Haymanti Saha

Plant protection with beneficial microbes is considered to be a promising alternative to chemical control of pests and pathogens. Beneficial microbes can boost plant defences via induced systemic resistance (ISR), enhancing plant resistance against future biotic stresses. Although the use of ISR-inducing microbes in agriculture seems promising, the activation of ISR is context-dependent: it often occurs only under particular biotic and abiotic conditions, thus making its use unpredictable and hindering its application. Although major breakthroughs in research on mechanistic aspects of ISR have been reported, ISR research is mainly conducted under highly controlled conditions, differing from those in agricultural systems. This forms one of the bottlenecks for the development of applications based on ISR-inducing microbes in commercial agriculture. We propose an approach that explicitly incorporates context-dependent factors in ISR research to improve the predictability of ISR induction under environmentally variable conditions. Here, we highlight how abiotic and biotic factors influence plant–microbe interactions in the context of ISR. We also discuss the need to raise awareness in harnessing interdisciplinary efforts between researchers and stakeholders partaking in the development of applications involving ISR-inducing microbes for sustainable agriculture.

Beneficial soil microbes can enhance plant growth and defense, but the extent to which this occurs depends on the availability of resources, such as water and nutrients. However, relatively little is known about the role of light quality, which is altered during shading, resulting a low red: far-red ratio (R:FR) of light. We examined how low R:FR light influences arbuscular mycorrhizal fungus (AMF)-mediated changes in plant growth and defense using Solanum lycopersicum (tomato) and the insect herbivore Chrysodeixis chalcites. We also examined effects on third trophic level interactions with the parasitoid Cotesia marginiventris. Under low R:FR light, non-mycorrhizal plants activated the shade avoidance syndrome (SAS), resulting in enhanced biomass production. However, mycorrhizal inoculation decreased stem elongation in shaded plants, thus counteracting the plant’s SAS response to shading. Unexpectedly, activation of SAS under low R:FR light did not increase plant susceptibility to the herbivore in either non-mycorrhizal or mycorrhizal plants. AMF did not significantly affect survival or growth of caterpillars and parasitoids but suppressed herbivore-induced expression of jasmonic acid-signaled defenses genes under low R:FR light. These results highlight the context-dependency of AMF effects on plant growth and defense and the potentially adverse effects of AMF under shading.