Halophilic Plant Growth Promoting Rhizobacteria as Producers of Antifungal Metabolites under Salt Stress

Abstract

Salinity is one of the major factors responsible for the deterioration of soils, making them unsuitable for agriculture. The application of halotolerant and halophilic plant growth promoting bacteria (PGPR: Plant Growth Promoting Rhizobacteria) with biological control activities as an inoculant of cultivated plants offers a biological alternative to the use of agrochemicals, particularly when subjected to salt stress. In this perspective, 70 bacterial strains were isolated from saline soils (sebkha) in arid and semi-arid areas of eastern Algeria. Three isolates were selected based on their ability to produce bioactive molecules allowing them to promote plant growth, such as hydrolytic enzymes, indole acetic acid, HCN, NH3, etc. Two of these isolates belong to the genus Serratia and the third is a halophilic Halomonas bacteria. These bacteria were identified based on the 16S rDNA sequence. Antagonism tests against phytopathogenic fungi were carried out. The identification of the antifungal molecules produced by these bacteria was done by high performance liquid chromatography. These bacteria are capable of inhibiting mycelial development against phytopathogenic fungi with rates reaching 80.67% against Botrytis cinerea, 76.22% against Aspergillus niger and 66.67% against Fusarium culmorum for Serratia sp. The strain Halomonas sp. inhibited mycelial growth by production of volatile substances of Aspergillus niger at 71.29%, Aspergillus flavus at 75.49% and Penicillium glabrum at a rate of 72.22%. The identification by HPLC of the antifungal molecules produced by these three bacteria reveals that they are polyphenols, which makes these strains the first rhizobacteria capable of producing phenolic compounds. Finally, pot tests to determine the effectiveness of these strains in promoting wheat growth under salinity stress (125 mM, 150 mM and 200 mM) were carried out. The results revealed that a consortium of two isolates (Serratia sp. and Halomonas sp.) performed best at 125 mM. But, at higher concentrations it was the halophilic bacteria Halomonas sp which gave the best result. In all cases, there was a significant improvement in the growth of wheat seedlings inoculated with the bacteria, compared to non-inoculated controls.