The Short-Term Effect of Microplastics in Lettuce Involves Size- and Dose-Dependent Coordinate Shaping of Root Metabolome, Exudation Profile and Rhizomicrobiome

The impact of four different particle sizes of polyethylene plastics, including microplastics and nanoplastics (MNPs) applied at four different concentrations was studied using lettuce (Lactuca sativa L.) as a model plant. The impact of MNPs was investigated with a focus on the tripartite interaction between the root metabolic process, exudation pattern, and rhizosphere microbial modulation using a multi-omics integrated approach. Our results showed that particle sizes and their concentrations significantly and distinctively modulated plant processes.Untargeted metabolomics suggested that the significantly affected pathways were fatty acids, amino acids, and hormones biosynthesis. Additionally, a related exudation of phenylpropanoids was observed. Rhizosphere bacterial α-diversity was reduced in a size-dependent trend for specific taxa. The omics data integration highlighted a correlation between Proteobacteria and Actinobacteria phyla and Bacillaceae family (Peribacillus simplex) and the exudation of flavonoids, phenolic acids, and lignans in lettuce exposed to increasing sizes of MNPs. This study provides a novel insight into the potential effects of different particle sizes and concentrations of MNPs on the soil-plant continuum, providing evidence about size- and concentration-dependent effects, suggesting the need for further investigation focused on medium- to long-term exposure.