Biostimulants applied to maize seeds modulate the enzymatic activity and metaproteome of the rhizosphere

Biostimulants are organic substances that can be applied to leaves, seeds or soil for enhancing plant growth and optimize productivity also in condition of stress-induced limitations. In this work, we combined mass spectrometry-based soil metaproteomics with a 18 multiplex enzymatic assay to investigate the effects of two different biostimulants (B1 and B2) on the rhizosphere of four-leaf stage maize plants. The rhizosphere of biostimulant-treated samples resulted in an enhancement of enzymatic activity related to phosphorus and glucose processes, highly induced in B1 sample. More than 700 unique proteins for each sample, identified by liquid chromatography-high resolution mass spectrometry, revealed an increase, especially for B2, in the representation of Gene Ontology biological process categories related to carbohydrate, organic substance and phosphorus metabolism, broadly matching the enzymatic activities. Furthermore, KEGG database analysis showed that proteins involved in antibiotic resistance were more abundant in both biostimulant-treated samples. The biostimulants therefore increased at a different extent the activity of the bacterial community, in particular species beneficial to plant growth, as Bradyrhizobium japonicum, Hyalangium minuturn, Stenotrophomonas rhizophila, Variovorax paradoxes and Paenibacillus macerans, without changing the composition of the microbiota. Our investigation highlighted for the first time that biostimulants modified directly and positively the bacterial activity of the rhizosphere, suggesting that they would be ideal for organic and sustainable farming.