Microbe-induced plant resistance against insect pests depends on timing of inoculation, but is consistent across climatic conditions

To cope with abiotic and biotic stressors, plants have developed mutualistic associations with beneficial soil microbes, but little is known about how (extreme) abiotic conditions impact on microbe-induce resistance to insect herbivores. Extreme temperatures are often accompanied by extremes in plant water availability, which together reduce plant growth and change plant physiology. There are potential consequences for increasing plant susceptibility to biotic stresses, and this poses a real challenge for plant productivity. We evaluated how the effects of beneficial soil bacteria ( Acidovorax radicis ) on barley plant growth and resultant resistance against aphid infestation ( Sitobion avenae ) were impacted by a single heatwave event across a plant water availability gradient. We also tested if timing of bacterial inoculation (before or after the heatwave) affected bacteria-plant interactions on aphids. We found that heatwaves affected plant biomass allocation from aboveground to belowground tissues. Inoculation with A. radicis led to reduction of aphid numbers, but depended on timing of inoculation, and led to stronger resistance when inoculations occurred closer to aphid infestation. Remarkably, microbe-induced resistance against aphids was consistent across heatwave and water availability treatments. This study provides evidence that beneficial plant-bacteria interactions may represent a potential solution for sustainable agricultural practices to enhance plant growth and response to insect pests under climate change. Future field trials should investigate the consistency of beneficial effects reported here for a better understanding of multispecies interactions in the context of global change.