Rhizosphere Tripartite Interactions and PGPR-Mediated Metabolic Reprogramming towards ISR and Plant Priming: A Metabolomics Review

SIMPLE SUMMARY: Plant growth-promoting rhizobacteria (PGPR) are rhizosphere-inhabiting eco-friendly bacteria that indirectly/directly enhance plant growth and development and are tipped as the ‘Messiah” of plant protection against environmental stresses. However, their large-scale use in agriculture is still minimal due to the limited knowledge on them. From a metabolomics perspective, we review the current knowledge on PGPR–plant chemical communication leading to plant priming/resistant induction, highlight complex tripartite chemical interactions in the rhizosphere, and show how metabolomics can help unravel these chemical communications. ABSTRACT: Plant growth-promoting rhizobacteria (PGPR) are beneficial microorganisms colonising the rhizosphere. PGPR are involved in plant growth promotion and plant priming against biotic and abiotic stresses. Plant–microbe interactions occur through chemical communications in the rhizosphere and a tripartite interaction mechanism between plants, pathogenic microbes and plant-beneficial microbes has been defined. However, comprehensive information on the rhizosphere communications between plants and microbes, the tripartite interactions and the biochemical implications of these interactions on the plant metabolome is minimal and not yet widely available nor well understood. Furthermore, the mechanistic nature of PGPR effects on induced systemic resistance (ISR) and priming in plants at the molecular and metabolic levels is yet to be fully elucidated. As such, research investigating chemical communication in the rhizosphere is currently underway. Over the past decades, metabolomics approaches have been extensively used in describing the detailed metabolome of organisms and have allowed the understanding of metabolic reprogramming in plants due to tripartite interactions. Here, we review communication systems between plants and microorganisms in the rhizosphere that lead to plant growth stimulation and priming/induced resistance and the applications of metabolomics in understanding these complex tripartite interactions.