Evidence of Autoinducer-Dependent and -Independent Heterogeneous Gene Expression in Sinorhizobium fredii NGR234

Abstract

ABSTRACTPopulations of genetically identicalSinorhizobium frediiNGR234 cells differ significantly in their expression profiles of autoinducer (AI)-dependent and AI-independent genes. Promoter fusions of the NGR234 AI synthase genestraIandngrIshowed high levels of phenotypic heterogeneity during growth in TY medium on a single-cell level. However, adding very high concentrations ofN-(3-oxooctanoyl-)-l-homoserine lactone resulted in a more homogeneous expression profile. Similarly, the lack of internally synthesized AIs in the background of the NGR234-ΔtraIor the NGR234-ΔngrImutant resulted in a highly homogenous expression of the corresponding promoter fusions in the population. Expression studies with reporter fusions of the promoter regions of the quorum-quenching genesdlhRandqsdR1and the type IV pilus gene cluster located on pNGR234bsuggested that factors other than AI molecules affect NGR234 phenotypic heterogeneity. Further studies with root exudates and developingArabidopsis thalianaseedlings provide the first evidence that plant root exudates have strong effects on the heterogeneity of AI synthase and quorum-quenching genes in NGR234. Therefore, plant-released octopine appears to play a key role in modulation of heterogeneous gene expression.