Plant Growth Promoting Rhizobacteria Are More Effective Under Water Stress: A Meta-Analysis.

Drought stress reduces crop yields worldwide. Plant growth promoting rhizobacteria (PGPR) may improve drought tolerance while reducing irrigation demand, but the extent of this benefit has not been quantitatively summarized. Using meta-analysis, we synthesized the effects of PGPR inoculation on plant performance under drought and well-watered conditions. We addressed: 1) to what degree do PGPR increase root mass, shoot mass and yield? 2) Do PGPR ameliorate the negative effects of drought stress? and 3) Which moderators introduce variation in plant response to inoculation? Across all treatments, plants were highly responsive to PGPR: root mass increased by 35%, shoot mass increased by 38% and yield increased by 29%. Furthermore, PGPR ameliorated the impacts of drought on root mass by 15%, shoot mass by 23%, and yield by 17%. PGPR responsiveness also varied according to plant functional group. We demonstrate that PGPR are highly effective for improving plant growth, with a greater effect under drought. This study illuminates the potential for PGPR to enhance food security, water conservation and sustainability.