Plant growth-promoting bacteria and nitrate availability: impacts on root development and nitrate uptake

Plant growth-promoting bacteria (PGPB) and NO3- availability both affect NO3- uptake and root architecture. The presence of external NO3- induces the expression of NO3- transporter genes and elicits lateral root elongation in the part of the root system exposed to the NO3- supply. By contrast, an increase in NO3- supply leads to a higher plant N status (low N demand), which represses both the NO3- transporters and lateral root development. The effects of PGPB on NO3- uptake and root development are similar to those of low NO3- availability (concomitant stimulation of NO3- uptake rate and lateral root development). The mechanisms responsible for the localized and long-distance regulation of NO3- uptake and root development by NO3- availability are beginning to be elucidated. By contrast, the signalling and transduction pathways elicited by the rhizobacteria remain totally unknown. This review will compare the effects of NO3- availability and PGPB on root morphogenesis and NO3- uptake, in order to determine whether interactions exist between the NO3--dependent and the PGPB-dependent regulatory pathways.