Quorum Quenching Activity of the PGPR Bacillus subtilis UD1022 Influences the Legume Symbiont Sinorhizobium meliloti Quorum Sensing Activity, Amanda Rosier
From Lauren Mosesso
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The use of plant beneficial organisms derived from the plant microbiome is gaining interest in agriculture to decrease dependence on pesticide and fertilizer use. It is understood that the application of individual plant growth promoting rhizobacteria (PGPR) increases plant health. However, limited research efforts have investigated complex interactions occurring between multiple species of plant beneficial bacteria in the rhizosphere, and how the outcome of those associations influence their ability to benefit the plant. Sinorhizobium meliloti engages in a unique form of inter-species interaction with legumes (symbiosis) and relies on a complex intra-species communication system call ‘quorum sensing’ (QS) for efficient nodulation of the plant through the production of polysaccharide biofilms. Using a tri-trophic model system of the legume Medicago truncatula A17 Jemalong, its mutualistic symbiont S. meliloti strain Rm8530 (hereafter Rm8530), and the PGPR Bacillus subtilis UD1022 (hereafter UD1022), we found that dual inoculation led to no enhanced plant growth or nodulation. Expression and functional analysis of Rm8530 QS genes showed that a UD1022 extracellular element impacts Rm8530 QS controlled biofilm production. Further genetic analysis revealed UD1022 produces the metallo-beta-lactamase YtnP which cleaves the structure of QS signal molecules. This inactivation of QS signals is known as “quorum quenching” (QQ). We showed purified YtnP protein effectively inactivated pure QS molecules while UD1022 ytnP¯ mutants failed to degrade QS molecules as compared to WT UD1022. This specific QQ interaction may contribute to ineffective nodulation through interfering with Rm8530 symbiotically active biofilm production. This work emphasizes that interactions between the PGPRs may influence their individual associations and activities on the plant root and could have greater implications in altering the ability of the PGPRs to positively affect plant health.