Water stress protection by the arbuscular mycorrhizal fungus Rhizoglomus irregulare involves physiological and hormonal responses in an organ-specific manner.

Arbuscular mycorrhizal fungi may alleviate water stress in plants. Although several protection mechanisms have already been described, little information is available on how these fungi influence the hormonal response to water stress at an organ-specific level. In this study, we evaluated the physiological and hormonal responses to water stress in above and below-ground tissues of the legume grass Trifolium repens colonized by the arbuscular mycorrhizal fungus Rhizoglomus irregulare. Plants were subjected to progressive water stress and recovery. Different leaf and root physiological parameters, as well as phytohormone levels, were quantified. Water-stressed mycorrhizal plants showed an improved water status and no photoinhibition compared to uncolonized individuals, while some stress markers like α-tocopherol and malondialdehyde content, an indicator of the extent of lipid peroxidation, transiently increased in roots, but not in leaves. Water stress protection exerted by mycorrhiza appeared to be related to a differential root-to-shoot redox signaling, probably mediated by jasmonates, and mycorrhization enhanced the production of the cytokinin trans-zeatin in both roots and leaves. Overall, our results suggest that mycorrhization affects physiological, redox and hormonal responses to water stress at an organ-specific level, which may eventually modulate the final protection of the host from water stress.