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 in 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.

Data collected:

• AMF colonization - calculated in percentage from ink-stained plant roots
• Plant Biomass - plants were oven dried and then weighed
• Plant Height - plant growth or height was measured in the early weeks of the growth after AMF inoculation using a measuring scale that measured the shoot length
• Primary metabolite/chemical analysis - Carbon and Nitrogen ratio was analysed using Element analyzer, plant total phosphorus was analysed using ICP - dried plant material was used
• Relative gene expression - RNA was extracted from flash-frozen leaf material and qPCR was performed. Relative expression was calculated using the delta delta CT method.
• Caterpillar bioassay - weight of caterpillar feeding on plant was measured every 3rd day for 2 weeks. This data was then used for calculating relative growth rate. Mortality (if a caterpillar survived or died) was also noted. Caterpillar survival was analysed using survival analysis.

All data was analysed using R studio

Microsoft Excel