Fungal endophytes help grasses to tolerate sap-sucking herbivores through a hormone signaling system

Background: Plants possess a sophisticated hormone signaling system, which includes salicylic acid (SA) and jasmonic acid (JA), to defend itself from herbivores. In addition, this immune system is modulated by nonpathogenic microbes that live asymptomatically within intercellular spaces of host grasses. We investigated the resistance triggered by defense hormones, and that provided by endophyte Epichloë gansuensis against aphid Rhopalosiphum padi, and how the endophyte regulates this tripartite interaction. Methods: Achnatherum inebrians plants, with and without the fungal endophyte Epichloё gansuensis, were grown in the greenhouse of the College of Pasture Agriculture Science and Technology, Yuzhong campus of Lanzhou University (temperature: 24±2 °C, moisture: 76±2% photoperiod:L16:D8 h), were subjected to exogenous application of SA (5 mM, 10 mM, 15 mM) and aphid Rhopalosiphum padi infestation (5 A./plant, 10 A./plant, 15 A./plant). Result: Endophyte induced plant tolerance to the aphid feeding and exogenous SA increased plant shoot biomass, root biomass, plant length and chlorophyll content. Endophyte colonization induces WRKY54 factor that overrides negative effects on plant growth and possibly suppresses SA accumulation. In addition, the endophyte promotes a level of JA that is antagonistic to the SA pathway. By disabling the SA mechanism for herbivore-mediated plant growth inhibition, the endophyte induces plant tolerance to herbivory.