Pseudomonas putida and Pseudomonas fluorescens Influence Arabidopsis Root System Architecture Through an Auxin Response Mediated by Bioactive Cyclodipeptides

Plant growth-promoting rhizobacteria modulate root development through different mechanisms. This work was conducted to evaluate the effects of root colonization by Pseudomonas putida and Pseudomonas fluorescens in biomass production, lateral root formation, and activation of auxin signaling in Arabidopsis thaliana. Selected strains of P. putida and P. fluorescens were tested for modification of DR5::uidA, BA3::uidA and HS::AXR3NT-GUS auxin-related gene expression, and to promote root hair and lateral root formation in WT and tir1-1, tir1-1afb2-1afb3-1, arf7-1, arf19-1, arf7-1arf19-1, and rhd6 mutants. Production of cyclodipeptides with possible roles in auxin signaling was also determined in P. putida and P. fluorescens culture supernatants by gas chromatography-mass spectrometry. P. putida and P. fluorescens stimulated lateral root and root hair formation and increased plant biomass, which correlated with an induction of the auxin response. Genetic analyses suggested that growth promotion involves auxin signaling as tir1-1, tir1-1afb2-1afb3-1, arf7-1, arf19-1, and arf7-1arf19-1 mutants showed decreased lateral root response to inoculation and because P. putida and P. fluorescens restored root hair development in the rhd6 mutant. It was also found that these bacteria produce the cyclodipeptides cyclo(L-Pro-L-Val), cyclo(L-Pro-L-Phe), and cyclo(L-Pro-L-Tyr), which modulates auxin-responsive gene expression in roots. Our results suggest a role of cyclodipeptides for bacterial phytostimulation.