Halotolerance of Indigenous Fluorescent Pseudomonads in the Presence of Natural Osmoprotectants

Abstract

Background: Salinity is one of the major factors affecting agriculture. To grow in saline environments, bacteria and plants have to adjust their turgor pressure by accumulating compatible solutes as glycine betaine and proline. Inoculation of plants of economic interest, mainly wheat, by Plant Growth Promoting Rhizobacteria such as Pseudomonas species is an effective biological approach for the recovery of soils affected by salt. Methodology: The halotolerance of indigenous Pseudomonas strains was tested in the presence of high salt concentrations. Under these stress conditions, the effect of natural osmoprotectant molecules elaborated by the halophyte A. halimus was observed. Results: In this study, 3 Fluorescent pseudomonads were isolated from wheat rhizosphere and one from the endophyte of Atriplex halimus. They were identified as P. putida AF2, P. aeruginosa RB5, P. fluorescensRB13 and P. aeruginosa EH4; they exhibited good PGPR activities. The growth of the strains was stimulated in the presence of 100 and 300 mM of NaCl. P. fluorescens CHA0 was inhibited at 500 mM; the remaining strains were affected by 800 mM.  Exogenous supply of glycine betaine and proline alleviated the stress. The extract of the halophyte A. halimus restored the growth of 3 strains.  NaCl/ 900 mM was strongly inhibitor of all bacteria. The restoration of the growth of P. aeruginosa RB5 and P. aeruginosa EH4 by glycine betaine or proline was significant. No osmoprotectant molecule could overcome stress imposed by 1000 mM. Conclusion: On the basis of their halotolrance and their ability to use natural osmoprotectant to restore their growth, the PGP fluorescent pseudomonads strains tested could be applied as inoculants of wheat for sustainable agriculture in salty soils.