Abstract
Plants have a variety of chemical and anatomical defences, whose strengths depend on biotic and environmental influences. We show here that root inoculation with belowground bacteria, filamentous gram-positive streptomycetes, can induce plant defence responses. Such induced plant responses can occur belowground in the roots, but also aboveground, in the leaves, and include priming (sensitizing) like characters. Streptomycetes have also evolved mechanisms to facilitate plant root symbioses, mycorrhiza and root nodulation. By promoting fungal growth and by decreasing plant defence responses, these bacteria promote mycorrhiza formation. This minireview covers our current knowledge on the complex interactions that take place between streptomycetes, plants and rhizosphere microbes.