Plant Growth Promoting Rhizobacteria characterization. Dissecting the role of Beijerinckia fluminensis in reducing salt stress in A. thaliana

Beneficial interactions between plants and certain bacterial species have long been recognized as capable of resulting in growth enhancement and exerting protective action on economically important crops. In this study, six different bacterial strains (Paenarthrobacter ureafaciens, Beijerinckia fluminensis, Pseudomonas protegens, Arthrobacter sp., Arthrobacter defluii and Arthrobacter nicotinovorans) were characterized in terms of growth promotion in the model plant A. thaliana and antimicrobial activity against three phytopathogenic fungi in vitro (Aspergillus flavus, Fusarium proliferatum and Fusarium verticillioides). The tested strains were positive for some plant growth-promoting traits (PGPs), such as indol-3-acetic acid (IAA), 1-amino cyclopropane-1-carboxylate deaminase (ACC-deaminase), siderophore production, and phosphate solubilization. Analysis of bacterial broths by ultra-high-performance liquid chromatography-mass spectrometry (UPLC-MS) and liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS/MS) confirmed the presence of potential PGP compounds, such as several types of siderophores, including desferrioxamine B and salicylic acid. PVr_9 strain (Beijerinckia fluminensis), which has shown a higher growth-promoting activity in A. thaliana as well as significant antifungal activity against all phytopathogenic fungi used, was also tested for its ability to reduce salt stress in A. thaliana in vitro. PVr_9-inoculated seedlings showed greater resistance to the adverse effects of salt than uninoculated seedlings in terms of morphological features. Furthermore, plants inoculated with PVr_9 had low contents of ROS, 8-oxodG, proline, soluble sugars and ABA. In plants inoculated with PVr_9, a significant decrease in Na+ was also observed in leaves and a corresponding increase in roots compared to uninoculated plants. qPCR analysis performed to test the expression of some genes involved in the response to salt stress revealed an increase in the expression of SOS1 and NHX1 in PVr_9-inoculated plants subjected to salt stress, while a decrease in the expression of several MYB genes involved in the response to stress signals was observed compared to uninoculated plants. These results open the possibility of using Beijerinckia fluminensis as a valuable biofertilizer and biopesticide also in field trials on agronomically important crops.