Systemic signaling in mycorrhizal and plant herbivore interactions

Abstract

Soil beneficial microbes are potential and recommendable biofertilizers instead of chemical fertilizers because they benefit plants in an economical and environmentally-friendly way. Apart from interacting with beneficial soil microbes, plants encounter biotic and abiotic stresses simultaneously or consecutively daily. Among these stress factors, insect herbivores are widespread biotic stresses leading to the loss of global crop production. In this thesis, I study systemic signaling pathways, such as jasmonic acid (JA), phosphate (Pi) and ethylene signaling pathways, in the tripartite interactions between an arbuscular mycorrhizal (AM) fungus (Rhizophagus irregularis), a legume plant (Medicago truncatula) and an insect herbivore (Spodoptera exigua). In chapter 3.1, I present a novel mechanism that AM fungi were involved in reshaping the plant defense strategy against herbivory. I found that AM symbiosis led to a strategy shift towards a tolerance-dominant pattern, and the AM-mediated Pi-uptake pathway played a vital role in this shift. On the one hand, AM symbiosis boosted Pi uptake and resulted in an over-compensatory plant growth upon herbivory. On the other hand, AM symbiosis facilitated herbivore performance, partly by counteracting the herbivore-induced JA production. To deeply understand the trade-off mechanism between growth and defense in plants, in chapter 3.2, I explored the relationship between JA and Pi signaling. First, the inhibition of root growth by MeJA was somewhat relieved by a chemical inhibitor jarin-1. Second, jarin-1 slightly improved Pi levels in leaves after herbivory. Third, I partially blocked the JA-Ile biosynthesis triggered by herbivory in leaves with the inhibitor jarin-1. Lastly, there was a reduced Pi starvation response in systemic leaves after jarin-1 and herbivore treatment. In chapter 3.3, I found the ethylene receptor MtEIN2 was probably involved in mycorrhizal-plant-herbivore interaction. Next, in chapter 6, MtEIN2 was further investigated to see if it participates in nitrate uptake pathway under low Pi or combined low Pi and nitrate conditions