The role of plant growth promoting rhizobacteria in sustainable and low-input graminaceous crop production

Plant growth stimulating rhizobacteria that improve the yield of graminaceous crops have been studied since the 1930s. Increases in crop yield have often been inconsistent, reflecting a lack of understanding by which PGPR exert their effects. Many PGPR are able to fix N2, which was initially assumed to boost crops by supplementing soil N. Subsequently, it became clear that for most free-living PGPR other mechanisms affecting root development and nutrient uptake can explain the increased crop yields. Endophytic bacteria have demonstrated some potential to contribute to the N budget of certain graminaceous crops but require more robust assessment of their potential. Here, we review the current state of our understanding of PGPR in graminaceous crop cultivation, identifying their potential contribution to more sustainable agricultural practices but also highlighting issues that need to be addressed before this technology can be appropriately assessed as a replacement for inorganic N addition.