups, the main metabolites in the olive xylem sap were mannitol, ethanol, glutamine, acetate and trigonelline, whereas K and Cl- were the main element and inorganic anion, respectively. Metabolomic profile varied when comparing olive plant age and genotype. The levels of glucose, fructose, sucrose and mannitol, choline, B and PO43 were significantly higher in adult trees than in plantlets for both olive genotypes, whereas NO3- and Rb content showed the opposite behavior. On the other hand, levels of aspartate, phenylalanine and Na were significantly higher in ‘Picual’ than in ‘Arbequina’ whereas Fe showed the opposite behavior but only for adult trees. Non-supervised hierarchical clustering analysis separated xylem sap composition firstly according to the plant age and then by the olive cultivar. Supervised PLS-DA analysis revealed that B, ethanol, Fe, Fructose, glucose, mannitol, sucrose and Sr were the most significative compounds discriminating adult trees from plantlets, whereas asparagine, aspartate, glutamate and phenylalanine or aspartate, arginine, ethanol and Sr were the most contributory compounds in the discrimination of both olive genotypes for adult trees or plantlets, respectively. Knowledge of the chemical composition of xylem sap will lead to a better understanding of the complex nutritional requirements of olive xylem-inhabiting microorganisms, including its vascular pathogens, and would allow the design of artificial growing media to improve culturing the olive microbiome.
Pentas longiflora Oliv. (Rubiaceae), a plant used in the treatment of Pityriasis Versicolor in Rwanda: Chemical composition and standardization of leaves and roots