Impact of phenanthrene on primary metabolite profiling in root exudates and maize mucilage

This study was conducted to assess the impact of polycyclic aromatic hydrocarbon on the composition of rhizodeposits. Maize was submitted to increasing phenanthrene (PHE) concentrations in the substrate (0, 25, 50, and 100 mg PHE.kg^-1 of dry sand). After 6 weeks of cultivation, two types of rhizodeposit solution were collected. The first one, called rhizospheric sand extract, resulted from the extraction of root adhering sand in order to collect mucilage and associated compounds. The second one, the diffusate solution, was collected by the diffusion of exudates from roots soaked in water. The impact of phenanthrene on maize morphology and functioning was measured prior to the analysis of the main components of the rhizodeposit solutions, by measuring total carbon, protein, amino acid, and sugars as well as by determining about 40 compounds using GC-MS and LC-MS. As maize exposure to PHE increased, different trends were observed in the two rhizodeposit solutions. In the diffusate solution, we measured a global increase of metabolites exudation like carbohydrates, amino acids, and proteins except for some monoglycerides and organic acids which exudation decreased in the presence of PHE. In the rhizospheric sand extract, we witnessed a decrease in carbohydrates and amino acids secretion as well as in fatty and organic acids when plants were exposed to PHE. Many of the compounds measured, like organic acids, carbohydrates, amino acids, or fatty acids, could directly or indirectly drive PAHs availability in soils with particular consequences for their degradation.