Changes in Root Exudates and Root Proteins in Groundnut-Pseudomonas sp. Interaction Contribute to Root Colonization by Bacteria and Defense Response of the Host

Selection and application of rhizobacteria, for improved plant health will benefit from a complete understanding of the plant-bacteria interaction. Root exudates (REs) are known to contain signal molecules that facilitate beneficial association of plants with microbes. We have selected a tentatively identified Pseudomonas sp. (RP2), from 126 groundnut (Arachis hypogaea L.)-associated bacterial isolates that significantly promoted growth of groundnut and also induced resistance against the stem rot pathogen Sclerotium rolfsii. REs were collected from 12 to 24days grown RP2-bacterized and non-bacterized plants and analyzed through gas chromatography coupled with mass spectrometer. Several organic acids, fatty acids, sugars, hydrocarbons, and alcohols were detected. In the untargeted multivariate analysis of the REs, relative content of eight compounds varied significantly on RP2 bacterization. Among these eight compounds, myristic acid, stearic acid, and palmitic acid, positively influenced the root colonization by RP2. Benzoic acid and salicylic acid, increased in RP2-bacterized REs, showed the highest growth inhibition of S. rolfsii. In root proteomics, 11 differentially expressed proteins were identified by 2D-gel electrophoresis followed by matrix-assisted laser desorption ionization-time of flight. Chitinase, thaumatin-like protein, ascorbate peroxidase, and glutathione S-transferase, known to have a role in plant defense against phytopathogens, were upregulated in RP2 interaction. Similarly, upregulation of enolase in roots is likely to improve plant growth in RP2-bacterized groundnut. We conclude that colonization of groundnut roots by RP2 resulted in exudation of metabolites that facilitated root colonization, suppressed fungal growth, promoted plant growth, and also increased the expression of defense-related proteins in the roots.