Phytohormones as integrators of environmental signals in the regulation of mycorrhizal symbioses

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. Contents Summary 1431 I. Introduction 1431 II. Hormonal balance regulates plant growth and defence 1432 III. Phytohormones control the arbuscular mycorrhizal symbiosis 1433 IV. Environmental conditions have an impact on the partner's interactions 1434 V. Mycorrhizal symbiosis alters hormone homeostasis in the host and its response to environmental challenges 1435 VI. Conclusions and outlook 1435 Acknowledgements 1435 References 1435