The nature of the interactionAzospirillum-Arabidopsisdetermine the molecular and morphological changes in root and plant growth promotion

Plant growth promoting rhizobacteria influence host functional and adaptive traits via complex mechanisms that are just started to be clarified.Azospirillum brasilenseacts as a probiotic bacterium, but detailed information about its molecular mechanisms of phytostimulation is scarce. Three interaction systems were established to analyze the impact ofA. brasilenseSp245 on the phenotype ofArabidopsisseedlings, and underlying molecular responses were assessed under the following growth conditions: (1) direct contact of roots with the bacterium, (2) chemical communication via diffusible compounds produced by the bacterium, (3) signaling via volatiles.A. brasilenseSp245 improved shoot and root biomass and lateral root production in the three interaction systems assayed. Cell division, quiescent center, and differentiation protein reporters pCYCB1;1::GUS, WOX5::GFP, and pAtEXP7::GUS had a variable expression in roots depending of the nature of interaction. pCYCB1;1::GUS and WOX5::GFP increased with volatile compounds, whereas pAtEXP7::GUS expression was enhanced towards the root tip in plants with direct contact with the bacterium. The auxin reporter DR5::GUS was highly expressed with diffusible and volatile compounds, and accordingly, auxin signaling mutantspin3,slr1,arf7arf19, andtir1afb2afb3showed differential phytostimulant responses when compared with the wild type. By contrast, ethylene signaling was not determinant to mediate root changes in response to the different interactions, as observed using the ethylene-related mutantsetr1,ein2, andein3. Our data highlight the diverse effects by whichA. brasilenseSp245 improves plant growth and root architectural traits and define a critical role of auxin but not ethylene in mediating root response to bacterization.