The adaptive metabolomic profile and functional activity of tomato rhizosphere are revealed upon PGPB inoculation under saline stress

Salinization of agricultural lands is of increasing concern at global scale due to its negative impacts on several physiological and developmental processes of most crops, that in turn can lead to threatening food security. Salt-tolerant plant growth-promoting bacteria (PGPB) represent an emerging strategy to protect plants and enhance the productivity of agroecosystems suffering from salinization. This study aimed at investigating the individual effect of two PGPB in alleviating salt-stressed tomato plants, as well as the rhizosphere functioning and metabolomic profiles under soil salinity and PGPB inoculation treatments. The Biolog Eco-Plate functional analysis and untargeted metabolomics approaches were used to this aim. Under saline conditions, plants inoculated with Pseudomonas 16S showed higher biomass (p < 0.05) than both uninoculated and Enterobacter 15S-inoculated plants, indicating that the former strain was efficient in alleviate the saline stress. Moreover, independently from PGPB inoculation, the microbial metabolism in tomato's rhizosphere was significantly increased by salinity, especially related to the catabolism of amines, carbohydrates, and polymers. The UHPLC/QTOF-MS approach allowed the identification of signature metabolomic patterns for the rhizosphere of tomato, with inflected accumulation of compounds imposed by either Pseudomonas 16S, Enterobacter 15S or saline stress. Crucial compounds from primary and secondary metabolism related to salinity stress or PGPB inoculation were detected, including low molecular weight osmolytes (e.g., amino acids), as well as anthocyanins, hydroxycinnamic acids, coumarins and saponins. An increase in the content of ROS-scavenging and antioxidant compounds in addition to the facilitation of Fe acquisition induced by Pseudomonas 16S, are suggested as mechanisms responsible for the higher biomass accumulation by tomato plants grown under saline stress.