I.II.III.IV.V.VI.References Summary For survival, plants have to efficiently adjust their phenotype to environmental challenges, finely coordinating their responses to balance growth and defence. Such phenotypic plasticity can be modulated by their associated microbiota. The widespread mycorrhizal symbioses modify plant responses to external stimuli, generally improving the resilience of the symbiotic system to environmental stresses. Phytohormones, central regulators of plant development and immunity, are instrumental in orchestrating plant responses to the fluctuating environment, but also in the regulation of mycorrhizal symbioses. Exciting advances in the molecular regulation of phytohormone signalling are providing mechanistic insights into how plants coordinate their responses to environmental cues and mycorrhizal functioning. Here, we summarize how these mechanisms permit the fine‐tuning of the symbiosis according to the ever‐changing environment.

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The response of plants to arbuscular mycorrhizal fungi involves a temporal and spatial activation of different defence mechanisms. The activation and regulation of these defences have been proposed to play a role in the maintenance of the mutualistic status of the association, however, how these defences affect the functioning and development of arbuscular mycorrhiza remains unclear. A number of regulatory mechanisms of plant defence response have been described during the establishment of the arbuscular mycorrhizal symbiosis, including elicitor degradation, modulation of second messenger concentration, nutritional and hormonal plant defence regulation, and activation of regulatory symbiotic gene expression. The functional characterization of these regulatory mechanisms on arbuscular mycorrhiza, including cross-talk between them, will be the aim and objective of future work on this topic.

Summary• We investigated the relationship between ABA and ethylene regulating the formation of the arbuscular mycorrhiza (AM) symbiosis in tomato (Solanum lycopersicum) plants and tried to define the specific roles played by each of these phytohormones in the mycorrhization process.• We analysed the impact of ABA biosynthesis inhibition on mycorrhization by Glomus intraradices in transgenic tomato plants with an altered ethylene pathway. We also studied the effects on mycorrhization in sitiens plants treated with the aminoethoxyvinyl glycine hydrochloride (AVG) ethylene biosynthesis inhibitor and supplemented with ABA. In addition, the expression of plant and fungal genes involved in the mycorrhization process was studied.• ABA biosynthesis inhibition qualitatively altered the parameters of mycorrhization in accordance with the plant's ethylene perception and ethylene biosynthesis abilities. Inhibition of ABA biosynthesis in wild-type plants negatively affected all the mycorrhization parameters studied, while tomato mutants impaired in ethylene synthesis only showed a reduced arbuscular abundance in mycorrhizal roots. Inhibition of ethylene synthesis in ABA-deficient sitiens plants increased the intensity of mycorrhiza development, while ABA application rescued arbuscule abundance in the root's mycorrhizal zones.• The results of our study show an antagonistic interaction between ABA and ethylene, and different roles of each of the two hormones during AM formation. This suggests that a dual ethylene-dependent ⁄ ethylene-independent mechanism is involved in ABA regulation of AM formation.

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