Understanding the mechanism of action of stress-acclimatized rhizospheric microbiome towards salinity stress mitigation in Vigna radiata: A focus on the emission of volatiles

Microbiome-based rhizospheric engineering has emerged as a promising approach towards attaining agricultural sustainability. While the top-down approach of acclimatizing the rhizospheric microbiome to stressors is relatively easy to perform, the mechanisms involved in plant growth promotion by directed evolution are yet to be understood. The present study aimed to understand the mechanism of stress management by a salt stressacclimatized microbiome, generated through indirect plant-mediated selection over multiple plant (Vigna radiata) growth cycles, with a focus on the emission of volatile compounds. A culture bank was generated by isolation of bacterial strains from the stress-acclimatised microbiome, using group-specific media. Characterization of the strains was performed by examining their potential plant growth-promoting (PGP) traits followed by phylogenetic affiliation using 16S rRNA sequencing. The strains exhibited strong tolerance to salinity stress when grown on artificial root exudate medium and V8 medium, at different salt concentrations. Based on PGP scoring, the eleven best performing bacterial strains were assessed for their ability to impact plant morphology and growth attributes in a split plate assay allowing only volatile compounds to be exchanged, using Arabidopsis as a model plant. An alteration in the seedling morphology of Arabidopsis was observed due to the production of volatile bioactive compounds by some of the strains. To delineate the specific volatiles produced by the bacterial strains responsible for such an impact on Arabidopsis seedlings, the volatiles emitted by selected strains were collected and analyzed by GC-MS. A correlation could be drawn between the specific volatiles produced by key bacterial members of the acclimatized microbiome e.g. Bacillus haynesii, Pseudomonas monteilii and Ochrobactrum soli, and their contribution towards alleviation of salinity stress in A. thaliana. A detailed analysis of the volatilomes emitted by stress-acclimatized bacterial strains revealed the identity of crucial metabolites, for instance hydrocarbons like 7 methyl-hexadecane, 2-(2-butoxyethoxy) ethanol or sulfur containing volatiles, which could at least partially contribute to plant growth promotion and stress tolerance. This work opens up new avenues for rhizospheric engineering as a way towards more resilient and sustainable crop production.