Arabidopsis MEB3 functions as a vacuolar transporter to regulate iron accumulation in roots
Iron is an essential nutrient for plant photosynthesis and development, but excess iron leads to stress. After absorption from the soil, plants store iron in roots and distribute it to shoots via long-distance transport. Vacuole serves as the iron storage organ in root cells, maintaining cellular iron homeostasis, and vacuolar iron transporter (VIT) family proteins have been identified as plant vacuolar iron transporters. However, the contribution of vacuolar iron transporters to the overall iron homeostasis of plants is not fully understood. Here, we show that MEMBRANE PROTEIN OF ER BODY 3 (MEB3), a VIT family member, is a vacuolar iron transporter involved in root-shoot iron distribution in Arabidopsis thaliana. Heterologous expression of Arabidopsis MEB3 in yeast restored the iron resistance phenotype of the vacuolar iron transporter deficient mutant ccc1, indicating that MEB3 regulates iron transport. In Arabidopsis, MEB3 was expressed in almost all tissues, albeit to higher levels in roots and seedlings, and the MEB3 protein localized to the tonoplast. At low iron concentration, meb3 knockout mutants accumulated less iron in shoots, suggesting that MEB3 promotes iron accumulation in shoots. Consistently, meb3 mutants exhibited reduced growth compared with the wild type upon transfer to iron-deficient medium. However, at high iron concentration, meb3 mutants accumulated more iron in shoots and less iron in roots than the wild type, indicating the impairment of proper iron distribution in meb3 mutants. These findings demonstrate that MEB3 is a vacuolar iron transporter involved in root-to-shoot iron distribution.