Proteome and Physiological Characterization of Halotolerant Nodule Endophytes: The Case of Rahnella aquatilis and Serratia plymuthica

Bacterial endophytes are bacteria that colonize internal plant tissues, in some cases promoting plant growth and in other cases protecting the plant from biotic stresses, such as pathogen attack, or abiotic stresses, such as salt stress and the presence of pollutants such as heavy metals. Bacterial endophytes were isolated from pea and fava nodules. The strains were identified and characterized for plant beneficial activities (phosphate solubilization, indoloacetic acid synthesis, and siderophores) and salt tolerance. Based on these data, four strains of Rahnella aquatilis and three of Serratia plymuthica were selected. To shed light on the mechanisms underlying salt tolerance, the proteome of the two best-performing strains (Ra4 and Sp2) grown in the presence or absence of salt was characterized. The number of proteins expressed by the endophytes was higher in the presence of salt. The modulated proteome consisted of 302 (100 up-regulated, 202 down-regulated) and 323 (206 up-regulated, 117 down-regulated) proteins in Ra4 and Sp2, respectively. Overall, proteins involved in abiotic stress responses were up-regulated, while those involved in metabolism and flagellum structure were down-regulated. The main proteins up-regulated in Sp2 were the DsbA protein, which is required for sulfur-binding formation in periplasmic proteins, while in Ra4 they corresponded to the soluble fraction of ABC transporters, with a role in the uptake of compatible solutes. Our results demonstrated a conserved response to salt stress in the two taxonomically related species.