Arbuscular Mycorrhiza Modulates Iron Distribution and Vacuolar Iron Transporter Expression in Tomato, Whereas Iron Limitation Reduces Mycorrhization

Plants have evolved highly efficient strategies to maintain iron (Fe) homeostasis. In this study, we investigate the impact of arbuscular mycorrhizal (AM) symbiosis on the Fe-deficiency response and ionomic profile of tomato plants, as well as how Fe availability affects AM symbiosis. Fe deficiency and AM colonization both reduced shoot Fe concentrations, while root Fe concentrations increased in AM plants. Notably, Fe accumulated in cortical cells colonized by arbuscules. We further show that Fe deficiency reduces expression of AM-related tomato genes (SlEXO84, SlRAM1, SlAMT2.2 and SlPT4) and of the fungal RiEF1α gene. These findings indicate that Fe availability is crucial for sustaining AM colonization and symbiotic functionality. Under Fe-limiting conditions, AM symbiosis enhances the Strategy I Fe acquisition pathway (SlFRO1, SlIRT1), an effect not observed under Fe-sufficient conditions. Four vacuolar transporter genes of the VIT/VTL family were identified in the tomato genome. Yeast complementation assays revealed that SlVIT1, SlVTL1, and SlVTL2 function as dual Fe/Mn transporters, whereas SlVIT2 appears to function as a Mn transporter. The high Fe demand of AM symbiosis is supported by the reduced expression of SlVIT1 and SlVTL1 in mycorrhizal roots. Ionomic analysis shows that AM colonization partially alleviates Fe deficiency-induced nutrient imbalances, highlighting its contribution to improved mineral homeostasis under Fe stress.