How root beneficial fungi modulate plant responses to insect herbivory: Insights from a transcriptome study in Plantago lanceolata

Not only insect-associated microbes, but also plant-associated microbes play an important role in plant-insect interactions. Inoculation of plants with beneficial microbes is increasingly recognized as a powerful tool to enhance plant defense against insect pests. Arbuscular Mycorrhizal (AM) fungi can sensitize the plant’s immune system, resulting in faster or stronger activation of jasmonic acid (JA)- signalled defences against leaf-chewing insects (priming). However, recent studies show wide variation in the effects of AM fungi on plant defense. We therefore studied how AM fungi affect a broader range of processes involved in plant-herbivore interactions.
Plantago lanceolata plants were subjected to a factorial combination of microbial inoculation (AM vs. mock) and herbivory (leaf herbivory by Spodoptera exigua vs. undamaged controls). Leaf and root samples were processed for RNA sequencing to analyse effects on plant gene expression and identify genes affected by both AMF and herbivory.
The fungus enhanced transcription of genes involved in abiotic and biotic stress tolerance in the roots, but did not prime plants for JA-signalled shoot defenses. On the contrary, the AM fungus suppressed induction of shoot terpenoids involved in direct and indirect defense and suppressed storage of carbon and nitrogen in shoots and roots, thereby negating the process of herbivore-induced resource sequestration, a common plant response to herbivory.
We conclude that plant microbes have multifaceted effects on the plant’s responses to herbivory, contributing to their broad range of effects on plant defense, which should be taken into account when developing products intended for microbially based pest control.