FIGURE 1.

Visual dissection* of the events during AMF colonization and virus infection: changes in hormones levels and related processes may enhance (left) or limit (right) viral infection leading to the final outcome of the complex tripartite interaction. In the early stage of mycorrhization, the increase of salicylic acid (SA) induces the priming of SA-dependent defenses, the major defensive pathway against viruses, and enhances the siRNA-mediated antiviral silencing. At the same time, abscisic acid (ABA) increases with both positive and negative consequences on plant defenses: it induces callose deposition on plasmodesmata, limiting cell-to-cell movement, suppresses SA signaling transduction, thus inhibiting defenses controlled by this pathway and weakens siRNA-antiviral system. In the late AMF colonization stage, the increase of jasmonate (JA) and ethylene (ET) induces the priming of JA- and ET-dependent defenses. JA has been shown to reduce viral symptoms at early infection stage but increase susceptibility in late infection stage; on the other hand, JA treatment decreases viral titre during geminivirus infection. ET antagonizes the pathway downstream the SA signaling and may be involved in symptom development, viral systemic movement and formation of necrotic lesions. However, spraying plants with the ET precursor 1-aminocyclopropane-l-carboxylic acid may reduce viral titre. Finally, mycorrhizal plants have been shown to improve aphid survival and increase attractiveness toward aphids parasitoids. PR: pathogenesis-related proteins, HR: hypersensitive response, ROS: reactive oxygen species, brown hexagones indicate viral particles (Lozano-Durán et al., 2011; Cameron et al., 2013; Alazem and Lin, 2015; Volpe et al., 2018).